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Amplitude modulation (AM) is a form of modulation in which the amplitude of a carrier wave is varied in direct proportion to that of a modulating signal. (Contrast this with frequency modulation, in which the frequency of the carrier is varied; and phase modulation, in which the phase is varied.) AM is commonly used at radio frequencies and was the first method used to broadcast commercial radio. The term "AM" is sometimes used generically to refer to the AM broadcast (mediumwave) band (see AM radio). Ampli ... Including: Amplitude modulation - Applications in radio o Amplitude modulation - AM vs. FM Amplitude modulation - Forms of AM Amplitude modulation - Example o Amplitude modulation - A more general example Amplitude modulation - Modulation index Amplitude modulation - Amplitude modulator designs o Amplitude modulation - Circuits o Amplitude modulation - Low level o Amplitude modulation - High level Read more here: » Amplitude modulation: Encyclopedia - Amplitude modulation amplitude modulation - applications in radio: Encyclopedia II - Amplitude modulation - Applications in radio A basic AM radio transmitter works by first DC-shifting the modulating signal,

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Page 1: Amplitude modulation

Amplitude modulation (AM) is a form of modulation in which the amplitude of a carrier wave is varied in direct proportion to that of a modulating signal. (Contrast this with frequency modulation, in which the frequency of the carrier is varied; and phase modulation, in which the phase is varied.) AM is commonly used at radio frequencies and was the first method used to broadcast commercial radio. The term "AM" is sometimes used generically to refer to the AM broadcast (mediumwave) band (see AM radio). Ampli ...

Including:

Amplitude modulation - Applications in radio o Amplitude modulation - AM vs. FM

Amplitude modulation - Forms of AM Amplitude modulation - Example

o Amplitude modulation - A more general example Amplitude modulation - Modulation index Amplitude modulation - Amplitude modulator designs

o Amplitude modulation - Circuits o Amplitude modulation - Low level o Amplitude modulation - High level

Read more here: » Amplitude modulation: Encyclopedia - Amplitude modulation

amplitude modulation - applications in radio: Encyclopedia II - Amplitude modulation - Applications in radio A basic AM radio transmitter works by first DC-shifting the modulating signal, then multiplying it with the carrier wave using a frequency mixer. The output of this process is a signal with the same frequency as the carrier but with peaks and troughs that vary in proportion to the strength of the modulating signal. This is amplified and fed to an antenna. Amplitude modulation - AM vs. FM. AM radio's main limitation is its susceptibility to atmospheric interference, which is heard as static from the receive ...

See also:Amplitude modulation, Amplitude modulation - Applications in radio, Amplitude modulation -

Page 2: Amplitude modulation

AM vs. FM, Amplitude modulation - Forms of AM, Amplitude modulation - Example, Amplitude modulation - A more general example, Amplitude modulation - Modulation index, Amplitude modulation - Amplitude modulator designs, Amplitude modulation - Circuits, Amplitude modulation - Low level, Amplitude modulation - High level

Read more here: » Amplitude modulation: Encyclopedia II - Amplitude modulation - Applications in radio

amplitude modulation - applications in radio: Encyclopedia - Automatic gain control Automatic gain control (AGC) is an electronic system found in many types of devices. Its purpose is to control the gain of a system in order to maintain some measure of performance over a changing range of real world conditions. A very common and typical example is the AGC used in AM radio. Such a receiver is essentially linear - that is, the output is proportional to the input. This is a necessary requirement because the information content of the signal is carried by the changes of amplitude of the carrier frequency. If the c ...

Read more here: » Automatic gain control: Encyclopedia - Automatic gain control

amplitude modulation - applications in radio: Encyclopedia - Very low frequency Very low frequency or VLF refers to radio frequencies (RF) in the range of 3 to 30 kHz. Since there is not much bandwidth in this band of the radio spectrum, only the very simplest signals are used, such as for radionavigation. Because VLF waves can penetrate water only to a depth of roughly 10 to 40 metres (30 to 130 feet), depending on the frequency and the salinity of the water, they are used to communicate with submarines nea ...

Including:

Very low frequency - Details of VLF submarine communication methods Very low frequency - PC-based VLF reception Very low frequency - List of VLF transmitters

Read more here: » Very low frequency: Encyclopedia - Very low frequency

Page 3: Amplitude modulation

amplitude modulation - applications in radio: Encyclopedia - Orthogonal frequency-division multiplexing Orthogonal frequency-division multiplexing (OFDM), also sometimes called discrete multitone modulation (DMT), is a transmission technique based upon the idea of frequency-division multiplexing (FDM). Orthogonal frequency-division multiplexing - Characteristics. An OFDM carrier signal is the sum of a number of orthogonal sub-carriers, with baseband data on each sub-carrier being independently modulated commonly using some type of quadrature amplitude modulation (QAM) or phase-shift keyin ...

Including:

Orthogonal frequency-division multiplexing - Characteristics o Orthogonal frequency-division multiplexing - Benefits o Orthogonal frequency-division multiplexing - Disadvantages of OFDM

Orthogonal frequency-division multiplexing - OFDM feature abstract Orthogonal frequency-division multiplexing - Usage

o Orthogonal frequency-division multiplexing - ADSL o Orthogonal frequency-division multiplexing - Wireless LAN o Orthogonal frequency-division multiplexing - Digital radio and television o Orthogonal frequency-division multiplexing - DVB-T's implementation of

COFDM o Orthogonal frequency-division multiplexing - DRM and Eureka-147's DAB

implementation of COFDM o Orthogonal frequency-division multiplexing - Ultra wideband o Orthogonal frequency-division multiplexing - Flash-OFDM o Orthogonal frequency-division multiplexing - BST-OFDM

Orthogonal frequency-division multiplexing - Ideal encoder Orthogonal frequency-division multiplexing - Mathematical Description Orthogonal frequency-division multiplexing - OFDM history

Read more here: » Orthogonal frequency-division multiplexing: Encyclopedia - Orthogonal frequency-division multiplexing

amplitude modulation - applications in radio: Encyclopedia - Audio level compression

Page 4: Amplitude modulation

Audio level compression, also called compression or limiting, is a process that manipulates the dynamic range of an audio signal. Compression is used in sound recording and live sound reinforcement fields to improve the perceived quality of audio. (This should not be confused with audio data compression, which reduces the data size of digital audio signals.) A compressor is the device used to create compression. Audio level compression - Controls. A compressor reduces the dynamic range ...

Including:

Audio level compression - Controls Audio level compression - Limiting Audio level compression - Side-chaining Audio level compression - Multiband compression Audio level compression - Common uses Audio level compression - Underlying electronics Audio level compression - Other uses

Read more here: » Audio level compression: Encyclopedia - Audio level compression

amplitude modulation - applications in radio: Encyclopedia - Alternator An alternator is an electromechanical device that converts mechanical energy to alternating current electrical energy. Most alternators use a rotating magnetic field. Different geometries - such as a linear alternator for use with stirling engines - are also occasionally used. In principle any AC generator can be called an alternator, but usually the word refers to small rotating machines driven by automotive and other internal combustion engines. Alternator - History. Alternating current generating ...

Including:

Alternator - History Alternator - Theory of operation Alternator - Automotive alternators Alternator - Radio alternators

Page 5: Amplitude modulation

Alternator - External articles and futher reading

Read more here: » Alternator: Encyclopedia - Alternator

amplitude modulation - applications in radio: Encyclopedia - Amateur Radio Direction Finding Amateur Radio Direction Finding is an amateur map and compass sport that combines the skills of orienteering and radio direction finding. It is a timed race in which individual competitors use a topographic map and a magnetic compass to navigate through diverse, wooded terrain while searching for radio transmitters. The rules of the sport and international competititions are organized by the International Amateur Radio Union. World wide, the sport is most often refered to by its English language acronym, ARDF, but ...

Including:

Amateur Radio Direction Finding - History Amateur Radio Direction Finding - Description of competition and rules

o Amateur Radio Direction Finding - Entry categories o Amateur Radio Direction Finding - Youth competitions o Amateur Radio Direction Finding - Local variations

Amateur Radio Direction Finding - Map and course details Amateur Radio Direction Finding - Equipment and clothing

o Amateur Radio Direction Finding - Transmitter equipment o Amateur Radio Direction Finding - Receiver equipment o Amateur Radio Direction Finding - Clothing o Amateur Radio Direction Finding - Other equipment

Amateur Radio Direction Finding - Variations

Read more here: » Amateur Radio Direction Finding: Encyclopedia - Amateur Radio Direction Finding

amplitude modulation - applications in radio: Encyclopedia - Amateur radio

Page 6: Amplitude modulation

Amateur radio, often called ham radio, is a hobby enjoyed by many people throughout the world. An amateur radio operator, ham, or radio amateur uses two-way radio to communicate with other radio amateurs, for recreation or self-edification. As of 2004 there were about 3 million hams worldwide with about 700,000 in the USA, 600,000 in Japan, 140,000 each in South Korea and Thailand, 57,000 in Canada, 70,000 in Germany, 60,000 in UK, 11,000 in Sweden, and 5,000 in Norway. Amateur radio - ...

Including:

Amateur radio - History Amateur radio - Amateur Radio Activities and Practices

o Amateur radio - Emergency and public service communications o Amateur radio - DXing QSL cards and awards o Amateur radio - Contesting o Amateur radio - Vintage Radio o Amateur radio - VHF UHF and microwave weak-signal operation o Amateur radio - Portable operations o Amateur radio - Low power operations

Amateur radio - Amateur radio licensing o Amateur radio - US Licensing o Amateur radio - International operation o Amateur radio - Privileges of the Amateur

Amateur radio - Governance and amateur radio societies o Amateur radio - Band plans and frequency allocations

Amateur radio - Amateur radio in popular culture Amateur radio - Publications

Read more here: » Amateur radio: Encyclopedia - Amateur radio

amplitude modulation - applications in radio: Encyclopedia - Sound effect For the album, see Sound Affects. Sound effects or audio effects are artificially created or enhanced sounds, or sound processes used to emphasize artistic or other content of movies, video games, music, or other media. In motion picture and television production, a sound effect is a sound recorded and presented to make a specific storytelling or creative point without the use of dialogue or music. The term often refers to a process applied to a recording, without

Page 7: Amplitude modulation

necessarily referring to the recordi ...

Including:

Sound effect - History Sound effect - In film Sound effect - In video games Sound effect - Recording effects Sound effect - Processing effects Sound effect - Aesthetics in film Sound effect - Techniques

Read more here: » Sound effect: Encyclopedia - Sound effect

amplitude modulation - applications in radio: Encyclopedia II - Frequency modulation - Theory If the signal to be transmitted is which is restricted in amplitude to be and the sinusoidal carrier is where fc is the carrier's base frequency in hertz and A is an arbitrary amplitude, the carrier will be modulated by the signal as in where, f(t) = fc + fΔx ...

See also:Frequency modulation, Frequency modulation - Applications in radio, Frequency modulation - Theory, Frequency modulation - Modulation Index

Read more here: » Frequency modulation: Encyclopedia II - Frequency modulation - Theory

amplitude modulation - applications in radio: Encyclopedia II - Frequency modulation - Applications in radio Edwin Armstrong presented his paper: "A Method of Reducing Disturbances in Radio Signaling by a System of Frequency Modulation", which first described FM radio, before the New York section of the Institute of Radio Engineers on November 6, 1935. Frequency modulation requires a wider bandwidth than amplitude modulation by an equivalent modulating signal, but this also makes the signal more robust against interference. Frequency modulation is also

Page 8: Amplitude modulation

more robust against simple signal amplitude fading phenomena. As a result, FM was chosen a ...

See also:Frequency modulation, Frequency modulation - Applications in radio, Frequency modulation - Theory, Frequency modulation - Modulation Index

Read more here: » Frequency modulation: Encyclopedia II - Frequency modulation - Applications in radio

amplitude modulation - applications in radio: Encyclopedia II - Rectifier - Applications One of the first applications of rectifiers was detection of amplitude modulated radio signals by a diode. In early Crystal radio receivers the diode was a simple piece of semiconductive mineral. Rectifier - Power transmission. AC is used for current transmission because it can easily be stepped up or down in voltage by a simple transformer. High voltage power lines transmit the same power at lower current (which causes lower heat) and it is then stepped down by substation transformers to the more manageab ...

See also:Rectifier, Rectifier - 'Half-wave' rectification, Rectifier - 'Full-wave' rectification, Rectifier - Applications, Rectifier - Power transmission, Rectifier - Rectifier output smoothing, Rectifier - Rectification efficiency

Read more here: » Rectifier: Encyclopedia II - Rectifier - Applications

amplitude modulation - applications in radio: Encyclopedia II - Radio control - Modern military and aerospace applications Remote control military applications are typically not radio control in the direct sense, directly operating flight control surfaces and propulsion power settings, but instead take the form of instructions sent to a completely autonomous, computerized automatic pilot. Instead of a "turn left" signal that is applied until the aircraft is flying in the right direction, the system sends a single instruction that says "fly to this point". The most outstanding example of remote r ...

Page 9: Amplitude modulation

See also:Radio control, Radio control - History, Radio control - Military applications in the Second World War, Radio control - Radio-controlled models, Radio control - Modern military and aerospace applications, Radio control - Industrial control

Read more here: » Radio control: Encyclopedia II - Radio control - Modern military and aerospace applications

amplitude modulation - applications in radio: Encyclopedia II - Quadrature amplitude modulation - Overview As with all modulation schemes, QAM conveys data by changing some aspect of a base signal, the carrier wave, (usually a sinusoid) in response to a data signal. In the case of QAM, the amplitude of two quadrature waves is changed (modulated or keyed) to represent the data signal. Phase modulation (analogue PM) and phase-shift keying (digital PSK) can be regarded as a special case of QAM, where the amplitude of the modulating signal is constant, with only the phase varying. This can also be extended to frequency modulation (FM) and frequency-shift keying (FSK), as ...

See also:Quadrature amplitude modulation, Quadrature amplitude modulation - Overview, Quadrature amplitude modulation - Ideal structure, Quadrature amplitude modulation - Transmitter, Quadrature amplitude modulation - Receiver, Quadrature amplitude modulation - Performance, Quadrature amplitude modulation - Definitions, Quadrature amplitude modulation - Rectangular QAM, Quadrature amplitude modulation - Odd-k QAM, Quadrature amplitude modulation - Non-rectangular QAM

Read more here: » Quadrature amplitude modulation: Encyclopedia II - Quadrature amplitude modulation - Overview

amplitude modulation - applications in radio: Encyclopedia II - Quadrature amplitude modulation - Ideal structure Quadrature amplitude modulation - Transmitter. The following picture shows the ideal structure of a QAM transmitter: First the flow of bits to be transmitted is split into two equal parts: this process generates two independent signals to be transmitted. They are encoded separately

Page 10: Amplitude modulation

just like they were in an ASK modulator. Then one channel (the one "in phase") is multiplied by a cosine, while the other channel ("in quadrature") is multiplied by a sine. This way there is a phase of 90° between them. They are simply added one to the other and sent through the real channel.

See also:

Quadrature amplitude modulation, Quadrature amplitude modulation - Overview, Quadrature amplitude modulation - Ideal structure, Quadrature amplitude modulation - Transmitter, Quadrature amplitude modulation - Receiver, Quadrature amplitude modulation - Performance, Quadrature amplitude modulation - Definitions, Quadrature amplitude modulation - Rectangular QAM, Quadrature amplitude modulation - Odd-k QAM, Quadrature amplitude modulation - Non-rectangular QAM

Read more here: » Quadrature amplitude modulation: Encyclopedia II - Quadrature amplitude modulation - Ideal structure

amplitude modulation - applications in radio: Encyclopedia II - Radio control - Military applications in the Second World War Radio control was further developed during World War II, primarily by the Germans who used it in a number of missile projects. Their main effort was the development of radio-controlled missiles and glide bombs for use against shipping, a target that is otherwise both difficult and dangerous to attack. However by the end of the war the Luftwaffe was having similar problems attacking allied bombers, and developed ...

See also:Radio control, Radio control - History, Radio control - Military applications in the Second World War, Radio control - Radio-controlled models, Radio control - Modern military and aerospace applications, Radio control - Industrial control

Read more here: » Radio control: Encyclopedia II - Radio control - Military applications in the Second World War

amplitude modulation - applications in radio: Encyclopedia II - Orthogonal

Page 11: Amplitude modulation

frequency-division multiplexing - Mathematical Description The low-pass equivalent OFDM signal is expressed as where {Ik} are the data symbols, N is the number of subcarriers, and T is the OFDM block time. The subcarriers spacing of 1 / T Hz makes the subcarriers orthogonal; this property is ex ...

See also:Orthogonal frequency-division multiplexing, Orthogonal frequency-division multiplexing - Characteristics, Orthogonal frequency-division multiplexing - Benefits, Orthogonal frequency-division multiplexing - Disadvantages of OFDM, Orthogonal frequency-division multiplexing - OFDM feature abstract, Orthogonal frequency-division multiplexing - Usage, Orthogonal frequency-division multiplexing - ADSL, Orthogonal frequency-division multiplexing - HomePlug powerline alliance, Orthogonal frequency-division multiplexing - Wireless LAN, Orthogonal frequency-division multiplexing - Digital radio and television, Orthogonal frequency-division multiplexing - DVB-T's implementation of COFDM, Orthogonal frequency-division multiplexing - DRM and Eureka-147's DAB implementation of COFDM, Orthogonal frequency-division multiplexing - Ultra wideband, Orthogonal frequency-division multiplexing - Flash-OFDM, Orthogonal frequency-division multiplexing - BST-OFDM, Orthogonal frequency-division multiplexing - Ideal encoder, Orthogonal frequency-division multiplexing - Mathematical Description, Orthogonal frequency-division multiplexing - OFDM history

Read more here: » Orthogonal frequency-division multiplexing: Encyclopedia II - Orthogonal frequency-division multiplexing - Mathematical Description

amplitude modulation - applications in radio: Encyclopedia II - Klystron tube - Reflex klystron In the reflex klystron, the electron beam passes through a single resonant cavity. The electrons are fired into one end of the tube by an electron gun. After passing through the resonant cavity they are reflected by a negatively charged reflector electrode for another pass through the cavity, where they are then collected. The electron beam is velocity modulated when it first passes through the cavity. The formation of electron bunches takes place in the drift space between the reflector and the cavity. The voltage on the reflector mu ...

See also:Klystron tube, Klystron tube - Two-chamber klystron, Klystron tube - Reflex klystron, Klystron tube - Multicavity klystron, Klystron tube - Multibeam klystron, Klystron tube - Floating drift tube klystron, Klystron tube - Collector, Klystron tube - Applications

Read more here: » Klystron tube: Encyclopedia II - Klystron tube - Reflex klystron

Page 12: Amplitude modulation

amplitude modulation - applications in radio: Encyclopedia II - Electronics engineering - Subfields Electronics engineering has many subfields. This section describes some of the most popular subfields in electronics engineering. Although there are engineers who focus exclusively on one subfield, there are also many who focus on a combination of subfields. For more information on each of the following, click the read more... link. Electronics engineering - Overview of electronics engineering. Electronics engineering involves the design and testing of electronic circuits that use the electronic properties of components such as resistors, capacitors, inductors

Radio transmitter designFrom Wikipedia, the free encyclopediaJump to: navigation, search

This article has multiple issues. Please help improve it or discuss these issues on the talk page. It may contain an excessive amount of intricate detail which may only interest a specific audience. Tagged since August 2008. It needs to be updated. Tagged since August 2009.

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Page 13: Amplitude modulation

Contents[hide]

1 Methods 2 Frequency synthesis

o 2.1 Fixed frequency systems o 2.2 Variable frequency systems

3 Frequency multiplication 4 Frequency mixing and modulation

o 4.1 AM modes 4.1.1 Low level and high level

4.1.1.1 Low level 4.1.1.2 High level

4.1.2 Types of AM modulators 4.1.2.1 Plate AM modulators 4.1.2.2 Screen AM modulators

o 4.2 Other modes which are related to AM 4.2.1 Single-sideband modulation

4.2.1.1 Filter method 4.2.1.2 Phasing method

4.2.2 Vestigial-sideband modulation 4.2.3 Morse

o 4.3 FM modes 4.3.1 Direct FM 4.3.2 Indirect FM

5 RF power amplifiers o 5.1 Valves

5.1.1 Advantages of valves 5.1.2 Disadvantages of valves

o 5.2 Solid state 6 Linking the transmitter to the aerial 7 EMC matters

o 7.1 RF leakage (defective RF shielding) o 7.2 Spurious emissions

7.2.1 Harmonics 7.2.1.1 Avoiding harmonic generation 7.2.1.2 Removal of harmonics with filters 7.2.1.3 Detection

7.2.2 Local oscillators and unwanted mixing products 7.2.3 Instability and parasitic oscillations

8 See also

9 References

Page 14: Amplitude modulation

Radio transmitter design is a complex topic which can be broken down into a series of smaller topics. A radio communication system requires two tuned circuits each at the transmitter and receiver, all four tuned to the same frequency.[1] The transmitter is an electronic device which, usually with the aid of an antenna, propagates an electromagnetic signal such as radio, television, or other telecommunications.

[edit] MethodsAt the beginning of the 20th century, there were four chief methods of arranging the transmitting circuits:[2]

1. The transmitting system consists of two tuned circuits such that the one containing the spark-gap is a persistent oscillator; the other, containing the aerial structure, is a free radiator maintained in oscillation by being coupled to the first (Nikola Tesla and Guglielmo Marconi).

2. The oscillating system, including the aerial structure with its associated inductance-coils and condensers, is designed to be both a sufficiently persistent oscillator and a sufficiently active radiator (Oliver Joseph Lodge).

3. The transmitting system consists of two electrically coupled circuits, one of which, containing the air-gap, is a powerful but not persistent oscillator, being provided with a device for quenching the spark so soon as it has imparted sufficient energy to the other circuit containing the aerial structure, this second circuit then independently radiating the train of slightly damped waves at its own period (Oliver Joseph Lodge and Wilhelm Wien).

4. The transmitting system, by means either of an oscillating arc (Valdemar Poulsen) or a high-frequency alternator (Rudolf Goldschmidt), emits a persistent train of undamped waves interrupted only by being broken up into long and short groups by the operator's key.

[edit] Frequency synthesis

[edit] Fixed frequency systemsFor a fixed frequency transmitter one commonly used method is to use a resonant quartz crystal in a Crystal oscillator to fix the frequency. Where the frequency has to be variable, several options can be used.

[edit] Variable frequency systems An array of crystals – used to enable a transmitter to be used on several different frequencies; rather than being a truly variable frequency

system, it is a system which is fixed to several different frequencies (a subset of the above). Variable-frequency oscillator (VFO) Phase-locked loop frequency synthesiser Direct digital synthesis

Page 15: Amplitude modulation

[edit] Frequency multiplication

Frequency doublerA basic design for a frequency doubler (screen grids, bias supplies and other elements are not shown).

Frequency triplerA basic design for a frequency tripler (screen grids, bias supplies and other elements are not shown).

For VHF transmitters, it is often not possible to operate the oscillator at the final output frequency. In such cases, for reasons including frequency stability, it is better to multiply the frequency of the free running oscillator up to the final, required frequency.

If the output of an amplifier stage is tuned to a multiple of the frequency with which the stage is driven, the stage will give a larger harmonic output than a linear amplifier. In a push-push stage, the output will only contain even harmonics. This is because the currents which would generate the fundamental and the odd harmonics in this circuit (if one valve was removed) are canceled by the second valve. In the diagrams, bias supplies and neutralization measure have been omitted for clarity. In a real system, it is likely that tetrodes would be used, as plate-to-grid capacitance in a tetrode is lower, thereby reducing stage instability.

In a push-pull stage, the output will contain only odd harmonics because of the canceling effect.

[edit] Frequency mixing and modulationThe task of many transmitters is to transmit some form of information using a radio signal (carrier wave) which has been modulated to carry the intelligence. A few rare types of transmitter do not carry information: the RF generator in a microwave oven, electrosurgery, and induction heating. RF transmitters that do not carry information are required by law to operate in an ISM band.

[edit] AM modesIn many cases the carrier wave is mixed with another electrical signal to impose information upon it. This occurs in Amplitude modulation (AM). Amplitude Modulation: In Amplitude modulation the instantaneous change in the amplitude of the carrier Frequency with respect to the amplitude of the modulating or Base band signal.

[edit] Low level and high level

[edit] Low levelHere a small audio stage is used to modulate a low power stage, the output of this stage is then amplified using a linear RF amplifier.

Advantages

The advantage of using a linear RF amplifier is that the smaller early stages can be modulated, which only requires a small audio amplifier to drive the modulator.

Disadvantages

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The great disadvantage of this system is that the amplifier chain is less efficient, because it has to be linear to preserve the modulation. Hence class C amplifiers cannot be employed.

An approach which marries the advantages of low-level modulation with the efficiency of a Class C power amplifier chain is to arrange a feedback system to compensate for the substantial distortion of the AM envelope. A simple detector at the transmitter output (which can be little more than a loosely coupled diode) recovers the audio signal, and this is used as negative feedback to the audio modulator stage. The overall chain then acts as a linear amplifier as far as the actual modulation is concerned, though the RF amplifier itself still retains the Class C efficiency. This approach is widely used in practical medium power transmitters, such as AM radiotelephones.

[edit] High level Advantages

One advantage of using class C amplifiers in a broadcast AM transmitter is that only the final stage needs to be modulated, and that all the earlier stages can be driven at a constant level. These class C stages will be able to generate the drive for the final stage for a smaller DC power input. However, in many designs in order to obtain better quality AM the penultimate RF stages will need to be subject to modulation as well as the final stage.

Disadvantages

A large audio amplifier will be needed for the modulation stage, at least equal to the power of the transmitter output itself. Traditionally the modulation is applied using an audio transformer, and this can be bulky. Direct coupling from the audio amplifier is also possible (known as a cascode arrangement), though this usually requires quite a high DC supply voltage (say 30 V or more), which is not suitable for mobile units.

[edit] Types of AM modulators

A wide range of different circuits have been used for AM. While it is perfectly possible to create good designs using solid-state electronics, valved (tube) circuits are shown here. In general, valves are able to easily yield RF powers far in excess of what can be achieved using solid state. Most high-power broadcast stations still use valves.

[edit] Plate AM modulatorsIn plate modulation systems the voltage delivered to the stage is changed. As the power output available is a function of the supply voltage, the output power is modulated. This can be done using a transformer to alter the anode (plate) voltage. The advantage of the transformer method is that the audio power can be supplied to the RF stage and converted into RF power. With anode modulation using a transformer, the tetrode is supplied with an anode supply (and screen grid supply) which is modulated via the transformer. The resistor R1 sets the grid bias, both the input and outputs are tuned LC circuits which are tapped into by inductive coupling. In series modulated amplitude modulation, the tetrode is supplied with an anode supply (and screen grid supply) which is modulated by the modulator valve. The resistor VR1 sets the grid bias for the modulator valve, both the RF input (tuned grid) and outputs are tuned LC circuits which are tapped into by inductive coupling.

Anode modulation using a transformer. An example of a series modulated amplitude modulation stage.

Page 17: Amplitude modulation

When the valve at the top conducts more than the potential difference between the anode and cathode of the lower valve (RF valve) will increase. The two valves can be thought of as two resistors in a potentiometer.

[edit] Screen AM modulators

Screen AM modulator.Under steady state conditions (no audio driven) the stage will be a simple RF amplifier where the grid bias is set by the cathode current. When the stage is modulated the screen potential changes and so alters the gain of the stage.

[edit] Other modes which are related to AMSeveral derivatives of AM are in common use. These are

[edit] Single-sideband modulation

Main article: Single-sideband modulationSSB, or SSB-AM single-sideband full carrier modulation, is very similar to single-sideband suppressed carrier modulation (SSB-SC)

[edit] Filter methodUsing a balanced mixer a double side band signal is generated, this is then passed through a very narrow bandpass filter to leave only one side-band. By convention it is normal to use the upper sideband (USB) in communication systems, except for HAM radio when the carrier frequency is below 10 MHz here the lower side band (LSB) is normally used.

[edit] Phasing methodThis method is an alternative method for the generation of single sideband signals. One of the weaknesses of this method is the need for a network which imposes a constant 90o phase shift on audio signals throughout the entire audio spectrum. By reducing the audio bandwidth the task of designing the phaseshift network can be made more easy.

Imagine that the audio is a single sine wave E = E° sine (ωt)

The audio signal is passed through the phase shift network to give two identical signals which differ by 90o.

So as the audio input is a single sine wave the outputs will be

and

These audio outputs are mixed in non linear mixers with a carrier, the carrier drive for one of these mixers is shifted by 90°. The output of these mixers is combined in a linear circuit to give the SSB signal.

Page 18: Amplitude modulation

[edit] Vestigial-sideband modulation

Main article: Vestigial-sideband modulationVestigial-sideband modulation (VSB, or VSB-AM) is a type of modulation system commonly used in analogue TV systems. It is normal AM which has been passed through a filter which reduces one of the sidebands. Typically, components of the lower sideband more than 0.75 MHz or 1.25 MHz below the carrier will be heavily attenuated.

[edit] Morse

Strictly speaking the commonly used 'AM' is double-sideband full carrier. Morse is often sent using on-off keying of an unmodulated carrier (Continuous wave), this can be thought of as an AM mode.

[edit] FM modesAngle modulation is the proper term for modulation by changing the instantaneous frequency or phase of the carrier signal. True FM and phase modulation are the most commonly employed forms of analogue angle modulation.

[edit] Direct FM

Direct FM (true Frequency modulation) is where the frequency of an oscillator is altered to impose the modulation upon the carrier wave. This can be done by using a voltage-controlled capacitor (Varicap diode) in a crystal-controlled oscillator or frequency synthesiser. The frequency of the oscillator is then multiplied up using a frequency multiplier stage, or is translated upwards using a mixing stage, to the output frequency of the transmitter.

[edit] Indirect FM

Indirect FM solid state circuit.Indirect FM employs a varicap diode to impose a phase shift (which is voltage-controlled) in a tuned circuit that is fed with a plain carrier. This is termed phase modulation. The modulated signal from a phase-modulated stage can be understood with an FM receiver, but for good audio quality, the audio is applied to the phase modulation stage. The amount of modulation is referred to as the deviation, being the amount that the frequency of the carrier instantaneously deviates from the centre carrier frequency.

In some indirect FM solid state circuits, an RF drive is applied to the base of a transistor. The tank circuit (LC), connected to the collector via a capacitor, contains a pair of varicap diodes. As the voltage applied to the varicaps is changed, the phase shift of the output will change.

Phase modulation is mathematically equivalent to direct Frequency modulation with a 6dB/octave high-pass filter applied to the modulating signal. This high-pass effect can be exploited or compensated for using suitable frequency-shaping circuitry in the audio stages ahead of the modulator. For example, many FM systems

Page 19: Amplitude modulation

will employ pre-emphasis and de-emphasis for noise reduction, in which case the high-pass equivalency of phase modulation automatically provides for the pre-emphasis. Phase modulators are typically only capable of relatively small amounts of deviation while remaining linear, but any frequency multiplier stages also multiply the deviation in proportion.

Sigma-delta modulation (∑Δ)

[edit] RF power amplifiers

[edit] ValvesFor high power systems it is normal to use valves, please see Valve RF amplifier for details of how valved RF power stages work.

[edit] Advantages of valves

Good for high power systems Electrically very robust, they can tolerate overloads for minutes which would destroy bipolar transistor systems in milliseconds

[edit] Disadvantages of valves

Heater supplies are required for the cathodes High voltages (risk of death) are required for the anodes Valves often have a shorter working life than solid state parts because the heaters tend to fail

[edit] Solid stateFor low and medium power it is often the case that solid state power stages are used. For higher power systems these cost more per watt of output power than a valved system.

[edit] Linking the transmitter to the aerialThe majority of modern transmitting equipment is designed to operate with a resistive load fed via coaxial cable of a particular characteristic impedance, often 50 ohms. To connect the aerial to this coaxial cable transmission line a matching network and/or a balun may be required. Commonly an SWR meter and/or an antenna analyzer are used to check the extent of the match between the aerial system and the transmitter via the transmission line (feeder). An SWR meter indicates forward power, reflected power, and the ratio between them.

See Antenna tuner and balun for details of matching networks and baluns respectively.

Page 20: Amplitude modulation

[edit] EMC mattersMain article: Electromagnetic compatibilityWhile this section was written from the point of view of an amateur radio operator with relation to television interference it applies to the construction and use of all radio transmitters, and other electronic devices which generate high RF powers with no intention of radiating these. For instance a dielectric heater might contain a 2000 watt 27 MHz source within it, if the machine operates as intended then none of this RF power will leak out. However, if the device is subject to a fault then when it operates RF will leak out and it will be now a transmitter. Also computers are RF devices, if the case is poorly made then the computer will radiate at VHF. For example if you attempt to tune into a weak FM radio station (88 to 108 MHz, band II) at your desk you may lose reception when you switch on your PC. Equipment which is not intended to generate RF, but does so through for example sparking at switch contacts is not considered here.

[edit] RF leakage (defective RF shielding)Main article: RF shieldingAll equipment using RF electronics should be inside a screened metal box, all connections in or out of the metal box should be filtered to avoid the ingress or egress of radio signals. A common and effective method of doing so for wires carrying DC supplies, 50/60 Hz AC connections, audio and control signals is to use a feedthrough capacitor. This is a capacitor which is mounted in a hole in the shield, one terminal of the capacitor is its metal body which touches the shielding of the box while the other two terminal of the capacitor are the on either side of the shield. The feed through capacitor can be thought of as a metal rod which has a dielectric sheath which in turn has a metal coating.

In addition to the feed through capacitor, either a resistor or RF choke can be used to increase the filtering on the lead. In transmitters it is vital to prevent RF from entering the transmitter through any lead such as an electric power, microphone or control connection. If RF does enter a transmitter in this way then an instability known as motorboating can occur. Motorboating is an example of a self inflicted EMC problem.

If a transmitter is suspected of being responsible for a television interference problem, then it should be run into a dummy load; this is a resistor in a screened box or can which will allow the transmitter to generate radio signals without sending them to the antenna. If the transmitter does not cause interference during this test, then it is safe to assume that a signal has to be radiated from the antenna to cause a problem. If the transmitter does cause interference during this test then a path exists by which RF power is leaking out of the equipment, this can be due to bad shielding. This is a rare but insidious problem and it is vital that it be tested for. Such leakage is most likely to occur on homemade equipment or equipment that has been modified. RF leakage from microwave ovens may also sometimes be observed, especially when the oven's RF seal has been compromised.

[edit] Spurious emissions Early in the development of radio technology it was recognised that the signals emitted by transmitters had to be 'pure'. For instance

Spark-gap transmitters were quickly outlawed as they give an output which is so wide in terms of frequency. In modern equipment there are three main types of spurious emissions.

The term spurious emissions refers to any signal which comes out of a transmitter other than the wanted signal. The spurious emissions include harmonics, out of band mixer products which are not fully suppressed and leakage from the local oscillator and other systems within the transmitter.

Page 21: Amplitude modulation

[edit] Harmonics

These are multiples of the operation frequency of the transmitter, they can be generated in a stage of the transmitter even if it is driven with a perfect sine wave because no real life amplifier is perfectly linear.

[edit] Avoiding harmonic generation

Note that B+ is the anode supply, C- is the grid bias. While the circuit shown here uses tetrode valves (for example 2 x 4CX250B) many designs have used solid state semiconductor parts (such as MOSFETS). Note that NC is a neutralization capacitor.

Note that B+ is the anode supply, C- is the grid bias. While the circuit shown here uses a tetrode valve (for example the 4CX250B) many designs have used solid state semiconductor parts (such as MOSFETS).

It is best if these harmonics are designed out at an early stage. For instance a push-pull amplifier consisting of two tetrode valves attached to an anode tank resonant LC circuit which has a coil which is connected to the high voltage DC supply at the centre (Which is also RF ground) will only give a signal for the fundamental and the odd harmonics.

[edit] Removal of harmonics with filtersIn addition to the good design of the amplifier stages, the transmitter's output should be filtered with a low pass filter to reduce the level of the harmonics.

[edit] DetectionThe harmonics can be tested for using an RF spectrum analyser (expensive) or with an absorption wavemeter (cheap). If a harmonic is found which is at the same frequency as the frequency of the signal wanted at the receiver then this spurious emission can prevent the wanted signal from being received.

[edit] Local oscillators and unwanted mixing products

Please help improve this This section has too much detail, repeating information found elsewhere, all unsourced, and seems to reflect just the personal experience of an editor by pruning it. Further information might be found on the talk page.

Imagine a transmitter, which has an intermediate frequency (IF) of 144 MHz, which is mixed with 94 MHz to create a signal at 50 MHz, which is then amplified and transmitted. If the local oscillator signal was to enter the power amplifier and not be adequately suppressed then it could be radiated. It would then have the potential to interfere with radio signals at 94 MHz in the FM audio (band II) broadcast band. Also the unwanted mixing product at 238 MHz could in a poorly designed system be radiated. Normally with good choice of the intermediate and local oscillator frequencies this type of trouble can be avoided, but one potentially bad situation is in the construction of a 144 to 70 MHz converter, here the local oscillator is at 74 MHz which is very close to the wanted output. Good well made units have been made which use this conversion but their design and construction has been challenging, for instance in the late 1980s Practical

Page 22: Amplitude modulation

Wireless published a design (Meon-4) for such a transverter [1] [2] . This problem can be thought of as being related to the Image response problem which exists in receivers.

Simple but poor mixerOne method of reducing the potential for this transmitter defect is the use of balance and double balanced mixers. If the equation is assumed to be

E = E1 E2

and is driven by two simple sine waves, f1 and f2 then the output will be a mixture of four frequencies

f1

f1+f2

f1-f2

f2

If the simple mixer is replaced with a balanced mixer then the number of possible products is reduced. Imagine that two mixers which have the equation {I = E1 E2} are wired up so that the current outputs are wired to the two ends of a coil (the centre of this coil is wired to ground) then the total current flowing through the coil is the difference between the output of the two mixer stages. If the f1 drive for one of the mixers is phase shifted by 180° then the overall system will be a balanced mixer.

Note that while this hypothetical design uses tetrodes many designs have used solid state semiconductor parts (such as MOSFETS).E = K . Ef2 . ΔEf1

So the output will now have only three frequencies

f1+f2

f1-f2

f2

Now as the frequency mixer has fewer outputs the task of making sure that the final output is clean will be simpler.

[edit] Instability and parasitic oscillations

If a stage in a transmitter is unstable and is able to oscillate then it can start to generate RF at either a frequency close to the operating frequency or at a very different frequency. One good sign that it is occurring is if an RF stage has a power output even without being driven by an exciting stage. Another sign is if the output power suddenly increases wildly when the input power is increased slightly, it is noteworthy that in a class C stage that this behaviour can be seen under normal conditions. The best defence against this transmitter defect is a good design, also it is important to pay good attention to the neutralization of the valves or transistors.

Page 23: Amplitude modulation

[edit] See also

The Wikibook Electronics has a page on the topic of Transmitter design

Radio transmitter designFrom Wikipedia, the free encyclopediaJump to: navigation, search

This article has multiple issues. Please help improve it or discuss these issues on the talk page. It may contain an excessive amount of intricate detail which may only interest a specific audience. Tagged since August 2008. It needs to be updated. Tagged since August 2009.

It may need a complete rewrite to meet Wikipedia's quality standards. Tagged since May 2009.

This article only describes one highly specialized aspect of its associated subject. Please help improve this article by adding more general information. The talk page may contain suggestions. (October 2009)

Radio portal

Page 24: Amplitude modulation

Contents[hide]

1 Methods 2 Frequency synthesis

o 2.1 Fixed frequency systems o 2.2 Variable frequency systems

3 Frequency multiplication 4 Frequency mixing and modulation

o 4.1 AM modes 4.1.1 Low level and high level

4.1.1.1 Low level 4.1.1.2 High level

4.1.2 Types of AM modulators 4.1.2.1 Plate AM modulators 4.1.2.2 Screen AM modulators

o 4.2 Other modes which are related to AM 4.2.1 Single-sideband modulation

4.2.1.1 Filter method 4.2.1.2 Phasing method

4.2.2 Vestigial-sideband modulation 4.2.3 Morse

o 4.3 FM modes 4.3.1 Direct FM 4.3.2 Indirect FM

5 RF power amplifiers o 5.1 Valves

5.1.1 Advantages of valves 5.1.2 Disadvantages of valves

o 5.2 Solid state 6 Linking the transmitter to the aerial 7 EMC matters

o 7.1 RF leakage (defective RF shielding) o 7.2 Spurious emissions

7.2.1 Harmonics 7.2.1.1 Avoiding harmonic generation 7.2.1.2 Removal of harmonics with filters 7.2.1.3 Detection

7.2.2 Local oscillators and unwanted mixing products 7.2.3 Instability and parasitic oscillations

8 See also

9 References

Page 25: Amplitude modulation

Radio transmitter design is a complex topic which can be broken down into a series of smaller topics. A radio communication system requires two tuned circuits each at the transmitter and receiver, all four tuned to the same frequency.[1] The transmitter is an electronic device which, usually with the aid of an antenna, propagates an electromagnetic signal such as radio, television, or other telecommunications.

[edit] MethodsAt the beginning of the 20th century, there were four chief methods of arranging the transmitting circuits:[2]

1. The transmitting system consists of two tuned circuits such that the one containing the spark-gap is a persistent oscillator; the other, containing the aerial structure, is a free radiator maintained in oscillation by being coupled to the first (Nikola Tesla and Guglielmo Marconi).

2. The oscillating system, including the aerial structure with its associated inductance-coils and condensers, is designed to be both a sufficiently persistent oscillator and a sufficiently active radiator (Oliver Joseph Lodge).

3. The transmitting system consists of two electrically coupled circuits, one of which, containing the air-gap, is a powerful but not persistent oscillator, being provided with a device for quenching the spark so soon as it has imparted sufficient energy to the other circuit containing the aerial structure, this second circuit then independently radiating the train of slightly damped waves at its own period (Oliver Joseph Lodge and Wilhelm Wien).

4. The transmitting system, by means either of an oscillating arc (Valdemar Poulsen) or a high-frequency alternator (Rudolf Goldschmidt), emits a persistent train of undamped waves interrupted only by being broken up into long and short groups by the operator's key.

[edit] Frequency synthesis

[edit] Fixed frequency systemsFor a fixed frequency transmitter one commonly used method is to use a resonant quartz crystal in a Crystal oscillator to fix the frequency. Where the frequency has to be variable, several options can be used.

[edit] Variable frequency systems An array of crystals – used to enable a transmitter to be used on several different frequencies; rather than being a truly variable frequency

system, it is a system which is fixed to several different frequencies (a subset of the above). Variable-frequency oscillator (VFO) Phase-locked loop frequency synthesiser Direct digital synthesis

Page 26: Amplitude modulation

[edit] Frequency multiplication

Frequency doublerA basic design for a frequency doubler (screen grids, bias supplies and other elements are not shown).

Frequency triplerA basic design for a frequency tripler (screen grids, bias supplies and other elements are not shown).

For VHF transmitters, it is often not possible to operate the oscillator at the final output frequency. In such cases, for reasons including frequency stability, it is better to multiply the frequency of the free running oscillator up to the final, required frequency.

If the output of an amplifier stage is tuned to a multiple of the frequency with which the stage is driven, the stage will give a larger harmonic output than a linear amplifier. In a push-push stage, the output will only contain even harmonics. This is because the currents which would generate the fundamental and the odd harmonics in this circuit (if one valve was removed) are canceled by the second valve. In the diagrams, bias supplies and neutralization measure have been omitted for clarity. In a real system, it is likely that tetrodes would be used, as plate-to-grid capacitance in a tetrode is lower, thereby reducing stage instability.

In a push-pull stage, the output will contain only odd harmonics because of the canceling effect.

[edit] Frequency mixing and modulationThe task of many transmitters is to transmit some form of information using a radio signal (carrier wave) which has been modulated to carry the intelligence. A few rare types of transmitter do not carry information: the RF generator in a microwave oven, electrosurgery, and induction heating. RF transmitters that do not carry information are required by law to operate in an ISM band.

[edit] AM modesIn many cases the carrier wave is mixed with another electrical signal to impose information upon it. This occurs in Amplitude modulation (AM). Amplitude Modulation: In Amplitude modulation the instantaneous change in the amplitude of the carrier Frequency with respect to the amplitude of the modulating or Base band signal.

[edit] Low level and high level

[edit] Low levelHere a small audio stage is used to modulate a low power stage, the output of this stage is then amplified using a linear RF amplifier.

Advantages

The advantage of using a linear RF amplifier is that the smaller early stages can be modulated, which only requires a small audio amplifier to drive the modulator.

Disadvantages

Page 27: Amplitude modulation

The great disadvantage of this system is that the amplifier chain is less efficient, because it has to be linear to preserve the modulation. Hence class C amplifiers cannot be employed.

An approach which marries the advantages of low-level modulation with the efficiency of a Class C power amplifier chain is to arrange a feedback system to compensate for the substantial distortion of the AM envelope. A simple detector at the transmitter output (which can be little more than a loosely coupled diode) recovers the audio signal, and this is used as negative feedback to the audio modulator stage. The overall chain then acts as a linear amplifier as far as the actual modulation is concerned, though the RF amplifier itself still retains the Class C efficiency. This approach is widely used in practical medium power transmitters, such as AM radiotelephones.

[edit] High level Advantages

One advantage of using class C amplifiers in a broadcast AM transmitter is that only the final stage needs to be modulated, and that all the earlier stages can be driven at a constant level. These class C stages will be able to generate the drive for the final stage for a smaller DC power input. However, in many designs in order to obtain better quality AM the penultimate RF stages will need to be subject to modulation as well as the final stage.

Disadvantages

A large audio amplifier will be needed for the modulation stage, at least equal to the power of the transmitter output itself. Traditionally the modulation is applied using an audio transformer, and this can be bulky. Direct coupling from the audio amplifier is also possible (known as a cascode arrangement), though this usually requires quite a high DC supply voltage (say 30 V or more), which is not suitable for mobile units.

[edit] Types of AM modulators

A wide range of different circuits have been used for AM. While it is perfectly possible to create good designs using solid-state electronics, valved (tube) circuits are shown here. In general, valves are able to easily yield RF powers far in excess of what can be achieved using solid state. Most high-power broadcast stations still use valves.

[edit] Plate AM modulatorsIn plate modulation systems the voltage delivered to the stage is changed. As the power output available is a function of the supply voltage, the output power is modulated. This can be done using a transformer to alter the anode (plate) voltage. The advantage of the transformer method is that the audio power can be supplied to the RF stage and converted into RF power. With anode modulation using a transformer, the tetrode is supplied with an anode supply (and screen grid supply) which is modulated via the transformer. The resistor R1 sets the grid bias, both the input and outputs are tuned LC circuits which are tapped into by inductive coupling. In series modulated amplitude modulation, the tetrode is supplied with an anode supply (and screen grid supply) which is modulated by the modulator valve. The resistor VR1 sets the grid bias for the modulator valve, both the RF input (tuned grid) and outputs are tuned LC circuits which are tapped into by inductive coupling.

Anode modulation using a transformer. An example of a series modulated amplitude modulation stage.

Page 28: Amplitude modulation

When the valve at the top conducts more than the potential difference between the anode and cathode of the lower valve (RF valve) will increase. The two valves can be thought of as two resistors in a potentiometer.

[edit] Screen AM modulators

Screen AM modulator.Under steady state conditions (no audio driven) the stage will be a simple RF amplifier where the grid bias is set by the cathode current. When the stage is modulated the screen potential changes and so alters the gain of the stage.

[edit] Other modes which are related to AMSeveral derivatives of AM are in common use. These are

[edit] Single-sideband modulation

Main article: Single-sideband modulationSSB, or SSB-AM single-sideband full carrier modulation, is very similar to single-sideband suppressed carrier modulation (SSB-SC)

[edit] Filter methodUsing a balanced mixer a double side band signal is generated, this is then passed through a very narrow bandpass filter to leave only one side-band. By convention it is normal to use the upper sideband (USB) in communication systems, except for HAM radio when the carrier frequency is below 10 MHz here the lower side band (LSB) is normally used.

[edit] Phasing methodThis method is an alternative method for the generation of single sideband signals. One of the weaknesses of this method is the need for a network which imposes a constant 90o phase shift on audio signals throughout the entire audio spectrum. By reducing the audio bandwidth the task of designing the phaseshift network can be made more easy.

Imagine that the audio is a single sine wave E = E° sine (ωt)

The audio signal is passed through the phase shift network to give two identical signals which differ by 90o.

So as the audio input is a single sine wave the outputs will be

and

These audio outputs are mixed in non linear mixers with a carrier, the carrier drive for one of these mixers is shifted by 90°. The output of these mixers is combined in a linear circuit to give the SSB signal.

Page 29: Amplitude modulation

[edit] Vestigial-sideband modulation

Main article: Vestigial-sideband modulationVestigial-sideband modulation (VSB, or VSB-AM) is a type of modulation system commonly used in analogue TV systems. It is normal AM which has been passed through a filter which reduces one of the sidebands. Typically, components of the lower sideband more than 0.75 MHz or 1.25 MHz below the carrier will be heavily attenuated.

[edit] Morse

Strictly speaking the commonly used 'AM' is double-sideband full carrier. Morse is often sent using on-off keying of an unmodulated carrier (Continuous wave), this can be thought of as an AM mode.

[edit] FM modesAngle modulation is the proper term for modulation by changing the instantaneous frequency or phase of the carrier signal. True FM and phase modulation are the most commonly employed forms of analogue angle modulation.

[edit] Direct FM

Direct FM (true Frequency modulation) is where the frequency of an oscillator is altered to impose the modulation upon the carrier wave. This can be done by using a voltage-controlled capacitor (Varicap diode) in a crystal-controlled oscillator or frequency synthesiser. The frequency of the oscillator is then multiplied up using a frequency multiplier stage, or is translated upwards using a mixing stage, to the output frequency of the transmitter.

[edit] Indirect FM

Indirect FM solid state circuit.Indirect FM employs a varicap diode to impose a phase shift (which is voltage-controlled) in a tuned circuit that is fed with a plain carrier. This is termed phase modulation. The modulated signal from a phase-modulated stage can be understood with an FM receiver, but for good audio quality, the audio is applied to the phase modulation stage. The amount of modulation is referred to as the deviation, being the amount that the frequency of the carrier instantaneously deviates from the centre carrier frequency.

In some indirect FM solid state circuits, an RF drive is applied to the base of a transistor. The tank circuit (LC), connected to the collector via a capacitor, contains a pair of varicap diodes. As the voltage applied to the varicaps is changed, the phase shift of the output will change.

Phase modulation is mathematically equivalent to direct Frequency modulation with a 6dB/octave high-pass filter applied to the modulating signal. This high-pass effect can be exploited or compensated for using suitable frequency-shaping circuitry in the audio stages ahead of the modulator. For example, many FM systems

Page 30: Amplitude modulation

will employ pre-emphasis and de-emphasis for noise reduction, in which case the high-pass equivalency of phase modulation automatically provides for the pre-emphasis. Phase modulators are typically only capable of relatively small amounts of deviation while remaining linear, but any frequency multiplier stages also multiply the deviation in proportion.

Sigma-delta modulation (∑Δ)

[edit] RF power amplifiers

[edit] ValvesFor high power systems it is normal to use valves, please see Valve RF amplifier for details of how valved RF power stages work.

[edit] Advantages of valves

Good for high power systems Electrically very robust, they can tolerate overloads for minutes which would destroy bipolar transistor systems in milliseconds

[edit] Disadvantages of valves

Heater supplies are required for the cathodes High voltages (risk of death) are required for the anodes Valves often have a shorter working life than solid state parts because the heaters tend to fail

[edit] Solid stateFor low and medium power it is often the case that solid state power stages are used. For higher power systems these cost more per watt of output power than a valved system.

[edit] Linking the transmitter to the aerialThe majority of modern transmitting equipment is designed to operate with a resistive load fed via coaxial cable of a particular characteristic impedance, often 50 ohms. To connect the aerial to this coaxial cable transmission line a matching network and/or a balun may be required. Commonly an SWR meter and/or an antenna analyzer are used to check the extent of the match between the aerial system and the transmitter via the transmission line (feeder). An SWR meter indicates forward power, reflected power, and the ratio between them.

See Antenna tuner and balun for details of matching networks and baluns respectively.

Page 31: Amplitude modulation

[edit] EMC mattersMain article: Electromagnetic compatibilityWhile this section was written from the point of view of an amateur radio operator with relation to television interference it applies to the construction and use of all radio transmitters, and other electronic devices which generate high RF powers with no intention of radiating these. For instance a dielectric heater might contain a 2000 watt 27 MHz source within it, if the machine operates as intended then none of this RF power will leak out. However, if the device is subject to a fault then when it operates RF will leak out and it will be now a transmitter. Also computers are RF devices, if the case is poorly made then the computer will radiate at VHF. For example if you attempt to tune into a weak FM radio station (88 to 108 MHz, band II) at your desk you may lose reception when you switch on your PC. Equipment which is not intended to generate RF, but does so through for example sparking at switch contacts is not considered here.

[edit] RF leakage (defective RF shielding)Main article: RF shieldingAll equipment using RF electronics should be inside a screened metal box, all connections in or out of the metal box should be filtered to avoid the ingress or egress of radio signals. A common and effective method of doing so for wires carrying DC supplies, 50/60 Hz AC connections, audio and control signals is to use a feedthrough capacitor. This is a capacitor which is mounted in a hole in the shield, one terminal of the capacitor is its metal body which touches the shielding of the box while the other two terminal of the capacitor are the on either side of the shield. The feed through capacitor can be thought of as a metal rod which has a dielectric sheath which in turn has a metal coating.

In addition to the feed through capacitor, either a resistor or RF choke can be used to increase the filtering on the lead. In transmitters it is vital to prevent RF from entering the transmitter through any lead such as an electric power, microphone or control connection. If RF does enter a transmitter in this way then an instability known as motorboating can occur. Motorboating is an example of a self inflicted EMC problem.

If a transmitter is suspected of being responsible for a television interference problem, then it should be run into a dummy load; this is a resistor in a screened box or can which will allow the transmitter to generate radio signals without sending them to the antenna. If the transmitter does not cause interference during this test, then it is safe to assume that a signal has to be radiated from the antenna to cause a problem. If the transmitter does cause interference during this test then a path exists by which RF power is leaking out of the equipment, this can be due to bad shielding. This is a rare but insidious problem and it is vital that it be tested for. Such leakage is most likely to occur on homemade equipment or equipment that has been modified. RF leakage from microwave ovens may also sometimes be observed, especially when the oven's RF seal has been compromised.

[edit] Spurious emissions Early in the development of radio technology it was recognised that the signals emitted by transmitters had to be 'pure'. For instance

Spark-gap transmitters were quickly outlawed as they give an output which is so wide in terms of frequency. In modern equipment there are three main types of spurious emissions.

The term spurious emissions refers to any signal which comes out of a transmitter other than the wanted signal. The spurious emissions include harmonics, out of band mixer products which are not fully suppressed and leakage from the local oscillator and other systems within the transmitter.

Page 32: Amplitude modulation

[edit] Harmonics

These are multiples of the operation frequency of the transmitter, they can be generated in a stage of the transmitter even if it is driven with a perfect sine wave because no real life amplifier is perfectly linear.

[edit] Avoiding harmonic generation

Note that B+ is the anode supply, C- is the grid bias. While the circuit shown here uses tetrode valves (for example 2 x 4CX250B) many designs have used solid state semiconductor parts (such as MOSFETS). Note that NC is a neutralization capacitor.

Note that B+ is the anode supply, C- is the grid bias. While the circuit shown here uses a tetrode valve (for example the 4CX250B) many designs have used solid state semiconductor parts (such as MOSFETS).

It is best if these harmonics are designed out at an early stage. For instance a push-pull amplifier consisting of two tetrode valves attached to an anode tank resonant LC circuit which has a coil which is connected to the high voltage DC supply at the centre (Which is also RF ground) will only give a signal for the fundamental and the odd harmonics.

[edit] Removal of harmonics with filtersIn addition to the good design of the amplifier stages, the transmitter's output should be filtered with a low pass filter to reduce the level of the harmonics.

[edit] DetectionThe harmonics can be tested for using an RF spectrum analyser (expensive) or with an absorption wavemeter (cheap). If a harmonic is found which is at the same frequency as the frequency of the signal wanted at the receiver then this spurious emission can prevent the wanted signal from being received.

[edit] Local oscillators and unwanted mixing products

Please help improve this This section has too much detail, repeating information found elsewhere, all unsourced, and seems to reflect just the personal experience of an editor by pruning it. Further information might be found on the talk page.

Imagine a transmitter, which has an intermediate frequency (IF) of 144 MHz, which is mixed with 94 MHz to create a signal at 50 MHz, which is then amplified and transmitted. If the local oscillator signal was to enter the power amplifier and not be adequately suppressed then it could be radiated. It would then have the potential to interfere with radio signals at 94 MHz in the FM audio (band II) broadcast band. Also the unwanted mixing product at 238 MHz could in a poorly designed system be radiated. Normally with good choice of the intermediate and local oscillator frequencies this type of trouble can be avoided, but one potentially bad situation is in the construction of a 144 to 70 MHz converter, here the local oscillator is at 74 MHz which is very close to the wanted output. Good well made units have been made which use this conversion but their design and construction has been challenging, for instance in the late 1980s Practical

Page 33: Amplitude modulation

Wireless published a design (Meon-4) for such a transverter [1] [2] . This problem can be thought of as being related to the Image response problem which exists in receivers.

Simple but poor mixerOne method of reducing the potential for this transmitter defect is the use of balance and double balanced mixers. If the equation is assumed to be

E = E1 E2

and is driven by two simple sine waves, f1 and f2 then the output will be a mixture of four frequencies

f1

f1+f2

f1-f2

f2

If the simple mixer is replaced with a balanced mixer then the number of possible products is reduced. Imagine that two mixers which have the equation {I = E1 E2} are wired up so that the current outputs are wired to the two ends of a coil (the centre of this coil is wired to ground) then the total current flowing through the coil is the difference between the output of the two mixer stages. If the f1 drive for one of the mixers is phase shifted by 180° then the overall system will be a balanced mixer.

Note that while this hypothetical design uses tetrodes many designs have used solid state semiconductor parts (such as MOSFETS).E = K . Ef2 . ΔEf1

So the output will now have only three frequencies

f1+f2

f1-f2

f2

Now as the frequency mixer has fewer outputs the task of making sure that the final output is clean will be simpler.

[edit] Instability and parasitic oscillations

If a stage in a transmitter is unstable and is able to oscillate then it can start to generate RF at either a frequency close to the operating frequency or at a very different frequency. One good sign that it is occurring is if an RF stage has a power output even without being driven by an exciting stage. Another sign is if the output power suddenly increases wildly when the input power is increased slightly, it is noteworthy that in a class C stage that this behaviour can be seen under normal conditions. The best defence against this transmitter defect is a good design, also it is important to pay good attention to the neutralization of the valves or transistors.

Page 34: Amplitude modulation

[edit] See also

The Wikibook Electronics has a page on the topic of Transmitter design

OverviewThe circuit was designed to be able to obtain signals via amplitude modulation where the sensitivity and selectivity is fairly good.

Terminologyo Amplitude Modulation AM – a method of emphasizing data on an alternating current waveform or radio wave wherein the carried transmitted

signal varies with reference to the amplitude; the highest frequency of the modulating data is typically less than 10 percent of the carrier frequency and the overall power signal varies depending on the amplitude of the modulating data

Circuit Explanation Three BC549 transistor were used to construct this circuit. These are low noise type small signal NPN Silicon epitaxial planar transistors which are subdivide

into three groups according to current gain. The circuit will operate under medium wave frequency but can work under higher frequencies if another type of

Radio portal

Page 35: Amplitude modulation

capacitor and tuning coil will be used. The composite pair of transistor Q1 (conducts on emitter follower) and Q2 (conducts on common emitter) results to

high gain and high input impedance while the third transistor executes a dual function by demodulating the RF carrier and amplifying the audio signal.

The overall performance of the circuit is influenced by the value and regenerative feedback of the 120K ohm resistor between the Q2 output and tank circuit.

İn the existence of excessive feedback, the circuit becomes unstable while insufficient feedback makes the circuit deaf. The selectivity and sensitivity of the

circuit varies with the value set on the resistor R1.

Taken from an old radio, a tuning capacitor and a ferrite rod was reused which can be tuned around 550-1600 KHz. The copper wire is wound around the

ferrite rod and rotating the rod would void some signals while improving the others. An external antenna, can be of several feet of flexible wire, may be

necessary if the location of the receiver is in an area of weak reception. The tuning capacitor’s moving plates should be connected to the base of Q1 while

the fixed plates to the junction of R1 and C1.   Useless oscillation and instability may occur if the connection of the capacitor is reversed.

Application Amplitude modulation may be utilized not only in radio communications but also in transmitting human voice electronically, where the voice waves on both

sides are modulating the voltage of the direct current loop connected to them by the telephone company. In AM radio, the voltage or amplitude of a carrier

with a fixed center frequency by the station is varied by the analog audio signal. Amplitude modulation is also used in digital data, like quadrature amplitude

modulation,   where both amplitude and phase modulation are used to create different binary states for transmission. AM is also used in modulating the light

waves in optical fibers.

Some of the advantages possessed by using amplitude modulation receiver are, simplicity of implementation, can be demodulated using a circuit consisting

of very few components and they are cheap as no specialised components are needed. The disadvantages that one may encounter are inefficient in terms

of its power usage, inefficient in terms of its use of bandwidth and prone to high levels of noise.

Source:www.zen22142.zen.co.uk/Circuits/rf/amrec.html

30 Metre QRSS Beacon UpdatesThe MEPT Beacon has gone through some steady changes over the past few days. Most importantly it is now in somewhat of a "finished" state, stable and boxed-up ready for real transmitting experiments. A fourth piece of foam surrounds the circuit for insulation once the top is closed, the open ends of this "oven" don't seem to hurt. Modules are (clockwise from the microcontroller board); The ATtiny13V based

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controller, the fine tuning and FSK modulator, the carrier oscillator and buffer, the amplitude modulator (on wall), the driver amplifier, the low pass filter (on wall), and finally the class-E power amplifier.

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Carrier OscillatorEssentially unchanged from the original circuit, except I have stripped out the diode switched modulation trimmer and replaced it with the frequency modulation and tuning board. Conventional Colpitts oscillator using a 2N3904 buffered by a J310 with a low-pass filter delivering 900 uW into 50 Ohms.

That 3.1 uH inductor in the filter is implemented by 32 turns on a T37-6 toroid. The 78 pF caps are two 39 pF in parallel.

The reverse-isolation of this circuit is pretty good. It might be used as-is for a QRPp beacon or with the driver amplifier for 14 dBm output which is quite usable.

Frequency Modulator and TuningA pair of 1N4007s are used as varactor diodes. I picked 1N4007s not because they are particularly well suited, but simply that I had some on the bench. One of the varactors is directly coupled to the xtal circuit through 1 nF so most of its capacitance change is seen, the other is coupled through a small trimmer to adjust its maximum effect. This gives two independent channels, one for relatively coarse frequency tuning, the other for FSK modulation. Thanks to John for his suggestion around this part of the circuit.

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The varactor modulators were characterised to design the bias network in the controller for best linearity and desired range of shift (etc).

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Fine was measured with the coupling trimmer quite loose, and subsequently tightened on final assembly to give a good maximum modulation width.

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The data looks characteristically parabolic as expected. I suspect the oddness of the fine is measurement error. The y coordinates are in Hz relative to 10.138000 MHz (x is reverse voltage) so it doesn't take much error in measurement to make the fine trace look lumpy.

Driver AmplifierThe driver amp is a simple 2N3904-based feedback amplifier delivering about 25 mW from the 800 uW output of the oscillator board. The amplifier input impedance is about 300 ohms when loaded with 50 at the output, this is a pretty serious mismatch for the previous stage's output filter (Return loss of about -2.7 dB). The gain from a 50 ohm source models at about 11 dB, but the circuit in practice measured

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about 15 dB transducer gain from using the oscillator as a signal source, probably because of the increased output of the oscillator with the lower loading offered by the input impedance (compared to a dummy load). DC input power is about 500 mW, so this stage is woefully inefficient, running at its limits, and should probably be redesigned. The supply line is keyed by the amplitude modulator to affect CW modulation.

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Amplitude ModulatorThe amplitude modulator is simply a BD140 PNP transistor with a 2N7000 switching its base voltage. A resistor network gives some envelope shaping to soften the keying. In theory PWM could be used for fine-grain power control from the microcontroller board, but I don't think I will attempt this.

Power AmplifierThe power amplifier is a single 2N7000 running Class-E. From its 25 mW drive it can deliver about 2 Watts output (33 dBm) through the output filter into 50 Ohms from a 13.8 Volt supply. It barely gets warm although it is biased very slightly on by the (selected for the device) resistor network at the gate to give good 50% duty cycle drive and sensitivity to the relatively small drive voltage. At the normal supply voltage (regulated 12.0 volts) it will deliver about 1.5 Watts - note also that optimal tuning changes noticeably with a change in supply voltage.

Impedance matching into the gate could be better but I suspect acheiving a good return loss would reduce the drive voltage amplitude and kill the output power, another active device would likely be required - in any case the impedance doesn't appear to upset the

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oscillator buffer LP filter even though the driver is partially "transparent" being a feedback amplifier. The mid-stage filter is largely redundant, but was useful for initial experiments, you might like to omit it.

I wrote a calculator to help design class-E amplifiers . The design was based around 1 Watt out from a 12 Volt supply and a loaded Q of 5. The result gives a load of about 75 Ohms, but empirical tests showed 50 Ohms was suitable loading giving a sightly worse efficiency and more voltage at the drain (without changing the shunt capacitor). The shunt capacitor value was decreased from the calculated value to match the output capacitance of the 2N7000 (about 18 pF), experimental measurements closely agreeing with theory. Even better efficiency is likely achievable, but the circuit works quite well as-is.

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The drain choke is 12 turns on an FT50-43, its precise value is not critical, it just acts as a current source and could be (and perhaps should be) much smaller - the calculator does make a (very tiny - often ignorable) adjustment for its admittance. The series resonator is 35 turns on an T68-6. The larger core was chosen because of the large circulating currents, but numeric analysis was not carried out, T50

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or perhaps even T37 might be OK? The trimmer is probably not the best for higher Q circuits, but at 5 it seems to work OK and doesn't get hot.

Low-Pass FilterThe filter is a Chebyshev with its final peak at the frequency of operation. It has pretty good measured characteristics, the centre capacitor was tweaked to tune out the effect of using E12 preferred values. Normal ceramic capacitors were used with no measurable ill effects.

The 3.1 uH inductors are 28 turns on T50-6 toroids.

The ControllerAn ATtiny13V is the brains of the MEPT beacon. The microcontroller code hasn't changed since the original build. The board also contains the resistor network for deriving the modulator and tuning signals from the "width" and "centre" pots, in addition to the green indicating LEDs which show the CW and FSK keying state.

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There are some elements omitted from these diagrams, for example the 5 and 8 volt three-terminal regulators, some filter capacitors, decoupling, supply filtering at entrance to the box, etc. All are extremely non-critical and conventional so I haven't detailed them here. Please post a comment if you want anything clarified.

Vertical AntennaThe antenna has also changed, I've built a matching network to feed my 3-metre vertical used in the 80 metre beacon on 30 metres. I hope to eventually build a diplexer and matching network that can feed this same chunk of metal with both beacon TXs concurrently - but much design work remains before I can attempt that.

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The matching inductor is 7.8 uH. A polyvaricon fine-tunes the match. Oddly enough the return loss into this simple network alone is quite good. This is suggestive of rather large ground losses... A lot more antenna work needs to be done, while I get excellent signals into David VK6DI and Bob VK7KRW's sites I am yet to be seen anywhere else by the online grabber network.

ResultsDavid VK6DI has been able to copy my beacon in all its various states of construction and stability. One experiment in particular used just the 25 mW output of the driver amplifier. Signal to noise measurements from this test indicate I should be just visible above his noise floor running only 25 uW!

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With the beacon output now at an easily measured ~30 dBm it is a simple matter to attenuate down for the real QRPp(p) experiments.

Bob VK7KRW has been seeing my signal as well. He is my first report from VK7, including the 80 metre beacon experiments. Next time I fire up the 80 metre beacon Bob will listen out for it as well.

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QRSS ReceptionMy attempts to receive QRSS have not been as successful as my TX work. My noise floor is *horrible*, combined with poor antennas this limits my chances. I did accidentally see VK6DI's 500 mW signal during measurement experiments on my beacon.

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2 comments.

Attachments

title type size

Oscillator Circuit Source application/postscript 14.584 kbytes

Frequency Modulator Circuit Source application/postscript 11.443 kbytes

Driver Amp Circuit Source application/postscript 12.719 kbytes

Amplitude Modulator Circuit Source application/postscript 11.658 kbytes

Power Amplifier Circuit Source application/postscript 12.412 kbytes

Low-Pass Filter Circuit Source application/postscript 10.173 kbytes

Controller Circuit Source application/postscript 12.063 kbytes

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AM Modulation Lab

I. OBJECTIVE

 Amplitude Modulation is one of the most basic of engineering techniques for which the frequency spectrum is the best tool for thinking about it. Concepts that may take pages of equations in the time-domain become simple and illuminating when viewed on the frequency axis. The objective is to create such confidence.

II. EQUIPMENT AND MATERIALS

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 For this lab, we used a function generator, scope with FFT capability, and an active multiplier circuit to be described herein.

III. BACKGROUND INFORMATION

There are many examples of a mismatch between the frequency content of a signal and the ideal frequency range of a medium through which the signal needs to be transmitted or on which the signal needs to be recorded. Radio, for instance, works much more efficiently at frequencies which are far above the range of voice and music. Magnetic recording media can retain magnetic signals at higher frequencies, but not so well at low frequencies. Techniques are necessary for shifting a signal to a frequency range that is more convenient and still be able to recover the original information later. One such technique, which is explored in this lab, is known as Amplitude Modulation (AM).

The essence of AM is the multiplication of the signal, m(t), with a ‘sinusoidal carrier’ signal centered in the frequency range to which one wants to shift the signal: m(t)cos(wct).

In the time domain, it can be difficult to see how multiplication achieves this unless one restricts oneself to the case of both carrier and signal being sinusoids. If m(t) = cos(wmt) then

cos(wmt)cos(wct) =[cos(wm + wc)t + cos(wm - wc)t]/2

 using the trigonometric identity: cosAcosB = [cos(A+B)+cos(A-B)]/2

 If the carrier frequency is much larger than the signal frequency, then both of the frequencies above will be near that of the original carrier frequency. The signal has been shifted in frequency as follows.

   

In the frequency domain, we can go much further. Recall that when two functions are multiplied in the time domain, the Fourier transform of the resulting signal is equal to the convolution of the Fourier transforms of the two original functions, i.e.,

  f1(t)f2(t) ó [F1(w)*F2(w)]/(2p)

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 where F1(w) and F2(w) are the Fourier transforms of f1(t) and f2(t) respectively. Normally, convolution causes revulsion and horror, but when one of the functions to be convolved is an impulse function, it becomes exceedingly simple which is our case since the carrier signal is: cos(wct) ó p[d(w + wc) + d(w - wc)]

 Using this convolution property we then obtained that :

m(t)cos(wct) ó [M(w + wc) + M(w - wc)]/2

 where m(t) ó M(w).

 IV. PROCEDURE AND RESULTS

 As an overview, here are the steps we took for this lab:

1.     Built a multiplier circuit

2.     Produced an AM signal at our assigned carrier frequency

3.     Measured the frequency spectrum of the AM output signal

4.     Went through some modules in a computer animated course on AM.

 Build the multiplier circuit shown below:

Figure 1. A circuit that can be used as a “multiplier” to produce Amplitude Modulation

 As you can see, this op-amp circuit is essentially in the configuration of an inverting amplifier. This is even clearer if you consider the JFET to be a variable resistor. The resistance of the JFET changes with the signal voltage at its gate.

For the gate input to your multiplier, supply a sinusoid as a carrier, of a frequency that should have been assigned in the lab lecture according to team. It will be one of:

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20kHz 95kHz

35kHz 110kHz

50kHz 125kHz

65kHz 140kHz

80kHz 155kHz

Josie and I were assigned the 50kHz carrier sinusoid.

  For simplicity, we selected the input signal that will modulate the carrier to be a sinusoid with a frequency of 500Hz. We applied this sinusoid to the drain of the JFET.

 

AM signal output

Displaying the output of the multiplier, it looked like an AM signal with possibly some noise as follows.

We were able to see the carrier frequency as the small sinewaves within the larger envelope (input signal) function. However, our op-amp gave little amplification in the positive region.

FFT of AMsignal

Displaying the FFT of the AM signal yielded the followinggraph

 

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As wehoped, we saw the fundamental spike at 50 kHz as the carrier. 500 Hz above and below the carrier werespikes representing the information signal.

·Reviewing the material in theAMFundamentals, it seemed helpful in justifying some of the characteristics weexperienced during the lab such as overmodulation. The major drawback to the program would haveto be the lack of sound.

 V.               CONCLUSION

 In conclusion, I have learned where there are manydifferent advantages of the FFT function when dealing with an Amplitude Modifiedsignal. It is good to know how toanalyze the different functions by the use of this powerful tool.