Advanced Radio And Radar

Radio

• There are various methods of communications. The voice being a main one.

• A transmitter and receiver are required.

• Voice – Transmitter• Ears – Receiver

• Speed of travel is quite slow in air: 340 m/s at 20ºC or 760 mph (the speed of propagation of sound).

• Sound will not travel through a vacuum.

• It needs a substance or “medium” to transmit the energy.

• The medium can also be liquid (water or mercury) or a solid (bar of steel or a brick wall)

• The energy in the molecules vibrate off each other in the same direction as the propagation.

• This is why sound travels further in solids as oppose to gases – there are more molecules.

• The direction of travel is a longitudinal wave

• Sound does not travel very far in air. The air acts like a shock absorber and the sound energy is converted into extremely small amounts of heat.

• So sound travels faster and much more efficiently in liquid and therefore, much further. Solid mediums even more so.

• Depending on a number of complex factors, radio waves can propagate through the atmosphere in various ways.

These include:• Ground waves• Ionospheric waves• Space waves• Tropospheric waves.

• As their name suggests, ground waves (or surface waves) travel close to the surface of the earth and propagate for relatively short distances at HF and VHF.

• Ground waves have two basic components; a direct wave and a ground reflected wave.

• The direct path is called line of sight.

• A radio communications system consists of a transmitter (Tx), to send the message and a receiver (Rx) to receive the reply.

• The link between the Tx and Rx is this time not sound energy, but electromagnetic (em) energy, - radio waves.

• Just like light from the sun, radio waves can travel not only through air, but also through a vacuum.

• They travel at the same extremely high speed.

• Both the transmitter and receiver use antennas to radiate and capture the radio signal.

What is electromagnetic energy?• When an alternating electric current flows in a wire,

both magnetic and electric fields are produced outside the wire.

• Some can be used for radio communications – radio waves.

• The frequency of the alternating current will determine the frequency of the ‘electromagnetic’ waves produced, and its power rating will govern the range of radiation.

• There is no theoretical limit to the frequency of ‘electromagnetic’ waves, and the expression "electromagnetic spectrum" has been coined to embrace all radiations of this type, which include heat and light.

• Electromagnetic radiation travels in waves in a similar fashion to sound waves travelling through air.

• The waves travel in all directions from their source rather like the pattern produced when a stone is dropped into the water in a still pond.

• Frequency - The number of complete vibrations or fluctuations each second (i.e. cycles per sec).

• Amplitude - The distance between O on the Amplitude axis and a crest.

• Wavelength - The distance between any two identical points in a wave (literally the length of one wave).

• Velocity - The speed with which the waves moves is given by the formula: v = f λ

• Microwaves are radio waves with frequencies higher than television signals.

• The wavelengths of microwaves are of the order of a few millimeters.

• We know that sound waves spread and bend around the corner of an obstacle.

• This is because the wavelength of sound wave is generally comparable to the size of the obstacle.

• Unlike a sound wave, a light wave keeps itself along a straight path.

• The light waves bend by only a small amount at the corners of the obstacles.

• This is because the wavelength of light waves is smaller as compared to the wavelength of sound waves.

• Therefore, lesser the wavelength of a wave, smaller is its bending at the corners of ordinary obstacles and greater the ability of the wave to follow a straight path.

• The wavelength of microwaves is very small as compared to the wavelength of radio waves. So, microwaves are better suited to beam signals in a particular direction

Modulation

• For the transmission of sounds such as speech and music, the sound waves are converted by a microphone into an oscillating electric current which varies at the same frequency as the sound wave.

• This is called an "audio-frequency" current.

• There are two ways to carry analog information with radio waves.

• These are Amplitude Modulation and Frequency Modulation (AM and FM)

• Amplitude modulation (AM) is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave.

• With AM, the amplitude of the wave is made to vary in accordance with the audio wave.

• It is modulated in amplitude by the signal that is to be transmitted.

• With Frequency Modulation, the frequency of the carrier wave is made to vary in accordance with the audio wave.

• It is modulated in frequency by the signal that is to be transmitted.

• AM radio ranges from 535 to 1705 kilohertz and FM radio ranges in a higher spectrum from 88 to 108 megahertz.

• A transmitter and receiver and required for both modulations.

The above scheme suffers from the following drawbacks:

• EM waves in the frequency range of 20 Hz - 20 kHz (audio-frequency range) cannot be efficiently radiated and do not propagate well in space.

• Simultaneous transmission of different signals by different transmitters would lead to confusion at the receiver.

• We need to devise methods to convert or translate the audio signals to the radio-frequency range before transmission and recover the audio-frequency signals back at the receiver.

• Different transmitting stations can then be allotted slots in the radio-frequency range and a single receiver can then tune into these transmitters without confusion.

• The frequency range 500 kHz to 20 MHz is reserved for amplitude-modulated broadcast, which is the range covered by most three band transistor radios.

• The process of frequency translation at the transmitter is called modulation.

• The process of recovering the audio-signal at the receiver is called demodulation

• A radio transmitter produces radio waves.• These waves are sent out through an antenna

to a receiver. • The transmitter is a source of electrical energy,

producing alternating current of a desired frequency of oscillation.

• A radio receiver receives these radio waves and converts them back into audio or visual information.

• The waves a received by an antenna once again.

• The process for a transmitter is the audio stage which increases the weak signal coming from the microphone.

• The modulator then modulates the audio signal onto the radio frequency carrier.

• Then at the frequency generator stage, the frequency is defined on which the transmitter will operate.

• finally the RF power amplifier stage, the power amplification of the radio signal is carried out.

• It makes the signal stronger so that it can be transmitted into the aerial.

• The process for the receiver is the tuning and RF amplifier stage which selects the signal we want to hear as there are many radio signals being transmitted on different radio frequencies.

• The RF amplifier increases the signal received from the air by the antenna.

• Antennas are connected to receivers by special wires known as feeders.

• The detection stage is next which the process is where the original modulating signal is recovered. It is the opposite of the modulation stage in transmitting.

• Then it’s the audio amplifier stage which is where the detected audio signal is increased to a level which can be played and heard.

• Master Oscillator - This generates a sinusoidal voltage (the carrier) at the required RF frequency (FO). Oscillators are often crystal-controlled to ensure good frequency stability.

• Buffer Amplifier - This isolates the oscillator from the power amplifying stage, and prevents instability occurring.

• Power Amplifier - This is used to increase the power of the signal to the required level before radiation from the aerial (AF).

• Amplifier - This amplifies the microphone signal to the desired level for output.

The modulation takes place in the power amplifier stage.

If the input frequencies to the modulator are FO from the oscillator and AF from the microphone, we find that the output of the power amplifier will consist of 3 frequencies:

• The carrier (FO ).• The carrier minus the tone frequency (speech)

(FO – AF).• The carrier plus the tone frequency (FO + AF).

• For example, if the audio frequency ranged from 300 to 3000 Hz and the carrier was 1 MHz, then the frequencies in the output would look like the diagram above.

• In the diagram you can see two sidebands to the carrier frequency, an upper sideband and a lower sideband.

• Some modes of operation use only one, and this is called single sideband (SSB) transmission.

• Transmitting only one sideband reduces the size and weight of the transmitter – important factors when talking about aircraft systems.

• The great drawback with the AM system is the need for such a large bandwidth (i.e. all frequencies including both sidebands, approximately 6KHz) in a limited frequency spread (30 KHz to 3 MHz i.e. Medium band).

• This means in reality that the AM system could only have 148 stations at any one time.

• Obviously, when many transmitters are crammed into a small band and overlap each other there is a big problem with signals from other transmissions breaking into the one you are using – this is known as "interference".

• To overcome this, the use of short-range frequency modulated systems has become popular.

• In those early models of receiver the problems encountered were noise (too much interference), poor amplification, limited selectivity, poor sensitivity (ability to remain on a station) and lack of fidelity (quality of sound).

• To overcome some of these problems, the superheterodyne (superhet) receiver was developed.

• Heterodyne is the term used to describe the mixing of one frequency with a slightly different frequency to produce something called "beats".

• If two notes of nearly equal frequency are sounded together, a periodic rise and fall in intensity (i.e. a beat) can be heard.

• For example, if an audio note of 48 Hz is sounded together with one of 56 Hz then the rhythmic beat of 8 Hz (56 - 48) would be heard.

• The basic block diagram of a basic superhet receiver is shown below.

• This details the most basic form of the receiver and serves to illustrate the basic blocks and their function.

• The way in which the receiver works can be seen by following the signal as is passes through the receiver.

• Front end amplifier and tuning block: Signals enter the front end circuitry from the antenna. This circuit block performs two main functions:

• Tuning: Broadband tuning is applied to the RF stage. The purpose of this is to reject the signals on the image frequency and accept those on the wanted frequency.

• It must also be able to track the local oscillator so that as the receiver is tuned, so the RF tuning remains on the required frequency.

• Amplification: In terms of amplification, the level is carefully chosen so that it does not overload the mixer when strong signals are present, but enables the signals to be amplified sufficiently to ensure a good signal to noise ratio is achieved.

• The amplifier must also be a low noise design. Any noise introduced in this block will be amplified later in the receiver.

• Mixer / frequency translator block: The tuned and amplified signal then enters one port of the mixer.

• The local oscillator signal enters the other port.

• The performance of the mixer is crucial to many elements of the overall receiver performance.

• It should be as linear as possible. If not, then spurious signals will be generated and these may appear as 'phantom' received signals.

• Local oscillator: The local oscillator may consist of a variable frequency oscillator that can be tuned by altering the setting on a variable capacitor.

• Alternatively it may be a frequency synthesizer that will enable greater levels of stability and setting accuracy.

• Intermediate frequency amplifier, IF block: Once the signals leave the mixer they enter the IF stages.

• These stages contain most of the amplification in the receiver as well as the filtering that enables signals on one frequency to be separated from those on the next.

• Filters may consist simply of LC tuned transformers providing inter-stage coupling, or they may be much higher performance ceramic or even crystal filters, dependent upon what is required.

• Detector / demodulator stage: Once the signals have passed through the IF stages of the superheterodyne receiver, they need to be demodulated.

• Different demodulators are required for different types of transmission, and as a result some receivers may have a variety of demodulators that can be switched in to accommodate the different types of transmission that are to be encountered.

• Audio amplifier: The output from the demodulator is the recovered audio.

• This is passed into the audio stages where they are amplified and presented to the headphones or loudspeaker

• Reception on the AM bands is limited in both quality of reproduction and bandwidth availability.

• FM systems are less likely to be affected by "noise" and give increased signal performance.

• The FM receiver circuitry is similar to the AM system but uses a discriminator (also called a ratio detector) in place of a demodulator.

• The discriminator is a circuit which has been designed to detect small differences in frequencies.

• These differences are converted to a voltage output that represents the AF component input.

• The first element in the process of receiving a radio message is the aerial.

• An aerial can vary from a length of wire supported off the ground to a complex array designed to select only certain frequencies, but whatever its shape, its purpose is to detect the tiny amounts of ‘em’ energy radiated from the transmitter.

• If an aerial in the form of a length of wire is placed into an electromagnetic field, tiny voltages are induced in it.

• These voltages alternate with the frequency of the ‘em’ radiation and are passed to the receiver circuitry for processing.

• The signal strength that the aerial inputs to the receiver is very tiny the order of 5 ì(micro) volts (0.000005 volts).

• Therefore the receiver circuits have to be extremely sensitive.

• The circuits must also isolate the wanted signal from all the unwanted ones being received, and this is achieved by using tuned circuits.

• A tuned circuit simply allows a single frequency to pass, thus filtering out all the unwanted signals.

• The best known version of a tuned circuit is the "crystal set" or "cat’s whisker" as it was called in the 1920’s and 30’s.

What is the speed of light ?• 3 x 108 ms-1• 3 x 106 ms-1• 30 x 109 ms-1• 30 x 101 ms-1

• 3 x 108 ms-1

The relationship between frequency (f), wavelength (l) and velocity of light (v) is given in the formula:

• velocity = frequency x wavelength (v = f x l)• velocity = frequency + wavelength (v = f + l)• velocity = frequency - wavelength (v = f - l)• Frequency = velocity - wavelength (f = v - l)

• velocity = frequency x wavelength (v = f x l)

If the velocity of radio waves is 3 x 108, what would be the value of l for frequency of 3 x 106 ?

• 1000m• 10m• 100m• 1m

• 100m

What does the abbreviation SSB stand for ?• Single Side Band• Single Silicone Band• Ship to Shore Broadcast• Solo Side Band

• Single Side Band

What is the purpose of an aerial on a receiver?• To convert the electromagnetic waves (‘em’) into

tiny voltages• To convert the electromagnetic waves (‘em’) into

large voltages• To convert the electromagnetic waves (‘em’) into

very large voltages• To convert the electromagnetic waves (‘em’) into a

constant voltage

• To convert the electromagnetic waves (‘em’) into tiny voltages

What does superheterodyne receivers make use of?

• Bleats• Boats• Beats• Bullets

• Beats

What do FM receivers use to demodulate signals?

• Distractor• Modulator• Discriminator• Disputer

• Discriminator