# EENG 3810 Chapter 4

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Amplitude modulation

### Text of EENG 3810 Chapter 4

• **EENG 3810 Chapter 4Amplitude Modulation(AM)

• **Chapter 4 Homework1.For an AM DSBFC modulator with a carrier frequencyfc = 200KHz and a maximum modulating signal frequency fm(max) = 10 KHz, determine :a. Frequency limits for the upper and lower sidebands.b. Bandwidth.b. Upper and lower side frequencies produced when the modulating signal is a single-frequency 6 KHz tone.

• **Homework Continued2. For the AM wave form above determine:

• **Homework Continued3.

• **Homework ContinuedRepeat steps (a) through (d) in Example 4 in these lecture slides for a modulation coefficient of 0.5.For an AM DSBFC wave with a peak unmodulated carrier voltage Vc = 20 Vp, a load resistance RL = 20 , and a modulation coefficient m = 0.8, determine the power of the modulated wave

• Homework Continued6.Determine the noise improvement for a receiver with an RF bandwidth equal to 100 KHz and an IF bandwidth equal to 20 KHz.

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• *Amplitude Modulation Transmission*

• **AM Generation

• **Frequency Spectrum of An AM Double Sideband Full Carrier (DSBFC) Wave

• **Example 1For an AM DSBFC modulator with a carrier frequencyfc = 100KHz and a maximum modulating signal frequency fm(max) = 5 KHz, determine :

a. Frequency limits for the upper and lower sidebands.b. Bandwidth.c. Upper and lower side frequencies produced when the modulating signal is a single-frequency 3 KHz tone.

• **Example 1 Solutiona.b.c.

• **Example 1 d. The Output Spectrum For An AM DSBFC Wave

• **Phasor addition in an AM DSBFC envelopeFor a single-frequency modulating signal, am AM envelop is produced from the vector addition of the carrier and upper and lower side frequencies. Phasors of the carrier, The upper and lower frequencies combine and produce a resultant component that combines with the carrier component.Phasors for the carrier, upper and lower frequencies all rotate in the counterclockwise direction.The upper sideband frequency rotates faster than the carrier. (usf > c)The lower sideband frequency rotes slower than the carrier. (usf < c)

• **Phasor addition in an AM DSBFC envelope

• **Modulation Coefficient

• **If the modulating signal is pure, single frequency sine wave and the modulation process is symmetrical, the % modulation can be derived as follows:

• **Peak Amplitudes of Upper and Lower Sidebands The peak change in amplitude of the output wave (Em) is equal to the sum of the voltages from the upper and lower sideband frequencies. Therefore,

• **Percent Modulation of An AM DSBFC Envelope (a) modulating signal; (b) unmodulated carrier; (c) 50% modulated wave; (d) 100% modulated wave

• **Example 2For the AM wave form above determine:

• **Example 2

• **Voltage Spectrum for an AM DSBFC Wave

• **Generation of an AM DSBFC Envelope Shown in The Time Domainsin(225t) cos(230t)+ cos(220t)summation of (a), (b), and (c)

• **Voltage of an AM DSBFC Envelope In The Time Domain

• **Example 3

• **Example 3 Continued

• **Output Spectrum for Example 3

• **AM envelope for Example 3

• **Power for Upper and Lower Sideband

• **Total Power for an AM DSBFC Envelop

• **Power Spectrum for an AM DSBFC Wave with a Single-frequency Modulating Signal

• **Example 4

• **Power Spectrum for Example 4

• **Single Transistor, Emitter Modulator

• **Single Transistor, Emitter Modulator (output waveforms )

• *Medium-power Transistor AM DSBFC Modulator*

• *High-power AM DSBFC Transistor Modulator*

• *Linear Integrated-circuit AM Modulator*

• *Block Diagram of a Low-level AM DSBFC Transmitter*

• *Block Diagram of a High-level AM DSBFC Transmitter*

• *Single-Sideband

• *Conventional DSFC-AM

• *Single-side Band Full Carrier (SSBFC)The carrier is transmitted at full power and only one sideband is transmitted.

• *SSBFC waveform, 100% modulation

• *Single-Sideband Suppressed Carrier (SSBSC)The carrier is suppressed 100% and one sideband is removed. Only one sideband is transmitted.

• *SSBSC waveform

• *Single-Sideband Reduced Carrier(SSBRC)One sideband is removed and the carrier voltage is reduced to 10% of its un-modulated amplitude.

• *Independent Sideband(ISB)A single carrier is independently modulated by two different modulating signals.

• *ISB waveform

• *Vestigial Sideband(VSB)The carrier and one complete sideband are transmitted, but only part of the other sideband is transmitted.

• *

• *Single-Sideband Generation

• *Balanced modulator waveforms

• *FET Balanced Modulator

• *AM DSBSC modulator using the LM1496/1596 linear integrated circuit

• *Amplitude Modulation Reception*

• *Simplified Block Diagram of an AM Receiver*

• *Simplified Block Diagram of an AM ReceiverReceiver front end = RF sectionDetecting the signalBand-limiting the signalAmplifying the Band-limited signalMixer/converterDown converts the RF signal to an IF signalIntermediate frequency (IF) signalAmplificationSelectivityAbility of a receiver to accept assigned frequency Ability of a receiver to reject other frequenciesAM detector demodulates the IF signal to the original signalAudio section amplifies the recovered signal.

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• *AM Superheterodyne Receiver Block Diagram *

• *Bandwidth Improvement (BI)Noise reduction ratioBI = BRF / BIF Noise figure improvementNFIMP = 10 log BIDetermine the noise improvement for a receiver with an RF bandwidth equal to 200 KHz and an IF bandwidth equal to 10 KHz.BI = 200 KHz / 10 KHZ = 20NFImp = 10 log 20 = 13 dB

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• *SensitivitySensitivity: minimum RF signal level that the receiver can detect at the RF input.AM broadcast receivers10 dB signal to noise ratio watt (27 dBm) of power at the audio output50 uV SensitivityMicrowave receivers40 dB signal to noise ratio5 mw (7 dBm) of power at the output

Aa

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• *Dynamic RangeDynamic RangeDifference in dB between the minimum input level and the level that will over drive the receiver (produce distortion).Input power range that the receiver is useful.100 dB is about the highest posible.Low Dynamic RangeCauses desensitizing of the RF amplifiers Results in sever inter-modulation distortion of weaker signals*

• *FidelityAbility to produce an exact replica of the original signal.Forms of distortionAmplitudeResults from non-uniform gain in amplifiers and filters.Output signal differs from the original signalFrequency: frequencies are in the output that were not in the orginal signalPhaseNot important for voice transmissionDevastating for digital transmission

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