Demodulation and Super Heterodyne Reciever

8/6/2019 Demodulation and Super Heterodyne Reciever

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Lecture 25Demodulation

and theSuperheterodyne Receiver

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2Couch, Digital and Analog Communication Systems, Seventh Edition 2007 Pearson Education, Inc. All rights reserved. 0-13-142492-0

Figure 429 Superheterodyne receiver.

m(t)


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Synchronous Demodulation

s(t)

2Cos(2fct)

LPF m(t)

Only method for DSB-SC, USB-SC, LSB-SCAM with carrier

Envelope Detection Input SNR >~10 dB requiredSynchronous Detection (no threshold effect)

Note the 2 on the LO normalizes the output amplitude


Figure 424 PLL used for coherent detection of AM.


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Superheterodyne Receiver

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Super-Heterodyne AM Receiver


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RF Filter Provides Image Rejection fimage=fLO+fif Reduces amplitude of interfering signals

far from the carrier frequency

Reduces the amount of LO signal thatradiates from the Antenna


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Figure 430 Spectra of signals and transfer function of an RF amplifier in a

superheterodyne receiver.


Figure 429 Superheterodyne receiver.


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Mixer and LO

The mixer produces

fSUM=fLO+fRF and fDIF=fLO-fIF The conventional AM radio uses the difference

frequency

The LO (Local Oscillator) tunes the radio so thatthe desired input frequency passes through theIF filters.

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Antenna, Mixer, LO


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IF Amplifiers and Filters The IF filters: The bandwidth is set wide enough to pass the transmitted signal

Provides adjacent channel rejection.

If we are tuned to 1400 KHz, the Adjacent channels are at 1390

KHz and 1410 KHz

This bandwidth determines the noise bandwidth of the receiver

The filter is optimized for IF frequency so all input signals passthrough the same filters. This simplifies filter and amplifier

design

The IF amplifier gain is variable to adjust for changes in the input

signal power level. The received signal level may vary from 60dB)

Note that an FM radio uses a limiting IF amplifier not a variablegain amplifier. See FM notes


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TABLE 42 FILTER CONSTRUCTION TECHNIQUES

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IF Amplifier


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Envelope Detector The envelope detector recovers the original m(t)

modulation and a DC voltage that is proportionalto the received signal carrier amplitude Ac.

The DC voltage is used to automatically adjustthe gain of the IF amplifier in a control loop(AGC- automatic gain control). This maintains aconstant recovered m(t) amplitude as the

receiver input signal level changes, otherwisethe volume would change as much as 60dB!


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Detector

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Detector, AGC, Audio


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IF and AGC

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IF and agc


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Converter

Image RejectionFrequency TranslationRF amplificationLO- tuning

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Converter


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Lecture 26-27Single Sideband

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Figure 54 Spectrum for a USSB signal.


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Single Side Band

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Single Side Band


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Single Side Band

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Single Side Band


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Single Side Band

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Single Side Band


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Single Side Band

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Single Side Band


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Figure 55 Generation of SSB.

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Single Side Band


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Single Side Band

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Single Side Band


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Figure 56 VSB transmitter and spectra.

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Demodulation and Super Heterodyne Reciever