Chapter 5 AM Receivers

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1 Chapter 5 AM Receivers Prepared by Prof.V.K.Jain 1

2 Lecture outcome After studying this lecture, you should be able to: Describe the basic superheterodyne system Choose suitable intermediate frequencies and calculate image rejection for receiver Explain the requirements for each stage in a receiver. Prepared by Prof.V.K.Jain 2

3 AM Demodulation AM demodulation is the reverse process of AM modulation AM demodulator converts the AM signal back to the original information signal To achieve this, the receiver must be capable of receiving, amplifying and demodulating an AM signal Lets look at a typical AM receiver block diagram Prepared by Prof.V.K.Jain 3

4 AM Receiver Prepared by Prof.V.K.Jain 4

5 AM Receiver RF section: Often called receiver front end, includes the following operations: detecting RF Signals, band limiting amplifying Mixer: Down converts the RF frequencies to intermediate frequencies (IF). IF section: Includes amplification and selectivity functions. AM detector: Demodulates the AM wave and converts it to original information signal. Prepared by Prof.V.K.Jain 5

6 Superheterodyne AM Receiver flo = frf + fif Prepared by Prof.V.K.Jain 6

7 AM Receiver Operation flo = frf + fif Prepared by Prof.V.K.Jain 7

8 AM Receiver Operation kHz to 1605kHz f = f + F Lo RF IF = f + 455kHz Rf 995kHz to 2055kHz Prepared by Prof.V.K.Jain 8

9 AM Receiver Operation Image frequency Image Frequency: is any frequency other than the selected carrier that, if allowed to enter receiver and mix with local carrier, will produce a cross product frequency equal to IF. 2f IF f IF f IF IF RF LO Image f = f + 2 f image RF IF Frequency Prepared by Prof.V.K.Jain 9

10 AM Receiver Operation Image-frequency rejection ratio Image rejection ratio is a measure of ability of preselector to reject the image frequency. Image rejection ratio is the ratio of voltage gain at input frequency at which receiver is tuned to the gain at image frequency. Asig IR = = 1+ Q x A image fimage x = f sig f f sig image 2 2 Prepared by Prof.V.K.Jain 10

khz, and 1055 khz, refer to Figure and determine: Image frequency & IR (consider Q = 100)

11 AM Receiver Operation Image-frequency rejection ratio For an AM broadcast-band superheterodyne receiver with IF, RF, and local oscillator frequencies of 455 khz, 600 khz, and 1055 khz, refer to Figure and determine: Image frequency & IR (consider Q = 100) f = f + 2 f image RF IF IR = + = Q x = 20log(211.3) = 46.5 db Prepared by Prof.V.K.Jain 11

12 AM Receiver Operation Automatic Gain Control (AGC) Prepared by Prof.V.K.Jain 12

13 SSB Receivers With SSBSC or DSBSC a product detector can be used. Since DSBSC is seldom used by itself, it would be treated as SSB with one sideband removed in IF before detection. Prepared by Prof.V.K.Jain 13

14 SSB Receivers e () t E sin(2 π f t) m c c ' ( π ) ( π ϕ) vt () = e () tesin 2 ft Esin 2 ft+ m c c c c { π ( ) ϕ π( ) ϕ } ' EE c c = em( t) cos 2 cos 2 2 fc fc t fc + fc t+ ' EE c c = em() t { cos( ϕ) + cos 2 ( 2fc) t } 2 π + ϕ ' EE c c vo() t = cos ( ϕ ) em() t 2 = αe () t m Prepared by Prof.V.K.Jain 14

15 Noncoherent SSB-BFO Receivers Ganged tuning Prepared by Prof.V.K.Jain 15

16 Noncoherent SSB-BFO Receivers An SSBSC signal has a suppressed carrier frequency in the IF of a receiver, of 2 MHz. It is modulated with two audio tones having frequencies of 1500 Hz and 2200 Hz. The receiver is mistuned so that the BFO is at MHz. What will be the output frequencies of the demodulator if the signal is LSB? MHz MHz 2MHz 1000 Hz 1700 Hz f Ganged tuning MHz Prepared by Prof.V.K.Jain 16

Noncoherent SSB-BFO Receivers For the BFO receiver shown in figure below, a received RF frequency band of 30 MHz to 30.

17 Noncoherent SSB-BFO Receivers For the BFO receiver shown in figure below, a received RF frequency band of 30 MHz to MHz, an RF local oscillator frequency of 20 MHz, an IF frequency band of 10 MHz to MHz, and a BFO frequency of 10 MHz, determine Ganged tuning Demodulated information frequency band. Demodulated information frequency band if the RF local oscillator frequency drifts down 0.001% Prepared by Prof.V.K.Jain 17

18 Noncoherent SSB-BFO Receivers 30 MHz MHz 10 MHz MHz Hz Ganged tuning 20 MHz 10 MHz Prepared by Prof.V.K.Jain 18

19 Noncoherent SSB-BFO Receivers 30 MHz MHz MHz MHz 200 Hz 5200 Hz Ganged tuning f LO = 20 MHz 200 Hz = MHz 10 MHz Prepared by Prof.V.K.Jain 19

20 Coherent SSB-BFO Receiver Prepared by Prof.V.K.Jain 20

21 Coherent SSB-BFO Receiver For the coherent SSB BFO receiver, an RF reduced carrier frequency of 30 MHz with USB extends from 30 MHz to MHz, an RF local oscillator of 20 MHz, an IF center frequency of 10 MHz, and BFO frequency of 10 MHz, determine: Demodulated information signal, Demodulated information band if the reduced RF carrier drifted upwards 60 Hz producing RF carrier of MHz and USB extends to MHz. Prepared by Prof.V.K.Jain 21

22 Coherent SSB-BFO Receiver 30 MHz MHz 10 MHz MHz 0 5 KHz 20MHz 10MHz Prepared by Prof.V.K.Jain 22

23 Coherent SSB-BFO Receiver MHz MHz MHz MHz 0 5 KHz MHz MHz Prepared by Prof.V.K.Jain 23

24 Points to remember Receiver must separate the desired signal from other signals and noise and then demodulate the signal. The most common receiver type is superheterodyne, which uses a mixer to transfer all incoming signals to a common IF. The RF amplifier is principal responsible for setting noise figure for receiver. Single-sideband receivers require a beat-frequency oscillator to reinsert the carrier and generally use product detector. ***** Prepared by Prof.V.K.Jain 24

Introduction to Receivers

Introduction to Receivers Purpose: translate RF signals to baseband Shift frequency Amplify Filter Demodulate Why is this a challenge? Interference Large dynamic range required Many receivers must be capable