Analog & Digital Communication

UNIT I

www.EEENotes.in

Super Heterodyne Receiver

www.EEENotes.in

Outline Block diagram of heterodyne receiver Descriptions of various sections of

receiver Receiver operation Various issues

Frequency conversion Local oscillator tracking Image frequency

www.EEENotes.in

Principles Frequency Spectrum Sharing (many

transmitters using one medium) Demodulating desired signal and rejecting

other signals transmitted at the same time

www.EEENotes.in

Source signal The source signal is audio Different sources have different spectrum

Voice (speech) Music Hybrid signals (music, voice, singing)

www.EEENotes.in

Different audio sources have different bandwidth W

Speech- 4kHz High quality music- 15kHz AM radio limits base-band bandwidth W

to 5kHz FM radio uses base-band bandwidth W

to 15kHz

www.EEENotes.in

Typical radio system Radio system should be able to receive

any type of audio source simultaneously. Different stations with different sources

transmit signals simultaneously. Different listeners tune to different stations

simultaneously.

www.EEENotes.in

Requirement The radio receiver has to be cost effective Requirements:

Has to work with both AM and FM signals Tune to and amplify desired radio station Filter out all other stations Demodulator has to work with all radio

stations regardless of carrier frequency

www.EEENotes.in

IF For the demodulator to work with any radio

signal, we convert the carrier frequency of any radio signal to Intermediate Frequency (IF)

Radio receiver design can be optimized for that frequency

IF filter and a demodulator for IF frequency

www.EEENotes.in

Basic radio receiver A radio receiver consists of the following:

A Radio Frequency (RF) section An RF-to-IF converter (mixer) An Intermediate Frequency (IF) section Demodulator Audio amplifier

www.EEENotes.in

This is known as the Superheterodyne Receiver

Two stages: RF and IF (filtering and amplification)

The receiver was designed by Armstrong

www.EEENotes.in

Transmission Bandwidth Transmission bandwidth: BT BT is the bandwidth occupied by a

message signal in the radio frequency spectrum

BT is also the carrier spacing AM bandwidth is BT = 2W

www.EEENotes.in

Important The minimum bandwidth of RF filter (front

end tuner) is BRF > BT

Passes the desired radio channel, and adjacent channels

www.EEENotes.in

Block diagramwww.EEENotes.in

Heterodyning Mixing of two different frequencies in a

non-linear device Translation of one frequency to another

using non-linear mixing Basically does multiplication job

www.EEENotes.in

Various sections RF section Mixer or converter section IF section Audio detector section Audio amplifier section

www.EEENotes.in

RF section RF section has pre-selector and amplifier

www.EEENotes.in

Pre-selector Band pass filter Broad tuned i.e. adjustable centre

frequency Blocks image frequency Reduces noise bandwidth Provides initial step to limit the receiver

bandwidth

www.EEENotes.in

RF amplifier Determines sensitivity of receiver First active device in receiver path Noise added at this stage should be as

minimum as possible Very important in deciding noise figure of

receiver

www.EEENotes.in

Mixer/converter section

OscillatorMixer stage

www.EEENotes.in

Local oscillator Wein bridge oscillator Hartley oscillator Clapp oscillator Selecting a particular oscillator circuit

depends on Stability Accuracy

www.EEENotes.in

Mixer Converts radio frequencies (RF) to intermediate

frequencies (IF) Non-linear device Heterodyning takes place in the mixer stage

RF to IF Carrier and side band frequencies converted

from RF to IF Envelope remains same Bandwidth remains same Typical IF is 455 KHz

www.EEENotes.in

Mixer or Balanced modulator Two inputs to mixer or balanced modulator Output of balanced mixer is product of two input

signals( )( )

( )[ ] ( )[ ]tfftfftftfVout

2121

21

2cos212cos

21

2sin2sin

+=

=

pipi

pipi

Output of mixer has both sum and difference of input frequenciesBy placing an appropriate filter either one of the terms can be selected

www.EEENotes.in

One input to mixer : from pre-selector, chosen station i.e. fc

Another input : from local oscillator, flo Output of mixer : (flo-fc) and (flo+fc) Choose flo always greater than fc by 455

KHz Then difference term will produce 455 KHz

irrespective of fc

www.EEENotes.in

IF sectionBand pass filterSelects only 455 KHz (differenceterm)Rejects sum term

IF amplifier

www.EEENotes.in

IF section Gives most of receivers gain and selectivity IF center frequency and bandwidth are

constant for all stations IF is lesser than the lowest available AM

station frequency Easier to construct amplifiers and filters at

lower frequencies IF amplifier more stable than RF amplifier

i.e. no break down oscillation 5 to 6 stages of IF amp, but only single RF amp

www.EEENotes.in

Detector section Converts IF signals to original source

information Also called audio detector Envelope detector

www.EEENotes.in

Audio amplifier section Several amplifiers cascaded Output is given to one or more speakers Number of amplifiers depends on output

power required

www.EEENotes.in

Basic receiver operation Demodulation process undergoes two or

more frequency conversions RF to IF then IF to source information RF and IF are relative terms

No absolute values In commercial broadcast AM receiver (535

KHz to 1605 KHz) IF may be 455 KHz

In commercial FM broad cast receiver IF may be 10.7 MHz

www.EEENotes.in

Frequency conversion Identical to modulator stage of transmitter Difference is we do down conversion

instead of up conversion

www.EEENotes.in

Principle of frequency conversion

The combination of any two sine waves through a nonlinear device produce the following frequency components:

1. A dc level2. Components at each of the 2 original

frequencies.3. Components at the sum and difference

frequencies of the 2 original frequencies.4. Harmonics of the original frequencies.

www.EEENotes.in

Examplewww.EEENotes.in

Gang tuning Both pre-selector and local oscillator are

tuned together When station is changed in AM receiver

Center frequency is changed in pre-selector Together local oscillator frequency is changed Difference between them is always 455 KHz

www.EEENotes.in

Local oscillator tracking When ever station is changed oscillator

frequency is also changed Oscillator frequency changes so that when

it combines with antenna signal at mixer stage it always produces IF (455 KHz for commercial AM)

This ability of oscillator is called tracking

www.EEENotes.in

www.EEENotes.in

Tuning ratio AM stations range from 540 KHz to 1600

KHz Pre-selector center frequency changed

from 540 KHz to 1600 KHz Tuning ratio is 2.96

Local oscillator frequency changed from 995 KHz to 2055 KHz Tuning ratio is 2.06

www.EEENotes.in

AM station range is:540 KHz to 1600 KHz 455 KHz IF may be obtained by using a

local oscillator which has rangefrom 85 KHz to 1145 KHz

ORfrom 995 KHz to 2055 KHz

1st method offers ratio of 13 2nd method offers ratio of 2

www.EEENotes.in

Remember

More tuning ratio more difficult to design an oscillator

www.EEENotes.in

Capacitance tuning ratio Resonant frequency proportional to (1/C) Capacitance in pre-selector changes by

8.8 Capacitance in oscillator changes by 4.26

www.EEENotes.in

Tracking error Local oscillator always should oscillate

455 KHz above the pre-selector frequency Difference between them is always 455

KHz If it deviates then the amount is called

tracking error Tracking error of 3 KHz is allowed

www.EEENotes.in

Tracking errorwww.EEENotes.in

Electronic tuning Instead of big ganged capacitors use

reverse biased diodes Reverse bias p-n junction Depletion layer changes with applied

reverse bias Depletion capacitance also changes Use this for tuning purpose Varactor diodes

www.EEENotes.in

Electronic tuningwww.EEENotes.in

Example We have selected an AM station whose

frequency is 600 KHz To get an IF of 455 KHz local oscillator

has to oscillate with 600 KHz + 455 KHz = 1055 KHz

www.EEENotes.in

600KHz

From antenna

1055KHz

605KHz

595KHz

www.EEENotes.in

Suppose 1510 KHz is also present at the input of mixer, then?

Difference between 1510 KHz and 1055 KHz is also 455 KHz

We get information from 600 KHz AM station as well as from 1510 KHz AM station

No way to differentiate them

www.EEENotes.in

Only absolute difference matters

www.EEENotes.in

Image frequency Image frequency is radio frequency that is

located in IF frequency above local oscillator frequency

fimage=flocal_osc+fIFSubstituing, flocal_osc=fRF+fIF

fimage=fRF+2.fIF

www.EEENotes.in

Solution to Image problem Go for higher IF

Puts image farther in spectrum Problem of choosing higher IF

Stability of IF amplifier poorer Trade off

Get rid of image and stability

www.EEENotes.in

Solution to Image problem If pre-selector does not allow image

frequency then there is no image problem How to make pre-selector do this? Reduce bandwidth of pre-selector How to reduce bandwidth of pre-selector? Go for higher Q factor

www.EEENotes.in

Image frequency rejection ratioIFRR=(1 + Q22)=(fim/fRF) (fRF/fim)IFRR(dB)=10 log(IFRR)

www.EEENotes.in

AM detectors Peak detector Non-coherent Non-linear device

diode Half-wave rectifier

with capacitor

www.EEENotes.in

Peak detector wave formswww.EEENotes.in

Positive peak detector

Irrespective of modulation depth the output average is always carrier peakThis fact is used in AGC

www.EEENotes.in

Detector distortionwww.EEENotes.in

Highest modulating frequency that can be detected using peak detector is given by

RCmf m pi2

11 2(max)

=

where m modulation indexRC time constant What about 100% modulation?

www.EEENotes.in

RF signal strength RF signal travels through space Signal strength varies depending on

environment also depending on transmitter location

If not taken care then speaker output (volume) changes randomly

www.EEENotes.in

Automatic gain control Takes care minor variations in signal

strength AGC automatically increases gain when

received signal is weaker and vice versa Monitors signal strength Adjusts gain of RF and IF amplifiers

automatically

www.EEENotes.in

AM receiver with AGCwww.EEENotes.in

Simple AGC

www.EEENotes.in

AGC operation Output of peak detector is used as feed

back Remember average voltage of peak

detector o/p only depends on carrier strength

Higher signal strength higher negative voltage at peak detector o/p

This reduces positive bias at Q1, and thereby gain

www.EEENotes.in

Problem of using AGC Assume radio station is not broadcasting

anything Then carrier strength is zero AGC increases the gain of amplifier

indefinitely Receiver amplifies its own noise and

outputs to speaker Quiet receiver when there is no signal

Squelch circuit does this

www.EEENotes.in