Receivers

Introduction to Receivers used in Communication

1. RECEIVERS
PRESENTED BY
A.ABHISHEK REDDY
ASSISTANT PROFESSOR, MECS.
Mobile No.:8121444996
1 SESSION 2

2. RECEIVERS
Radioreceiverisanelectronicequipmentwhich
pickupsthedesiredsignal,rejecttheunwanted
signalanddemodulatethecarriersignaltoget
backtheoriginalmodulatingsignal.
2 SESSION 2

3. Function of Radio Receivers
Intercept the incoming modulated signal
Select desired signal and reject unwanted signals
Amplify selected R.F signal
Detect modulated signal to get back original
modulating signal
Amplify modulating frequency signal
3 SESSION 2

4. Design of Receiver
The radio receiver has to be cost effective
Requirements:
–Has to work according to application as for AM
or 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
4 SESSION 2

5. Classification of Radio Receivers
Depending upon application
AM Receivers-receive broadcast of speech or
music from AM transmitters which operate on
long wave, medium wave or short wave bands.
FM Receivers –receive broadcast programs from
FM transmitters which operate in VHF or UHF
bands.
5 SESSION 2

6. CONTD…
Communication Receivers-used for reception of
telegraph and short wave telephone signals.
Television Receivers-used to receive television
broadcast in VHF or UHF bands.
Radar Receivers–used to receive radio detection
and ranging signals.
6 SESSION 2

7. CONTD…
Depending upon fundamental aspects
Tuned Radio Frequency (TRF)Receivers
Super-heterodyne Receivers
7 SESSION 2

8. RECEIVERS
Tuned Radio Frequency (TRF) Receiver:
Composed of RF amplifiers and detectors.
No frequency conversion
It is not often used.
Difficult to design tunable RF stages.
Difficult to obtain high gain RF amplifiers
8 SESSION 2

9. Super-hetrodyne Receiver
Downconvert RF signal to lower IF
frequency
Main amplifixcation takes place at IF
Communication Receiver
Downconvert RF signal to twoIF frequency
9 SESSION 2

10. TRF (Tuned Radio frequency) RECEIVER
10 SESSION 2

11. SESSION 211

12. TRF receiver includes an
–RF stage
–a detector stage
–and an audio stage.
TwoorthreeRFamplifiersarerequiredtofilterand
amplifythereceivedsignaltoalevelsufficientto
drivethedetectorstage.
12 SESSION 2

13. RF section (Receiver front end)
used to detect the signal
band limit the received RF signal
and amplifying the received RF signal.
AM detector
Demodulates the AM wave and converts it
to the original information signal.
Audio section
Used to amplify the recovered signal13 SESSION 2

14. Advantages of TRF
TRF receivers are simple to design and allow the
broadcast frequency 535 KHz to 1640 KHz.
High sensitivity.
No problem of tracking & alignment
14 SESSION 2

15. Disadvantages of TRF
At the higher frequency, it produces difficulty in
design.
It has poor audio quality.
Drawbacks
–Instability
–Variation in BW
–Poor Selectivity
15 SESSION 2

16. INSTABILITY
Duetohighfrequency,multistageamplifiersare
susceptibletobreakingintooscillation.
AsgainofRFamplifierisveryhigh,asmallfeedback
fromoutputtoinputwithcorrectphasecanleadto
oscillations.
Correctphasemeansapositivefeedbackandittakesplace
duethroughstraycapacitances
Asreactanceofstraycapacitancesdecreasesathigher
frequenciesresultinginincreasedfeedback.
Forcingthedevicetoworkasanoscillatorinsteadofan
amplifier.
16 SESSION 2

17. VARIATION IN BANDWIDTH
Thebandwidthisinconsistentandvarieswith
thecenterfrequencywhentunedoverawide
rangeofinputfrequencies.
Asfrequencyincreases,thebandwidth(f/Q)
increases.Thus,theselectivityoftheinputfilter
changesoveranyappreciablerangeofinput
frequencies.
17 SESSION 2

18. Example
Suppose required BW=10KHz
We have f1=545KHz,f2=1640KHz
Q1= f1/BW= 54.5 ,
Q2=f2/BW=164
But practically Q is limited upto 120
Considering Q limit 120 , BW changes to13.6 KHz
( as BW=f2/Q2=1640/120)
So Adjacent channel is picked up resulting in variation in
bandwidth.
18 SESSION 2

19. POOR SELECTIVITY
–The gains are not uniform over a very wide
frequency range.
–Due to higher frequencies ability to select
desired signal is affected.
–Suffers from OVER and UNDER selective
problems
Due to these drawbacks TRF are rarely used.
19 SESSION 2

20. SUPER HETRODYNE RECEIVER
TheshortcomingsoftheTRFreceiverareovercome
bythesuperheterodynereceiver.
20 SESSION 2

21. Heterodyne–tomixtwofrequenciestogetherina
nonlineardeviceortotransmitonefrequencyto
anotherusingnonlinearmixing.
Alsoknownasfrequencyconversion,high
frequencydownconvertedtolowfrequency.(IF)
Asuperheterodynereceiverconvertsallincoming
radiofrequency(RF)signalstoalowerfrequency
knownasan“IntermediateFrequency”(IF).
21 SESSION 2

22. DRAWBACKS OVERCOMED
–Stability–ashighfrequencyisdownconvertedto
IFthereactanceofstraycapacitanceswillnot
decreaseasitwasathigherfrequenciesresulting
inincreasedfeedback.
–NovariationinBW-asIFrangeis438to465
KHz(incaseofAMreceivers)mostly455KHz
,appropriateforQlimit(120).
–Betterselectivity-asnoadjacentchannelsare
pickedduetovariationinBW.
22 SESSION 2

23. 23 SESSION 2

24. SESSION 224

25. RF AMPLIFIER
Tuned voltage amplifier that selects and amplifies
signals from 540KHz to 1650KHz with BW of 10KHz
Main functions:
A.To give high gain
B.To have good SNR and better sensitivity
C.To have good image frequency rejection & IF rejection capability
D.To have better adjacent channel selectivity
25 SESSION 2

26. LOCAL OSCILLATOR
Variable oscillator ranging from 0.995MHz to
2.105MHz.
Provide predetermined fixed frequency called
Intermediate Frequency signal(455KHz).
Always LO frequency is kept higher than the signal
frequency.
26 SESSION 2

27. MIXER
It is a nonlinear device to convert radio frequent
to IF and also generates sum and difference
frequency components in addition to the original
frequency.
The output of the mixer is always a IF signal
which is modulated by the original message
signal.
Mixer output is designed to have a 3db BW of
10KHz to accommodate all side –frequencies of
the AM signal
27 SESSION 2

28. IF AMPLIFIER
It is a tuned voltage amplifier of high gain.
Consists of 1or 2 stages of IF amplifier and
band pass filter to achieve gain and
selectivity.
Provide good sensitivity and selectivity
28 SESSION 2

29. DETECTOR
Extracts the modulating signal from AM
signal.
In commercial broadcast receivers ,
envelope detectors are used which require
minimum of 1V amplitude for proper
operation.
Designed to provide linear operation and
avoid distortion
29 SESSION 2

30. AGC ( Automatic Gain Control )
–AdjusttheIFamplifiergainaccordingtosignallevel(tothe
averageamplitudesignalalmostconstant).
–AGCisasystembymeansofwhichtheoverallgainofradio
receiverisvariedautomaticallywiththevariationsinthe
strengthofreceivedsignals,tomaintaintheoutputconstant.
---AGCcircuitisusedtoadjustandstabilizethefrequencyof
localoscillator.
TypesofAGC–
NoAGC
SimpleAGC
DelayedAGC
30 SESSION 2

31. AUDIO AMPLIFIER
The detected modulating signal is amplified
to drive the loudspeaker.
31 SESSION 2

32. FREQUENCYCONVERSION
inthemixerstageisidenticaltothefrequencyconversioninthe
modulatorexceptthatinthereceiver,thefrequenciesaredown-
convertedratherthatup-converted.
–Inthemixer,RFsignalsarecombinedwiththelocaloscillator
frequency
–Thelocaloscillatorisdesignedsuchthatitsfrequencyof
oscillationisalwaysaboveorbelowthedesiredRFcarrierbyan
amountequaltotheIFcenterfrequency.
–ThereforethedifferenceofRFandoscillatorfrequencyisalways
equaltotheIFfrequency
32 SESSION 2

33. Theadjustmentforthecenterfrequencyofthepre-selectorandthelocal
oscillatorfrequencyaregang-tune(thetwoadjustmentsaretiedtogether
sothatsingleadjustmentwillchangethecenterfrequencyofthepre-
selectorandatthesametimechangethelocaloscillator)
whenlocaloscillatorfrequencyistunedabovetheRF–highside
injection
whenlocaloscillatorfrequencyistunedbelowtheRF–lowside
injection
Mathematicallyexpressed:
Highsideinjection
LowsideinjectionIFRFlo fff  IFRFlo fff 
33 SESSION 2

34. 34 SESSION 2

35. EXAMPLE OF TUNING PROCESS
Select an AM station i.e. 640KHz
Tune the RF amplifier to lower end of AM band say 1095KHz
Mix 1095 KHz and 640 KHz which produces the following signals :
i.1.095 MHz local oscillator frequency
ii.640 KHz AM station carrier frequency.
iii.455 kHz difference frequency
iv.1.735 MHz sum frequency.
Bcz of narrow bandwidth , the IF amplifier rejects all other frequencies
except 455KHz. This reduces the risk of interference from other
stations . This is the key to the superheterodyne.
The process of tuning the LO to a predetermined frequency for each
station throughout the AM band is known as “TRACKING”.
35 SESSION 2

36. COMPARISON
TRF Receiver
No frequency conversion
No IF frequency
Instability , variation in
BW and poor selectivity
due to high frequencies
Difficult to design tunable
RF stages.
Rarely used
Super hetrodyne Receiver
Frequency conversion
Downconvert RF signal to
lower IF frequency
No instability,variation in
BW and poor selectivity
as IF introduced.
Main amplifixcation takes
place at IF
Mostly used
36 SESSION 2

37. THANKU
SESSION-237