18 FM_Demodulators-Foster_Seeley_and_Ratio_Detector.pdf

FM

1. 1
Foster-Seeley
Discriminator
Prof.Dr.G.Aarthi,
Associate Professor, SENSE

2. Foster-Seeley (Phase) Discriminator
•Inthisallthetunedcircuitsaretunedtothe
samefrequency.
•BalancedSlopeDetectorcircuitwithsome
changesisused.
•Thiscircuityieldsfarbetterlinearitythan
slopedetection.
2

3. Phase Discriminator
3

4. The capacitances Cc,C1 and C2 are chosen such that they are short
circuits for the IF center frequency.
The IF signal is fed directly across L3. V
L3V
IN
Incoming IF is inverted 180 degree by T1 and divided equally between
L
aand L
b.
So Voltage across diode= V
IN+ Secondary Voltage/2
Phase Discriminator
4

5. WhenIFcenterfrequency(f
c)=f
o,thensecondarycurrentI
sisin
phasewithSecondaryvoltageV
sand180
o
outofphasewithV
L3.
V
Laand V
Lbare 180
o
out of
phase with each other and
90
o
out of phase with V
L3.
AtResonant Frequency
5

6. V
D1=V
L3+V
Laand V
D2=V
L3+V
Lb
The voltages across the diodes D
1and D
2are equal.
Therefore,I
1andI
2areequal.
C
1andC
2chargetoequal
Magnitudevoltagesexcept
Withoppositepolarities
V
out=V
c1-V
c2=0V
AtResonant Frequency
6

7. If IF(f
c)>f
othen X
L>X
C, the
secondary tank circuit impedance
becomes inductive.
Is lags V
sby some angle θwhich is
proportional to the magnitude of
the frequency deviation.
The vector sum of voltage across
D
1>D
2
C
1charges and C
2discharges and
V
out=V
c1-V
c2=+veVoltage
IF goes above Resonance
7

8. If IF(f
c)<f
othen X
L<X
C, the
secondary tank circuit
impedance becomes capacitive.
I
sleads V
sby some angle θ
which is proportional to the
magnitude of the frequency
deviation.
The vector sum of voltage
across
D
1<D
2
C
1discharges and C
2charges
and
V
out=V
c1-V
c2= -veVoltage
IF goes below Resonance
8

9. Vector Diagrams
9

10. Voltage Vs Frequency Response
10

11. Phase Discriminator
•Itismucheasiertoalign,astherearenowtwotuned
circuitsandbotharetunedtothesamefrequency.
•Linearityisquitebetter,ascircuitrelieslesson
frequency&moreonprimary-secondaryphase
relation,whichisquitelinear.
•Onlydrawbackis,thereisnoprovisionforamplitude
limiting.
11

12. Ratio-Detector
•RatiodetectordemodulatorismodifiedFoster-
Seeleycircuitinordertoincorporateamplitude
limiting.
•InFoster-Seeleydiscriminatorthesumofvoltages
V
D1+V
D2shouldremainconstant.
•Theirdifferenceshouldvaryduetovariationininput
frequency.
12

13. Ratio-detector
•Butpracticallyspeakinganyvariationinthe
amplitudeofinputsignal,alsohasimpacton
sumofV
D1+V
D2,leadingtodistortion.
•Ratio-detectorcircuiteliminatesthisvariation
ofV
D1+V
D2,andperformsthefunctionof
amplitudelimiteralso.
13

14. Ratio-Detector
Changes made in Foster-Seeley discriminator:
•One of The diodes has been reversed.
•A large capacitor has been placed between
points, from where output was taken.
14

15. Ratio-Detector
15

16. Ratio-Detector
Aswithfosterseeleydiscriminator,aratiodetectorhasasingle
tunedcircuitinthesecondary.(operationissimilar).
DiodeD2isreversedsothatcurrentcanflowaroundthe
outermostloopofthecircuit.
16

17. Ratio-Detector
Afterseveralinputcycles,shuntcapacitorCschargesto
approximatelythepeakvoltageacrossthesecondarywindingof
T1.
ThetimeconstantforRsandCsissufficientlylongsothatrapid
changesintheamplitudeoftheinputsignal,havenoeffecton
theaveragevoltageacrossCs. 17

18. Ratio-Detector
C1andC2chargeanddischargeproportionaltofrequency
changesintheinputsignalandarerelativelyimmuneto
amplitudevariations.
AtResonance,theoutputvoltageisdividedequallybetweenC1
andC2andredistributedastheinputfrequencyisdeviated
aboveandbelowresonance.
18

19. Ratio-Detector
At resonance ,Voutis not equal to 0V, but rather to one half of
the voltage across the secondary windings of T1.
19

20. Voltage Vs Frequency Response
20

21. Ratio-Detector Advantages
•Amplitude limiting is possible.
•Linearity is quite good as compared to others.
So quite often used in high quality receivers.
21

22. Ratio-Detector Dis-advantages
•Undercriticalnoiseconditions,suchassatellite
receivers,wheredemodulatornoiseperformance
becomesverysignificant,eventhisdemodulatoris
foundwanting.
•Undertheseconditionsmoreadvanced
demodulatorssuchasPhaseLockedLoopareused.
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