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

2. Foster-Seeley (Phase) Discriminator
• In this all the tuned circuits are tuned to the
same frequency.
• Balanced Slope Detector circuit with some
changes is used.
• This circuit yields far better linearity than
slope detection.
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3. Phase Discriminator
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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. VL3 VIN
Incoming IF is inverted 180 degree by T1 and divided equally between
La and Lb.
So Voltage across diode= VIN + Secondary Voltage/2
Phase Discriminator
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5. When IF center frequency (fc)=fo, then secondary current Is is in
phase with Secondary voltage Vs and 180o out of phase with VL3.
VLa and VLb are 180o out of
phase with each other and
90o out of phase with VL3.
At Resonant Frequency
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6. VD1=VL3+VLa and VD2=VL3+VLb
The voltages across the diodes D1 and D2 are equal.
Therefore, I1 and I2 are equal.
C1 and C2 charge to equal
Magnitude voltages except
With opposite polarities
Vout=Vc1-Vc2=0V
At Resonant Frequency
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7. If IF(fc)>fo then XL>XC, the
secondary tank circuit impedance
becomes inductive.
Is lags Vs by some angle θ which is
proportional to the magnitude of
the frequency deviation.
The vector sum of voltage across
D1>D2
C1 charges and C2 discharges and
Vout=Vc1-Vc2=+ve Voltage
IF goes above Resonance
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8. If IF(fc)<fo then XL<XC, the
secondary tank circuit
impedance becomes capacitive.
Is leads Vs by some angle θ
which is proportional to the
magnitude of the frequency
deviation.
The vector sum of voltage
across
D1<D2
C1 discharges and C2 charges
and
Vout=Vc1-Vc2= -ve Voltage
IF goes below Resonance
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9. Vector Diagrams
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10. Voltage Vs Frequency Response
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11. Phase Discriminator
• It is much easier to align, as there are now two tuned
circuits and both are tuned to the same frequency.
• Linearity is quite better, as circuit relies less on
frequency & more on primary-secondary phase
relation, which is quite linear.
• Only drawback is, there is no provision for amplitude
limiting.
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12. Ratio- Detector
• Ratio detector demodulator is modified Foster-
Seeley circuit in order to incorporate amplitude
limiting.
• In Foster-Seeley discriminator the sum of voltages
VD1+VD2 should remain constant.
• Their difference should vary due to variation in input
frequency.
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13. Ratio-detector
• But practically speaking any variation in the
amplitude of input signal, also has impact on
sum of VD1+VD2, leading to distortion.
• Ratio-detector circuit eliminates this variation
of VD1+VD2, and performs the function of
amplitude limiter also.
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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.
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15. Ratio-Detector
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16. Ratio-Detector
As with foster seeley discriminator, a ratio detector has a single
tuned circuit in the secondary.(operation is similar ).
Diode D2 is reversed so that current can flow around the
outermost loop of the circuit.
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17. Ratio-Detector
After several input cycles , shunt capacitor Cs charges to
approximately the peak voltage across the secondary winding of
T1.
The time constant for Rs and Cs is sufficiently long so that rapid
changes in the amplitude of the input signal, have no effect on
the average voltage across Cs. 17

18. Ratio-Detector
C1 and C2 charge and discharge proportional to frequency
changes in the input signal and are relatively immune to
amplitude variations.
At Resonance, the output voltage is divided equally between C1
and C2 and redistributed as the input frequency is deviated
above and below resonance. 18

19. Ratio-Detector
At resonance ,Vout is not equal to 0V, but rather to one half of
the voltage across the secondary windings of T1.
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20. Voltage Vs Frequency Response
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21. Ratio-Detector Advantages
• Amplitude limiting is possible.
• Linearity is quite good as compared to others.
So quite often used in high quality receivers.
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22. Ratio-Detector Dis-advantages
• Under critical noise conditions, such as satellite
receivers, where demodulator noise performance
becomes very significant, even this demodulator is
found wanting.
• Under these conditions more advanced
demodulators such as Phase Locked Loop are used.
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