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MAYURBHANJ SCHOOL OF ENGINEERING � LAXMIPOSI ,BARIPADA,757107

DEPARTMENT- E&TC ENGG.
SEMISTAR- 5^TH
SUBJECT-A & D Communication
TOPIC – 3 - Angle Modulation
NAME OF TOPIC –FM Demodulation
PREPARED BY - U S Panda (Sr. Lect. E & TC Engineering)
AY – 2021-2022, WINTER-2021

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FM Demodulation

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Methods for Frequency Demodulation

What are Frequency demodulators ?
They produce output voltage whose instantaneous amplitude is directly proportional to the instantaneous frequency of the input FM wave.
Methods:

Balanced Slope Detector
Foster Seely Descriminator
Phase locked loop demodulator.
Ratio-detector. (old)

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Basic Idea

where = demodulated output signal (Volts)

= demodulator transfer function (Volts per Hertz)

= difference between the input frequency and the center frequency of the demodulator (Hertz).

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Balanced Slope Detector

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Advantages and Limitations

Advantages:

(i) This circuit is more efficient than simple slope detector.

(ii) It has better linearity than the simple slope detector.

Limitations:

(i) Even though linearity is good, it is not good enough.

(ii) This circuit is difficult to tune since the three tuned circuits are to be tuned at different frequencies, and

(iii) Amplitude limiting is not provided.

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��Foster-Seeley Discriminator� (Phase Discriminator)

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Foster-Seeley Discriminator

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Foster-Seeley…

Advantages:

using procedure is simpler than balanced slope detector, because it contains only two tuned circuits and both are tuned to the same frequency .
Better linearity, because the operation of the circuit is dependent more on the primary to secondary phase relationship which is very much linear.

Limitations:

It does not provide amplitude limiting. So in the presence of noise or any other spurious amplitude variations, the demodulator output respond to them and produce errors.

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Ratio Detector

Similar to the Foster-Seeley discriminator .

(i) The direction of diode is reversed.

(ii) A large capacitance C_s is included in the circuit.

(iii) The output is taken different locations.

Advantages:

Easy to align.
Good linearity due to linear phase relationship between primary and secondary.
Amplitude limiting is provided inherently. Hence additional limiter is not required.

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Ratio Detector

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Phase-locked Loops

PLLs when fed with an FM signal directly produce an output signal that is proportional to the message signal.
PLL has low cost and superior performance even at low SNR (signal-to-noise ratio)
Where do we take the output? Compare with the case of carrier acquisition.

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Different Phase Detector Characteristics

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Performance Comparison of FM Demodulators�

S.No.	Parameter of Comparison	Balanced Slope detector	Foster-Seeley (Phase) discriminator	Ratio Detector
(i)	Alignment/tuning	Critical as three circuits are to be tuned at different frequencies	Not Critical	Not Critical
(ii)	Output characteristics depends on	Primary and secondary frequency relationship	Primary and secondary phase relation.	Primary and secondary phase relation.
(iii)	Linearity of output characteristics	Poor	Very good	Good
(iv )	Amplitude limiting	Not providing inherently	Not Provided inherently	Provided by the ratio detector.
(v)	Amplifications	Not used in practice	FM radio, satellite station receiver etc.	TV receiver sound section , narrow band FM receivers.

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