1. Analog Communications
Dr. M. Venu Gopala Rao
A.M.I.E.T.E, M.Tech, Ph.D(Engg)
Cert. in R.S.T ( City & Guild’s London Institute, London)
F.I.E.T.E, L.M.I.S.T.E, I.S.O.I., S.S.I., M.I.A.E.
Professor, Dept. of ECE, K L University

2. FM Generation

3. Indirect FM Generation Armstrong Method

4. Indirect FM (Armstrong)Generation
Step1 Generation of NBFM

5. Step2: Frequency Multiplication
Memoryless Non-linear operation
Band Pass Filter (BPF):
WBFM Generation

7. Example
Design an WBFM transmitter to transmit audio signals containing frequencies in the range 100 Hz to 15 kHz. The NBFM is supplied with a carrier wave of frequency f1 = 0.1 MHz by a crystal controlled oscillator. The desired FM wave at the transmitter output has a carrier frequency of fC = 100 MHz and frequency deviation 75 kHz.

8. Example
Let
Then
To produce
Frequency Multiplication
Two stages

9. Two stages

10. Direct FM Generation / Parameter Variation Method

11. 1. FM Reactance Modulators

12. The gate voltage
Then the drain current
Impedance
Assuming that
the impedance

13. When the modulating signal is applied, the gate to source voltage varied accordingly, causing proportional change in gm . As a result the frequency of oscillator tank circuit is a function of the amplitude of the modulating signal and the rate at which it changes is equal to fm .

14. 2. Varactor Diode Modulator
The capacitance of a varactor diode is inversely proportional to the reversed biased voltage amplitude.
where
For m(t) , the capacitance
where

15. Frequency stabilized FM modulator
The carrier frequency is not obtained from a highly stable oscillator.

16. Limitations of direct methods of FM generation
Difficult to obtain a high order of stability in carrier frequency because tank circuit consists of L and C.
The crystal oscillator can be used for carrier frequency stability, but frequency deviation is limited.
The non linearity produces a frequency variation due to harmonics of the modulating signal hence there are distortions in the output FM signal.

17. FM Demodulators

18. 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).

19. 1. Slope Detector

20. Slope Detector . . .
Advantages: The only advantage of the basic slope detector circuit is its simplicity. Limitations: (i) The range of linear slope of tuned circuit is quite small. (ii) The detector also responds to spurious amplitude variations of the input FM. These drawbacks are overcome by using balanced slope detector.

21. Balanced Slope Detector

22. Balanced Slope Detector

23. Balanced Slope Detector
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.

24. Foster-Seeley Discriminator (Phase Discriminator)

25. Foster-Seeley Discriminator

26. Foster-Seeley… Advantages:
Tuning 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.

27. Ratio Detector Similar to the Foster-Seeley discriminator .
(i) The direction of diode is reversed.
(ii) A large capacitance Cs 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.

28. Ratio Detector

29. 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
(ii)
Output characteristics depends on
Primary and secondary frequency relationship
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.

30. Main Points of Angle Modulation
An angle-modulation signal is a nonlinear function of the modulation, and consequently, the bandwidth of the signal increases as the modulation index increases.
The discrete carrier level changes, depending on the modulating signal, and is zero for certain types of modulating waveforms.
The bandwidth of a narrowband angle-modulated signal is twice the modulating signal bandwidth (the same as that for AM signaling).
The real envelope of an angle-modulated signal is constant and consequently does not depend on the level of the modulating signal.

31. End