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 mvgr03@kluniversity.in

FM Generation

Indirect FM Generation Armstrong Method

Indirect FM (Armstrong)Generation Step1 Generation of NBFM

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

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.

Example Let Then To produce Frequency Multiplication Two stages

Two stages

Direct FM Generation / Parameter Variation Method

1. FM Reactance Modulators

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

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 .

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

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

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.

FM Demodulators

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).

1. Slope Detector

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.

Balanced Slope Detector

Balanced Slope Detector

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.

Foster-Seeley Discriminator (Phase Discriminator)

Foster-Seeley Discriminator

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.

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.

Ratio Detector

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.

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.

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