Frequency Modulation And Demodulation

Introduction to frequency modulation and demodulation:

Modulation

Frequency modulation is a system in which the amplitude of the modulated carrier is kept constant, while its frequency and rate of change are varied by the modulating signal. Lets assume for the moment that the carrier of the transmitter is at its resting frequency (no modulation) of 100 MHz and a modulating signal is applied. The amplitude of the modulating signal will cause the carrier to deviate (shift) from this resting frequency by a certain amount. If amplitude of the modulating signal is increased the deviation will also increases more. If modulating signal is removed, the carrier signal shifts back to its resting frequency (100 MHz). From this example it can be seen that the deviation of the carrier is proportional to the amplitude of the modulating voltage. The shift in the carrier frequency from its resting point compared to the amplitude of the modulating voltage is called the deviation ratio.

Types of Frequency Modulation

Frequency modulation is classified on the basis of the bandwidth of the FM signal. The bandwidth of an FM signal depends on the deviation Kff(t). When the deviation is high, the bandwidth will be large, and vice-versa. Thus, for a given f (t), the deviation, and hence bandwidth will depend on frequency sensitivity Kf.

(i) Narrowband FM: When Kf is small, the bandwidth of FM is narrow. Hence known as Narrowband FM. Bandwidth of Narrowband FM is same as that of AM (amplitude modulation).

(ii) Wideband FM: When value of Kf is high, then the FM signal has a wide bandwidth, hence known as Wideband FM. Ideally bandwidth of wideband FM is infinite.

Frequency Demodulation

The process of extracting a modulating signal from a modulated carrier is known as demodulation. Electronic circuits that perform the demodulation process are called FM detectors. The detector performs the extraction in two steps:

(i) It converts the frequency modulated signal (FM) signal into a corresponding amplitude modulated signal (AM) signal by using frequency-dependent circuits, i.e. circuits whose output voltage depends on input frequency. Such circuits are called as frequency discriminators.

(ii) The original modulating signal f (t) is recovered from this AM signal by using a linear diode envelope detector.

A simple R-L circuit can be used as a discriminator, but this circuit has a poor sensitivity as compared to a tuned LC circuit. The FM discriminators suffer from the threshold effect in the presence of excessive noise. The threshold can be improved by pre-emphasis and de-emphasis circuits.

by:Omkar Nayak

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