The effects of random jitter in the carrier frequency of an FSK signal

Abstract

Experimental studies have been carried out on the effect of random jitter in the carrier frequency, on the demodulated output signal-to-noise ratio of an FSK signal. A mathematical expression is derived for the power density spectrum of an FSK signal in the presence of random variations in the carrier frequency. Several curves are drawn to illustrate this effect for different modulation indices, when the modulating signal is a penny-toss type square wave. A particular class of pseudo-random binary signal, called the 'n' signal, is used for modulating the carrier. The statistical properties of this signal closely approximate those of the random binary signal. It is shown that the variations in the carrier frequency can be simulated by adding random noise to the binary modulating signal. A large number of curves have been drawn giving the output signal-to-noise ratio as a function of RMS frequency jitter in the carrier, using a few typical values of the modulation index and of the bandwidth of the IF filter. Study is also made of the effect of the rate of variations in the carrier frequency on the output signal-to-noise ratio. Based on the experimental results, typical values have been derived for the modulation index and the bandwidth of the IF filter, at which the system has maximum tolerance to the jitter in the carrier frequency.