DEMODULATION OF FM SIGNALS BY A DISCRETE PHASE LOCKED LOOP

Abstract

This paper discusses the discrete-phase locked loop for demodulation of an incoming frequency modulated analog signal imbedded in white noise. The discrete samples admitted into the system adoptively, represent an estimate of the message at the discrete instants of time which can be improved further with a correction factor. The work is in two parts: (1) even without the noise, perfect demodulation is not possible. To arrive at the best estimate, requirements are specified regarding the ratio of carrier frequency to maximum frequency deviation, taking into consideration the lock range and the Nyquist sample rate. A simulation study is performed to specify the bounds for the estimate to be accurate enough. (2) With the above considerations, the receiver is analyzed with noise. For high carrier to noise ratio, it is shown that the performance is at least as good as an analog discriminator. The only requirement being that the condition specified in (1) is more stringent. Digital computer solutions of the system check the above and provide a qualitative picture of the threshold behavior comparable to analog feedback discriminators.