Variable-delay, sine-cosine non-coherent demodulator

Abstract

A variable-delay, sine-cosine non-coherent demodulator recovers digital data angle-modulated upon a radio-frequency (RF) carrier signal, by frequency conversion of the carrier signal to a selected IF frequency, which is ideally equal to one-fourth of a sample frequency supplied to the demodulator. The IF signal is sampled at the sample frequency, and the samples are then sorted to provide a pair of baseband quadrature-phased I and Q signals. Each of the I and Q signals is delayed by a variable first delay T.sub.1 to provide first-delayed in-phase I.sub.1 and first-delayed quadrature-phased Q.sub.1 signals. These signals are then delayed by a second delay T.sub.2 to provide twice-delayed in-phase I.sub.2 and twice-delay quadrature-phase Q.sub.2 signals. The sum of the two delays is a constant related to the sample frequency, which itself is related to the data bit rate, but the first delay T.sub.1 is variably dependent upon the frequency offset between the IF signal and the sample signal. The various signals are applied to a sine (discriminator) detector and to a cosine (differential) detector, with the output of a selected detector being filtered and processed to provide the recovered data. The detected data output is also utilized to control the first and second delays and to provide, in coordination with the discriminator detector output, automatic frequency control of an oscillator, in the frequency converter means, to minimize the difference between the IF signal frequency and the desired submultiple of the sample frequency.