Hard limiting acquisition technique for psk carrier detector

Abstract

A carrier recovery scheme for a PSK demodulator through which side locking onto an interference-created sideband is avoided includes a hard-limited upstream of a phase-locked loop which removes jammer-induced AM from the input signal. As the loop sweeps through the frequency range of interest, a lock detector, coupled to the phase-locked loop will lock onto or acquire only the true carrier and thereby enable demodulation of the PSK information signal. In a digital implementation of the invention, received PSK input signals are converted into digital format by a multi-level quantizer, after being down-converted to an IF signal and subjected to gain control. The output of the quantizer may be a multi-bit, parallel word indicative of the changing amplitude of the IF signal which is to be sampled at the receiver sampling rate and converted into a digitally encoded representation of a received IF signal sample. Successive digitally encoded samples are coupled through matched in-phase (I) and quadrature (Q) channel filters and applied to a phase-locked loop. The phase-locked loop is, in turn, coupled to a lock detector which controls the operation of the hard-limiter. The phase-locked loop is also coupled to the quantizer to control the rate at which quantized input signals are sampled. Prior to acquiring the carrier, the output of the lock detector is such that the quantizer effectively hard-limits the PSK input to one of two levels, so that it is essentially polarity sensitive, rather than both polarity and magnitude sensitive. Once the carrier has been acquired, the lock detector removes the hard-limiting action of the quantizer and causes the quantizer to operate in its normal multi-level mode. Although removal of the hard-limiting action of the quantizer permits AM sidebands to return, the loop has already locked onto the true carrier, so that the sidebands will not be swept to cause false locking.