Abstract: A frequency-tracking feedback circuit for acquiring a communication carrier signal from a relatively noisy environment. The circuit includes a digital phase-frequency detector, a limiter, a mixer, and a voltage-controlled oscillator, connected in a long control loop in such a manner that an input band of signals is translated in frequency until the desired carrier is centered within a relatively narrow acquisition window. The characteristics of the loop are such that acquisition may be achieved even when the total power of the noise in the input signal band exceeds the carrier power, provided that the noise is relatively uniformly distributed over the input band. A technique is also disclosed for compensating from any unbalance in the noise spectrum, so that relatively low carrier-to-noise ratios can be tolerated.

Abstract: Digital circuitry, and a corresponding method for its operation, for detecting frequency or phase-angle differences between two digital input signals. The detector includes a differentiating circuit to provide signals indicative of the times of occurrence of a selected feature of the input signals, such as a pulse edge, and a pair of memory devices, such as flip-flops, which can be set by respective input signals. The detector further includes a feedback circuit operative to clear both flip-flops in the event that the input signals would be effective to place both of them in a set condition. The feedback circuit operates in a parallel, rather than a series timing relationship with input circuitry that sets the flip-flops, and the performance of the detector is thereby significantly improved, as evidenced by a smaller dead zone in its phase output characteristic for a given frequency of operation, or a capability of operation at higher frequencies for a given degree of degradation.