Symbol sign directed phase detector

Abstract

A phase detector that uses symbol sign information to aid the phase error detection process to determine a phase error of a complex signal such as a vestigial side band (VSB) signal. The phase detector comprises a subtractor having one input coupled to the I signal (i.e., an unquantized digital value that is sometimes referred to as a quasi-analog signal) and a second input coupled to the data output of a slicer (quantizer) representing the magnitude of a sample point within a symbol (i.e., a digital value representing one of M levels of the VSB signal). The unquantized signal is subtracted from the quantized I signal to produce a &Dgr;I signal. The I signal is also processed to determine the sign of the present symbol. A before-after symbol processor determines the sign values of a previous (before) symbol and a later (after) symbol and compares those signs to the sign of the present symbol. A change in signs from the before symbol to the after symbol will produce a substantial imaginary component in the phase error signal that can easily be tracked by the phase tracking loop. If the signs of the symbols (before-to-after) do not change, then the imaginary component of the phase error will be very small and nearly impossible to track and compensate. As such, the loop is disabled when the before-after processor detects such a symbol sign combination. As such the symbol sign information is used to “correct” or “gate” the phase error. This phase error signal is used in a digital television receiver to extract a data signal from a VSB signal for demodulation.