Passband equalizers with filter coefficients calculated from modulated carrier signals

Abstract

A digital communications radio receiver, as for television, digitizes a final intermediate-frequency signal offset from zero frequency no more than a few megahertz. This digitized final I-F signal is passed through an adaptive equalizer before being synchrodyned to baseband in the digital regime, which adaptive equalizer has its weighting coefficients determined by comparing I-F signal as received with possible multipath with ideal I-F signal free from multipath distortion. Decision feedback for adjusting the filtering coefficients of the equalizer is described, which is based on the ideal baseband signal being re-modulated onto final I-F carrier and compared to the received signal as translated in frequency to provide the final I-F signal. A training-signal method for adjusting the filtering coefficients of the equalizer is described, which characterizes the reception channel by comparing prescribed portions of the received signal as translated in frequency to provide the final I-F signal to final I-F signal segments stored at the receiver. Equalization of the final I-F signal based on error signals derived from modulated I-F signal, rather than from demodulated baseband signal, allows different equalization of the upper sideband and the lower sideband of the final I-F signal. This facilitates receiver passband tilts as well as multipath distortion being compensated for.