Bandpass tracker for DTV receiver, with VSB I-F signal filtered for more symmetrical AFPC Loop response

摘要

Digital radio receivers for digital television signals often employ bandpass trackers in which the bandwidth of the automatic-frequency-and-phase-control (AFPC) loop is determined in the baseband by lowpass filtering after demodulating the intermediate frequency signal. If the I-F signal is vestigial-sideband in nature, and if baseband symbol code is designed to have a spectrum reaching at least substantially to zero-frequency, low-frequency modulation components tend to give rise to phase jitter in the AFPC loop. This is owing to the asymmetry of the modulation sidebands near carrier frequency. To reduce such phase jitter, a filter is introduced before the AFPC detector for reducing asymmetry of channel response in a narrow-frequency band including the pilot carrier signal.