QAM demodulator with rapid resynchronization function

Claim 1

A QAM demodulator including a transversal equalizer and a demodulating section, said transversal equalizer including a transversal filter, said transversal filter compensating for intersymbol interference involved in an incoming quadrature-amplitude modulated signal applied thereto, said demodulating section receiving the output of said transversal filter and producing data and error signals, said demodulating section including a sine wave generator which produces a sine wave in response to asynchronism induced in said demodulating section, said demodulating section comprises: first means coupled to said sine wave generator, said first means producing a first logic signal indicating whether or not said demodulator is in synchronism; second means coupled to said sine wave generator, said second means producing a second logic signal according to the sine wave applied thereto, the period of said second logic signal being equal to the period of said sine wave, and the duty cycle of said second logic signal being adjustable, and said transversal equalizer comprising: switch means having its output coupled to said transversal filter and having a first input coupled to said second means, said switch means intermittently applying a first constant voltage to said transversal filter in response to said second logic signal; third means receiving the data and error signals and said first logic signal, and determining whether or not a message point of the incoming signal is deviated from the normal position thereof into a predetermined region of a signal space diagram, said third means outputting a third logic signal indicating whether or not the message point is deviated into said predetermined region; and fourth means coupled to a second input of said switch means, said fourth means responding to said first and third logic signals and controlling the operation of said transversal filter together with said switch means such that, (a) in the event that said demodulating section is in synchronism, said transversal equalizer is allowed to operate normally, (b) in the event that said demodulating section falls into asynchronism, if a deviated message point is detected within said predetermined region, said transversal filter intermittently operates and receives said first constant voltage between the intermittent operations, and (c) in the event that said demodulating section falls into asynchronism, if a deviated message point is outside said predetermined region, said transversal filter receives alternately said first constant voltage and a second constant voltage from said fourth means.