Abstract:
A linear phase detection (PD) has a variable gain that is regulated in function of the monitored transition density of the input signal (DAT). The transition density is sensed by a circuit (Q3', Q4', R2, C2, Ipd) that generates a signal (V2) corresponding to a time averaged common mode component of the differential current signal (OUT+, OUT-) output by the phase detector (PD).
Abstract:
A phase detector input with a generally oscillating signal, and with a clock signal for outputting a differential signal representing the phase difference between the oscillating signal and the clock signal, comprises
a first differential pair of transistors (Q3,Q4) driven by the clock signal and by its inverted replica (CK and CKN) for generating the differential signal (OUT+,OUT-) corresponding to the currents respectively flowing in the transistors of the first differential pair; at least an auxiliary differential pair of transistors (Q1,Q2) driven by the oscillating signal and its inverted replica (DAT and DATN) having its common current node coupled to corresponding current nodes of the first differential pair; a current generator (Ipd) biasing all the differential pairs. If there are long periods of time during which the oscillating signal does not switch, the precision of the frequency of the recovered clock may worsen progressively. This problem is solved by providing the phase detector with a feedback loop for regulating the current delivered by the current generator that monitors the transition density of the generally oscillating input signal and increases the bias current of the differential pairs when the transition density decreases. The output differential signal is thus generated with a greater gain thus making the VCO that is present downstream, adjust more promptly the frequency of the recovered clock.