PURPOSE: To ensure both the fast response and the signal purity of a phase locked loop by inserting diodes connected in parallel with their polarities opposite to each other as a nonlinear circuit between the stages of the 1st and 2nd LPFs.
CONSTITUTION: The diodes D1 and D2 connected in parallel with their polarities opposite to each other as a nonlinear circuit are inserted between the stages of the 1st and 2nd LPFs 20 and 21. When the phase is locked, no current virtually flows and therefore the impedance of each diode is very high. Then the time constant of the rear end 2nd LPF is increased with its cut-off frequency reduced. This greatly contributes to the smoothing of both pulse and noise components outputted from a phase comparator 1. When the frequency is switched, the comparator 1 functions to change the frequency of a voltage control oscillator 2. Thus the DC potential differences are produced at the points before and after a loop filter. Then one of both diodes is forwardly biased and its impedance is reduced. As a result, the capacitor of the rear end 2nd LPF is quickly charged and discharged and the fast response is ensured for a phase locked loop.
NAKAGAWA JUNICHI
AKAIKE KAZUO
KOKUSAI ELECTRIC CO LTD