Phase-locking loop with variable bandwidth filter

Claim 1

In a signaling system including a transmission channel carrying an incoming wave of predetermined frequency, a phase comparator provided with a first and a second input, said first input being connected to said channel for receiving said incoming wave therefrom, an adjustable local oscillator delivering to said second input a matching oscillation of substantially the same frequency as said incoming wave, and circuit means including a low-pass filter connected in a signal path between an output of said phase comparator and a control input of said oscillator for applying to the latter a control voltage varying with a phase difference between said incoming wave and said matching oscillation to modify the frequency of said matching oscillation in a sense tending to reduce said phase difference, said filter comprising an operational amplifier provided with a capacitive feedback circuit, the improvement wherein said circuit means includes nonlinear impedance means in said feedback circuit rising in magnitude with increasing values of said control voltage for enlarging the effective bandwidth of said circuit means during an acquisition period in comparison with steady-state operation by lowering the effective series resistance of said signal path in the presence of higher values of said control voltage.