Browse Prior Art Database

Original Publication Date: 1979-Jun-30
Included in the Prior Art Database: 2004-Apr-01

Publishing Venue

Xerox Disclosure Journal


A sampling phase locked loop functions as a carrier detector to supply an pin-phase" carrier frequency signal in response to an incoming amplitude modulated pass band signaL To that end, a phase discriminator 11 compares the pass band signal against a reference signal supplied by a voltage controlled oscillator 12 to generate a pulse-like error signal which, in turn, is applied to a filter 13 via a switch 14. The free running frequency of the voltage controlled oscillator 12 is at least approxi-mately equal to the anticipated carrier frequency of the pass band signal so that the error signal has a polarity dependent on the sense of any phase different between the reference and pass band signals and a duty ratio dependent on the extent of the phase difference. The switch 14 is closed when a pass band signal is present, so that the filter 13 develops a control voltage for the voltage controlled oscillator 12 in response to the error signal, thereby increasing or decreasing the frequency of the reference signal until the reference signal is brought into phase with the pass band signal. However, in the absence of a pass band signal, the switch 14 is opened to prevent rapid change of the control voltage for the voltage controlled oscillator 12, Thus, once an in-phase relationship is established between the reference signal and the pass band signal, periodic brief updating of the control voltage for the voltage controlled oscillator 12 is all that is required to obtain an essentially constant -in phaseucarrier frequency signal therefrom,