Detection of discrete frequencies - using phase-locked loop and automatic phase synchronisation of VCO

摘要

A circuit recognising discrete frequencies uses a phase-locked loop contg. a phase comparator, a low pass filter connected to its output and a v.c.o. They enable very rapid correction of the initial phase condition of the loop. The initial loop phase condition is corrected by automatic synchronisation of the phase of the voltage controlled oscillator with that of the input signal. The oscillator phase synchronisation takes place outside the phase-locked loop either before or after the loop and uses the input signal edges.