Detection of discrete frequencies using phase locked loop - with low pass filter corner frequency regulated by comparator and threshold switch

摘要

A circuit for detection of discrete frequencies has a phase locked loop contg. a phase comparator, a low pass filter, and a voltage controlled oscillator. It achieves high resolution over a wide frequency range, has rapid disturbance-immune response, and does not require extremely stable oscillator frequency. The second phase comparator drives a second low-pass filter which drives a threshold switch whose output controls the limit frequency of the low pass filter. A cut-off circuit connected to the first comparator output is controlled by the threshold switch.