IC555 Phase Lock Loop Ckt,
the left one is reference oscillator and the other is VCO to capture reference frequency. As an ideal, XNOR gate is used as phase detector to sense the phase difference of two frequency and result the vtg to RC low-pass filter to filter out noise.
This DC vtg is fed back to VCO to control its frequency till the VCO frequency get to be rhythmic with incoming signal.
By varying the values of R and C at filter ckt, Capture FREQUENCY range can be adjusted.
The reference frequency can be varied by adjusting the rheostat.
Current range of locked frequency----
-- from 10.7kHz to 13kHz
