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A simple PLL with a digital XNOR mixer, simple low pass filter, and a 555 based VCO. Takes a minute to settle. Tuned to best lock on to a 1 MHz signal. The mixer will output high when the signal and feedback are in phase, low when out of phase, and a center voltage when 90 degrees out of phase with 180 degree ambiguity.
Personal note: the PLL seems to best lock on when the reference voltage for the VCO is set up such that the mixer will output the voltage that will set the VCO to the desired lock-on frequency. For example, this circuit is set up so that a 90 degree phase shift results in 2.5 volts at the mixer output, and the 555 reference voltage is set such that 2.5 volts produces 1 MHz. Since +- 90 degrees shift outputs the same voltage at the mixer, the circuit doesn't have a proper reference in order to lock phases if trying to lock to 0 degrees or 180. If set to lock at 90 degrees, the circuit has a reference to move to, since 0 and 180 degrees phase is unambiguous in this circuit.
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