I kludged up a very simple power line interface (3VAC transformer -> 10K resistor -> diode -> PICPET) and fed the (unfiltered) output into a PICPET. The PICPET was clocked using a 10 MHz TTL oscillator. I used Lady Heather to capture and analyze the data. It was measuring the time interval between 60 cycles on the input waveform and calculating the frequency error, phase, and xDEVs. It worked surprisingly well. Overnight the freq varied over a 0.0475 Hz range. Average freq was off by -0.005 Hz