FET phase comparator

I am a volunteer at a museum. My project requires a magnitude/phase comparator. The supply is half rectified 115VAC 400Hz. The input is half rectified 0-58VAC at 0 or 180 phase relative to the source. Both supply and signal are referenced to ground. When the input signal is in phase with the supply the output is proportional to the input. The output needs to range from 0-5VDC, and feeds an op amp (i.e. hi input impedance).

I think I need a high voltage small signal FET arranged as a source follower, with a voltage divider on the input. That is about the limit of my circuit ability with FETs. Any suggestions as to the source resistor and capacitor values, the gate resistor values, and the FET number?

--jon

hi,
The way I read your post is:
You want two outputs.?? Magnitude and Phase difference.

One to give a 0v to +5v output for the 0 to 58Vhw rectified DC and the other output of 0v to +5v for a 0deg to 180deg phase difference between the 'fixed' 115Vhw rectified 400Hz.?

Is this correct.?

Thanks for the reply. There is only one output: 0-5VDC proportional to the magnitude of the input signal when it is in phase with the supply.

Perhaps the confusion is that I tried to boil down the problem to much. What I described is one half of the actual circuit....

Imagine there are two half rectified supplies: one at 0 phase (in-phase) and one at 180 phase (out-of-phase). Each supply feeds a source follower-like circuit, as previously described, with the same half rectified input signal driving both "sides". The input signal is an "error" signal. It varies continuously in magnitude indicating the size of the error, but its phase is either 0 or 180, indicating the direction of error.

An op amp then measures the continuous difference of the outputs between the two sides, with an op amp output of 5VDC indicating the input signal is max magnitude in-phase (0 phase), 2.5VDC indicating zero magnitude input, and 0VDC indicating max magnitude out-of-phase (180 phase).

BTW: In googling around I found a fet, ZVN4424A, with a Vds of 240. Seems that this should work. Any suggestions as to what the source follower source resistor should be?

--jon

ericgibbs said:

hi,
The way I read your post is:
You want two outputs.?? Magnitude and Phase difference.

One to give a 0v to +5v output for the 0 to 58Vhw rectified DC and the other output of 0v to +5v for a 0deg to 180deg phase difference between the 'fixed' 115Vhw rectified 400Hz.?

Is this correct.?

Click to expand...

What he's asking for is a servo error signal decoder/driver. DEMODULATORS IN THE SERVO SYSTEM

Figure 1-28.