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Comparing a "phase" at 3MHz

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Infraviolet:
Another separate trouble in my current project... Hopefully this should be the last nasty surprise of the high frequency world which I discover in the course of this project.

I've got a 3MHz sine wave signal (the output of the 3MHz amplifier I was working on, thanks for the help there) and I need to compare whether it is roughly in phase, or in anti-phase with a 3MHz square wave. That is to say I'm trying to check whether the 3MHz sine wave is in phase with the square wave, or inverted relative to it.

In an ideal world I'd use op amps, but these signals are proving too fast for any I have to hand.

I've also failed with comparators (LM393), seems the comparators I have are too slow to operate from the signal.

I tried using a 4066 analog switch IC with the square wave on the control pin and then feeding the sine wave in to one of the switched pins with the other switched pin pulled to ground and meaurements taken there. But the 4066 seems to induce huge distortions in signals this fast, capacitance somewhere I think judging by the shapes of the severaly distorted waveform.

I also considered a 4066 for sample and hold purposes at the time of a peak, but again the distortion it introduced was enormous, large than even the strongest signal.

I thought of trying to massively amplify the sine wave and then use logic gates, but the amount of amplification needed would be very large indeed, as I need to be able to know if it is in phase or antiphase from signal strengths of +/- 20mV peak to peak to 1.4V peak to peak. Logic gate ICs look like a good solution or these speeds, but I'd somehow have to get the signal up to logic levels of voltage, for any input signal size. Is there any easy way to make a crude comparator (input signal versus a carefully selected static voltage rail) from transistors?

What should I try, the trouble is that at a 3MHz frequency everything either I'm used to can't keep up (op amps...), and capacitors cease to be something to damp out fast vibrations and instead become plain short circuits.

What avenues of approach have I got for this problem? All the things I can think of only work at lower frequencies.

Thanks

P.S. to be clear the phase is only ever in phase or in antiphase, its not quite perfectly so its a few tens of degrees out, but this situation is still evry much a digital output of in or anti, not a continuous output of phase difference.

radiolistener:

--- Quote from: Infraviolet on June 10, 2023, 12:41:17 pm ---I've got a 3MHz sine wave signal (the output of the 3MHz amplifier I was working on, thanks for the help there) and I need to compare whether it is roughly in phase, or in anti-phase with a 3MHz square wave. That is to say I'm trying to check whether the 3MHz sine wave is in phase with the square wave, or inverted relative to it.

--- End quote ---

just use second channel on oscilloscope. But first check and calibrate it with your cables, because there is a phase delay in the cable, so if you use cables with different electrical length they will have different phase delay, you're needs to take it into account. Use square wave with a sharp edge for phase delay calibration in order to get more precise result.

Since you're want to compare phase between sine wave and square wave, I suggest to use trigger for sine wave, so it's zero crossing will be exactly at the center of the oscilloscope screen and you can easily find delay between screen center and square wave edge.

tggzzz:
It would help if you mentioned the Vac and Vdc of the signals.

Comparators are slower responding with small differential signals (i.e. small Vac). Probably the simplest solution is to get faster comparators.

Alternatively, if your two input signals are V1 and V2, then create an inverted version of one of them, say V2I. Then, depending on the phase, either V1+V2 or V1+V2I will be roughly constant and the other will exhibit larger voltage swings.

dmills:
Diode ring mixer?

Fast comparator and nand gate then lowpass will give you a voltage proportional to phase difference.
Lots of ways to do this, just remember that your 3MHz square wave will have a mess of harmonics out to several tens of MHz and you need to respect that in the layout. 

Infraviolet:
radiolistener, sorry, should have been mroe clear, I've already done that with the scope for checking the signal, this is about doing it in a circuit such that it outputs some simple (varying only when the relative phases of the square wave and the sine wave change, not varying as fast as the square and sine actually oscillate) digital or analog output signal.

tggzz, I can make an inverted version of the square wave pretty easily, but low pass filtering the V1+V2 and V1+V2Inv would surely gvie the same longer term average voltage for both. Also, can you recommend and faster comparator types which are relatively "generic" (that is to say other types with same pinout and performance if they go out of stock)?

dmills, the diode ring mixer, can that be run with capacitors rather than transformers to provide DC blocking? Also, would it fail whenever the variable amplitude sine wave signal has amplitudes under a diode drop (yes I can get some Schottky to make it smaller than the usual 0.7V, but schottky's still have a drop) in size?

Thanks

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