Match function generator output phase to line phase

Question

I am new here so my apologies if I am missing pertinent details. I am trying to cancel a 60Hz EM field by sending a small 60Hz AC current through a loop of wire that I am generating with a function generator (HP33120A). It works extremely well when the two signals are out of phase. The problem is that line phase (or the phase of the field I want to cancel) is fluctuating enough that eventually the waveforms align and add. I have tried to create a Schmitt trigger with a LM741 opamp to externally trigger the function generator but I need to have a burst of 10 or more cycles to have a consistent output and it adds significant field noise away from the 60Hz peak.

So, my question is, is there a good way to match the output of a function generator to line phase? Or maybe, is there a good way to phase shift the line signal so that I can use it directly?

I am no expert but, maybe my trigger is poorly designed? Is it possible to purchase a Schmitt trigger in a chip that will function better than a home designed one? the rise time on the trigger signal is quite high, on the order of microseconds. Is it possible that this is limiting my ability to have a burst of only one cycle?

Answer 1

I'd avoid using a 741 unless you already have them, not anything else, and are tight with money. Otherwise for Schmidt triggers, if I don't toss one together with discrete transistors, I'll grab a CMOS hex inverting Schmidt trigger chip, from the ancient 4000 series - the 40106. There are others. Also, any cheap common comparator chip will do, needing a positive feedback resistor for hysteresis.

Whatever the trigger, the AC signal needs to be clean. If it's ultimately coming from a wall socket, stepped down and safely isolated of course, it'll have all kinds of hash, spikes, noise due to all the things plugged in everywhere near and far. Use a low pass filter, an LC filter if you don't mind the space for suitable sized parts, or RC.

But step back and consider the bigger design problem. What should happen if there's a sudden small phase shift or change in amplitude in the input 60Hz? Is it a sine wave you want, some other shape, or whatever waveshape the external field has? You want your generated one to follow along immediately, it sounds. Why have an oscillator (waveform generator) involved? Can you sense the external 60Hz with a coil, then amplify it, inverted, to drive your wire loop?