FPO Frequency Issues [Moog EM Theremin]

Posted: 4/1/2023 8:44:06 PM

Joined: 12/15/2022

I'm building the Moog EM Theremin (schematic on p.3) and already have the power circuit. I'm working on the Fixed Pitch Oscillator (FPO) now, which is meant to produce a 260kHz signal. Below are the results I get from my oscilloscope.

When sampling at 100kHz (which should be too low to detect the expected 260kHz signal), I get the first waveform, which is very stable at 60Hz and 1V p-p. Is this the resonant frequency of the circuit? When I increase the sample rate to 100Mhz (which should be able to detect the 260kHz FPO output), I get a nice regular sine wave, but it jumps around and my oscilloscope can't determine its frequency. Turning the potentiometer for pitch adjustment does absolutely nothing and I can remove the inductor from the circuit entirely and there is no change whatsoever.

Is this an issue with my oscilloscope not being able to detect the signal or did I build the circuit wrong? What is that 60Hz waveform I'm seeing? My power circuit and oscilloscope (which is an ADALM2000 hooked up to my laptop) are both about 9" from the FPO circuit.

Posted: 4/2/2023 4:01:37 PM

From: 60 Miles North of San Diego, CA

Joined: 10/1/2014

I see power supply issues. Temporarily use batteries and a good earth ground.


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