2019-09-27 Phase Shift Oscillator For RF Signal Generator

2019-09-27 PSO

I tack soldered the parts together for a PSO, and made modifications to get it go work at 500 Hz. My goal is to get a steady tone to use to amplitude modulate the RF signal generator that I have built (see this blog). The sine wave from this PSO is beautiful – the most beautiful I’ve ever seen from a PSO. Usually the PSO’s sine wave is clipped because there is no automatic gain control and the amplitude grows until it is clipped.

Once I got it working at the right frequency I unsoldered the parts and put them on the same PC board that is holding the RF signal generator. I got done and it works. But every time I get close to it, the zone wave drops to zero. It’s like a proximity detector that drops out of oscillating when I get close to it. Evidently I chose part values that are putting the circuit right at the point of oscillating, the circuit is getting just the right amount of positive feedback to oscillate. But when anything changes the feedback to just a tiny bit less, the oscillations can’t sustain themselves and they drop to zero.

So I increased the 3.3k collector load resistor to 3.9k, then 4.3k. The waveform is slightly clipped at the bottom but that’s okay. The oscillations are much more stable, they don’t die when I put my hand near it. But the oscillations rapidly stop when I turn on the RF oscillator. There are two .22 uF bypass capacitors across the supply to filter out the RF. Apparently there is still enough RF to disturb the PSO. So I’m thinking about using some RF chokes in the supply lines.

I put a 1 mH choke in the supply line to the PSO, and it seemed to help. the PSO stays running. Now I have to do the power amp and modulator circuit after the RF oscillator.

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