I built a RC-servo, worked well, except it picked up
>the local AM radio station, so I disconnected the
>control voltage input from the Rx circuitry and it
>was still there even more! The servo arm was
>under control of the AM station. I doubt I could
>design that on purpose. Using "trial and error"
>I clipped in a cap that fixed it, but I was a bit haunted
>because I didn't really understand the anomally.
>Ken
When you disconnected the control voltage, which was a low impedance source, the free input saw only its own impedance, likely much higher. In effect, the control voltage low impedance had been shunting out the "antenna" of the input. The cap you added was an even better shunt at RF (but not at DC for the control voltage), which stopped the AM station.
Best regards,
Bob Masta DAQARTA v4.00 Data AcQuisition And Real-Time Analysis
formatting link
Scope, Spectrum, Spectrogram, Sound Level Meter FREE Signal Generator Science with your sound card!