What errors in a pcb layout for audio circuit cause radio reception?

[danjel]

I have a pcb designed and built that is for a synthesizer voltage controlled low pass filter. The circuit works perfectly (all the controls, i/o jacks etc.) except for one problem: when I have nothing plugged into it (no audio source being filtered) I can hear a lot of radio noise. When I move one of the knobs it is changing the radio blips (almost like tuning).

What are common errors on a pcb layout or circuit design that would cause it to act like a radio tuner? ground loops? high impedence amp inputs?

 

[audioguru]

If the circuit is clipping then the distortion harmonics might go up as high as radio frequencies. A simple RC lowpass filer on the output might fix it.

The circuit might be oscillating at a radio frequency because an input wire or ground might be too close to an output wire or ground.

 

[carbonzit]

If the circuit is clipping then the distortion harmonics might go up as high as radio frequencies. A simple RC lowpass filer on the output might fix it.

The circuit might be oscillating at a radio frequency because an input wire or ground might be too close to an output wire or ground.

Both the things you mentioned would explain why the circuit would work as a radio transmitter (=RFI source), but not as a radio receiver, which I believe is what the O.P. is actually asking about.

 

[danjel]

problem solved! I was using ssm2164 quad vca ICs and I forgot to include the mandatory 500ohm+560p RC filter on the input of each VCA. Once I added those all noise eradicated.