I have a 3-stage BJT audio amp for an electret mic, for inputting into an AM TX. And I'm trying to get the bandpass filters between stages to work better for audio. They are LC and RC , set up for 60Hz to 15-20kHz. Some of the circuit is on a protoboard, and the filters and emitter bypass caps are on a breadboard.
And I have a lot of FM radio everywhere at every node, around -63 to -55dBV most of the time. In open air my probe gets around -75dBV while the GND is still on the circuit GND. Without the GND clipped on it's back around -65 to -55dBV. (but my AWG and other scope probe were still GNDed on that circuit)
1 thing I just tried was putting a 1uH inductor in series from the PSU to my circuit Vcc point. It only has 0.2 ohm resistance, and I still have my 5V DC. The total circuit current, when idling, with the mic disconnected, and no oscillations under 100MHz anyways, the idle current is 14mA, which is about what it should be with the LED on the board.
But with the 1uH inductor from the PSU, my idle current drops to 8mA, and the audio won't work either, it's gone from upto 3-4Vpp, to nothing, just the -55to-65 dBV noise floor.