Put a GOOD ground on your receiver. You can easily be depending on the
ground through the power supply and the rectifiers are modulating that
ground, whether linear supply or switching supply. A switching supply
still has a diode rectifying 60 Hz, just it steps up the frequency for
switching so the transformer can be tiny. The hum modulation of the
ground is a standard problem with direct conversion receivers. Its made
worse by poor oscillator isolation from the audio amplifier with
imperfect balance in the detector.
73, Jerry, K0CQ
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.