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
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Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.
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