DickCappels
- Joined Aug 21, 2008
- 10,170
I ran into something like this. It was an FM band transmitter that had hum in the output no matter how I conditioned the power supply. Even cascading linear regulators and using huge capacitors didn't didn't make much difference. In desperation I powered it from a battery -no hum. Then I connected (only) the ground lead of the power supply -the hum was back!
It turned out to be a well-understood phenomenon. The rectifiers in the power supply were being switched on and off by the AC voltage and as they switched on and off the radiation from the transmitter was changing. It was like putting a 120 Hz switch on the ground plane of an antenna. The two approaches that seemed to help the most were to put ceramic capacitors across the power supply diodes and to put a bifilar choke between the power supply and the transmitter, or quadfilar in the case of feeding audio from an AC powered source.
http://home.computer.net/~pritch/shortwav.htm
It turned out to be a well-understood phenomenon. The rectifiers in the power supply were being switched on and off by the AC voltage and as they switched on and off the radiation from the transmitter was changing. It was like putting a 120 Hz switch on the ground plane of an antenna. The two approaches that seemed to help the most were to put ceramic capacitors across the power supply diodes and to put a bifilar choke between the power supply and the transmitter, or quadfilar in the case of feeding audio from an AC powered source.
http://home.computer.net/~pritch/shortwav.htm