Reducing Interference

 Referring to the discussion of interference from an air conditioner to ship-to-shore radio (see here)

The easiest way, and without modifying the circuit board, is to add external filters so I carried out some simple tests to determine the best ones for the job. I tried four different ferrite rings but of these only three were worth considering. These had been salvaged from computer monitors some time ago. Two were similar shaped cylinders and one was a ferrite ring. Below are the results. The experimental lash-up was a spectrum analyser with a link from its tracking generator fed to the input via a pair of cables coupled together by a BNC plug to wire link to BNC plug. To test each ferrite the wire link was opened and the ferrite slid over the wire.

 

 The test frequency was around 160MHz which is in the marine VHF band.

The analyser was adjusted to show a marked frequency of 160.66MHz and different ferrites fitted to judge their effect compared with the base level of -22dBm.

 

Attenuation was 6.7dB
 

 

Attenuation was 7.8dB
 

 

Attenuation was 4.6dB
 

 Next I looked for a specific radiated signal close to 160MHz emanating from the circuit board.

   

 Above is the method of signal detection. The whip aerial sheathed in plastic tubing is connected to the test receiver and the 4-turn coil is connected to the spectrum analyser. As was done in early tests the receiver is set to SSB reception which lets me determine if the signal is coming from the circuit board: touching the HF transformer shifts the received signal by a few KHz whilst other carriers don't vary. The signal chosen for tests is 160.102MHz. This is something like the eleven hundredth harmonic of the power supply basic frequency and is remarkably stable. Opposite top the marker is set to 160.10MHz. The scan is 160.0MHz to 160.2MHz
 

 Below, I placed the pickup coil in a plastic sheath and placed it over the mains input wires.

The frequency of the signal was 160.104 and its level around minus 104dBm with the baseline at minus 127dBM.

   

 Next, I opened the mains wires and fitted a ferrite ring, the one providing the best attenuation during initial tests. For the test I used a mains filter which has suitable faston connectors for the existing mains lead. You can see around 5dB improvement from the ferrite and about another 9dB from the mains filter. Finally a check to find the highest level of interference. This makes me think that the interference isn't from the power supply circuitry at all, it might be coming from the microprocessor. The 160MHz signal might be a harmonic of the microprocessor crystal and is being capacitively coupled to the mains wiring.?
   
   
   
   

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