Referring to the discussion
of interference from an air conditioner to ship-to-shore radio
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.?