You're making my brain hurt thinking about these things! Switching DC on
and off amounts to a square wave which, as we all know, is made up of an
infinite series of sine waves, i.e., DC to light, and somewhere in there is
the RF spectrum.

From my days as an Air Force radio tecnhician, before becoming an edumacated
AF ossifer and pilot, my experience with receivers indicated that just a
couple of micro volts at the antenna input would break squelch. Now that
was from a 5 watt transmitter many, many miles away (line of sight); and
your source is merely inches away. A capacitor of the correct value acts as
a short circuit to RF at the tuned frequency whereas an inductor (RF choke)
acts like a high resistance (at the tuned frequency). Either or both of
these of the proper values and connected in the proper way will greatly
reduce or eliminate RF interference.

In my case, with the turn indicator causing the interference, I rummaged
through an electronics parts bin (actually a jelly jar) and found a
ready-made RF choke which consisted of a torroid with two wraps of wire
giving 4 loose ends. I connected one wrap (about 4 or 5 turns) in series
with +12v and the other in series with the ground to the instrument.
Problem solved.


"Martin Gregorie" wrote in message
...
On Sun, 16 Dec 2012 17:12:24 -0700, Dan Marotta wrote:

Been a long time since EE school, but, IIRC, "switching on and off"
creates a LOT of RF noise.

Even at those low voltages and currents?

But what does solving the problem by fitting a simple capacitor across
the supply say about the relative amounts of RV vs. electrical noise?
Bear in mind that there's around 50cm of unshielded cable between the
capacitor on the switch and the T&B - cable that was supplied with it.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
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