On 12/27/2010 4:14 PM, Jimk8mr@aol.com wrote:
> The transmitter section was being grossly overloaded
> with HF RF from 20 meters, causing it to put out RF garbage at who knows
Great story! Many thanks.
One thought, however. I STRONGLY object to use of the word "overload"
to describe every case of RFI to equipment, because it's
counterproductive. That is, it doesn't get interference fixed, because
it doesn't get to the fundamental circuit mechanisms that couple RF into
the circuitry that generates (or detects) the trash.
In fact, most RFI detection is NOT caused by overload of a gain stage,
but rather by simple detection in the non-linearity at the turn-on
corner of a semiconductor junction somewhere in the signal chain! It
takes a LOT more signal to overload that gain stage.
So back to those coupling mechanisms, which are 1) Pin 1 Problems in the
equipment (that is, mis-termination of cable shields); 2) Poor shielding
of equipment, and poor circuit board layout within equipment; 3) poor
shielding of interconnect wiring; 4) Use of untwisted wiring for signal
conductors 5) excessive bandwidth (or inadequate bandwidth limiting) of
inputs and outputs.
My guess is that the dominant coupling was common mode, exciting #1. If
that's true, it could probably be killed with a multi-turn ferrite choke
on one or more of the leads going to that XM rcvr.
BTW -- your technique of reducing TX power to find out how much
attenuation it takes to get rid of the interference is a VERY good one
-- it tells you that you need at least 16dB of suppression at the victim
device to kill the interference.
73, Jim K9YC
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