Practically all modern avionics boxes have
multiple oscillators inside, unlike earilier
generation stuff. This does mean boxes installed
near or adjacent to each other can be a problem
unless each box has adequate internal shielding.
I know of no proof of this, but I theorize that
installations in plastic instrument panels in
plastic planes are more suceptible. It follows
that one's chosen "stack" should be wired and
bench-checked for interference prior to cutting
panel holes, or at least I plan on doing that.
Regarding comm xmit interference on GPS, it is
true that there are certain comm freq's whose
harmonics can degrade GPS reception. Comm
transmissions are short, so the problem is
transitory and can be latent to the user,
depending on how the GPS is programmed. I tried a
hand-held comm on the offending freq's, and even
an inch from a Garmin GPS-III, while the
signal-strength "thermometers" went haywire, she
never unlocked even after 20 seconds of xmit. At 2
feet (even one foot, allowing for the lower xmit
pwr), the GPS signal strength meters barely
moved. So, the further away the better. And, as
last resort, a sectional chart?
Neat idea to use the scanning feature to hunt for
interference freq's heard on the comm. Just watch
for the ones that may be always out there (alien
spacecraft?), even when the box under test is off.
Regards,
Fred Fillinger, A063
Jerry Davis wrote:
>
> I have GPS interferance between some GPS units and Icom radios. I think its
> due to the processor and clock. Dave mentioned waving a handset around the
> unit. Also try auto scaning the handheld to see if it stops and locks on the
> GPS inteferance. Most only play up on a few frequencies. You can have radio
> and GPS installed side by side for hours and not know you have a problem
> until one day you go to a new field that just happens to use a frequency
> that is particularly susceptable.
>
> Jerry
>
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