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Old November 3rd 03, 11:47 PM
Jim Weir
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I respectfully disagree with this statement. There is precious little that is
engineered as well as a commercial radio station. Even sloppy maintenance
wouldn't get the sidebands this out of whack...and besides, those sidebands
would have to be from the FM band, and we are talking about 20% away from the
carrier. Ain't no way.

I live next door (literally) to a 10kW AM transmitter, and occasionally I tune
the spectrum analyzer to their frequency and put an 80 dB notch filter on the
center of the carrier. With a 120 dBm noise floor, I **still** can't see any
splatter outside their allocated bandwidth.

While aircraft radios are generally good with filters, also, there comes a limit
to any design when the volts coming in the front end are just too much for the
microvolt amplifiers. It is called fundamental front end overload, and we are
all susceptible to it.

Both Microair and King use varactor tuned filters in the front end for
selectivity. While these are good design choices, you must remember that a
varactor is nothing more than a diode, and a varactor is one of the best
multipliers in the world. Whap that front end diode with a couple of volts and
you've got a little comb generator second to none.

I think we'd best look elsewhere for the solution.


Jim




"Bill Daniels"
shared these priceless pearls of wisdom:

-I think the commercial stations are poorly controlling their sideband
-radiation and some of their RF power is spilling onto the aviation band.
-Aircraft radios are generally well designed and reject signals that are not
-intended for them, but they can't do anything about spurious signals from
-sloppy maintenance at a commercial radio stations.

Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com