>> I found a very narrow heterodyne
What do you mean by "very narrow heterodyne?" Is it a single CW carrier?
Why do you say it is very narrow? Is is so super pure that you can tell
it has less noise around the center carrier than typical, so it is
narrower than other CW carriers?
Or does the audio note it produces change more quickly than "normal"
signals as you adjust your local oscillator (tuning) control? If the
audio note changes more quickly than normal, then it is a birdie.
Birdies are products generated by undesired mixes of components,
harmonics for instance, of the various local oscillators in your
receiver. If one of the components mixing together to generate the
birdie is a harmonic of the local oscillator controlled by the main
tuning, then the audio tone it produces will change 2X, 3X, 4X, or
whatever harmonic number it is, as fast as a normal signal. When local
oscillators in receivers were not locked to stable reference
oscillators, these spurious responses would warble, tweet or chirp as
the LO frequency changed due to mechanical, thermal or power supply
instabilities. ( I have not found a historical reference to back this
up, but I think this is why they came to be called "birdies". With local
oscillators nowadays locked to super stable reference oscillators,
birdies no longer chirp, they just whistle.)
>> just one sideband
Any real signal should be heard with the receiver operating in either
sideband mode. If a birdie or other spurious response is detected only
in one sideband, then it would most likely have something to do with a
spur or harmonic of the BFO in an analog product detector type of
receiver. Use a different BFO frequency and the birdie moves. The Orion
is a DSP radio, so it may have to do with aliasing that occurs in one
sideband detection mode and not the other.
>>
>> from 28,027,618 to 28,027,627 Hz. (With my step set normally at 10 Hz,
>> it was audible only at 28,027,620 Hz.) Anyone know its source?
>>
The first thing I would do is try to determine whether it is an
internally generated product of the receiver, involving no outside
sources. Disconnect the antenna and see if it goes away. Since others
have said their identical mode receiver does not hear it, then it could
be a spurious response of some real signal that is strong in your area
and not where the other guy tried to hear it. If it does involve a
signal coming from outside on the antenna port, then a lot can be
learned by determining how the strength of the spurious response varies
as known amounts of antenna input attenuation are added. Does adding a
10 dB pad to the antenna input make the spurious response drop 10 dB, 20
dB or 30 dB?
DE N6KB
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