On Sat, 2009-12-19 at 16:55 -0500, John K3GHH wrote:
> Ken Brown wrote:
> >>> 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?
> Ken, on UCW the CW note (tuning up the band) is first heard as a
> high-pitched tone at 28,027,618 Hz, falling in pitch until it disappears
> at ...627 Hz (not audible at 628). On LCW, tuning up the band again, it
> is first heard as a low-pitched tone at 619 Hz and the last frequency at
> which it is audible as a high-pitched tone is 627 Hz. My "very narrow"
> adjective is relative to CW signals I've been copying for 50 years: I've
> never heard a signal only 9 Hz (per sideband) wide.
What you are seeing is NOT a very narrow CW signal, you are seeing a
signal tuned by a very high harmonic of the receiver LO. If it was an
analogue receiver sweeping past the 500 HZ IF filter while tuning the
dial 9 Hz would say you are hearing a beat note from about the 55th
harmonic of the VFO and some other fixed oscillator harmonic. In the DSP
receiver that might be fine tuning at 15 KHz. 28,027,618 / 55 is 509,593
Hz. And that might actually be a spur from the processor in the fine
tuning section or a digital spur in the DSP code. In an Omni VI that
might be a beat happening at 275 MHz but this isn't an Omni VI with a 5
MHz analogue VFO. Though the Orion has much the same front end analogue
parts in places. The 9 MHz first IF of many TenTec radios has lead to a
few "reliable" birdies, you have identified one in the Orion that many
would miss because it tunes so fast. At least one crystal frequency in
the Corsair and Omni is not exactly on the theoretical integral MHz
because avoiding an in band birdie is most important and the digital
display can handle such adjustments in crystal frequency.
Might the Orion with its 455 KHz second IF and 15 KHz third IF, be using
an oscillator about 470 KHz for very fine tuning. And wouldn't
28,027,618 / 59 be about 475 KHz while / 60 would be 467 KHz. Yet that
fine tuning ought to be all in DSP at the 15 KHz IF I'd think.
Its probably not something to be detected if tuning in 10 Hz steps
either, only in 1 Hz steps which I presume are both valid Orion options.
It probably would be worth the bother to clean all the cable connections
between boards as well as the board to mother board connectors, and the
tighten any board mounting screws to be sure of all grounds. But its an
internal birdie, all multiple conversion radios have them. Some have had
noisy mixers to hide more of the birdies, tweets, or bleeps.
> I do not know the
> difference between judging that the signal is "very narrow" and that
> "the audio note... changes more quickly than 'normal'"; the latter is
> how I judge the former.
>
> > 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?
> >>>
> >>>
73, Jerry, K0CQ
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