Dr. Gerald N. Johnson wrote:
> 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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>
Thanks, Jerry... your comments are consistent with what I had suspected,
though I lack the technical knowledge to explain it as you have.
It's so narrow, and on a band I don't frequent anyway, that it's hardly
a bother.
Thanks again, and 73...
--
John, K3GHH
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