The OMNI C manual I have downloaded from TenTec shows a table of LO
injection frequencies to the signal mixers on page 3-13. It shows 9-9.5
for the 18 MHz band. That's guaranteed to produce the spur. It means a 4
MHz crystal would be supplied for that band. To cure the spur also
inverts the side band selection and means the LO injection needs to tune
from 27 to 27.5 MHz from using a 22 MHz crystal and bandpass filter on
the crystal mixing board tune to 27 MHz.
On page 3-15 the manual hints that mixer balance and drive level will
affect spurious outputs, but they suggest adjusting to minimize a
receive spur at 1.950 MHz. And the schematic on that page shows a 4 MHz
crystal for 18 MHz and a bandpass filter circuit difficult to adapt to
27 MHz when the highest frequency it has to tune to originally is 20.5
MHz for the 29.5 MHz band segment.
I'm sure the 18 MHz transmit spur was fixed at some point in time, but I
don't know which radio was fixed first or if some Omni C were fixed.
Omni A and Omni D original and series B in the downloaded manuals have
no provisions for 18 and 24 MHz bands at all. An example of fixed is in
the Corsair. There fixed tuned bandpass filters are switched for each
band. And the mixer board is twice as wide to hold them all. In the Omni
C and before the mixer output filter is a single transformer that is
resonated for the 10m injection frequencies with fixed capacitors and
the bandswitch adds more C for lower frequencies. In the Corsair the
crystal for 18 MHz is 22 MHz as is required to cure the spur. The manual
does admit SSB-N is lower sideband on 18 MHz, but doesn't say why.
I said the same things in that 2003 discussion.
73, Jerry, K0CQ
On 1/29/2011 12:47 AM, Mike Hyder -N4NT- wrote:
> My Omni-C puts out big-time spurs on 17 Meters. I fear Ten-Tec's assurances
> that the Omni-C is OK on that band may be wishful thinking in many or most
> cases. I'm the guy who reported spurious output on the order of two watts
> KEY UP on the 17 Meter band with my Omni-C. You can look back in the
> archives in 2003 and find a big discussion (if you haven't done that).
> 73, Mike N4NT
> ----- Original Message -----
> From: "Dr. Gerald N. Johnson"<email@example.com>
> Sent: Friday, January 28, 2011 5:08 PM
> Subject: Re: [TenTec] WARC band crystal part numbers
>> Yes the 17 meter spur was fixed at some point. Yhe way know its fixed is
>> when you have to go to the inverted side band to ge USB on 17m. Because
>> the fix is to mix the 5 MHz PTO with 22 MHz to make a 27 MHz TX mixer
>> injection LO rather than mixing the 5 MHz PTO with 4 to make a 9 MHz LO
>> injection which has the harmonic problem. There might be a little bit
>> from the SSB signal but its typically 10 or 20 dB weaker than the LO
>> going into the mixer and so less of a harmonic generator. So the 17
>> meter crystal will be 22 or 11 MHz instead of 4 for the old scheme.
>> I haven't bought from ICM lately but $25 from Tentec sounds like a
>> decent buy with no questions on what the specs are.
>> 73, Jerry, K0CQ
>> On 1/28/2011 2:20 PM, Bill Acito W1PA wrote:
>>> Thanks, Jerry.
>>> Despite reports that these were no longer available, I spoke with Paul
>>> at the factory, and he indeed had these (12 and 17M) for the 580 Delta
>>> I nudged him about the 17m issue, and he said the Delta and the OMNI-C
>>> not have this issue, only the Omni-A/B.
>>> Is that correct? (haven't had a chance to search the archive yet, but I
>>> something about "putting out a watt or two, even on receive")
>>> Bill W1PA
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