[TenTec] Omni VI Plus freq. readout
Robert & Linda McGraw K4TAX
Sun, 29 Sep 2002 22:09:42 -0500
Pete has some interesting comments. Keep in mind that the 7 - 10 ppm on 20
meters is almost 100 Hz and the 10 ppm is 142 Hz error. Now in this writers
opinion, that's a lot. As to the 2 - 3 ppm well that still says the rig is
28 to 42 Hz off. Eh, so what. As to those rigs with the "frequency
counter" being internal, keep in mind that the received frequency is the
compilation of the various oscillators both variable and fixed to manage
from the input frequency through the multiple conversion process. Including
the LSB or USB oscillator for injection into the product detector. Any of
these being off a few Hz can make for several Hz error one way or the other.
Most interpolate the frequency and read only one oscillator. On the other
hand, reading transmit frequency is OK and easy on CW but a challenge on SSB
as the actual carrier is typically suppressed (we hope) some 50 to 60 dB or
more below rated output.
Now, here's a point I pick. If WWV is 10.0000 MHz and the crystal
calibrator is say 2 Hz above 10.0000 MHz the two signals produced are the
sum and difference hence 2 Hz and 20.0000 MHz + 2 Hz. I doubt that most
audio paths in our radios will pass a 2 Hz signal and most assuredly the
human ear can't hear 2 Hz. In reality, we hear the "beat" which is really
the phase addition and subtraction in terms of time domain. Hear we can
extract a difference as the rotation of phase cancellation is in the order
of 2 Hz or so.
For what it's worth, I prefer to measure TX frequency in the CW mode using a
counter of sufficient known accuracy. Knowing the CW offset ( which may not
be exactly 800 Hz) as it is tweakable via C10 on the TX Audio Board and
handled by the DSP system based on one's preferred CW offset for receive.
It should be 9.000400 MHz) then I can presume that I know the TX frequency.
In general, I find that my OMNI VI+ is always closer than 10 Hz of where
it's supposed to be. All is well if the LSB oscillator, adjusted by C16,
is 9.000000 MHz , USB oscillator, adjusted by C12, is 9.003000 MHz, FSK
Space oscillator, adjusted by C13, is 9.002295 MHz and FSK Mark oscillator,
is adjusted by C18, is 9.002125 MHz. Should you want to see where the SSB
"carrier frequency" is located, just tweak the carrier null pot, R10, on the
L.O. Mixer Board a bit to let a bit of carrier through for the external
counter to measure. When finished, don't forget to do a carrier null
procedure. Here again I caution, IF YOU DON'T HAVE THE EQUIPMENT TO DO
THIS CORRECTLY.........DON'T DO IT!
In the case of the OMNI VI+, if you examine the L.O. Mixer board you will
see 10 crystals, Y1 - Y10, that are used for the respective bands. There is
no adjustment to "trim" any of these. From this, one can expect to see some
band-to-band errors. Just the nature of a crystal mixer rig.
Frankly, I don't find the Kenwood's, Yaesu's or Icom's to be any better in
terms of actual frequency vs. displayed frequency.
Now, lets get back to hammin' and quit twiddling over 5 or 10 Hz.
----- Original Message -----
Sent: Sunday, September 29, 2002 9:10 PM
Subject: Re: [TenTec] Omni VI Plus freq. readout
> Except for a few rigs with a built in frequency counter, I haven't found
> rig that's on its indicated frequency. Most rigs are within 7-10 ppm, a
> have been within 2-3 ppm, but they have all been more or less "off."
> I don't have a "professional" counter but my ears say the crystal
> is within 2 Hz of WWV at 10 mHz, the B&K I do have says the calibrator is
> dead on, and that all three of my VI's, one plus and two option 3's, are
> solid and within perhaps 30 Hz of the indicated transmit frequency.
> Of course, besides the direct method of transmitting and checking the
> frequency, you can hook a counter up to the speaker, tune in WWV in CW
> and check the audio frequency over time. This is a pretty solid check of
> receive frequency stability as well.
> I sure wish Izzy had gone some other way. I'd be at the 'fest.
> 73 all Pete Allen AC5E
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