[TenTec] Omni VI Freq calibration

[Bob McGraw - K4TAX]

One method I've used to check that LSB and USB are receiving the correct 
frequency is to use a soundcard, receive audio from the radio and your 
favorite computer program such as FLDIGI, MixW3 or HRD/DM780.  I used 
HRD/DM780 for the following.

Tune the radio to WWV in either USB or LSB mode.   WWV will be alternately 
transmitting tones of either 600 Hz or 440 Hz  or 500 Hz. depending on the 
minute of the hour.  Set the software for CW mode. Select the tone on the 
waterfall and read the frequency.  Do not use the "slide rule" readout but 
the actual frequency displayed in Hz.  The slide rule will not give adequate 
resolution to frequency.

Adjust the tuning on the radio such that the frequency display is the 
correct value to the Hz depending the frequency being transmitted by WWV. 
AFC on the software should be ON.  This will then track the decoded tone. 
Then without changing the tuning of the radio, switch to the opposite mode. 
If the frequency stays the same then the USB and LSB modes are correctly 
decoding the WWV signal.  If the frequency is changed, then the error in Hz 
is displayed and the USB and LSB modes are not exactly tracking.


CASE STUDY:
In my case this afternoon, the Omni VII on 10 MHz WWV has to be adjusted 5 
Hz high {10.000.005} for the two tones to be absolutely correct.  Then 
switching between LSB and USB there is less than 1 Hz difference.

On 15 MHz WWV the Omni VII has to be adjusted 9 Hz high for the tones to be 
decoded at the correct frequency.  Again the difference between LSB and USB 
is less than 1 Hz.

On 5 MHz WWV the Omni VII has to be adjusted 3 Hz  high for the tones to be 
decoded at the correct frequency.  Again the difference between LSB and USB 
is less than 1 Hz.

>From the info above one can see the multiplication of error increased 
mathematically as frequency increases.

CONCLUSION:  I'm not willing to change the TCXO for this small error for I 
figure 99.999999999% of the hams will never know the difference.  This 
procedure should get one to an accuracy of about +/- 1 Hz.

As Ken said earlier, to get better accuracy, one needs a very accurate 
frequency counter capable of better than 1 Hz resolution.


73
Bob, K4TAX








----- Original Message ----- 
From: "Ken Brown" <ken.d.brown at hawaiiantel.net>
To: <tentec at contesting.com>
Sent: Saturday, August 30, 2014 3:26 PM
Subject: Re: [TenTec] Omni VI Freq calibration


>
>> The OCXO is the reference for all bands.  As to individual bands, there's 
>> a crystal {10 of them total} being one for each band.  The crystal 
>> oscillator is supposed to be locked to the precision reference by a PLL. 
>> It may be that the PLL and varactor diode is not able to pull the crystal 
>> on frequency.  All of this is on the Xtal Osc. - L.O Mixer Board.
> Yes the band selecting crystal oscillators are supposed to be locked to 
> the OCXO. If they go out of lock they do not stop producing a signal, so 
> the radio will still work, but be off frequency. In that case there will 
> typically be a warbling character to received signals as the oscillator is 
> trying to lock up to the reference.
>>
>> I have noted the absolute frequency will vary from band to band by a few 
>> Hz. And unless the BFO frequencies are not absolutely correct, you may 
>> find a slight frequency discrepancy from mode to mode.
> Yes you really should compare the zero beat dial setting of USB versus LSB 
> on some stable carrier. If the dial readout is not equal for LSB and USB 
> when adjusted for zero beat then at least one of your BFO frequencies is 
> incorrect. (note that you need to adjust PBT to allow a zero beat 
> frequency to pass through the second IF, and even the 9 MHz IF filter 
> should attenuate a zero beat signal significantly, so hearing a zero beat 
> signal is tricky.) If you use a known frequency carrier such as WWV to 
> compare USB and LSB, you still cannot be certain if either of the BFO 
> frequencies is correct, because an incorrect BFO frequency can be 
> (incorrectly) compensated for with an improperly adjusted OCXO.
>
> The BFO frequencies are best adjusted using a frequency counter that has 
> an accurate timebase.
>
> DE N6KB
>
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