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[TenTec] Using WWV to calibrate an Omni VI... one method

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Subject: [TenTec] Using WWV to calibrate an Omni VI... one method
From: johnclif@ix.netcom.com (John Clifford)
Date: Sun Aug 17 14:21:09 2003
Hello Scott,

To verify/check for any soundcard error, you can use the sidetone.  It is
generated by the DSP and is independent of the TXCO/crystal oven.  Just run
DigiPan/MixW and verify that the sidetone is where it is supposed to be...
if not, then your soundcard is in need of calibration.  Like George, I have
found my integral soundcard chip to be spot-on.  Note that MixW has
instructions for calibrating your soundcard that you can use to check/verify
soundcard calibration without using your rig.

Re calibrating an Omni VI/VI+ using WWV, I didn't/don't use George's method,
but one similar to it.

[Note that this method assumes that the BFO crystal trimmer caps are set so
that the resultant BFO frequencies are at factory specs (unless you've
dinked with them or something in the BFO circuit, e.g., crystals, trimmer
caps, resistors, has been replaced, this is a safe assumption).  You will
need a frequency counter to set these, or another rig that you know is
calibrated to transmit and receive a CW signal on exactly the same frequency
(you can get around the lack of a frequency counter if you can trim your rig
to transmit and receive CW on the same frequency using a separate rig as a
reference... but that is an involved procedure so we'll assume that the BFO
board is aligned to spec).]

Undo the screws on your top but leave it on your radio, and then let your
radio warm up for at least 10 minutes, and after that time transmit into a
dummy load for 5-10 seconds at 25 watts.  Then, do the following:

1) Set the radio to CW

2) Tune to 10 Mhz

3) Tune the sidetone(i) on by holding the CW mode key down, and adjust the
VFO until you zero-beat the sidetone.  You can use DigiPan or MixW or
another program that shows a waterfall as a guide to help you get close if
you are tone deaf.

Once you are as close to zero-beat as you can get, the difference between
your displayed frequency and 10 Mhz is your master frequency reference
calibration error.  Now,  you need to adjust the 10 Mhz master reference
frequency so that the dial reads 10 Mhz when WWV is zero-beat.

4) Tune your rig back to 10.000 Mhz.

5) You will adjust the TXCO/oven similarly to the way you did the dial... by
holding CW in so that the sidetone is on, and then adjusting the TXCO/oven
until the sidetone is EXACTLY zero-beat to WWV.  Unlike the VFO which has up
to a 10 Hz error due to granularity, the TXCO/oven trimmer caps will let you
get to under 1 Hz.

Adjust the TXCO via the built-in trimmer cap (screw slot is visible on top
of the TXCO).  If you have a crystal oven, you will need to adjust C31, the
oven trimmer cap, on the Logic board.  If you use a metal screwdriver, you
will need to back away from the trimmer cap because the frequency will
change slightly with the presence/absence of a metal screwdriver.

Carefully remove the top so you don't touch anything inside the radio.  This
gives you access to the trimmer cap.

When you get close to an exact zero and the two tones come very close in
frequency, you will hear the differential beat between the sidetone and WWV
carrier start to 'sync'... rather than hear a quick beat note you will hear
a 'wow-wow-wow...' and the length of each 'wow' will get longer and longer.
When you hear less than one 'wow' per second you are within one hertz, and
it is possible with some judicious adjusting to get dead on.

6) Button your rig back up, and there you are.

A good check on your BFO trimmer cap settings is to have the VFO set to 10
Mhz and then switch back and forth between USB and LSB.  There should be no
change in the tone you are receiving from WWV.  If there is, then your BFO
board needs calibrating, and you will need a frequency counter or accurate
separate rig.


(i) Why do I use the sidetone?  Because it is an easily-verified signal
(using MixW/DigiPan), doesn't depend on the calibration of the rig's master
frequency reference, and give an easy way to ensure you are exactly on
frequency with WWV.


I'm open to any suggestions for corrections and/or improvements on this

 - jgc

John Clifford KD7KGX

Heathkit HW-9 WARC/HFT-9/HM-9
Elecraft K2 #1678 /KSB2/KIO2/KBT2/KAT2/KNB2/KAF2/KPA100
Ten-Tec Omni VI/Opt1
Alinco DR-605TQ
Icom T90A

email: kd7kgx@arrl.net

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