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Using WWV to Calibrate an Omni VI

(note: before performing frequency calibration, you need to make sure the BFO oscillators are correctly aligned/calibrated per the procedure in the manual; also, be prepared for C31 to have aged and to need replacement - N1EU)

From the Ten-Tec Reflector August 17, 2003

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.

  • 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 method.

John Clifford KD7KGX

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