I have set the sound card up using the
WWV modem and have it adjusted for the proper PPM offset on receive.
I followed the frequency calibration procedure that Icom
recommends for the IC-746PRO, adjusting WWV at 10 MHz for a zero beat.
Then fldigi was used in the "Freq Anal" mode to track the WWV carrier
at 10 MHz. In this mode the decoder is merely a very narrow band
AFC tracking filter. The filter bandwidth is set the 2 Hz and the
tracking time constants to about 5 seconds. Future releases will
probably make both of these user adjustable. When the signal is
being tracked the digiscope (right hand display) will be a horizontal
line. If the signal is very noisy and tracking difficult the
digiscope will jump and become wavy. You can see from the above
image that I am tracking about 0.22 Hz high on WWV. I have
repeated this measurement at various times during the day and on
various days with nearly the same result. So I am comfortable
with knowing that my local oscillator is just a little low (that is why
the Frequency reads high). ARRL has announced a FMT (frequency
measurement test) to take place on 17 Nov (16 Nov for N.A. time zones).
Check the latest QST for info or visit the www.arrl.org web site.
You should be able to make a submission to the FMT using this
technique. Make corrections to the FMT transmission based upon
your WWV measurement. You may have to adjust for other local
oscillator effects as well. If you have some good ways to measure
and correct for these I would be glad to share them with the other
fldigi users.