From: George, W5YR
Dated: July 26, 2003
Subject: Re: K2 Frequency Alignment
I made up an inductive "probe" by winding a few turns of wire
along the end
of a length of coax and putting shrink-wrap over it. The coax then connects
to my PRO2 receiver input where I can either use the 4 MHz signal directly
or use the 20 MHz WWV harmonic.
I lay the insulated coil end anywhere near the microprocessor chip and pick
up enough signal for the purpose without noticeably pulling the oscillator
frequency, per a precision counter also on the line.
The PRO2 has the sensitivity to allow the very small pickup signal to be
used. I use DigiPan as an audio spectrum analyzer to measure the 4 MHz
signal by tuning the receiver to 4.001 MHz and looking at the nominal 1000
Hz trace on the waterfall. DigiPan will read the actual frequency to within
an indicated 0.1 Hz.
I previously calibrate the PRO2 with WWV by first measuring the WWV audio
tone in AM mode to verify the soundcard/program operation. Any deviation
from 400 Hz or whatever is noted as a correction factor. Then I tune to WWV
at 10.001 MHz and look at the 1000 Hz trace. Any departure from exactly 1000
Hz +/- the soundcard factor is the dial calibration error of the PRO2 which
is then factored into the 4 MHz calibration. I usually keep the PRO2 to
within 0.5 Hz of WWV as measured in this manner.
I find that using the 20 MHz WWV signal to be far and away the most
accurate, but the other approach comes close enough to allow a fair degree
Considering the many strengths and advantages of the K2, I am willing to be
lenient with respect to dial calibration accuracy, especially since the
architecture of the receiver simply will not permit much better than +/- 20
Hz at best on some bands and much more on others.
If dial calibration is of importance, I compare the K2 with the PRO2 and
note the difference. I then use that as a correction factor for the K2 for
that band and at that time.
The inductive coupling method is, I believe, the best way to tap off some 4
MHz signal with the least disturbance to the actual oscillator frequency