From: John, KI6WX

Dated: April 17, 2003

Subject: Re: Off Frequency K2

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Over the last two years, I've become somewhat of an expert on the K2
oscillator design.  It is possible to get your K2 to consistently read
frequencies to within 20 Hz.  It does take a little work and care.

First you need to understand the design of the K2.  The VFO tunes in steps
that vary in size depending on frequency.  On 10 meters, these steps are
around 10 Hz.  On 20 meters they are around 7 Hz, and on 160 meters they are
about 3 Hz.  The firmware picks the nearest one to the desired frequency, so
we have a possible error of 5 Hz.

The Cal PLL function uses the 4 MHz oscillator to determine the PLL
reference frequency.  Even if you are very good at adjusting this frequency,
you will still get an error of at least 5 Hz at the frequency you are
calibrating it.

The BFO frequency is tuned in step sizes that range from 20 to 35 Hz
depending on the VFO frequency.  The CAL FIL function measures these
frequencies, but again this will be subject to the calibration error of the
4 MHz oscillator.

Finally, the K2 drifts some with temperature.  If you do your CAL PLL at one
temperature and run it at another temperature you will get an additional
frequency error.  If you carefully control all of these, you can get an
accuracy of about 20 Hz.  I don't think you can do much better given that
the K2 does not use a complex PLL as used in many other rigs.  This buys you
a much better dynamic range due to better oscillator performance, lower
current consumption, and a smaller part count, but it comes at a cost in
frequency accuracy.

To calibrate a K2, you first need to adjust the 4 MHz oscillator to be
exactly at 4 MHz.  I had tried the fifth harmonic zero beat to WWV several
years ago and decided it didn't have enough accuracy.  I found the best way
to do this was to set the K2 on 10 meters and measure the VFO frequency with
both an internal and external counter.  The K2 must be at its internal
operating temperature, so you either need to do this in a warm room or let
the K2 warm-up with the top cover in place (this can take an hour to fully
warm-up).  You can then remove the top cover and quickly adjust the 4 MHz
oscillator.

You then need to replace the K2 cover and let it again stabilize at its
normal internal operating temperature.  At this point you can run a CAL PLL.

Finally, you need to run a CAL FIL for each filter bandwidth and mode.  You
must adjust the CAL FIL frequency so that it will be recorded as a new value
even if you don't change that frequency.  This adjusts the K2 so that the
firmware knows the exact frequency of the BFO and the VFO can correctly
compensate for the exact BFO frequency.

If you do all of these steps carefully, you can get 20 Hz frequency accuracy
on 10, 15, & 20 MHz WWV.  It will probably be a little worse on 10 meters.

-John
KI6WX