[Elecraft] K2 Frequency Accuracy

[John Grebenkemper, KI6WX]

From: "Sherman Banks (W4ATL)" <w4atl@arrl.net>
> If I measure my frequency error on 7000 KHz and make the proper
calculation
> of my frequency error, then Cal PLL and Cal Fil (moving the BFO one count
> and then back) I get pretty good accuracy on 80 and 40 M throughput the
> band.  But the other bands seem to be off.  20 M is 50 Hz low.  15M is
about
> 80 Hz low.  10M error increases to about 120 Hz.  80M is right on.  Ver 2
> firmware. Is this normal?  Should I try to measure the error on the
highest
> band for more accuracy?
>
The absolute frequency error in the K2 will vary significantly by band if
the 4 MHz oscillator is not precisely adjusted to 4 MHz.  This error occurs
because the BFO and the VFO frequencies are determined by the 4 MHz
oscillator during CAL FIL and CAL PLL.  A second uncertainty is created
because the BFO step size is in the range of 20 to 30 Hz.

If you measure the frequency error in both LSB and USB (or CW and CWR), you
can compute the error for both the VFO and BFO which is very useful to
determine why the error is occuring.  Assuming that your data is for LSB on
80 and 40 meters and USB on 20, 15, and 10 meters, it fits quite well for
the 4 MHz oscillator to be 17 Hz high (this offset predicts -59 Hz on 20
meters, -89 Hz on 15 meters, and -119 Hz on 10 meters).

For those who are interested in the expected frequency error, I've computed
the following table.  There is roughly a 20 Hz uncertainty due to the finite
step size of the BFO and VFO.  The frequency error table assumes that the 4
MHz oscillator is 10 Hz high in frequency.

Freq       Mode      Error (Hz)
3500       LSB         +09
7000       LSB         +18
14000     USB         -35
21000     USB         -53
28000     USB         -70

It is very difficult to set the 4 MHz oscillator to a better accuracy than
~10 Hz, plus this oscillator does drift at a measured rate in my K2 of -1 Hz
per degree F increase in temperature.  The K2 internal temperature increases
by about 10F over a 1 hour warmup period in receive mode which will shift
this oscillator by -10 Hz as the K2 warms up.  The most accurate way to set
this oscillator is to drive the K2's internal counter with a known accurate
frequency source in the 20 to 30 MHz range.  You may also try to set the
fifth harmonic (20 MHz) of this oscillator so that it is at exactly the same
frequency as the 20 MHz WWV signal.  You have to remove the K2's antenna to
hear the fifth harmonic.

Finally, I think it is unreasonable to expect a K2 to have an absolute
frequency error better than the above table.  My K2 can regularly achieve an
absolute frequency accuracy of 40 Hz on 15 MHz WWV.  The K2 was not designed
with a temperature compensated master oscillator and a 1 Hz frequency
synthesizer.  While there are radios that have this level of performance,
they tend to be expensive, heavy, and power hungry.  Very few communication
modes require an absolute frequency accuracy of a few Hz.

-John
 KI6WX