[Elecraft] To PLL mod or not

[lhlousek]

As I understand it, characterizing the drift of a K2 isn�t quite as simple as seeing what happens at a single frequency
like 10.000 MHz.

The operation frequency of the K2 is determined by a combination of Local Oscillator (VCO/PLL) and BFO frequencies so if
one or the other drifts, the operating frequency will drift.  If both drift, the total drift can be better or worse than
the individual drifts depending on whether they add or subtract.  Whether they add or subtract changes when you go from
17 meters and below, where the operating frequency is equal to the LO minus the BFO frequency, to 15 m and above, where
the operating frequency is equal to the LO plus the BFO.

The magnitude of the contribution to drift due to the BFO is more or less constant from band to band since it is always
operated at the nearly the same frequency (around 4913 KHz or ~ 5MHz).  However, because of the wide tuning rang of the
LO (~ 6.7 to 24 MHz), its contribution depends on the band. This wide range is achieved by using a PLL to multiply a
VXCO (running near 12 MHz) so the drift depends on the multiplier.

For example, on 40 meters the LO is 7 + 5 = 12 MHz. The PLL multiplier is ~1 so LO drift will be equal to the VXCO
drift.  At 28 MHz, the LO is 28 - 5 = 23 MHz.  Since the VXCO operated at 12 MHz, the PLL multiplier is ~ 1.9 and the
contribution to drift from the VXCO will be nearly twice as big.

To make things interesting, the amount of temp drift of the LO or BFO is dependent on where you are in the tuning range
of their respective VXCOs.  Changing filters or sidebands changes the BFO frequency and so can affect its contribution
to drift.  The LO VXCO is used to tune over successive 5 KHz segments of a band so the contribution to drift due to the
LO will depend on where you are in each 5 KHz segment.  And, due to the PLL multiplier, the tuning range of the LO VXCO
ranges from about 2.6 KHz on 10 meters to about 9 KHz on 180m so the contribution due to where in the 5 KHz segment you
are is also band dependent!

The end result is that the overall magnitude and the direction of the drift can vary a lot depending on band, side band,
and over 5 KHz tuning intervals.

Drift in the BFO not only affects operating frequency but also affects where the bandpass of the filter lies with
respect to its setpoint.  If you are using a narrow 150 Hz CW filter centered on 750 Hz and the BFO drifts 200 Hz, a CW
signal zero beat to 750 Hz will no longer pass through the filter.

The drift of my K2 without the KPA100 was a non-issue.  However, with the KPA running at 100W , I was getting no
significant drift on 40 meters but upwards of 400Hz drift during a QSO on 15 meters after a long over.  I was one of the
guinea pigs for the new L33 toroid and (wound two of them myself) and the single VCO Xtal and these changes together
have reduced the drift to acceptable levels (100 to 200 Hz during a QSO) an all bands.

Lou W7dzn