[Elecraft] K2: VFO Drift and Temperature-Compensated PLL ReferenceUpgrade

[Ron D'Eau Claire]

Michael, N9BDF, wrote:

My K2 sits in the basement where it is currently seeing stable ambient
temperatures around 67F. When using 10 MHz WWV as a reference, I see
approximately 40 Hz of drift (10,000.03 to 9,999.99) over a 30 to 45 minute
period from a cold start. I am not transmitting during this period, I assume
this is just due to a slight heating (can't be much of a temperature change)
of various resistors, capacitors, inductors, and varactors. This is clearly
in-specification as it is much less than the 100 Hz that is specified in the
manual. ;^)...

Temperature-Compensated PLL Reference Upgrade documentation says that you
can check the VFO drift by transmitting for a while to raise the temperature
of the radio 15 or 30 degrees F _AFTER_ you have allowed a warm-up period of
30 or more minutes. This appears to infer that there is a difference in the
nature between warm-up drift and post warm-up drift. If so, can someone
explain what that difference is?

-------------------------------------------------

Yes, the additional heating caused by the RF power amplifier running
(especially if you have a K2/100) causes additional drift. Also, even with
the mods installed, there is a little drift in the BFO and PLL reference
oscillators and they tend to drift such that their errors add on the higher
frequency bands and cancel on the lower frequency bands. 

My K2 (S/N 1289) showed a drift on 15 meters (where the drift of the
oscillators add to make it worst-case) of up to 240 Hz from a cold start
before the mods. 

After the mods, I see about 50 Hz drift from a cold start for about 15
minutes or so. Then, worst case, I see up to 50 Hz additional drift if I
really "hammer" the K2 at 100 watts out for a long-winded CW transmission
(20 minutes or so). That's on 15 meters. On  the lower bands, the drift is
very close to zero, thanks to the canceling effect of the BFO and PLL
reference oscillator.

I believe my Reference Oscillator mod is typical. It is a "breadboard"
version built out of discrete components mounted on the bottom of the main
board that was used in testing the design for the production version of the
circuit. It worked FB, so I never bothered to replace it with the
"production" mod. 

Ron AC7AC