The K3EXREF circuit is essentially a AFC (sort of) where a highly
accurate 10-MHz reference is used to measure the frequency of the
TCXO and send a frequency correction "word" to control the TCXO so
that the error from 49.380 MHz is removed.
As I understand it this comparison is done about every
4-seconds. The TCXO drifts with temperature so it will definitely
change when you first power on the K3. This drift will slow at some
point and hover around a offset level (mine is about 49.380.080. The
TCXO is actually tuned to correct from that so REF*CAL is only
showing the amount of offset that is being corrected. Output of the
TCXO is not 80-Hz high. It would be if the EXREF did not correct it.
I have the K3/10 so only have the 12w PA which does not add as much
heat in transmit as the 100w PA. So that lessens the amount of
temperature drift of the TCXO. The TCXO-3 is better than the std
TCXO so the improvement the EXREF makes is better with it. On
transmit on 28-MHz using the EXREF, I see approx. +/- 2 Hz frequency
error with the TCXO-3. This is equivalent to 0.1 ppm. Without the
EXREF my error is five time larger or about +/- 14 Hz.
There is still a slight amount of frequency drift since the TCXO is
operating normally in between the times the 10-MHz reference checks
and corrects frequency error. This explains why a small amount of
drift still occurs vs what happens with a phase-locked
oscillator. The result is that the frequency will appear to look
like a ramp waveform over the 4-second interval between corrections.
The reason for choosing this form of frequency control is it
preserves the excellent low phase noise of the K3. PLL are difficult
to make with as low phase noise.
If any of this is incorrect, perhaps one of the Elecraft engineers
will correct what I wrote.
73, Ed - KL7UW
http://www.kl7uw.com
"Kits made by KL7UW"
Dubus Mag business:
dubus...@gmail.com
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