[Elecraft] K3EXREF question

[Edward R Cole]

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
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