[Elecraft] Off Frequency K2

[Ron D'Eau Claire]

There are two things to consider, Ed.=20

First, the K2 readout has a potential error built in of up to about 30 =
Hz,
due to the resolution of the Digital to Analog Converters (DACs) used in =
the
design.=20

Secondly, the local oscillator is not a "fully synthesized" oscillator. =
It
uses a VXO to tune across 5 kHz segments of the band and a Phase Locked =
Loop
to shift the frequency in 5 kHz segments. So within each 5 KHz tuning
segment, the frequency is not directly related to the crystal =
oscillator.=20

This has great advantages in the design of the K2 such as the extremely =
low
phase noise in the local oscillator, but it produces some unavoidable
inaccuracy in the dial readout.

I assume that you ran CAL PLL immediately after you set the 4 MHz =
oscillator
'dead on' 4.000 MHz. If not, that might give you a little tighter dial
accuracy. Even though the DAC's resolution allows up to a 30 Hz error, =
it
does not mean that you can't get closer by being "lucky" when CAL PLL
measures the frequencies. But it's essential that you re-run CAL PLL =
right
after setting the 4 MHz oscillator on frequency.=20

What's going on when you run CAL PLL, as I understand it, is that the K2
uses the 4.000 MHz oscillator as the time base in a frequency counter to
measure the VXO frequency at various points in it's tuning range. The =
tuning
voltage associated with each VXO frequency is stored in a "look up =
table" in
memory.=20

Then, when you are tuning your K2 in normal operation, the controller is
looking up the tuning voltage that was written in memory for the =
frequency
you are requesting by where you set the tuning dial. Frequencies between
those stored in memory are 'interpolated' or estimated by comparing the
nearby frequencies that were measured. A frequency control voltage is =
sent
to the VXO based on these calculations.=20

The accuracy of that voltage is based on the following:=20

1 - How accurately the VXO frequency was measured when you last ran CAL =
PLL
(which is why it is so important to have the 4 MHz oscillator set as =
close
as possible when you do that)=20

2 - The accuracy of the interpolation done if the frequency you are =
tuning
in is between frequencies stored when you ran CAL PLL.=20

3-  The accuracy of the Digital to Analog Converters that convert the
digital value to an analog voltage to tune the VXO.

Finally, the accuracy of the local oscillator signal is determined by =
how
stable the VXO is over time!  This is an issue because if the VXO has =
any
drift, it will not return to exactly the same frequency it showed when =
CAL
PLL was run even though the DAC might send it exactly the same tuning
voltage. And, as stable as a VXO is, they DO drift a little with =
temperature
and time.=20

Adding all these issues, a 50 Hz error doesn't seem all that surprising =
even
after a good warm-up!

Ron AC7AC
K2 # 1289

-----Original Message-----
From: elecraft-admin@mailman.qth.net =
[mailto:elecraft-admin@mailman.qth.net]
On Behalf Of n5gf
S
Looking for a helpful suggestion.   My K2 #3332 WILL NOT calibrate on
frequency.  ... the readout reads 50 + Hz away from the actual frequency
being received ... But that just isn't right and this radio should =
calibrate
much closer than it does.

73  de N5GF  Ed  K2 #3332 =20