[Elecraft] Calibration of BFO

[Ron D' Eau Claire]

Dave, K9DC, wrote:

> Actually, I believe the adjustment increment of the BFO is in 20 Hz
> steps. The VFO steps are 10 Hz. My comment about shifts less than 1 Hz is
> referring to frequency shifts when stepping through the four filter
> bandwidths. It can be less than 1 Hz.  It also will drive you
> nuts getting
> there. Sometimes just entering the filter alignment mode, making a change
> then returning back to the same setting and exiting will get it right on,
> sometimes it makes it worse. I've not discoverd the key to getting it
> right except to keep repeating the procedure... and then don't mess with
> it once you get it right.

As I recall a discussion a couple of years ago, the 12-bit Digital to Analog
Converters (DAC's) used in the K2 permit a 10 Hz resolution. That is, they
can't come up with a control voltage that will set the oscillator any closer
than that.

If you get lucky, the DAC will pick a value that hits closer, but it's
purely a matter of luck. The system can't tell the difference. And if you do
anything at all to make it store the value again, it may easily store a new
value that translates into a frequency error of up to 10 Hz.

I've always assumed that the reverse would be true when operating the rig.
Whenever the digital value representing a frequency was recalled from
memory, it would be translated into an analog voltage by the DAC, but only
with an accuracy that would place it within 10 Hz of the original spot. So
it would not be possible to store a value that would always be more accurate
than 10 Hz. I've always noted a 10 Hz step in at least one filter.

Sometimes when setting up the filters they sounded and looked 'exactly on'
but when I operated the rig later the 10 Hz frequency error would be back.
Never more than that, but it's always been there no matter how often I did
the alignment.

Ron AC7AC
K2 # 1289