[Elecraft] Calibration of BFO - FWD from KI6WX

[Tom Hammond NØSS]

Folks:

The following message was posted by John Grebenkemper, KI6WX, but for some 
reason it was being rejected by John's e-mail server. So, I post it on his 
behalf. - Tom N0SS

John Grebenkemper, KI6WX, writes:

----- Original Message -----
From: "Ron D' Eau Claire" <rondec@easystreet.com>
To: <elecraft@mailman.qth.net>
Sent: Monday, February 04, 2002 8:39 AM
Subject: RE: [Elecraft] Calibration of BFO
 >
 > 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.

The PLL reference that controls the VFO frequency is tuned by a 12 bit DAC,
which has about 3000 steps that are useful.  For the worst case tuning on 10
meters, this will yield a frequency resolution of about 8 Hz.  It is much
better on the lower bands, getting down to about 2 Hz on 80 meters.  The BFO
tunes with an 8 bit DAC and will have a tuning step size between 23 Hz to 32
Hz in the frequency range of interest.  The curve is pretty similar whether
you are using the solenoidal L33 or the toriodal L33.

There are two cases of interest when running the radio in SSB mode.  The
first is to use the same filter for both transmit and receive.  In this
case, both oscillators are left unchanged and the radio should transmit on
exactly the same frequency on which it was receiving unless there is some
effect that pulls one of the VCXO's.  I have not tried this experiment, so I
don't know if there are any effects that would pull a VCXO frequency between
transmit and receive.

In the second case, a different filter is used to receive.  Unless you use
the exact same BFO frequency, this will result in a frequency difference in
transmission.  From trying to understand what the firmware does, I believe
that Wayne adjusts both the BFO and VFO frequencies to get as close as he
can to the same transmission frequency, but there will always be some error.
Furthermore, the firmware assumes that the frequencies are the same as when
you last did a CAL PLL and CAL BFO.  These will almost certainly have
drifted if those measurements have not been done very recently.

The best way to avoid offset frequencies is to use the same filter for
transmit and receive, or to set the BFO for a different receive filter to
the same BFO frequency as the transmit filter.  If you do the latter, be
sure to reset both the transmit and receive BFO's at the same time so the
firmware knows they are at the same frequency.

-John
  KI6WX