[Elecraft] K3 pwr out on digimodes

[Leigh L. Klotz, Jr. WA5ZNU]

Joe W4TV wrote:
> Closed loop power controls are not appropriate to SSB systems because time
> constants sufficient to maintain constant power across the
> IF passband for digital operation would result in frequency
> dependent clipping and/or compression in voice operation.

Most digimode programs include a tune button which outputs a tone at the
selected output; psk's idle is a simple two-tone.  One option (and I
expressed this a year or so ago to the Elecraft folks) would be to offer
an IMD calibration at the IF level for a known two-tone response, to
determine whether at least that stage is being overdriven.  The DSP should
have a fairly good idea at that point.  If the two-tone signal used is a
PSK idle tone at the selected AF on the waterfall display, a rig control
command to read the IMD calculated would be just peachy.  Digimode
programs could then have a dedicated button to send an idle tone, read the
IMD, and adjust the power.  Regular communications could then commence.

A further advantage of this approach is that the actual power out could be
measured during this calibration period from either the built-in wattmeter
or the rig control command, and reported in real time, and thus the knob
could (or again, rig control) could be used to achieve the output power
desired by the user.

Elecraft's response was that perhaps the IMD calculation could be done on
the computer side; I'm not clear whether the new monitor out function
makes that possible or not, but if it does, that would be an approach that
could be made to work today, modulo the limitation that the IMD calculated
from the sound card will be a little worse due to the additional ADC of
the sound card, whereas a digital report from the K3 IF would be accurate.
 (And not to mention that an overdriven line input would result in a
drastically wrong IMD, but that's likely to have been already taken care
of by the digimode RX setup.)

In summary, I believe it's possible to provide closed loop control for
both power and drive/IMD readings for modes such as PSK, but not during a
QSO where the signal characteristics vary; instead provide enough CAT
commands and signal quality measurements from the K3 RF OUT and IF OUT
stages so that and digmode authors are willing to provide a "TX CAL" tune
button.  (Doing in the k3 utility wouldn't be helpful because the precise
audio settings used by the digimode program for TX are important.)

I'm willing to step up and try this for fldigi.

Leigh/WA5ZNU