[Elecraft] K3 pwr out on digimodes

[Joe Subich, W4TV]

>  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.

Unfortunately, the PA will be overdrive far before the IF. 
Doing any kind of IF IMD calibration will provide almost 
no benefit since the IF appears to be clean at almost any 
level (measured on the XVTR IF output). 

> Digimode programs could then have a dedicated button to 
> send an idle tone, read the IMD, and adjust the power. 

Digimode programs can already be programmed to send an 
idle signal and read the Power Meter (BG get) if the 
display is set to the proper mode.  That would allow the 
software to adjust for a specific power level.  However, 
that requires K3 specific software. 

It’s also very possible to adjust the drive for a specific 
ALC level - again using BG if the meter is set to read 
ALC instead of Power.  However, the important parameter 
in the K3 is probably power output rather than ALC level. 

73, 

   ... Joe, W4TV 
  




> -----Original Message-----
> From: Leigh L. Klotz, Jr. WA5ZNU [mailto:Leigh at WA5ZNU.org] 
> Sent: Friday, January 16, 2009 2:18 PM
> To: lists at subich.com
> Cc: 'Julian, G4ILO'; elecraft at mailman.qth.net
> Subject: Re: [Elecraft] K3 pwr out on digimodes
> 
> 
> 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
> 
> 
>