[Elecraft] K3: Still unable to set power correctly in DATA A(sigh)

[Joe Subich, W4TV]

> > Rich VE3KI posted some measurements done with the PSKMeter 
> that show poor IMD above about 50W.

The K3 wattmeter is essentially an average responding device. 
PSK31 has a relatively high peak to average ratio ... as much 
as 6 dB or more depending on the particular data but generally 
running at least 3 dB at idle.  Since the KPA3 will clip at an 
output power much above 110 watts, indicated power output on 
PSK31 idle should never exceed 50 watts with 30 to 40 watts 
being a "safe" value.   

> I've suggested he re-run the tests with a 3dB pad to compare 
> apples-to-apples and reduce the effect of non-linear response 
> in the PSKMeter, but Rich has suggested that perhaps the peak 
> power is exceeding 100W when the TX PWR setting is above 50W.

I've made similar measurements to those of VE3KI using a 
soundcard and software detectors on a separate receiver while 
monitoring the K3 power output with a factory calibrated LP-100. 
In fact, the K3's peak power output is around 110 watts when 
the indicated (average) power is about 50 watts with PSK31 
idle modulation.  Increasing the power output simply results 
in extreme overdrive of the KPA3 (and/or the IPA since I did 
not make separate measurements below the 12 watt level).  The 
excess drive results in a higher average power output due to 
clipping in the amplifier chain.  The clipping results in 
greatly increased IMD products.

Similar sets of measurements should be made for EVERY digital 
mode - particularly modes that use phase shift and/or multi-tone 
modulation.  Every one of those modes will have unique peak to 
average ratios and duty cycles.  Increasing the power output 
beyond the point of peak clipping (more specifically the 1 dB 
compression point) will, at best, only cause IMD and interference 
to other users and will, at worst, result in decoding errors in 
reception.  

Since the K3 power control is based on average power and not 
instantaneous (absolute) peak power, the power level (both 
requested and reported) can not be relied on as a gauge of 
proper operation unless the user knows with certainty how much  
headroom is required to handle the peak power without compression
or clipping. 

73, 

   ... Joe, W4TV 
 




> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Leigh 
> L. Klotz, Jr WA5ZNU
> Sent: Saturday, January 03, 2009 4:03 PM
> To: Julian, G4ILO
> Cc: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] K3: Still unable to set power 
> correctly in DATA A(sigh)
> 
> 
> There's a thread on a related topic (PSK TX IMD) on the 
> Elecraft K3 list 
> at Yahoo Groups.
> Rich VE3KI posted some measurements done with the PSKMeter that show 
> poor IMD above about 50W.
> I've suggested he re-run the tests with a 3dB pad to compare 
> apples-to-apples and reduce the effect of non-linear response in the 
> PSKMeter, but Rich has suggested that perhaps the peak power is 
> exceeding 100W when the TX PWR setting is above 50W.  Your 
> observations 
> would seem to confirm that.
> 
> Leigh/WA5ZNU
> > I recently installed the latest firmware and ran the TX Power 
> > calibration wizard - nice work. However while running WSPR (a sound 
> > card data mode) I was somewhat annoyed to find the K3 power meter 
> > showing about twice the output power I had selected. Checks 
> using my 
> > own power meter confirmed that the K3 is putting out about 
> twice the 
> > amount of power specified by the power control when DATA A is used 
> > (though the exact difference seems to vary a bit from band to band.)
> >
> > Using TXG VCE is NOT the answer, as the output power in SSB (and in 
> > CW) is regulated correctly. Using the maximum TXG VCE 
> -3.0dB I still 
> > can't quite get the power down to what it should be, but if 
> I do that, 
> > SSB output is too low.
> >
> > Forgive the note of frustration but I am frankly very 
> annoyed and fed 
> > up that after all this time and numerous reports of this 
> issue from me 
> > and several other people going back several months, the K3 is still 
> > not capable of performing a simple basic function like 
> regulating the 
> > output power to the level the operator requests. I am seriously 
> > considering selling the K3 at this point, as I am losing confidence 
> > that Elecraft has any intention of fixing this issue.
> >
> > -----
> > Julian, G4ILO. K2 #392  K3 #222.
> > http://www.g4ilo.com/ G4ILO's Shack   
> http://www.ham-directory.com/ Ham
> > Directory    http://www.g4ilo.com/kcomm.html KComm for 
> Elecraft K2 and K3 
> >   
> 
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