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

[Joe Subich, W4TV]

> The point is it was providing the correct power output levels 
> before the last couple of firmware revisions.  Something 
> changed that made it go from putting out approximately the 
> right power level, to putting out WAY TOO MUCH (i.e. causing 
> splatter) power.

Not true ... in spite of careful calibration at both 5 and 50 
watt levels neither of my K3s has ever produced exactly the 
power requested on any band other than 20 meters, on any mode 
other than CW and at any power other than 5 or 50 watts.  The 
variations can be as much as 3 dB on SSB depending on the peak 
to average ratio of the driving audio and amount of compression 
used.  

Setting a power control calibrated for CW and expecting that 
level to be accurate for PSK31 or any other arbitrary digital 
modulation is complete folly and utter hubris.  Among other 
issues the wattmeter does not know if you are setting peak 
or average power and it does not know what the average power 
should be for the arbitrary data and arbitrary modulation.  

For the K3 to accurately control the power level in all modes 
the its directional coupler would probably need to be improved 
by at least an order or magnitude.  The response flatness would 
need to be improved by at least an octave and the detectors would 
need to be substantially more linear ... and that's just for CW. 
To handle arbitrary audio inputs takes some knowledge of the 
peak to average ratio as well as the peak duration (duty cycle) 
in order to select the proper detector time constants.  Every 
voice every  digital mode and every arbitrary data stream will 
have different peak to average ratios and duty cycles.  Do you 
expect the DSP to calculate those values on the fly?  Perhaps 
the power metering should be derived from a DSP based spectrum 
analyzer? 

Accept the fact that the power level control is relative.  If  
Wayne and Lyle can improve on it that's great.  However, trying 
to hold .1 dB or even 1 dB accuracy from 1 mW to 120 W, 1.8 MHz 
to 54 MHz with arbitrary modulation sources in a reasonably 
priced amateur transceiver is a fool's errand. 

73, 

   ... Joe, W4TV 
 



> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Steve Ward
> Sent: Saturday, January 03, 2009 2:54 PM
> To: elecraft at mailman.qth.net
> Subject: [Elecraft] K3: Still unable to set power correctly 
> in, DATA A(sigh)
> 
> 
> "Joe Subich, W4TV" Wrote:
> 
> >So?  Reduce the power you request to get the required output.
> >What other radio has a "calibrated" power output control?  
> 
> >The wattmeter in the K3 is relative ... there is a documented
> >power vs. frequency slope.  Testing seems to indicate there 
> >are also nonlinearities with changes in peak to average ratio 
> >and as the power level moves away from the calibration points. 
> >The directional coupler in the K3 does not have the high level 
> >of directivity and compensation of a product like the Telepost 
> >LP-100 nor does it have the high quality detectors. 
> 
> >If you want a bloody laboratory grade meter, buy a laboratory
> >grade power meter.
> 
> The point is it was providing the correct power output levels 
> before the last couple of firmware revisions.  Something 
> changed that made it go from putting out approximately the 
> right power level, to putting out WAY TOO MUCH (i.e. causing 
> splatter) power.
> 
> Setting TXG VCE to -3 dB gets DATA A output almost to where 
> it should be, but of course adversely affects SSB.
> 
> This is NOT about the accuracy of the wattmeter, its about 
> overdriving the PA.
> 
> 73,
> Steve
> AD7OG
> K3 #1544
> 
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