[Elecraft] K3 Beta firmware rev. 2.57

[Julian, G4ILO]

Joe Subich, W4TV-3 wrote:
> 
> 
> Traditional ALC operates by overdriving the final amplifier. 
> It senses grid current Which is an indication of overdrive 
> and uses that to turn down the drive.  Previous generations 
> of solid state radios turned to sensing output power for the 
> level reference.  However, those "after the fact" ALC circuits 
> have two serious issues ... first overdrive must have already 
> occurred to generate an ALC signal and overshoot is certain 
> but there is also no protection against overdrive and distortion 
> in the low level intermediate stages. 
> 
> The K3 generates its ALC in the early stages (the DSP) using 
> a feed forward technique that delays the RF to allow the 
> control signal proper effect.  By controlling the level early 
> in the transmission chain, the K3 ALC prevents overshoot,   
> protects against distortion due to RF compression or clipping 
> after the "masking filter" or "channel filter" and eliminates 
> distortion due to ALC "hunting" (the effects of ALC constantly 
> overcorrecting then under correcting).  Instead of using a 
> short ALC time-constant (which causes the "hunting" distortion 
> in conventional designs), the K3's power level control can 
> almost have a fixed gain with only a slow time constant for 
> fine adjustment.  
> 
> However, since the power control really sets the average power 
> (e.g. CW), there must be some compensation for differences 
> in the peak to average power ratios of various voices, the 
> level of compression/clipping in use and different modulation 
> types (particularly some data modes with very high peak to 
> average ratios) so that operations with minimal clipping or 
> modes with high peak to average power ratios do not overdrive 
> the driver and PA on peaks even though the "average" power is 
> well below the level that has been set. 
> 
> 

Thank you for the excellent explanation, Joe.

If that's the case, how does someone who does not have a peak reading
wattmeter set up the gain so that the PEP output is equal to the amount of
power set on the power control? I can feed a sine wave in and adjust the
gain so that the RMS output is correct. But if I understand you correctly,
the actual peak output will be dependent on the peak to mean ratio of the
signal being fed in, so the only way to set the peak power on any mode that
modulates the amplitude of the carrier is to calibrate TXG VCE using a scope
or a meter that can tell you accurately what the peak power is?

In fact, if the power control is really setting the average power, then if I
turn off the compressor because I'm talking to someone who is already
getting me S9, won't the effect of the slow time constant ALC then be to
increase the gain to bring the average power level (and consequently the
peak power level) up, because the peak to mean ratio with compressor off
will be greater than when it is on?

-----
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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