[Elecraft] K3 Beta firmware rev. 2.57

[Joe Subich, W4TV]

> Perhaps I'm just being dim, but why is TXG necessary at all? 
> Why can't you just turn the mic gain up until you have plenty 
> of drive, and then use the power control to set the output 
> power using the ALC? 

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. 




> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Julian, G4ILO
> Sent: Saturday, October 25, 2008 6:17 PM
> To: elecraft at mailman.qth.net
> Subject: RE: Re: [Elecraft] K3 Beta firmware rev. 2.57
> 
> 
> 
> 
> 
> Joe Subich, W4TV-3 wrote:
> > 
> > 
> > I don't know ... it seems to work properly here.  If you elect
> > to not make it global, it may be necessary to provide separate 
> > TXG settings for voice (SSB), AM and AFSK A/DATA A (DTA) ... or 
> > at least SSB/Data and AM.
> > 
> > 
> Perhaps I'm just being dim, but why is TXG necessary at all? 
> Why can't you just turn the mic gain up until you have plenty 
> of drive, and then use the power control to set the output 
> power using the ALC? That's all I recall having to do on other radios.
> 
> -----
> 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 
> -- 
> View this message in context: 
> http://n2.nabble.com/Re%3A-K3-Beta-firmware-rev.-2.57-tp137495
4p1377026.html
Sent from the Elecraft mailing list archive at Nabble.com.

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