[Elecraft] K3 pwr out on digimodes

[Joe Subich, W4TV]

> I think Joe may be thinking of how older radios used to work, 
> where the power control varied the gain of the TX stages and 
> ALC did only start to operate once the rated output of 100W 
> or whatever had been reached.

Older (tube) radios did not have "power level controls."  They 
were operated at their design power and ALC reduced the gain 
to prevent overdriving the final amplifiers.  

Later radios added additional comparators to the ALC system 
to sense absolute power in an earlier stage and adjust for a 
relative power level but that is not ALC it is power control. 

The K3 operates much like the older radios except that the 
level is controlled at the output of the DSP (before the IF 
filter) and not the grid of the final amplifier.  The ALC 
adjusts the DSP so that when the power control is set for 5.0 
or 50 Watts the output power is 5.0 or 50 Watts at one specific 
frequency point in the SSB filter.  The DSP then adjusts its 
(peak) output as needed to maintain the correct peak level 
based on the assumption that the gain of the RF chain does 
not change.  

The 5.0 and 50 watt calibration levels are significant ... 
they are far enough below the compression point of the 
power amplifier to allow for slight gain changes within 
the IF passband.  

Again, you are simply asking the ALC and power control circuits 
of the K3 to do something they can not, and were not designed 
to do.  The RF chain is not flat across the IF passband; as 
long as the technology uses mixers and narrow filters it will 
never be flat.  If you want to be able to "click to tune" 
without manually adjusting power levels get a Flex-5000 or 
a transmitter that uses "on frequency" RF generation.  

73, 

   ... Joe, W4TV 
 

   


> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Julian, G4ILO
> Sent: Saturday, January 17, 2009 10:27 AM
> To: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] K3 pwr out on digimodes
> 
> 
> 
> 
> 
> Ian White GM3SEK wrote:
> > 
> > 
> > A sane engineer surely would; but the huge majority of transceivers 
> > are
> > trying to use the ALC loop for too many different functions 
> including 
> > gain levelling between bands, peak limiting and also power control.
> > 
> > [snip]
> > 
> 
> I seem to have read this before - perhaps in one of your 
> RadCom columns?
> 
> Certainly I am making the assumption that ALC is used to 
> control the power
> to the level the operator has set, and that a responsible PSK 
> operator has
> chosen a level of perhaps half the rated maximum SSB output 
> or even less. I
> think Joe may be thinking of how older radios used to work, 
> where the power
> control varied the gain of the TX stages and ALC did only 
> start to operate
> once the rated output of 100W or whatever had been reached.
> 
> Since we are talking about digimodes here, and since the K3 
> like most other
> modern radios has a separate mode setting for data modes, it is surely
> possible for the ALC to act more slowly for data than for 
> SSB, because the
> amplitude of a data signal does not vary much, if at all, and 
> the amount of
> gain reduction needed over the course of a transmission will 
> probably be
> more or less constant once the initial level has been set.
> 
> Again, I must reiterate that this is what the K2 appears to 
> do. At the start
> of the first data transmission of the day you would see the 
> RF output level
> climb or fall until the selected value was reached, after 
> which the radio
> appeared to remember the setting used so that subsequent transmissions
> always started at whatever power had been set.
> 
> -----
> 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/K3-pwr-out-on-digimodes-tp2163956p2173365.html
> Sent from the Elecraft mailing list archive at Nabble.com.
> 
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