[Elecraft] K3 pwr out on digimodes

[Joe Subich, W4TV]

> If the filter response varies across the passband, which it 
> does, the ALC will vary the amount of gain used to try to 
> keep the RF level to the amount it is set at, whether the 
> signal is being generated near the edge of the passband or at 
> the center.

This is your fundamental misunderstanding of ALC.  Traditional 
ALC does NOT (and can not) increase gain - it can only DECREASE 
gain to bring the grid/base/gate current down to the level at 
which the final amplifier is not being overdriven.  DSP based 
ALC is similar, it can only adjust the modulator (DSP) output 
to the level determined to be appropriate at the single point 
in the IF filter at which the power calibration (5W/50W) was 
made.  

ALC (and power control) can not compensate for differences in  
gain across the passband - in other words, non-flat response. 
If you wanted to compensate for that non-flat response, it 
would be necessary to calibrate the power (5W/50W) at regular 
intervals across the passband and build a gain vs. frequency 
table.  The DSP would then need to measure the frequency of 
any narrow band modulation and adjust the assumed gain of the 
power control circuit based on the gain of the IF chain at 
that frequency.  

> This is important to the PSK31 operator who does 
> not want to tune every signal to the center of the passband 
> sweet spot before hitting TX in order to get maximum power out.

The PSK31 operator must adjust his drive (mic gain or power 
control) with ANY transmitter if he changes his "subcarrier" 
frequency.  Even with conventional ALC if the drive is set 
to the ALC threshold when operating at the point of maximum 
gain, the output will be reduced when operating at other places 
in the passband (particularly when operating at the edges of 
the passand).  

Try this experiment with your FT-817 ... set the power level 
to the maximum, turn off any compression, set your soundcard 
to 1700 Hz and the level for 15 Watts out of the FT-817 with 
NO ALC.  Now, without making any other changes measure the 
power output with a 200 Hz tone and a 3200 Hz tone.  Is it 
identical AT ALL THREE points?  

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: Friday, January 16, 2009 12:52 PM
> To: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] K3 pwr out on digimodes
> 
> 
> 
> 
> 
> Joe Subich, W4TV-4 wrote:
> > 
> > 
> > the purpose of ALC is not to control the
> > shape of the passband across the SSB filter it is to prevent a 
> > SSB signal in total from overdriving the power amplifiers.
> > 
> > 
> 
> I agree with you. As applied to a voice SSB signal or even a 
> wideband data mode that is completely correct.
> 
> But in practise with a narrowband signal like PSK31 the 
> effect will be to keep the output RF at the same power, 
> whatever audio frequency the signal is being generated at, 
> because only a narrow band of frequencies is being used at 
> any one time.
> 
> If the filter response varies across the passband, which it 
> does, the ALC will vary the amount of gain used to try to 
> keep the RF level to the amount it is set at, whether the 
> signal is being generated near the edge of the passband or at 
> the center. This is important to the PSK31 operator who does 
> not want to tune every signal to the center of the passband 
> sweet spot before hitting TX in order to get maximum power out.
> 
> -----
> 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: 
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