[Elecraft] K3 pwr out on digimodes

[Julian, G4ILO]

Joe Subich, W4TV-4 wrote:
> 
> 
> 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.
> 
> 

No, I do not misunderstand what ALC does. Perhaps I am just not very good at
explaining what I am trying to say. I realize that ALC can only reduce the
gain. That is why it is necessary to set the audio drive level so that you
get a little bit of ALC across the whole passband range that you want to
use. Then, the ALC will be able to reduce the gain by the necessary amount
so that the RF output is constant across that passband range.



> 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). 
> 

No. This misses the whole point of the convenience of "click and call"
operating with programs like Digipan and HRD. I never suggested that the
drive must be set to the ALC threshold when operating at the point of
maximum gain. It must be set so that you get some ALC across the whole of
the passband you wish to use (probably corresponding to no more than the
-3dB bandwidth points, beyond that and the steepness of the filter slopes
would cause unacceptable distortion of even a narrowband signal such as
PSK31.) Then you will achieve constant power across that range, because the
ALC will be able to apply varying amounts of gain reduction to achieve that
constant power level. The K2 can do this. Why can't the K3?



> 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? 
> 

The FT-817 is not capable of 15W. But that is beside the point. What would
this experiment prove? I know as well as you do that it would not be
identical. That's why I am saying that some form of level control needs to
be in operation across the whole of the passband you wish to use. A small
amount of ALC with a slow time constant will not harm the IMD in PSK31 mode.
The K2 demonstrates this.

In SSB mode the K3 ALC provides the level control I am describing. If I turn
CMP off and switch the audio source to LINE IN it works exactly as I expect
it to work in data modes (apart from not disabling EQ, which is a different
issue.) In DATA A mode it does not do this, so you must constantly tweak the
drive level whenever you change frequency if you wish to transmit a specific
power level.

If this is how the K3 is supposed to operate then I shall put it up for sale
right away, because it is not something that people who use data modes a lot
want to do. (Well, perhaps there are some, because if I have learned nothing
else from my participation in ham forums like this, it is that some people
actually like doing things the hard way.)

-----
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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