[Elecraft] K3 pwr out on digimodes

[Ian White GM3SEK]

Peter Connors, F5VNB wrote:
>>
>> Absolutely not!  When conventional rigs are generating ALC
>> their final amplifiers are ALREADY in compression and they
>> are generating IMD in the PSK31 signal.
>
>A bit sweeping, Joe. Surely any sane engineer would put the onset of 
>system ALC at a level *before* the PA goes into compression? (Such 
>systems would not include grid current derived ALC from some class AB2 
>PA of yesteryear.)
>
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.

That is NOT how ALC was originally intended to work. The basic idea was 
invented and refined by the Collins company more than 30 years ago, but 
was intended only to handle *small, dynamic* variations in SSB output 
power (mostly due to filtering a clipped SSB signal, which re-introduces 
a  few dB of amplitude variation).

But ALC is also being used to achieve much larger output variations, 
notably for the front-panel RF PWR control. The ability to vary the 
output power all the way from 100W down to say 1W requires a very high 
gain in the ALC loop. As well as producing instability and overshoots, 
such a high loop gain ensures that any activation of the ALC will cause 
hard limiting - with obvious consequences for IMD.

SM5BSZ has published a  strong critique of the way that most rigs MISuse 
the ALC  loop. Instead of ALC helping to prevent IMD, it has become a 
major potential *source* of IMD. SM5BSZ's criticisms are backed up  by 
measurements on a range of different rigs:

Real life Dynamic Range of Modern Amateur Transceivers
http://www.sm5bsz.com/dynrange/dubus205/dubus205.htm

Speech Processing for SSB Transmitters
http://www.sm5bsz.com/dynrange/dubus405/dubus405.htm

The conclusion of SM5BSZ's papers is that rig designers need to stop 
demanding the impossible from the dynamic ALC loop, and go back to the 
original Collins concept. The big variations required for power control 
are "static" settings that do not require a dynamic control loop. They 
should be achieved by calibrating and then pre-setting the TX exciter 
gain to give the required power level for each band. Then the ALC can 
return to its original role of handling small dynamic variations, which 
only requires a low-gain ALC loop with a much lower potential for 
re-introducing IMD.

Does this sound familiar? It should, because the K3's power control and 
ALC system has been very much influenced by the ideas from SM5BSZ and 
originally from Collins.

Only Elecraft know the intimate details of how these concepts have been 
implemented in DSP code, and the extent to which the K3's ALC differs 
from traditional rigs. The bars on the K3's ALC display have been 
programmed to show something very different from the ALC meter in a 
traditional rig.

Bottom line: don't make assumptions about ALC behavior in the K3, based 
on the widespread misuse of ALC by other rigs.



-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek