[Elecraft] VP6DX -- the true story ?

[Eric Scace K3NA]

Privet Igor --

   As best I can remember, we used MCU v1.66 and DSP 1.52.

   I didn't use the AGC because I did not need it.  The point of 
automatic gain control is to adjust the gain of the various stages of 
the receiver in order to
   a) avoid distortion/overdriving a stage, and
   b) bring signals up/down to a comfortable listening level.

   (b) is not relevant to an operator who is working a pileup.  (b) is 
relevant to an operator who is monitoring a frequency with one station 
transmitting.

   For operating a pileup, there are a variety of tools the brain uses 
to distinguish the many signals:
   -- pitch (CW)
   -- style of speaking (speech) or keying (CW)
   -- artifacts; e.g., auroral flutter, chirp, etc.
   -- strength (all).
AGC tends to reduce the difference in signal strength, and so removed 
valuable information.

   In situations where static crashes interfere with reception, AGC hang 
time on a loud static crash also increases the length of time that a 
specific static crash interferes with reception.

   I used headphones with good audio isolation between my ears and the 
rest of the world around me.  That allows me to set receiver gain levels 
with the underlying antenna/band noise just above my threshold of 
hearing... and to use at least 80 dB of my hearing range for listening.  
In this quiet listening environment, I don't need AGC.

   Even in a less-than-quiet listening environment, if a band is just 
open weakly (e.g., 12m to Europe), the range of signal strengths in the 
pileup can be smaller: maybe less than 30 dB between band noise and the 
strongest signal.  So AGC isn't needed here either.

   My ideal AGC in these situations is one that only makes changes in 
receiver gain when a stage in the receiver is about to be over-driven 
(e.g., the A/D converter)... and removes those changes relatively 
quickly.  Even then, it might be fine to allow the receiver to be 
over-driven (a static crash contains no information).  If a signal I 
want to copy is over-driving the receiver, the best solution often is to 
reduce the RF gain manually during the duration of the time when I want 
to copy that station.  If that station is just "interference" (e.g., a 
loud USA station on 80m CW calling VP6DX, when I want to work northern 
Scandinavia and northwest Russia/western Asia during the brief opening), 
I have other controls (filter bandwidth, notch) than might be better to 
use that gain reduction (automatic or manual) that could suppress the 
desired weak signals.

   So, almost any AGC system is inappropriate for a DXpedition or 
content environment... as long as the receiver and one's own ears have 
enough dynamic range to handle all the signals presented to it.  The K3 
has more dynamic range than other receivers.

73,
   -- Eric K3NA

on 08 Mar 13 Thu 02:26 Igor Sokolov said the following:
>>   Almost all of the operators ran the K3 with AGC off (all modes).
>>
>> -- Eric
>
> Eric, can you explain what did you not like about AGC in K3?
> Did you notice any problems with DSP being permanently on in K3 when 
> listening to heavy pile ups?
>
> 73, Igor UA9CDC
>