[Elecraft] VP6DX -- the true story ?

[Igor Sokolov]

Thanks Eric,
I understand you think AGC in high dynamic range radio like K3 is not a 
necessity for the op with high dynamic range ears.
Actually the range should be close to 90-95 db on nowadays bands where 59+40 
signals co-exist with 1S unit signals very often. I also figured from your 
post that other ops from  VP6DX team choose not to use AGC not because of 
some sort of problem existed in that version of the firmware but purely 
because they prefer adjusting RF gain manually to squeeze out the most from 
the pile up.
What about the DSP then. I experienced that myself operating from 8Q7 in the 
past and many others confirmed my impressions that modern DSP based radios 
are no good when pulling call signs from the pile up of many stations when 
the average level of that pileup is close to the MDS of the radio. For 
example we could not use Orion in the expedition on a weak pile up on 10 and 
15 meters while the same operators could easily pick out calls using IC775. 
Therefore I would be happy to have a radio where DSP could be switched on or 
off depending on the situation. Did you notice the above mentioned effect 
while using K3?

73, Igor UA9CDC


> 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
>>