Posted by
Guy, K2AV on
Feb 07, 2010; 3:11pm
URL: http://elecraft.85.s1.nabble.com/K3-in-a-cw-pileup-needs-work-tp4523884p4529477.html
I don't believe so. I'm recommending using only just enough RF/IF
gain to produce moderately low audio from ambient noise on a clear
frequency. You can set the AGC threshold anywhere you want according
to taste. Some are using NO AGC and the audio limiter to expand
"threshold" to the audio limiter level.
I'm proposing that there is a moderation point between one extreme of
ambient noise shoved up into the AGC reduction range and the other of
no AGC at all. And that changing RF gain during a pile up can
maximize amplitude discrimination.
Particularly for contests, NOT reducing the RF/IF gain per band on the
lower bands just shoves the ambient noise up to a roar where the AGC
winds up REDUCING wanted signals down to the level where noise has
been BROUGHT UP, reducing the apparent signal to noise as perceived by
the ear/brain combination to zero, a researched and documented
phenomenon that has been quite appropriately reported by some here as
"mush". This uncomfortable state is amplified in a pile up where most
of the discrete signals become "noise" for hearing purposes, and the
already used up threshold in the AGC forces them all to the same
level. "Mush" is a very good description.
(One experiment I recall is asking someone to identify individual
conversations out of a monaural recording of a school cafeteria at
lunchtime -- only the very loudest can be discerned.)
AGC under squished circumstances can make "mush" all by itself without
any help from a purported (but never carefully measured and reported)
IMD "problem".
The AGC threshold engages at a fixed voltage out of the RF/IF string
for each step. Only the protective hardware AGC is operative before
this point. If your setting of PRE/ATT/RFGAIN places ambient noise at
what should be reserved for an S3 signal, you have subtracted that
from your selected threshold range, no matter what your threshold
preferences. And at worst case have already engaged AGC for the
ambient noise.
Threshold should allow one to have some range where any level
difference in competing signals come through to allow ear/brain to
tell them apart, AND still have a top where leveling kicks in to
protect the ears.
73, Guy.
On Sun, Feb 7, 2010 at 9:19 AM, Barry N1EU <
[hidden email]> wrote:
>
> If you simply raise AGC Threshold, you will have to lower AF Gain or risk
> overly loud audio from stronger signals. So what you would effectively be
> doing is translating upwards the K3's dynamic range into higher rf levels.
> This is NOT good practice - you want to keep the dynamic range operating at
> lower rf levels (thus all the talk of gain throttling).
>
> 73,
> Barry N1EU
>
>
> Guy, K2AV wrote:
>>
>> This is true, but seems to be a VERY hard point to get across. Running
>> excess gain in RF/IF strip is at the root of some number of
>> complaints.
>>
>> On Sun, Feb 7, 2010 at 8:22 AM, Barry N1EU <
[hidden email]> wrote:
>>> If gain were throttled (e.g., via ATT on, PRE off, reduced RF Gain),
>>> wouldn't AGC Threshold be effectively raised?
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