Re: Attenuator and RF Gain Settings
Posted by
Don Wilhelm-4 on
Feb 03, 2010; 12:28am
URL: http://elecraft.85.s1.nabble.com/Attenuator-and-RF-Gain-Settings-tp4503505p4504749.html
For those who can gather information from graphs, take a look at the AGC
behavior Jack Smith has plotted. The information is at
http://www.cliftonlaboratories.com/elecraft_k3_agc_and_s-meter.htm#AGC_SLP_and_AGC_THR.
73,
Don W3FPR
Don Wilhelm wrote:
> Brian,
>
> That is an illusion - yes the K3 gets more quiet (on noise) if the AGC
> Threshold is reduced.
> BUT what that indicates is that the AGC is being activated by the noise
> and the K3 is reducing the gain due to AGC action.
> The lowest AGC Threshold that is usable for any given level of
> atmospheric noise is the threshold setting just higher than the setting
> which begins to reduce the noise. In other words, listen to a spot on
> the band where there are no signals, then start at the highest Threshold
> setting of 008.
> Reduce the value of the AGC Threshold until you sense that the noise
> level starts to go down - then move the Threshold up to the next level.
>
> That setting will produce the most usable sensitivity for that band,
> that antenna, and at that particular point in time (atmospheric noise
> levels do change with time). If the resulting noise level is
> bothersome, then take other steps to reduce that response - Preamp off,
> ATT ON, and reduce the RF Gain (in that order) until you can just barely
> hear the noise. That will result in the most sensitive setting for the
> receiver.
> Remember (as Jim Brown just posted) - that atmospheric noise is just
> another 'signal' to the receiver that is to be amplified (it is coming
> in on the antenna port). One must condition the receiver to place that
> noise level at (or just above) your threshold of perception to reduce
> your fatigue level when listening to any band. That is what the preamp,
> attenuator, and RF Gain controls are for.
>
> After a bit of experimenting, you may come to the point where you have
> achieved a compromise set of AGC Threshold and Slope setting that work
> well in all but the most demanding situations. Leave them set at that
> point and control the receiver response with the normal controls of
> preamp, ATT, and RF Gain. For those digging for signals beneath the
> noise level (yes, some ops have very good ears for that), they will have
> to suffer with hearing some of the noise as well - just how much is an
> individual decision.
>
> Again, as Jim Brown pointed out - noise arriving from the antenna is
> just another signal source - some signals will be above that noise level
> and others will be below it. Normally we can only copy signals that are
> above the noise level, so why do some insist on setting "all controls to
> the right" and having the receiver amplify the noise as well as the
> signals - it is all in the quantity called "(signal + noise) to noise
> ratio".
>
> The only way to further reduce noise is to use noise reduction
> techniques which search through the signal plus noise to find something
> that appears to be coherent and amplify it. That is what the K3 NR
> algorithms do. How well it works depends on the type of noise present
> at the time, and the K3 offers several settings to handle the differing
> situations - do expect distortion with the more aggressive settings, but
> if they are able to dig a signal out of the noise for us, perhaps we can
> put up with the distortion to complete the contact. This is for use in
> extreme situations, and not for normal use when we want more casual copy.
>
> 73,
> Don W3FPR
>
>
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