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TT Orion 565 on 160m & optimal setting - part V. by YT1NT (AGC threshold comments)

27. února 2008 v 12:01 | Petr, OK1RP |  Operation topics
As I promised last time there I am posting the compilation 
of the comments by Sinisa regarding the AGC threshold 
and principles uses on the Orion 565 receiver.    
I hope it will help to Orion's owners to understand all of 
the neccesary details to be able to effectively use AGC 
setting on their Orions.    
see text as follows:    
 Richard Detweiler wrote:    
> When the noise drops down as the agc threshold is increased,
> what is happening, is the noise is dropping below the point 
> at which the AGC will try to amplify.    
   When AGC Threshold is increased, gain is    
   immediately reduced in the same proportion.    
   No automatic action here, and AGC never "tries"    
   to do anything else apart from maintaining    
   constant volume.    
   There is only one AGC Threshold,    
   and, for any given threshold value,    
   signals above it are attenuated    
   to maintain constant volume.    
   No attempt is made to amplify signals    
   below the threshold, an no attempt is made    
   to improve S/N ratio or anything else,    
   apart from maintaining constant volume.    
> This is a Good thing,  because hopefully the weak signal 
> just above the noise is not below that point and it will then 
> amplify only the signal of interst,    
   This reminds me of a myth about Orion's AGC    
   being fundamentally different from usual    
   implementations, i.e. being able to reduce    
   noise and amplify desired signals.    
   Unfortunately, there is no such a miracle.    
   At instants when both signal and noise are present,    
   signal to noise ratio does NOT depend at all on AGC    
   Threshold setting. And AGC action tends to reduce,    
   NOT increase the apparent signal to noise ratio,    
   as perceived with intermittent signal.    
   Orion's AGC action does not bring anything fundamentally new,    
   although the level of user control is greater than usual.    
   The user may or may not achive optimum settings,    
   depending on skill and willingness to play with controls.    
   By setting the AGC Threshold just above the    
   signal + noise level, AGC is essentially    
   turned off, with two benefits:    
     a) intermodulation between the signal and the noise    
        through AGC action (acting as undesired amplitude modulation)    
        is avoided;    
     b) our aural system doesn't get confused by fast and    
        frequent changes of gain.    
   The sole remaining AGC function is that of    
   protecting our ears from unexpected    
   strong signals. Not a miracle, but useful.    
   Sinisa  YT1NT, VE3EA    
That's all regarding the Orion 565 receiver setting at this moment. 
When I will have some another information then I will post them. 
Do not hesitate to email me if You will have another usefull informations 
or experiences with that tcvr on Topband.   
73, Petr OK1RP   

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