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Re: [Amps] Checking for IMD

To: <garyschafer@comcast.net>, <amps@contesting.com>
Subject: Re: [Amps] Checking for IMD
From: "Joe Subich, W4TV" <w4tv@subich.com>
Date: Mon, 14 Aug 2006 14:55:05 -0400
List-post: <mailto:amps@contesting.com>


Gary writes: 

> It is no poorer than tuning the receiver to another frequency 
> to listen for IM products. The same kind of noise and interference 
> will exist on other frequencies too. 

Gary, there is one problem with your analysis.  Most modern rigs 
have an 8 to 15 KHz wide filter in the front end (first IF).  
This filter is symmetric around the "carrier."  

When switching to the opposite sideband the fundamental tones 
are still inside that filter.  However if one stays in USB 
and tunes down by 3 to 5 KHz, the fundamental frequencies 
are down on the slope (or outside) the front-end (roofing) 
filter and much less likely to generate additional IMD in the 
receiver.  By putting the fundamental tones outside the front 
end filter,  the receiver is also more sensitive to the IMD 
and can potentially be used without AGC allowing more accurate 
measurement. 

73, 

   ... Joe, W4TV 

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