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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 16:59:52 -0400
List-post: <mailto:amps@contesting.com>

> But if we tune outside the range of the roofing filter it is 
> said that we will miss some of the lower order close in IM 
> products. :>)

That is possible depending on the frequencies and tone spacing, 

> I have some receivers with no roofing filters and they do 
> fine as long as you keep the signal levels down (attenuator) 
> so as not to overload the receiver.

That depends entirely on the receiver gain distribution and 
AGC performance.  Many receivers use AGC to turn down front
end gain to prevent receiver generated IMD in the presence of 
strong signals.   

> If receiver IM performance is that great a problem how do we 
> even manage to hear anything intelligible on the bands. Seems 
> there are some mighty strong signals out there that our receivers 
> have to deal with.

The difference is "close in" vs. off frequency IMD.  Many "modern" 
transceivers have as much as 20 dB worse internal IMD when the 
signals generating the IMD are inside the roofing filter (e.g., 
2 - 5 KHz away) vs. signals > 10 or 15 kHz away.  The problem 
is generally the inability of the second or third mixer and the 
IF amplifiers to handle the wide dynamic range without AGC to 
prevent overload. 

> Even if there is some receiver IM generated it only needs to 
> be a few db below what we are looking for to be masked.

However, absent AGC to prevent overload, the dynamic range of 
many IF strips (particularly where the primary filtering is 
provided by DSP) is less than 30 or 40 dB.  If you are looking 
at - 35 dB 3rd order and - 40 dB 5th order, that is pushing the 
real dynamic range of the IF.   

> By the way most spectrum analyzers have filters with a rather 
> broad shape factor compared to our receivers and seem to do 
> just fine.

Most spectrum analyzers have much better IF strips (and fewer 
conversions) that the typical amateur receiver.  They are 
optimized for dynamic range and not selectivity ... they have 
to be.  

73, 

   ... Joe, W4TV 

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