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Subject: [AMPS] IMD
From: (Tom Rauch)
Date: Wed, 19 Sep 2001 09:01:06 -0400
> What is the limitation, the close-spaced IM of the typical receiver,
> or is it actually a limitation in Spectrum lab's FFT algorithm (I
> haven't tried resolving closely spaced signals with spectrum lab)?

Since you would be depending on the entire receiver, including 
detector and audio amplifiers and even the sound card, it would 
likely be very poor.

I'd expect most systems to be in the 40dB IMDR or less range, 
and you'd also have a harmonic distortion because the audio would 
be back at baseband.    

> I have done some very crude tests here with 1 KHz tone spacing
> and using the CW filter in my R4C and a step attenuator to resolve
> level of IM sidebands coming from my rig. The levels seem about right,
> but I have never done any tests to validate this (I don't have a
> spectrum analyzer here). 

Different situation, because you are using a narrow filter in the 
second IF of the receiver. With an R4C, that puts the 3rd order 
IMDR at about 45 dB or more depending on the vintage of the 
receiver. Early R4C's are FT101 "quality" with close spaced 
signals, which is pretty poor. 

You can see measurements of R4C's and other receivers on my 
web page. Stock R4C's are not so good, but even so are still better 
than most transmitters. Unless you have a Sherwood 600Hz 
roofing filter or rework the second mixer, the very poor second 
mixer comes into play.  

> This brings up another question that I have been meaning to ask. When
> testing amplifiers how does ARRL generate the two-tones. Are they high
> level combining  the output of two trasceivers each generating a CW
> signal, or do they combine tones at a low level and run them thru an
> ultra clean "golden" PA?

As far as I know, they have a special RF source that drives PA's. 
But for transceivers, it has to be an audio source.

I'm more interested in how they use the equipment, because that is 
where most errors occur. You can get near perfect results with a 
very small and non-special setup if you know how to use it, and if 
you don't the best gear in the world will give poor readings.
73, Tom W8JI 

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