[Amps] Re: IMD

[Richard]

>Rich,
>
>How did you come up with 4.5 kc offset? 

This is the separation needed to stop the Tx filter skirt from 
overlapping the Rx filter skirt.  Thus, one hears only genuine, bona 
fide, feculence without any fundamental.  For a double filter radio, the 
steeper skirts allow an offset of c. 3.6kHz - which yields a better 
measurement.  

>I am assuming that you are using the
>receive filter on the same side band with that offset? With that setup I 
>would
>think that you would only see the products of the higher audio frequencies 
>and not the lower ones as they may not fall that far away. 

True, however, including the fundamental in the measurement - due to 
filter passband overlap - is seemingly a greater source of error.  

>Wouldn't multiple tone IMD testing give a more accurate picture?
>
Not in my experiences.  A human voice gives the dynamics a workout. 

>Although I do agree that it gives a good snapshot of excess bandwidth. I do
>kind of the same thing but I just switch side bands on the receiver and note
>the difference. However a guy with lots of lows has more energy on the other
>side band. But it still is a good quick check of bandwidth.
>
Agreed, but a double-filter receiver would be needed - and you would be 
measuring unwanted sideband energy instead of intermod products. 

cheers, Gary

-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K, 
www.vcnet.com/measures.  
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