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Re: [Amps] Checking for IMD

To: <amps@contesting.com>
Subject: Re: [Amps] Checking for IMD
From: "Tom W8JI" <w8ji@w8ji.com>
Date: Mon, 14 Aug 2006 12:09:00 -0400
List-post: <mailto:amps@contesting.com>
> Think about that again. If you are listening to upper side 
> band with a 3 KHz
> filter and you tune down 3 KHz in frequency with the dial 
> you have moved the
> band pass the same amount lower in frequency as you would 
> if you switched
> side bands with the side band switch. The only difference 
> is that the
> carrier will be on the other side of the filter.

1.) The passband of the filter is 3000Hz.

2.) The offset is 300Hz from the carrier.

3.) The sideband switch now moves the start of the receiver 
filter 600Hz lower than the start of the transmitter filter. 
We miss the low frequency tone mixes that are often 
problematic, and we also miss the high frequency tone mixes 
that are close spaced because we never checked above the TX 
signal.

4.) If I moved the receiver down 3000Hz and kept the same 
sideband, that would not happen. The upper edge of the 
receiver filter would hit the lower edge of the TX filter.

I don't see the big deal about moving the dial. Someone 
could check by moving up the SSB filter BW and down. .

I'm not saying the test is useless, just that anyone 
proposing or using a test should know what the test is 
measuring and what it isn't measuring. It's always a good 
idea to think about a new test procedure and understanding 
how it all works and explaining limitations and what it 
actually measures before going public.This is a pretty poor 
method for over the air measurements because of noise 
bandwidth problems.

73 Tom



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