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[AMPS] minimum distortion detectable

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Subject: [AMPS] minimum distortion detectable
From: (Tom Rauch)
Date: Tue, 18 Sep 2001 14:57:22 -0400
There was some illusion that the goal is to put the IMD 40 dB down 
so it is the minimum detectable distortion to the human ear.

That is not the problem.

The problem is in an RFPA the distortion generates additional 
frequencies that appear outside the passband of the transmitter. In 
this case, we want the IMD to be below the minimum detectable 
level on a receiver.

I often can find signals 70dB or more out of my noise floor, so that 
means 3rd order IMD of less than that amount is a problem.

Let me give an example. A W5 in the Dallas area has a TS-2000 
that he operates on 160 meters. When I listen west on a quiet day, 
he is 50 dB out of my noise. I can hear his TS-2000 for about 
15kHz up and down, when I use a receiver that has a 110dB 3rd 
order IMD dynamic range at 2 kHz spacing. The bandwidth I see 
here agrees with ARRL and RSGB measurements of the TS-2000.

The splatter of his friend, who is just as loud and using a better 
radio, is barely detectable 5kHz up and down.

The biggest problem we face is we depend on other radioamateurs 
to give us reports, and if they have a high noise level compared to 
the signal the noise will mask transmitter problems. They also can 
be using poor receivers making things look worse than they are.

The real thing we are interested is not the on-channel distortion we 
can hear, but the distortion products that are up and down the band.
73, Tom W8JI 

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