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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