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[AMPS] Transceivers high 3rd order IMD

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Subject: [AMPS] Transceivers high 3rd order IMD
From: (Martin Gary)
Date: Fri, 19 Feb 1999 10:19:26 -0500
>Thanks to all who replied. What is the standards for military
>communications SSB transmitters. I have heard the US coast guard  fire
>up on 13mhz with 10kw and the signal was huge, but just so clean and
>crisp. Also has anybody used or played around with modern commercial
>transceivers, do they have superior 3rd order IMD specs.
>Someone once said that in western Europe, marine and other commercial
>transceivers must meet -40db  below pep. Russian military equipment is
>just lousy, since most use 4cx250s/350s, they are impractical anyway
>for ham use.

I used to work on HF transmitters for the Navy. Military transmitters have
to meet the following standards:

You test the transmitter by injecting two tones on ssb - 1000 and 1625 Hz.

3rd order IMD -  -38 dB below the two primary tones.
Carrier suppression has to be -50 dB below the two tones.
Opposite sideband suppression has to be -50 dB below the two tones.

Military transmitters are very clean but they waste a lot of power. The amps
are usually operated class A.
The transmitters that I worked on had two 8122's driving two 4cx1500a's. The
8122's ran at 320 ma static plate current.
I think the spec says that max plate current for that tube is 300 ma.  Never
understood why they did that. Must have had to in order to keep the 3rd
order IMD below -38 dB...

Anyway, Its been ten years since I worked on them... I might have a couple
of details wrong...

Martin - N3HRT

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