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[AMPS] Tube IMD/6146/4cx10000

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Subject: [AMPS] Tube IMD/6146/4cx10000
From: (Peter Chadwick)
Date: Fri, 19 Feb 1999 10:58:30 -0000
Further to Rich's comments, I believe you'll really need negative feedback
whatever tube you use. However, you are getting down to looking at the
levels in the whole exciter.

To get carrier rejection, the balanced modulator is normally run at a
reasonably high level of drive, so the IMD can't be neglected there. Neither
can the 3td order IMD of the audio stages. Then there's the compressor, and
all the subsequent signal processing in the tx. Usually, you make the PA the
dominant IM producer, but if your design spec is -25dB on tone (-31dB rel
PEP), then exciter IMP's of around -35dB to -40dB rel to 1 tone of a 2 tone
signal are adequate. If you want -46dB relative to tone out of the system,
you probably will end up limited by the exciter portion before the 6146.

If signal levels into the SSB filter are around 0dBm, (220mV in 50 ohms)
then you can typically expect about 60dB down IMD products from the filter
alone. Interestingly, my measurements (admittedly some years ago) showed
that you could expect 3rd order products between 60 and 70dB down at 200mV
rms per tone pretty much regardless of filter impedance - I went from 50
ohms to 2K ohms on a number of filters, and they all tended to turn in
similar results.

So there's a real question of how far it's worth going on PA any case, IMP's of around 60 dB down could get drowned
in the phase noise, as well as the 'wide band' noise of the low level


Peter G3RZP

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