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Subject: [AMPS] IMD
From: (Tom Rauch)
Date: Tue, 18 Sep 2001 07:39:18 -0400
> As evidenced by the published test figures on, for example,  the
> TS2000. Interesting, since modern MOSFET PA's are supposedly very
> linear.

The TS2000 is not a good example. It is exceptionally poor. It is 
one step removed from class C performance. 

We have to remember a two-tone test does not show IMD caused 
by poor dynamic power supply regulation.

Also AB1 or 2 tetrodes get nasty FAST compared to a GG triode 
when overdriven or misadjusted.
> If the linear at full rated power is capable of -30dB rel PEP IMD
> products, then the total IMD power is likely to be (neglecting phasing
> and compensating distortion effects) 27dB down on PEP.

That's right, IMD can add. So the PA must be many times cleaner 
than the exciter to not make things worse.  
> case the transceiver) needs to be a lot better. That suggests that
> either it needs to run at a lower power output, demanding more gain
> from the amplifier, or needs to be 10 to 20dB better on IMD at the
> rated output. Many transceivers probably need to run around the 25
> watt level rather than the 100 watt level for acceptable overall IMD.
> However, Part 97.317 (a) 3 of the the FCC rules gives us a problem, as
> it requires a 50 watt (mean) drive minimum for an amplifier.

Reducing drive power from an exciter does not always make things 
better. Quite often it actually makes things worse.

73, Tom W8JI 

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