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Subject: [AMPS] IMD
From: (Peter Chadwick)
Date: Tue, 18 Sep 2001 07:58:19 +0100
Rich said:

>Few AB1-grid driven. or AB2-GG amplifiers have as much distortion as 
>modern transceivers. 

As evidenced by the published test figures on, for example,  the TS2000.
Interesting, since modern MOSFET PA's are supposedly very linear.

However, is not that (amplifier IMD less than transceiver IMD) as it should be?
The power/gain distribution in a transceiver/transmitter driving a linear is not
generally optimum. A transceiver is designed (possibly using the word a little
loosely!) to give a certain output at a certain IMD level - usually around 100
watts and  hopefully around -30dB on PEP. (Note: at this stage in the argument,
the difference between the total power in the IMD products and the single sided
IMD power suggested by measurement of the 3rd and 5th order IM ratio will be

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.

If however, you are looking for the final stage in the chain to determine the
overall IMD at the output, then the  'driver' (in our 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.

This means that we really need much (not just a little bit) cleaner PA stages at
the 100 watt level in transceivers. Possibly the 200 watt PAs do show 6dB better
IM ratio at 100 watts, which will help.

Instead, we have the situation where the driver fixes the IMD level, and the PA
should be linear enough that it doesn't degrade the transceiver's performance.

The 'turn the knobs to the right' syndrome is just operator incompetence - but
regrettably, there's a lot of that about! ALC is considered the cure all for
that, but I believe we're all aware of that fallacy......I wonder if a very long
decay time on ALC would mitigate this? Very fast attack, and then several
seconds decay - you'd still get the splatter on the attack, but the gain would
be reduced for several seconds before it 'spitched' again. Not that Joe Lid
would like it.......


Peter G3RZP

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