>
>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.
>
// FETs are more linear than bipolars, however, both need RF-NFB to
reach the limit of human ability to detect distortion -- i.e. 1 part in
10,000, or c. -40db below peak amplitude.
>However, is not that (amplifier IMD less than transceiver IMD) as it should
be?
// A 20x (13db) difference seems a bit much.
>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
>neglected).
>
>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.
>
// Which means that CBers need to buy a ham-radio radio in order to
drive their CBer-modified ham-radio 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?
// Surely, if the decay time were 24-hours.
>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.......
>
// 10-4 Rogers on that good buddy.
>
cheerio, Peter
- R. L. Measures, 805.386.3734, AG6K, www.vcnet.com/measures.
end
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