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Subject: [AMPS] IMD
From: (2)
Date: Tue, 18 Sep 2001 06:47:35 -0700
>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 

//  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 
>loosely!) to give a certain output at a certain IMD level - usually around 
>watts and  hopefully around -30dB on PEP. (Note: at this stage in the 
>the difference between the total power in the IMD products and the single 
>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 
>power output, demanding more gain from the amplifier, or needs to be 10 to 
>better on IMD at the rated output. Many transceivers probably need to run 
>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 
>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 
>The 'turn the knobs to the right' syndrome is just operator incompetence - 
>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 
>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,  

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