[1000mp] Amps & IMD Low power

Tue, 16 Jul 2002 09:11:42 -0400

I agree with Tom, most of the transceivers we have tested have had the
lowest IMD at about half power. At lower power, it can get a bit better,
stay about the same or, if there are "crossover" type distortions, get a bit
worse.

Mike Tracy and I just talked about it and we can easily add a half-power IMD
sweep to the IMD plots we put in the expanded reports, so he will start
doing it on 80 and 20 meters.  I don't want to do the tests on all bands
because those few minute-tests here and there can add up, but each of the
expanded reports in the future should contain some half power tests.

73, 
Ed Hare, W1RFI
ARRL Lab
225 Main St
Newington, CT 06111
Tel: 860-594-0318
Internet: [email protected]
Web: http://www.arrl.org/tis

> -----Original Message-----
> From: Tom Rauch [mailto:[email protected]]
> Sent: Monday, July 15, 2002 8:35 PM
> To: [email protected]
> Subject: Re: [1000mp] Amps & IMD Low power
> 
> 
> 
> 
> > Hi all
> > The IMD distortion on 12 volt final radio's goes UP as power goes
> > down. 
> 
> That isn't what I have seen. 
> 
> I've found most radios have the best IM performance at about half 
> factory rated power. You don't want to go too low, you certainly 
> never want to go too high.
> 
> At low power, cross-over distortion is a problem. At high power, the 
> driver and output devices get nasty. The problem is often not even in 
> the PA transistors. My IC751A's all improved greatly when I replaced 
> the driver transistors with higher power devices. 
> 
> I've seen some radios where contesting people crank the internal 
> power limit pots up to get more power. That is just begging for IM 
> problems on the TX, and may explain why some contesting stations have 
> such wide SSB signals. Anyone who cranks the factory internal power 
> limit controls up thinking the radio will work OK is making a 
> mistake.
> 
> Transistors, unlike tubes, need lots of headroom. They can never be 
> operated cleanly when power approaches the saturated power. About 50-
> 75% of saturated power seems to be the point where they begin to get 
> really nasty.
> 
> 73, Tom W8JI
> [email protected] 
> 
> _______________________________________________
> List Moderator: Richard Lubash N1VXW
> 1000mp mailing list
> [email protected]
> To Change Options or Unsubscribe:
> http://mailman.qth.net/mailman/listinfo/1000mp
> 