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Re: [Amps] pre-distortion

Subject: Re: [Amps] pre-distortion
From: "Roger (K8RI)" <>
Date: Sun, 19 Oct 2014 03:42:54 -0400
List-post: <">>

Warren, Thank you very much for this note. By profession and degree I'm a software engineer although having retired in 97, I'm afraid I've fallen far behind on both the hardware and software. I retired at 57 so I could go play, but regret not having kept up.

My last class in the masters program was the design and analysis of algorithms. IOW, putting the algorithms into mathematical formulas and running comparisons. Although I claim a Math minor, I do not claim to be good at it <:-))

Pre-distortion has interested me greatly, but I'm afraid I need to hit the books again although I'm not sure what reading would best serve me.

I would think pre-distortion would involve a great deal of math in the algorithms to analyze the signals

With existing rigs, the band width required is not there that's required for the process, although with the SDRs, if the source code and algorithms are available, I would think they would lend themselves nicely to this process.

Years ago, my cousin (an Electrical Engineer specializing in machine code and now a SK) asked me about SDRs before anyone had openly heard about them. He was interested in starting something up and wondered if I'd be interested. I thought for a moment about old timers and their resistance to change and replied, "Nah, I doubt there'd be a place fo it" Shows how good my predictions for the future are, <:-))


Roger (K8RI)

11:29 PM, Warren C. Pratt wrote:

I sometimes monitor this list and I see that the topic of pre-distortion is appearing recently in several posts. I wrote the pre-distortion software that was released a few months ago as part of the openHPSDR (open High Performance Software Defined Radio) program. I'll offer just a few observations:

** It seems to me that it's probably much easier to optimally implement pre-distortion in a DDC/DUC (digital up-conversion / digital down-conversion) software defined radio than in other radios. There are simply very few places that errors can creep into the required processing. The required precision and accuracy increase exponentially as the desired reduction in IMD increases. So, having few error sources and the benefit of double-precision floating point become important.

** 50V LDMOS amplifiers are probably the "most correctable" solid state amps. Tube type amps also correct very well. 13.8V amps are more difficult due to "memory effects."

** No problem to correct an entire amplifier chain, at legal limit. No extra hardware or software is required. One just feeds back a sample of the output from the last stage to calculate the correction.

** You cannot successfully correct by pre-distorting the MIC input to a radio UNLESS you have VERY wide bandwidth from the MIC through the entire transmitter. The IMD frequencies that you want to correct must be within the bandpass of the correction path.

At Ham Radio Friedrichshafen this past June, I gave a talk (approx. 30 min.) on the openHPSDR solution for pre-distortion. It's posted on the openHPSDR web site. If any are interested, here's a link:

"*Warren Pratt,*NR0V, 2014.Digital Predistortion linearizes RF amplifiers <>(MP4)"

This link is posted on the <> website on the Publications page. The video includes a few examples of amplifiers where this technology has been applied.

I hope this helps explain a little more about this technology and sparks more interest in finding ways to reduce IMD!

Warren  NR0V

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