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, <:-))
73
Roger (K8RI)
11:29 PM, Warren C. Pratt wrote:
Greetings,
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 <http://video.openhpsdr.org/HRF2014/PureSignal1.2.mp4>(MP4)"
This link is posted on the <openHPSDR.org> 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!
73,
Warren NR0V
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