Warren C. Pratt
warren at wpratt.com
Sat Oct 18 23:29:57 EDT 2014
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
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!
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