[Elecraft] PA with predistortion

[John Marvin]

I'm not sure how easy this would be to adapt to an existing product like 
the K3 or KX3. Note that the primary change is in the transceiver and 
not the PA, i.e. this is not something you would add to the KXPA100 or 
KPA-500, other than perhaps the head end of the PA output sampler. The 
attenuator part of the sampler could also go in the PA or that could go 
in the feedback input that would have to be added to the transceiver. In 
a self contained transceiver (i.e. with a built in PA) this could all be 
contained within the transceiver, although it would still be nice to 
provide an external feedback path for those with additional 
amplification (e.g. a KPA-500). This is what Apache-Labs is doing with 
the not yet released ANAN-200D. Earlier HPSDR based products based on 
the Hermes board or Angelia board are also capable of supporting 
predistortion, but it requires some changes, along with an external RF 
sampler of some type (although some people are getting decent results 
from just the crosstalk in the ANAN-100D case, although some of the 
settings need to be calibrated on a per band basis since the crosstalk 
level tends to change with frequency).

In order to do this you need to add a feedback path in the transceiver 
and the transceiver needs to support full duplex operation, i.e. either 
the feedback path needs to connect to an additional receiver or the T/R 
circuit needs to connect the feedback path to the receiver while the 
transmitter is connected to the PA/antenna. The DSP then needs to be 
able to monitor both the outgoing transmit signal and the incoming 
properly attenuated signal from the PA output. This can all be built 
into the transceiver for the "barefoot" case (I'm not sure barefoot 
applies in the case of a K3/100!). Of course the DSP needs to be capable 
of computing the required predistortion based on the feedback input, 
accounting for delay in the transmit path, and applying it to the 
transmitted signal.

I will also note that the results are best with higher voltage PA 
designs, e.g. LDMOS 50V amps. But the results can still be quite good 
for well designed 13.8v amps. I'm working on getting this to work with 
my HPSDR Hermes and my KXPA100, although it is a low priority task right 
now. I need to verify that the feedback signal from the KXPA100 is 
reasonably stronger than crosstalk within the transceiver, which 
contains a Hermes board and the 5W driver amp from the Hardrock 50 kit, 
and yet it can't be so strong that it overloads the ADC.

When it comes to IM3 distortion, applying correction from earlier in the 
transmit path can still result in some improvement, i.e. the distortion 
from earlier stages just gets amplified and added to new distortion 
products at each stage. When I looked at the KXPA100 test results that 
Elecraft submitted to the FCC, I noticed that the input test signal 
wasn't all that great. I wondered how much of the IM3 product at the 
KXPA100 output was contributed by the KXPA100 and how much was just the 
IM3 distortion from the input amplified by the KXPA100.

Regards,

John
AC0ZG

On 2/25/2014 10:31 AM, Sverre Holm (LA3ZA) wrote:
> "Apache Labs become the first Amateur Transceiver manufacturer to include
> Pre-distortion technology within their PA’s for ultra-clean TX".
>
> This is something that Elecraft should have all the possibilities to follow
> up on: using the DSP to clean up the PA.
>
> See  http://blog.wsplc.com/?p=1701 <http://blog.wsplc.com/?p=1701>
>
>
>
>
> -----
> Sverre, LA3ZA
>
> K2 #2198, K3 #3391,
> LA3ZA Blog: http://la3za.blogspot.com,
> LA3ZA Unofficial Guide to K2 modifications: http://la3za.blogspot.com/p/la3za-unofficial-guide-to-elecraft-k2.html
> --
> View this message in context: http://elecraft.365791.n2.nabble.com/PA-with-predistortion-tp7584706.html
> Sent from the Elecraft mailing list archive at Nabble.com.
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