Official Position on Adaptive Predistortion

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  • Updated 5 months ago

Periodically the topic of Adaptive Predistortion comes up for discussion on this community.  Our team has been consistent in our position on this subject but sometimes it is useful to restate what has been said many times before in the interest of clarity.  Here is a restatement of the facts:

  1. The FLEX-6000 Series hardware has the necessary connections and internal RF port to port isolation to support adaptive predistortion software.  We have verified that transmitter output to receiver input isolation (e.g. ANT1 to RXA) is >80 dB on all bands including 6m, which is more than ample to accommodate adaptive predistortion using an external amplifier in the loop.

  2. FlexRadio remains interested in adaptive predistortion technology but we have not in the past nor will we now make a time frame commitment.  Our internal engineering resources are 100% allocated to complete our published road map for v1.x releases through v2.0.  We believe that the features committed in those releases are currently of higher priority to the vast majority of our customers than is adaptive predistortion.  

  3. We periodically review and rank our software backlog and feature requests.  We look at the ranked priorities and compare the development magnitude for each feature to the available engineering resources within a release cycle.  Based on those factors, we make decisions on what features make the cut in the release planning process.  So far adaptive predistortion has not made the cut.  That doesn't mean it won’t make the cut in a future release.
73 and thanks for your support,
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Gerald - K5SDR, Employee

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Posted 5 years ago

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KY6LA - Howard, Elmer

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APD would be fabulous if it could be applied to digital modes. 

Unfortunately the current implementation works best for analog modes such as SSB where the benefit is mainly to other local stations not hearing your splatter... while its a good practice most people will not be aware of it.

I heard Warren Pratt's NR0V original presentation (I posted it above) at Friedrichshafen in 2014 and his most recent update for version 2.0 of APD at the Friedrichshafen SDR Academy in 2018.  It was quite exciting to listen to his progress but the big elephant in the room for me is Digital

Most Digital modes are by definition non linear. 

So adding additional non-linearity to fix the spurious response is very difficult.  If one were able to develop an algorithm to do so for say FT8 then the elimination of spurious signals would likely have significant improvements for the people trying to copy your signal  - Hence very exciting if it were possible....which it likely is..

Ultimately this, like most of Warren's work, is a science project.
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Rick Peterson

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Howard, KY6LA

For an example of the benefits of APD for an amateur digital mode, see:

This link shows the FreeDV+ signal before and after activating PureSignal (FreeDV plus Video is a multimedia HF mode combining digital voice and video, employing 66 subcarriers total).

Without APD, this mode would produce unacceptable splatter on adjacent frequencies.  So APD is being applied to amateur digital modes with great benefit.

I'm looking forward to FlexRadio's implementation of APD.

Regards, Rick, WA6NUT
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As it is Flex radios are already some of the cleanest radios on the market. So lets see... If the 6000 series radio has extremely low IMD at the rated 100W output and you connect a legal limit amp to it then run the radios power at let say 10w to 50w to get lets say 1000w output from the liner then it should be even cleaner with a ton of power.

Or just run the radio standalone with less than 100W and you will have a cleaner signal.

Now in the future the 6000 series will have the APD feature enabled and it will only get better.

As for me I keep my power as low as necessary and make sure my SWR is almost flat to make contacts so I don't every worry about having a dirty signal.

I’m just glad to have one of the finest and "Cleanest" radios on the market today!

Photo of Varistor


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As actual test data shows, Flex radios are not any different than similar DUCs:

Cleaner transmitters are good for everyone, particularly during major contests when the bands are wall to wall full of stations.
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Mark K1LSB

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The 6700 test in that link was conducted 4 years ago using SSDR V1.3.8.  You're comparing those results to the ICOM 7600 tested only a month ago.  And generally speaking, the 6700 2-tone TX IMD numbers are still several dB better than the IC-7610.  I'd be interested in seeing the same tests run again on the 6700 running a current version of SSDR.
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Actually John running radios below there rated power output can at times cause the IMD to be worse than if run at or near full power.
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Bill -VA3WTB

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The radios has many under the hood tweeks done since that old test that effect IMD performances. Most Flexes I see now can have an IMD as low as -40
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Gene - K3GC

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The Question, IMO, isn't whether Flex is in trouble without APD (I feel definitely that Flex is quite healthy).

The main question is how much market would Flex gain with APD?  Also, a cleaner signal is good Ham citizenship.
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Bill -VA3WTB

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Two things come to mind.
First, I don't think they have lost a lot of market share,,Flex radios offer so much that they sell because of what they do,,and what they can do in the future. Flex has things no other company has.
And the same question can be asked about all ham radio companies, Icom for instance has two SDR radios out without APD and are doing just fine without it. And they don't seem to plan on it ever.

Secondly, Flex radios are very good ham citizens because they are one of the cleanest radios on the market. -40 is really good, so bothering others is not an issue,,unless the radio is setup poorly.

And considering the engineering put into the Flex amp to make it really clean shows their commitment to clean signals.
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The latest QST has an excellent article by Rob Sherwood that should be a call to action on predistortion.  Hopefully the market will respond to this the way it did to Rob and the ARRL's focus on receiver performance over the last 10 - 15 years or so.  Some SSB signals I see during major contests look like the pyramids of Giza on the waterfall.  I know it will take a while to cycle all the old poor performing transmitters out of use once good ones begin to appear, but we will never get better if we don't start.  I would think predistortion would be a competitive advantage.

Keith - AC9S
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Stan - VA7NF

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Some contesting stations (CW and SSB) intentionally produce dirty signals - because it keeps other stations away from "their" frequency resulting in better reception.
As long as there is an advantage of dirty signals taking "poor performing transmitters out of use" will not happen.  On CW it also makes a more noticeable signal, and in many cases more readable.
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Yeah - I know.  It is a really poor practice and I suppose if we were to be true to principle we would avoid working those guys - at least in S&P.
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Mark Erbaugh

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I think contest organizers should recognize this practice and penalize this kind of behavior. Until they do, if a dirty signal gives a station an advantage, why wouldn’t a competitor have a dirty signal? After all, if your goal is to “win,” why wouldn’t you do everything within the rules (and even slightly beyond so long as you don’t get caught) to give yourself an edge?
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HCampbell WB4IVF

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According to Flex noise mitigation and pre-distortion are #1 and #2 customer priorities, per Flex’s comments on the customer priority survey results: 

 “.... the survey was very helpful in revealing the relative importance of noise mitigation and pre-distortion. We have heard these messages loud and clear and have already begun discussions on next steps to make improvements in these areas of performance.”

 So there is light at the end of the tunnel for those of us who would like to see progress in these areas, although I’m still a bit disappointed that Automatic AGC-T in some form didn’t make the cut. 


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Mark K1LSB

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What "survey"?  I was never aware of any survey..
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Bill -VA3WTB

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Were have you been Mark, that is all people talked about for weeks on here. The survey Flex invited us to fill out?
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Mark K1LSB

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Let me repeat, what "survey"?  I received no invitation (or any notice at all, for that matter) from Flex or anyone else.