Adaptive Predistortion

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I see another SDR group is all abuzz concerning the OpenHPSDR Groups new Adaptive Predistortion breakthrough of obtaining greater than 50db IMD3 from a 13.8V 100Watt PA TX system and congrats to them for their work in helping clean up the TX side of things re signal purity. Is the development of similar Predistortion software on the agenda for the Flex 6000 systems at some point? Not a current show stopper for me.... just curious.
regards
Larry K1UO
Flex6700
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K1UO - Larry

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Posted 5 years ago

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Barry N1EU

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Steve - N5AC, VP Engineering / CTO

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Official Response
When we designed the FLEX-6000 we had adaptive predistortion in mind so we included a tap from the transmitter that feeds back into the receiver. This gives us simultaneous access to the desired and actual transmit spectrum in the FPGA so that we can implement linearizarion. We also have sensors for temperature, voltage and load match of the PA as these can be used to improve linearizarion. The work that needs to be done for the FLEX-6000 is to experiment with algorithms and input sources to determine what gets us the most bang for the buck and then finalize the implementation.

The FLEX-6000 already has the highest precision DAC of any amateur transmitter and excellent IMD numbers so we decided to focus first on features that the community is waiting for and return later for implementation of predistortion. Predistortion in the cellular business is critical because they are putting many carriers in a single PA and spectral purity when you're doing this is critical. But for ham radio, it is more of a "nice to have" and "nice neighbor" sort of thing -- it's not going to prevent someone from copying YOUR signal and the FLEX-6000 spectral purity is well above the legal limits. Barring significant hype that makes this a marketing issue, we'll look into this after finishing the items on the road map that you, our customers, have told us are important for you.

Since we thought ahead, you will not need to use an external device to tap the actual transmit signal and feed it back into the receiver when the time comes. Incidentally, our VP/GM of our government business, Graham KE9H, has considerable experience with predistortion having run a company that focused on predistortion in cellular PAs. He also developed PAs for HPSDR.
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K4EAR

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I vote to get us down the roadmap asap and get the 6xxx at the feature level of PSDR. The beta testers for HPSDR's/Anan 100D's pre-distortion have a little feather in their cap for now.
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Ken - NM9P, Elmer

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My 6500 already has the best sounding, cleanest transmitter I have ever used. I can wait for additional improvements until all the other enhancements are finished. But I am very glad to see that FRS has already planned for its future implementation.
This really IS cutting edge.
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K1UO - Larry

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Thanks to Steve at Flex for "filling us in" on this aspect of their future plans for the fine 6K series...and remember guys.. I know its early but dont be afraid to ask questions whether the plans are for the future or not.. I, as much as anyone, want the so called basic features as much as the next guy and the sooner the better. I could care less about having FM capabilities but I am not saying to drop it for something I want at the expense of someone elses feature set.. I am looking forward to Diversity reception but Im sure there are many who could care less about that.... Its the great thing about this Flex architecture...so many many possibilites.... exciting times for sure.
regards
Larry K1UO
P.S. What are the claimed numbers nowdays anyhow.. Im sure they are still evolving since the initial firmware releases.
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Barry N1EU

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The external tap potentially allows predistortion software to lower the distortion in the output of a linear amplifier as well as the exciter - the OpenHPSDR predistortion effort is evidently allowing for this.
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Sergey, R5AU

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Well, in case implemented already capability on HW side i guess some features was planned at beginning and in the internal roadmap already. Just 5 cents - predistortions on side of the TOP radio should be, BTW deversity we would like to have early a bit
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Ed baker

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Yes I agree . Flex audio is just great . However implementation of this approach or planned add on accomplishes something more .

What it does accomplish by reducing the IMD or opposite side band it places more energy into the band pass .Linear non distorted audio product Besides your "Neighbor" loving it This means cleaner,Clearer . audio signal . Did I mention Stronger !!!

Have seen and listened to an on the air demo . Absolutely the nicest audio transmission to date

Flex . OK the highest precision DAC of any amateur transmitter .
Flex is a industry leader The 6500 is a game changer .

Then I can only guess that if you implemented this approach (Well within your hardware environment ) -70 DB would be the norm not the -20 WOOPS
(To be fixed in 1.1 )
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K4EAR

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I've heard it myself also with an ESSB Anan 100D station without any low frequency grunge blowbye in the other sideband!
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David Decoons wo2x, Elmer

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The unwanted sideband suppression in 1.0.24 is horrible!
Hopefully this will be fixed even if adaptive predistortion is down the road a bit...

Dave
wo2x
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Sergey, R5AU

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Hi David,
did you measured it ? What is value ?
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David Decoons wo2x, Elmer

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Just around 25 dB down. Very bad.

On 75 meters LSB I was listening 5 kHz above another 6500 user with my rx set to 50 - 3000 Hz. He was talking to someone running a newer Icom.. Both stations were running about 10 over S9. I could hear the buckshot from the 6500 no problem and nothing from the Icom.

Most of the guys are using 50 Hz for tx low cut filter.

Our group has four 6500s and all are reported to have bad unwanted sideband suppression with the 1.0.24 beta. I've seen screenshots of the 6500 I have here vs my Kenwood TS-990 with samr tx bandwidth and same equipment attached. All of us are using tube amps, not solid state.

1.0 is MUCH cleaner.

Screenshots were sent to Steve Hicks from one of the users.

Dave
wo2x
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Steve - N5AC, VP Engineering / CTO

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This was an issue in the Beta version (v1.0.24) which has been corrected.
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Sergey, R5AU

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Yes David, on my f6700 i have the same assesment with unwanted SB, i hope FRS will repair it in 1.1, we need to have a best stable radio

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