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Distorted WSJT FT8 transmit signal

I am using a Flex 6400, SmartSDR v3.2.39 and WSJT v2.5.4.
I can receive and decode signals and I can make contacts however I see that my transmit signal is distorted and have gotten reports of the distortion. I have not been able to make any adjustments to clear up the distortion. The attached screen print, Distorted Signal, shows the distortion. This is the signal when transmitting a message. The attached screen print Tune TX 1500hz shows the signal when tuning on 1500hz. This looks good however the interesting thing is that if I move the transmit offset to 1501hz it becomes distorted, see attachment Tune TX 1501hz. This problem occurs on multiple bands and either into an antenna or dummy load. I have also attached the DAX and Radio Panel screenshots.

Has anyone had the same type of issue and what was done to correct it?

If you need more information on my set up please let me know.

Here are some of the settings that I have for WSJT and Flex
WSJT-x v2.5.4
---Rig: FlexRadio 6xxx, Poll inteerval 1s
---Cat Control Network Server: localhost:5002
---PTT Method: CAT
---Mode: None
---Split Operation: None
---Input: DAX Audio RX 1 (FlexRadio Systems DAX Audio) Mono
---Output: DAX Audio TX (FlexRadio Systems DAX TX) Mono
Flex Radio 6400
-SmartSDR version 3.2.39
---TX Stream: TX Gain 48
---Mic Stream: Mic Record Gain 48
---RX Streams Slice A: RX Gain 34

Thanks Dave KW1DX


  • Al K0VM
    Al K0VM Member ✭✭✭


    I see nothing inherently wrong with your settings except there should be no need to have the Mic Stream enabled in DAX. The PA would indicate that the sidebands are likely more than 60 db below your desired signal and should have no impact on other stations coping your signal.

    One is tempted to assume that what you see the in PA during transmit is you actual signal, however what you see may also include RX overload artifacts due to the strong signal from the TX. In actual fact the TX signal is likely better than what you see in the TX PA. As a test, select RXA or XVTRA for receive antenna and then look at the PA TX signal.

    That said, If infact the sidebands are real the other folks on the band would appreciate a clean up. But I am at a loss as to how they could be generated.

    AL, K0VM

  • Geoff AB6BT
    Geoff AB6BT Member ✭✭✭

    Have you tried restarting DAX?

    It may not be your problem but I need to restsart DAX frequently. I ususlly test with the "Tune" button in WSJT-X. By monitoring the transmit signal I can tell immediately if a restart is necessary.

  • KW1DX
    KW1DX Member ✭✭
    I have restarted DAX, restarted SDR and rebooted, still the same
  • Geoff AB6BT
    Geoff AB6BT Member ✭✭✭

    Well I guess we have different problems.

    Just sharing what works for me...

  • Mike-VA3MW
    Mike-VA3MW Administrator, FlexRadio Employee, Community Manager, Super Elmer, Moderator admin

    I opened a support ticket for this.

  • KW1DX
    KW1DX Member ✭✭
    Thanks for opening the ticket
  • DG1KDD
    DG1KDD Member
    Hi guys, thanks for this interesting conversation becuase I have exactly the same problem with my Flex6600 since very long time. I have the same spikes shown in the tune screenshots of KW1DX and I also have a distorted TX signal in FT8. It is still within the legal limit of >30dB lower than main signal but very anoying for people here in the next kilometers. Please can you share the solution here or was it a defekt in the Flex?
    many thanks in advance nd 73
    DG1KDD, Dirk
  • KW1DX
    KW1DX Member ✭✭

    Dirk, I had to return the radio to Flex for them to look at it. They still have it so no resolution as of yet.

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