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DAX Signal sidebands

Mark Erbaugh
Mark Erbaugh Member ✭✭
edited June 2020 in SmartSDR for Windows
I had my first lengthy PSK31 ragchew with my new Flex 6300 this morning. I am running SSDR 1.3.8 and using DAX to interface with FLDIGI.

When I started out, I narrowed the filter so that the other station was the only one on FLDIGI's water fall. His signal was bright yellow. There was a little blue on either side and then just black. It looked pretty much like the waterfall in SSDR. His signal was clean - FLDIGI reported a -28 dB IMD. In the middle of the QSO, the display on FLDIGI changed. It was still copying his 100%, but now there were several sideband on either side of him. I used the DAX control panel to reduce the gain and the sidebands got darker, but didn't go away.  The signal on the SSDR waterfall didn't seem to change.

I'm thinking what I see is some artifact of digital filtering, but is it an FLDIGI, DAX or SSDR issue?

Happy Thanksgiving.
Mark


Answers

  • Jim KJ3P
    Jim KJ3P Member ✭✭✭
    edited April 2020

    If PSK31 is anything like JT65, excessive sidebands usually means that the received station is running excessive power (or is very nearby, or propagation is unusually good). The "sidebands" are probably not real, rather, an artifact of the program's decoding algorithms, as are multiple decodes of the same signal.

    The JT gurus have said that narrowing the bandwidth as you did is unnecessary. But, perhaps like you, I usually use TNF to notch out strong nearby signals...probably unnecessary too with the Flex, but just an old analog mindset that I can't break!

    I'm told that reducing the receiver AGC and/or RF gain is a better solution.

    Happy digiting.

       --Jim KJ3P

  • Ed, K0KC
    Ed, K0KC Member ✭✭
    edited June 2020
    There apparently is a corruption problem with DAX that has been addressed previously in this forum. I normally use the JT modes and I see this every day. The interim solution for me is to turn the DAX channel that I am using off and then back on quickly. The problem is more apparent on strong signals, but is a DAX problem since I can clear it up by the cycling just mentioned.

    Flex is aware of the problem...I only noticed it after I started using 1.3.8. Hopefully this will be fixed in 1.4.

    Ed, K0KC
  • pa0bie
    pa0bie Member
    edited November 2014
    We have seen the same problem with strong signals in WSPR-X. So we suspected the DAX.
  • Michael - OZ1MDF
    Michael - OZ1MDF Member
    edited December 2016
    My best guess is that your issue is actually the same as https://community.flexradio.com/flexradio/topics/peculiar-dax-issue 
  • Patrick
    Patrick Member ✭✭✭
    edited June 2016
    I confirm and see this problem mostly on JT65/9 signals of -10 or more.  And yes it can be fixed by cycling the DAX channel involved.  


    Pat
  • Patrick
    Patrick Member ✭✭✭
    edited June 2020
    Thanks for the link.  I do not think that this is an individual problem as I think this is a problem in the coding of the FPGA.  The FPGA may have a feed back leakage that is causing the oscillation/ringing. It takes some amount of operation time for this situation to occur.  Just my guess.  It seems to have to start at the origin of the audio stream.  Also been using the JT65 SW for a while under PowerSDR without this problem. 

    Pat

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