DAX Signal sidebands

Mark Erbaugh

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

Jim KJ3P

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

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

We have seen the same problem with strong signals in WSPR-X. So we suspected the DAX.

Michael - OZ1MDF

My best guess is that your issue is actually the same as https://community.flexradio.com/flexradio/topics/peculiar-dax-issue 

Patrick

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

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