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Strange Display AM-SSB

Hello folks

I have a FLRX 6400 radio, and the Smart SDR software running in a happy WIN 10 puter. I am preparing for the winter and Amateur radio on the lower bands.

I am seeing strange waveforms on the spectral display in the TRANSMIT mode. I have attached screen shots. You will see my freq set for 1.880khz. There are two blips of RF below and above my chosen frequency. The shots will show this is happening on AM and SSB. Audio should not be overdriving the final PA. Am I doing something wrong? There's a capture where there is no modulation and 2 others showing this anomoly with modulation AM & SSB

Thanks Fred KC4MOP


  • sdrwow
    sdrwow Member ✭✭

    Another comment.......The Flex is terminated right into a dummy load. It is not going through any other device.

    RF diect to Dummy load


  • VE7ATJ_Don
    VE7ATJ_Don Member ✭✭✭

    Hi Fred... what are you modulating the AM/SSB signal with? Is it your voice or is it a tone? If a tone, what frequency?

  • sdrwow
    sdrwow Member ✭✭

    Thanks.......modulation is just my voice making a long steady ahhhhhhhh


  • Trucker
    Trucker Member ✭✭✭
    edited September 2022

    I have been told by an FRS employee that the transmit display gets it's data before the filters in the PA. So, you won't get an accurate representation of your transmitted signal. I used your settings for AM and LSB and have similar results with what looks like spurs above and below the center frequency. On AM with carrier only, I see spikes like your second photo. But, not as pronounced as yours. When I modulate the signal, then I get the **** signals in place of the spikes. From what I was told, those harmonics are filtered and for the most part do not actually get transmitted. Or, are so weak that they do not cause interference. Why FRS chose to tap the signal path where they did, for the transmit display is a mystery to me as it is not accurate.

    If you have someone nearby that has a Flex or Anan radio that can monitor your transmitted signal, you could get a better idea of what is or is not, going on. Otherwise, a Spectrum Analyzer with enough attenuation, can show your actual transmitted signal. I have a TinySA that I have thought about using to see my signal better. But, I do not yet have a way to attenuate the signal enough to not damage or destroy the TinySA.



  • sdrwow
    sdrwow Member ✭✭

    thanks for a very interesting outlook. I can try to identify the ball park freq above and below and figure out a way to monitor those "spurs" maybe with one of those RF inline devices that give a "tap" of attenuated RF to go to a receiver to be able to compare how low they are. I just don't think I have seen this before on my radio display. It would be interesting to test on other bands.

    I DO have a KIWI SDR and I could attenuate the RF out of the FLEX and see if those "birdies / spurs" are heard.

    Thanks for getting with the brain folks who design this equipment.


  • KD0RC
    KD0RC Member, Super Elmer Moderator

    Hi Fred, your 6400 is a very capable device and can show you your transmitted signal without additional equipment. The Full Duplex (FDX) feature will allow you to get a nice spectrum of your actual signal as transmitted.

    Ken, NM9P has some great videos on YouTube of how to set this up. In a nutshell, open a second panadapter and put the slice on the same frequency as the first pan. Set the receive antenna to be different from the transmit antenna. Turn FDX on, and turn your transmit power down to a watt and transmit into a dummy load.

    The second pan will now show an accurate view of your transmitted signal.

  • Dan Trainor
    Dan Trainor Member ✭✭✭

    Yes, these false visual artifacts outside the BW slice surprised me as well when I first started using the 6400. I was convinced they were real, as why would anyone display graphics if they are not representative to reality. Makes no sense. But, it is the graphical decision they made.. If they did it correctly, there would be no RF visualization outside the BW slice or at least, at a minimum, the visualization would roughly represent the actual transmitted RF signal. de WA1QZX

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

    Good discussion

    If you have a look at the scale on the right side, you'll see these signals are very very weak. -140db

    They are also not being transitted.

    You will often see artifacts like this with a variety of modes. The next thing you want to check is how strong it is.


  • sdrwow
    sdrwow Member ✭✭

    hello to ALL

    Thanks for the useful replies. I followed the Youtube vid for FDX and the "spurs" are not on the RF output. A very handy tool for the Flex to see what's going on with the output and monitor the audio. Just deal with the latency.

    Another discovery was that the bandscope was zoomed OUT to see more information in the "slice" and that's when I saw the "spikes / spurs" Whatever. I must have changed the out-of-the-box setting for the bandscope for something else before the Summer started and outdoor projects took command. When I first bought the radio I never saw those spikes.

    By being able to monitor the out put for AM mode; I can tweek the audio and my processing for a nice large "controlled " sound. I regret getting rid of a nice processor for AM audio. The Schlockwood SW200.....A Jim Wood design. The guy behind the Inovonics 222 processor.

    Thanks much


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