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Dirty signal or SmartSDR oddity?

KM4SII Member
edited March 2023 in FLEX-6000 Signature Series
Hello all,

This evening I noticed that when transmitting (Flex 6300 at 100w) through my delta loop, it *appears* that my signal is quite a bit less clean as compared to when I transmit through my dipole. I checked my signal on a remote and it looked/sounded ok to me, however. I have read some earlier discussions here that indicated that what SmartSDR shows is not always what is going out over the air, so I wanted to get the opinion of those more knowledgeable than I.

Here is a video showing the behavior I am describing: https://youtu.be/nM9e1oYL14s

Is this cause for concern or is this perhaps just a display oddity? I have no ability to check my signal with an SDR locally as I am several hours away from the station. Any thoughts would be greatly appreciated!

Best Answer

  • Ken Wells
    Ken Wells Community Manager admin
    Answer ✓

    1) You have the Preamp turned on for 20 Meters, which is adding to your apparent receive signal issues.

    Also, the effective coupling to the receive detector increases as the frequency goes up.

    If you turn the preamp off for 20 meters, and adjust the scale of the panadapter so that you show the whole transmit signal (much of it is off screen to the top) and adjust the horizontal and vertical scales of the panadapter so that they show the same thing for each band, you will not notice as much difference between the two.

    2) Here is a statement that Gerald published several years ago about "spurious" signals on the panadapter:

    " I can understand your concern about what you are seeing on the panadapter while transmitting, but rest assured what you are seeing is not actually being transmitted.

    To generate the spectrum display when transmitting, we couple the SCU (the analog to digital converter) or receiver to the DAC (digital to analog converter) that is responsible for generating the low-level transmit signal so that you see the transmit spectrum in near real-time. Whenever a signal is generated by a DAC, there are always Bessel sidebands of the fundamental frequency produced by the mathematics used by the DSP algorithms. What you are seeing on the panadapter are these Bessel sidebands as well as some additional low-level RF coupling within the radio.

    Because we use the SCU (receiver) to show the spectrum by coupling it to the DAC, you are viewing the spectrum before the DSP and any other final filtering used by the PA which is why you are seeing the sidebands on the spectrum display. 

    The radio has a DSP transmit filter for each mode. Before the low-level RF is amplified by the PA (power amplifier), these sidebands are filtered out of the signal, so they are never amplified by the radio. 

    In addition, the PA in the radio has final band pass filters that prevent the transmission any spurious emissions that would exceed FCC regulations."


  • KM4SII
    KM4SII Member
    The behavior seems to be most pronounced on 12m. Here is a screenshot... I turned up the "black" value a bit so that the "noise" around the carrier can be seen on the waterfall.
  • Ha Gei
    Ha Gei Member ✭✭✭

    It´s right, what you see is not the pure outgoing signal, it´s rather a reception of it an IMD products are added on the reception too. Here it looks quite different depending on antennas in use and/or which antenna connector used.

    Better let someone look from the distance or use an SDR receiver for a check on air.

    Both picture and video look normal to me



  • KM4SII
    KM4SII Member
    edited March 2023
    Thank you very much Ken. That is exactly the kind of answer I was looking for. Your explanation was very helpful, and it is good to know that everything is functioning a-okay!

    Mason - KM4SII

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