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Flex PSK31 signal purity

KD0RC Broomfield, COMember, Super Elmer Moderator

I have been playing with PSK31 again after not messing with it for a few years. I wondered just how clean my signal was, and got these results:

An IMD of -31 dB at 100 W on 6 meters!

Test conditions:

Flex 6400 at 100 W out to beam, no external amplifier

Fldigi set to PSK31, idling signal

DAX TX Gain set to 50%, Peak Level shows approximately -3 dB

There is a lot of discussion about how to set signal levels for the digital modes. On legacy radios, we always had to watch the ALC and keep audio levels low enough to just barely move the ALC meter. This usually meant running a bit less power to keep a clean signal. With the Flex, it looks like just keeping the peak level indicator out of the red does the trick!

NOTE: I did NOT hook my spectrum analyzer directly to my transmitter. I transmitted on my beam antenna up 50 ft, and used a few inches of wire for a pickup antenna on the spectrum analyzer. I started at 1 W, moved to 10 W, added some attenuation, and went for 100 W. That way I did not risk damaging my SA front-end.


  • Butch
    Butch Member ✭✭✭

    I use a KK7UQ IMD Meter and a Telepost LP-700 Digital Station Monitor to monitor my PSK31 drive level.

    Are you using a Rigol DSA815-TG SA? I hope to add one of those to my test bench next year.

    Butch KF4HR


  • KD0RC
    KD0RC Broomfield, COMember, Super Elmer Moderator

    Hi Butch, that is exactly what I used! Does your IMD meter give similar results to what I got with my 815?

  • David Decoons, wo2x
    David Decoons, wo2x Member, Super Elmer Moderator

    I haven’t tested with PSK-31 but with FT8 and SmartSDR 3.2.39 I find optimal output level to be the power slider in WSJT-X all the way up and DAX TX set to 48. This keeps the peak level indicator yellow and actual level just below zero. In Node Red I have an actual level readout from radio internal input level meter plus I have a peak “LED” in Node Red that will turn from green to red if the level exceeds 0.

    At and above zero digital clipping starts to occur in the DSP audio chain in the radio.


    Dave wo2x

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