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Re: PSK31 IMD Ensemble RxTx

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  • warrenallgyer
    Mike While PSK31 is unlikely to generate true IMD since it is a single tone modulation scheme, what the listener was probably complaining about was a very wide
    Message 1 of 4 , May 22, 2013
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      Mike

      While PSK31 is unlikely to generate true IMD since it is a single tone modulation scheme, what the listener was probably complaining about was a very wide signal that is usually the product of over-driving the audio into distortion at some point in the audio chain.

      The adjustment you need to make is to assure proper audio levels out of the PC and at the input to the rig.

      You have several cascaded audio gain controls; probably one in the PC control panel, one in the EMU, one in fldigi (?), and one in rig itself(?). My guess is one of these is turned up too high and overloading the next stage down the line.

      Without knowing what kind of monitoring equipment you have access too it is hard to tell you exactly how to set it up. One trick you might try: While transmitting into dummy load and monitoring power output, reduce each the audio gains one control at a time. You should see a direct correlation between the audio gain setting and the output power. If one of the gains is set too high and causing clipping then reducing it will not immediately affect the power out.... and you have found your prime suspect.

      Finally, a thank you to the op who reported this to you. Too many of us are reluctant to criticize another's signal and this leads to lots of bad signals on the air.

      Warren Allgyer - 9V1TD

      --- In softrock40@yahoogroups.com, "Mike Saeger" <mike.saeger@...> wrote:
      >
      > I worked a station on 20m PSK31 and was told that I had a lot of IMD on my signal. I'm using FLdigi and Genesis PowerSDR with virtual audio and virtual serial cables.
      >
      > Any recommendations on what to adjust to eliminate the IMD reports? I'm driving a TAPR PennyWhistle FET amp about 10W output into an Ameritron AL-811 about 50W out. The PennyWhistle doc says to keep the power output to under 18 watts for best linearity.
      >
      > My sound card is a Creative Labs E-MU 0204 sample rate: 96Khz
      >
      > Mike N9MS
      >
    • David Turnbull
      ... My sound card is a Creative Labs E-MU 0204 sample rate: 96Khz ... The E-MU 0204 easily overdrives the RXTX Ensemble past linearity. It is designed for
      Message 2 of 4 , May 22, 2013
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        On Tue, May 21, 2013 at 10:55 PM, Mike Saeger <mike.saeger@...> wrote:
        I worked a station on 20m PSK31 and was told that I had a lot of IMD on my signal.
        My sound card is a Creative Labs E-MU 0204 sample rate: 96Khz

        The E-MU 0204 easily overdrives the RXTX Ensemble past linearity. It is designed for 4.0Vp-p. Here's some sound device output levels I measured.

        2.40V iMic
        4.00V MackBook Pro Built-In (Intel HDA)
        4.00V Peaberry SDR (PCM3060)
        5.76V E-MU 0204 stereo output jack

        Here's a nice page about PSK31 IMD: http://www.n7yg.com/files/IMD.htm

        73 David AE9RB


      • warrenallgyer
        Great article David and spot on. My post about PSK31 being a single tone mode was obviously way off base. I hate to admit it but I read it as WSPR instead of
        Message 3 of 4 , May 22, 2013
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          Great article David and spot on.

          My post about PSK31 being a single tone mode was obviously way off base. I hate to admit it but I read it as WSPR instead of PSK31... probably because I was staring at a WSPR waveform at the time.

          Apologies to Mike.

          Warren Allgyer - 9V1TD

          --- In softrock40@yahoogroups.com, David Turnbull <dturnbull@...> wrote:
          >
          > On Tue, May 21, 2013 at 10:55 PM, Mike Saeger <mike.saeger@...> wrote:
          >
          > > I worked a station on 20m PSK31 and was told that I had a lot of IMD on my
          > > signal.
          >
          > My sound card is a Creative Labs E-MU 0204 sample rate: 96Khz
          > >
          >
          > The E-MU 0204 easily overdrives the RXTX Ensemble past linearity. It is
          > designed for 4.0Vp-p. Here's some sound device output levels I measured.
          >
          > 2.40V iMic
          > 4.00V MackBook Pro Built-In (Intel HDA)
          > 4.00V Peaberry SDR (PCM3060)
          > 5.76V E-MU 0204 stereo output jack
          >
          > Here's a nice page about PSK31 IMD: http://www.n7yg.com/files/IMD.htm
          >
          > 73 David AE9RB
          > http://AE9RB.com/
          >
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