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Re: setting proper deviation

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  • Doug Reed
    Do you have a computer with a sound card? You could install one of the sound card audio analyzer programs or any program with a scope-type display and use it
    Message 1 of 1 , Dec 4, 2006
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      Do you have a computer with a sound card? You could install one of the
      sound card audio analyzer programs or any program with a scope-type
      display and use it as an aid to setting deviation. Some of those
      programs have audio generator functions as well.
      > http://www.brownbear.de/
      > http://www.tech-systems-labs.com/test-software.htm

      What I did for a long time was use an audio voltmeter to read the audio
      voltage output of a VHF/UHF scanner. But you still needed something to
      "calibrate" it. It is difficult to find a constant tone so the voltmeter
      would have a stable display. What I did was use the DTMF output from my
      HT as a standard level, but you can't even do that if you don't know
      what that deviation is. And DTMF isn't accurate either because it is a
      Dual Tone signal.

      You have to be very careful what tone you provide while setting the
      deviation. You have to set the deviation for the HIGHEST frequency tone
      in the signal. Because of audio pre-emphasis in the transmitter, if your
      target is 3.2KHz deviation, that should be the deviation at the space
      (high) frequency. Deviation at the mark (low) frequency will be around
      1.7KHz, 6dB per octave lower. If you use mixed tones, the average as
      shown by voltmeter will be somewhere in the middle.

      The best choice is to modify a scanner to get audio from the
      discriminator in the radio, or use the 9600 baud audio output from a
      "9600 ready" ham rig. Run the audio to your computer sound card and
      watch for the peak deviation on public safety transmissions in the VHF
      band. Those transmitters should be very well adjusted for 5KHz peak
      deviation. Once you know what the peak voltage is for 5KHz, you can set
      your TX for 3.2KHz peak deviation using either space tone or mixed
      tones. Or adjust the mark tone for 1.7KHz deviation and assume the space
      tone is right.

      According to Gary N8UR, "Lets not forget Layer 1", it is actually
      preferable if the space tone is slightly louder (has more deviation)
      than the mark tone. This was a factor for old TNC designs. This means
      you would want even less deviation on the mark tone when the space tone
      is adjusted correctly. Some of this is an attempt to make up for radios
      with poor de-emphasis circuits, some makes up for the TNC tone detector
      design.
      > http://www.febo.com/packet/index.html

      73, Doug Reed, N0NAS.


      > Posted by: "Stephen Brown Jr" stephen.brown75@... sbrown6079
      > Date: Sun Dec 3, 2006 9:46 am ((PST))
      >
      > What's the best way to set deviation without a scope? Would using the tune
      > feature of the calibrate command be reliable? Looks like a pretty nifty
      > feature from what I have read about it, but I'm not sure if that will give
      > me the desired results.
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