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16793Re: [tracker2] Interoperability of FoxTrak tracker a nd Argent Data's OT3

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  • Matthew Cook
    Aug 17, 2014
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      Kennith I'd also like to read your paper, you can find my email address on QRZ.com also.

      I think that many people overlook a very subtle point regarding pre-emphasis and de-emphasis circuits; 

      Question:  what else do they introduce other than level distortion that is frequency dependent?

      So why when you take the sum of all of this mysterious factor from modulator to demodulator at both of your tone frequencies would you care?

      The other clue is the original Bell-202 standard went into great pains to discuss things like level, tone frequency error, symmetry and twist.  It is helpful to understand what in a RF chain can cause each of these things to vary, including how the FM is being generated within the modulator (there are two methods one adds to the problem the other does not).

      It's also sometimes helpful to find a copy of a commercial Land Mobile Radio (LMR) spec and then compare this to your amateur radio.  Why were the LMR radio specs more strict than any Ham transceiver ?  The answer isn't immediately obvious, but when you stop to think about this and realise what it is and why you have to control it, then my statement "Now you are aware that packet signals weren't supposed be fed through pre-emphasis and de-emphasis circuits?" might be more apparent.

      I am sorry for deliberately being vague in my response above, but I was asked this same series of questions as a junior Engineer by my peers many many years ago.  It made me think for a while before the answer dawned.  I'd hate to give away the surprise.   YMMV.

      73's

      Matthew
      VK5ZM


      On 9 August 2014 05:44, owenvk@... [tracker2] <tracker2@yahoogroups.com> wrote:
       

      I am interested to read your paper... but I cannot find your email address. (Mine is on QRZ.com.)

      I also have searched for 'standards' documentation for the physical layer, but not been successful... hence the reason for asking for a reference for a quite strong statement re pre/de-emphasis usage.

      In my view, a flat channel end to end is needed, and that does not preclude the use of pre/de-emphasis, but it does preclude a mixed channel where pre/de-emphasis is implemented at one end only.

      No link is perfectly flat, in my experience radio manufacturers fiddle the audio response to enhance the sensititivity specs.

      I tested two radios recently to find why one was kinder to grossly overdriven AFSK signals.

      The receivers were measured to evaluate the accuracy of the de-emphasis circuit. The test was performed with a FM signal of 3kHz deviation and modulating frequencies of 2kHz and 1kHz, and the audio output voltage for each was read and the de-emphasis calculated:

      • Icom IC-2200H – 7.2dB
      • Yaesu FT-2800M – 4.3dB

      The de-emphasis should be 6dB, the Icom is over de-emphasised 1.2dB over the octave, and the Yaesu under de-emphasised 1.7dB over the octave, about 3dB less than the Icom over an octave.


      There is a measurable difference in the receivers, and an observable one as the FT-2800M is kinder to overdriven AFSK signals (and most are in my experience).




      Owen




      --
      Matthew
      VK5ZM
      0487 653 245
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