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DDS control line bypass capacitors

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  • de_n4bb
    Recognizing that DDS control line signals were relatively high speed, I figured that selecting a feedthru capacitor value to use in my new Hammond box for
    Message 1 of 1 , Mar 2, 2006
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      Recognizing that DDS control line signals were relatively high speed,
      I figured that selecting a feedthru capacitor value to use in my new
      Hammond "box" for the RF section of an IQPro would be a non-trivial
      matter. After four different tests yesterday, I have concluded it
      is, indeed, a non-trivial matter!

      Starting condition (prior to test 1): Working IQPro

      Test #1, .01uF feedthrus = Failure
      Test #2, .05uF feedthrus = Failure
      Test #3, 9 pin sub-min with .001uF's from pins to ground = Failure
      Test #4, 9 pin sub-min alone (.001uF's removed) = Success

      I repeated tests 1, 2, and 4 with two DDS-60 (NJQRP) units. One DDS-
      60 used the PC as a controller (WA6UFQ software), the other used
      Steve Weber's MS-DDS-60 controller. Results were the same as above.

      The RF section of the IQPro did not respond to any controller
      commands using the bypass capacitors. The DDS-60 would usually "set-
      up" on some frequency, but typically not the right one.

      Controller-to-RF unit cable was a lenght of ribbon cable (under 4
      inches), except in the case of the PC parallel port connection which
      used a 36 inch length of Belden 3 conductor shielded cable. The .01
      feedthrus were new 4-40 units from EMI Filter Company. The .05
      feedthrus and the .001's disc ceramics were old, but unused "junkbox"
      parts manufactured by Erie.


      Tom Baker, N4BB
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