DDS control line bypass capacitors
- 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