[ARDF] My PicCon "script"

[Dale Hunt, WB6BYU]

Marvin Johnston wrote:
...
> 
> I just took a look at the schematic for your PicCon and I see it uses a
> ceramic resonator instead of a crystal (as you mentioned and I
> overlooked.) You can replace the ceramic resonators with a crystal and a
> couple of small caps (Byon suggests about 6 pf) which might give you
> better long term accuracy. Since I am not an electrical engineer either,
> you might email Byon and ask him about this. All of my PicCons uses
> crystals. I rather suspect that the crystal will give more temperature
> stability than the ceramic resonator but someone else will have to
> verify that one :).


    Ceramic resonators have less initial accuracy AND much greater
    temperature sensitivity than a crystal.  They would be fine for
    a single-transmitter hunt where "one minute on, one minute off"
    doesn't need to be horribly accurate.

    Just pulled out my Digi-Key catalog and found some ceramic
    resonators: they are rated for frequency tolerance +/- 0.5%,
    temperature stability +/- 0.3%.  If you figure a drift of
    one second out of 200 for the frequency tolerance, that is
    about 18 seconds per hour.  So one minute drift in 3 hours
    is possible, even without considering temperature differences.

    A random crystal in the catalog lists a frequency tolerance
    (at 25C) of +/- 30ppm, and a frequency stability with 
    temperature of +/- 50ppm.

    My experience using the Montreal Controllers with a crystal
    time base is that I had to use 22pF NP0 feedback capacitors.
    20pf was far enough off that the overlap became noticable
    by the end of a practice session.  (I think the international
    standard is not more than a few seconds, and one of mine
    was about 10 seconds after 3 or 4 hours.)  Using crystals
    from the same lot and matching NP0 capacitors gives me
    adequate accuracy out to about 5 or 6 hours.

    So my first suggestion is to replace the ceramic resonators
    with crystals. Likely you will need to add a couple capacitors
    from each end of the crystal to ground if the ceramic resonators
    have three pins.  That probably will suffice for most hunts,
    but if you still have a problem then replacing one of the
    feedback capacitors with a trimmer will allow you to synchronize
    them much more exactly.  A frequency counter with readout to
    the nearest Hz should do the job.

    If you don't want to replace the resonators, sync them all with
    WWV some time and see which one keeps best time.  Then add a
    small trimcap across one side or the other of each of the others
    and you should be able to get them close together.  This may be
    adequate for most practice sessions if all the transmitters are
    at about the same temperature.

       - Dale WB6BYU
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