[ARDF] Piccon mods

[Jay Hennigan]

On Mon, 14 Nov 2005, Kenneth E. Harker wrote:

>      I've modified one of my piccons by desoldering the ceramic resonator
> and replacing it with a 3.579545 MHz crystal and two small 22pF caps.
> See my photos:
>
> http://www.wm5r.org/piccon/
>
>      Unfortunately, the controller is still running slow - at the
> rate of about 2.3 seconds per hour.  This is an order of magntiude
> improvement compared to the worst I experienced with the ceramic
> resonator, but it's still not good enough.

That's 638 parts per million, fairly significant for a crystal.  Have
you modified more than one PicCon, and if so, how well do the clocks track
with each other?

> The IARU rules specify
> that the maximum deviation of the transmitting periods is only five
> seconds for the entire competition (which could be eight or nine
> hours.)

Is that absolute or with respect to each other?

>      Any ideas?  Should I try changing the caps to a different value?
> Would I increase the value? (27 pF? higher?)   Decrease the value? (18pF?
> lower?)  Do I need to add resistance anywhere?

You would go lower to increase the frequency.  I'd try 10 pf or so.  The
crystal you're using is calibrated for a load capacitance of 18 pF from
the Mouser catalog.  Theoretically, this would mean that each capacitor
would be 36 pF as they're considered series connections to the crystal.
Stray caacitances may be a factor as well.  Also consider that the divisor
may not be exact.  3.579545 is kind of an oddball number to divide into
seconds.

See if the circuit will oscillate with the capacitors removed.  It may
not depending on stray capacitances.  If it does, run it for an hour or
two and measure the clock drift.  Hopefully it will be on the high side.
Then you can interpolate the needed value easily between 0 and 22 pF.

See:  http://www.fairchildsemi.com/an/AN/AN-340.pdf  and also
http://www.cirrus.com/en/pubs/appNote/an33.pdf  for some discussion on
this design.  It looks as if the previous PicCon was parallel-mode as
it had a shunt resistor instead of capacitors to ground.

A color TV set tuned to a network broadcast, a PicCon, and an 80M
receiver in close proximity to each other should give you an idea how
close you are by listening for zero-beat.  The 3.579545 colorburst from
a network broadcast transmitter is highly accurate.  You won't know if
you're high or low however.  Assume low if the clock is slow.

>      I am mildly annoyed that I have to do any of this just to get a
> timing controller that can keep proper time.

I hear you, the cheapening of the PicCon by removing the crystal was a
pretty boneheaded idea for what was previously a highly regarded product
among the ARDF community.

--
Jay Hennigan - CCIE #7880 - Network Administration - jay at west.net
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