[ARDF] Piccon mods

[Kenneth E. Harker]

On Mon, Nov 14, 2005 at 11:27:36AM -0800, Jay Hennigan wrote:
> 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?

I've only modified one piccon so far.

The ceramic resonator is marked as 3.58 whereas the crystal is 
3.579545 MHz.  

I suppose I should look at the circuit for the "old style" piccons which 
also used 3.579545 MHz crystals and see what they used.
 
> > 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?

This is the exact wording in the IARU R1 rules, Part B, Appendix A 
(http://www.ardf-r1.org/html/ardfrules26b_rev1.htm#BA1):

3.4       The maximum deviation of the transmitting periods is FIVE SECONDS 
against the official time during the whole competition. The maximum 
transmitting overlap of two transmitters is FIVE SECONDS.

> >      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.

I'll try that.
 
> 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
> WestNet:  Connecting you to the planet.  805 884-6323      WB6RDV
> NetLojix Communications, Inc.  -  http://www.netlojix.com/

-- 
Kenneth E. Harker WM5R
kenharker at kenharker.com
http://www.kenharker.com/