[ARDF] Piccon mods
Kenneth E. Harker
kenharker at kenharker.com
Mon Nov 14 12:34:51 PST 2005
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/
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