[time-nuts] Thought experiment on a low cost timing board
Tom Van Baak
tvb at leapsecond.com
Fri Feb 24 17:53:04 EST 2006
> From: "Tom Clark, K3IO (ex W3IWI)" <K3IO at verizon.net>
> I have done several GPSDOs using the NAVMAN receiver, so I add a few
> comments to the discussion:
> 1. Both the G3RUH
> (http://www.jrmiller.demon.co.uk/projects/ministd/frqstd.htm) and
> I2PHD (http://gpsdo.i2phd.com/) designs use 74HC390 divider chips;
I also want to thank G3RUH and I2PHD for the links. I
tend to get caught up with such high performance that
I overlook simpler (though somewhat less performing)
solutions like those.
> I also tried them. What I found was that the 74HC390 dividers had very
> strong temperature sensitivity, amounting to well over 100 nsec with a
> mild diurnal room temperature change. Of course this is due to the
> 'HC390s being a cascaded series of ripple counters. To go from 10 MHz
> to 10 kHz, you end up with 12 CMOS stages being cascaded.
> My solution was to replace the 'HC390 change with the elegant
> PIC-based divider chain invented by Tom van Baak. This uses the 10 MHz
> signal as the PIC's clock, and the tight code based on a fixed number
> of wait states makes a fully synchronous divider. I was unable to
> measure ANY temperature effects. The PIC requires less real estate
> than the 'HC390s and is also quite cost effective.
For those of you interested -- the free PIC source code
for the divider is at:
Also I think Brooks Shera keeps a copy of the code at:
Although written years ago for the PIC16C84 it works with
little modification to almost any PIC. Remember to turn on
the HS osc fuse, turn off the WDT fuse, and use a PIC part
that handles a 10 MHz clock.
> In short -- The Jupiter-T is a good timing receiver.
> Hope these factoids helped -- Tom
Yes, thanks much. Always a pleasure to hear from you.
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