[time-nuts] WTB: GPSDO

Tim Lister listertim at gmail.com
Tue Mar 14 18:33:15 EDT 2017

On Fri, Mar 10, 2017 at 2:35 PM, Chris Albertson
<albertson.chris at gmail.com> wrote:
> A GPSDO is not hard to make.  All you need is some way to compare the
> phase of two signals, an XOR gate can do that.  Then a small $2
> process moves the control voltage on the crystal.    I tried one to
> build the simplest GPSDO that could still work.   Got the parts count
> down to about four or five and the cost well under $10 plus the OXO
> which was about $20.  The simplest dumb one I could make keeps about
> e-10.  Not great but enough for many uses.   I compared to my
> Thunderbolt and I could see the phase advance and retreat.  Just a
> little most sophistication and I likely could do much better but my
> goal was to prove to myself that a GPSDO could be build VERY simply
> with cheap parts

Hi Chris, that's good news that a GPSDO is that easy to make (at least
a basic one) as that is exactly my medium term plan ! The issue of
course is having something to test the newly built GPSDO against... I
got one of the rehoused Trimble UCCM-based GPSDOs off ebay a while ago
but haven't been super happy with it. It's quite a bit less sensitive
than more modern GPS receivers and it often struggles to get even 1
satellite with the indoor patch antenna. At one point both red alarm
LEDs came on and stayed on despite power cycles - I eventually fixed
that by taking it apart and finding and hitting a reset button on the
board. Currently although I can talk to the unit over serial and it
seems to respond, Lady Heather is not seeing any output from it.

Combined these things don't give me a great deal of confidence that
this unit will act as a stable master reference. I was wondering if a
second GPSDO like Russ linked to would work better (I have a ublox
LEA-6T GPS already which I plan to use as the basis of the homebuilt
GPSDO and it consistently sees many more satellites than the UCCM
with a similar indoor antenna)  or put the money to getting an outdoor
antenna mounted (don't feel happy drilling holes in the house myself)
by someone. Do 2 GPSDOs tell you much more or just that each is
different and you need a third to adjudicate ? (I can see a slippery
slope looming from here...)


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