[time-nuts] WTB: GPSDO

Bob Camp kb8tq at n1k.org
Tue Mar 14 20:24:23 EDT 2017


> On Mar 14, 2017, at 6:33 PM, Tim Lister <listertim at gmail.com> wrote:
> 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 !

Actually it’s much easier. Just put a DVM on the XOR once a week and 
adjust your oscillator with a screwdriver. It saves *lots* of time and money.


> 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...)
> Cheers,
> Tim
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