[time-nuts] WTB: GPSDO

Mark Spencer mark at alignedsolutions.com
Mon Mar 20 17:26:28 EDT 2017


Bob's comment about adjusting an oscillator from time to time aligns well with my limited experience in the time nuts hobby.    Once I realized that in practice my better OCXO's were typically more than stable enough for my intended uses things became much simpler.   I also realized that I could utilize my collection of time interval counters to compare my chosen reference to other references (including a GPSDO) while also comparing the chosen reference to the "Device Under Test."  I realize this isn't likely an approach that a commercial lab would use but for my hobby use it seems to work ok for me.

I've more or less shelved my plans to discipline one of my high end OCXO's via a home brew GPSDO scheme.

Mark Spencer

> On Mar 14, 2017, at 5:24 PM, Bob Camp <kb8tq at n1k.org> wrote:
> Hi
>> 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.
> Bob
>> 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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