[time-nuts] WTB: GPSDO
kb8tq at n1k.org
Mon Mar 20 18:17:58 EDT 2017
A good OCXO run continuously should get down to < 0.1 ppb / week. Doing a tweak
every Saturday is likely enough to keep it in that range. The *big* advantage is that you
have the ADEV of the OCXO without any scruffy stuff from the control loop getting in the
way. If your objective is to run something like a frequency counter, you probably are better
off with the trimmed OCXO.
> On Mar 20, 2017, at 5:26 PM, Mark Spencer <mark at alignedsolutions.com> wrote:
> 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:
>>> 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...)
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