[time-nuts] Advise on building a DIY GPSDO?

Bob Camp kb8tq at n1k.org
Wed Apr 6 07:42:03 EDT 2016


Hi

One thing to keep in mind: 

If you are looking at stuff like sawtooth on most (but not all) GPS modules, the
counter you use to take the data does not have to be some sort of monster
machine. An old 5334 or 5335 will let you see the basic structure. If you spend time 
shopping, neither one is terribly expensive. If you need one “right now” you will
pay a lot for the availability (like > 3X the price). 

There are also things like the PicTIc that have way more resolution than you would
need. They are not in the same league as a 53132 either. They will give you quite
a bit more resolution than the 5334. 

Lots of choices, no big need to spend a boatload of money. 

Bob

> On Apr 6, 2016, at 3:00 AM, Hal Murray <hmurray at megapathdsl.net> wrote:
> 
> 
> albertson.chris at gmail.com said:
>> I may never get around tooth's as the Rb is very good all by itself to the
>> limit of my ability to measure it.
> 
> Wait longer.  :)
> 
> If your Rb doesn't have a PPS, make one.  One of tvb's divider chips is 
> probably the simplest approach.  You can probably do it with an Arduino if 
> you have one handy and like writing that sort of code.
> 
> Compare the PPS from the Rb with the one from your GPS.  It's probably 
> simplest if your Rb PPS is offset a bit from the GPS PPS.  The idea is to 
> make the difference always have the same sign.
> 
> Feed both PPS pulses to some equipment that can measure the time difference.  
> Collect data.
> 
> If you have a typical low cost (non GPSDO) GPS device, you should be able to 
> make graphs like the ones that show a hanging bridge.  Look at a sawtooth 
> region, not a bridge.  Pick the top or the bottom or the middle or whatever 
> you can remember.  Call that the offset.  Wait an hour collect more data.  
> Has your offset shifted?  If not wait longer...
> 
> You are measuring the average frequency of the Rb.  It might be temperature 
> sensitive.  Can you see the offset change over a day?  (That assumes you turn 
> the heat down at night...)  Or maybe you are seeing the GPS night/day shift.
> 
> 
> -- 
> These are my opinions.  I hate spam.
> 
> 
> 
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