[time-nuts] Advise on building a DIY GPSDO?
EWKehren at aol.com
EWKehren at aol.com
Wed Apr 6 09:37:29 EDT 2016
Having been a time nut for almost 50 years knot knowing it I like to add
my thoughts. Starting in the late 60's I won second place at IEEE student
contest for my counter design using 7 segment lamp and Motorola RTL in the
70's I buried a Sulzer using bank coin bags filled with sand to close the
hole and application notes at TI using TI LED and IC's, in the 80's bought an
Efraton FRK and did my first DO using a Tracor Omega converted to 60 KHz
and a Philbrick DAC. I was the u processor using every 6 month using the
Tracor to recalculate the time for pulses to the DAC counter. I had then and
still do now have problems with computers and processors. When Brooks came
out with his GPSDO i bought 10 A&A boards and started dialog with Brooks.
Thanks to Corby I joined time nuts and continued work with Richard McCorkle.
One of the results is a GPSDO for FE 5680, 5650 and 405 that is readied for
release by Cash Olsen. It took extensive testing fine tuning and more
testing. During that time I also had the opportunity to communicate with pro's
on that subject and came away even for the pro's it is not a walk in the
woods. Tom's 2008 comparison mentioned in this thread
http://leapsecond.com/pages/gpsdo is very informative it clearly shows the relationship between OCXO
and digital loop. Few HP units are as good as his and there is a wide
distribution when it comes to 10811's. The tbolt shows clearly the impact of
the fact that it changes the frequency to correct the 1 pps.
Miller is very impressive and we have done one using u-lox-6 and a MV89
with similar results mainly intended for Ham's. Working on a 100 MHz version
for Ham uwave work
Summary true GPSDO is complex and long time work, Miller is simple and low
cost and maximum bang for your buck.
One more comment on u-blox. In my opinion unless you plan on doing saw
tooth correction the T version is a waste of money. Also if you plan on using
the Miller circuit buy a $ 10 version and use a PIC to program in the higher
output frequency a version that stores it in EEROM costs three times as
much relying on the battery unless it is always powered up may be a problem
once you are out in the field and have no PC to change from the 1pps default
Bert Kehren
In a message dated 4/6/2016 4:20:14 A.M. Eastern Daylight Time,
hmurray at megapathdsl.net writes:
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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