Well, thanks, everyone, for the information. I appreciate the help.
First, I am presently not up to adding another project to my long list
of projects. I get whiplash every time I walk into the lab. Building a
GPSDO sound like fun. Perhaps down the line.
I figured I should add some information about myself: I am an electrical
engineer (currently employed) with a lot of digital/Verilog experience
and a fair bit of analog experience (but less than my digital
experience) and quite a bit of software experience, all of this from
working for about thirty-eight years on various embedded systems.
Currently, I shy away from writing code just because I don't enjoy it
much and have done too much professionally. But I know that eventually I
will need to write code in my lab. Presently, I am in the process of
restoring some older ham radio gear, but I became sidetracked from that
by the necessity to repair a bunch of vintage test equipment which
effort has somehow taken on a life of it's own.
What I need right now is a frequency standard that is accurate enough to
use as a reference for things like calibrating test gear. I also want to
"play" with one before I build one. Just going through all of the
educational material is a daunting task. I figured I'd combine an
interest with GPSDOs in general with a need for an accurate enough
"standard" (I use the term loosely here) to get some instruments
calibrated.
Thanks again for all the information!
Cheers,
DaveD
I had forgotten about the LTE-lite; I should add that to the list of
choices. I'm tending towards either a 10 MHz version of that or the
Lucent boxes.
On 12/14/2014 8:00 AM, Bob Camp wrote:
Hi
On Dec 14, 2014, at 12:47 AM, Chris Albertson <albertson.ch...@gmail.com> wrote:
I tried to see just how simple, low cost and self contained I could make a
GPSDO. I started with the Lars Walenius design then removed everything I
could from it. I replace all the software with just a small loop with
about a dozen lines of code so it would be easy to understand.
My goal was to make something that could be built and tested using just
basic equipment. The question is of course "How do you know the unit is
making a 10 MHz signal if you don't already have a 10MHz reference to
compare it to?" Well you can assume that your 1PPS reference is accurate.
Except that the GPS PPS is *not* perfect, far from it. It’s only reasonably
accurate over very long time spans. Over short spans the pps moves around a lot.
Then you count and make sure you see EXACTLY 10,000,000 oscillator cycles
per each PPS.
If you do a tight lock (“EXACTLY”) against a GPS PPS that is moving +/- 10 ns,
your frequency will swing +/- 1x10^-8 every second
Count both for a few days and verify the ratio remains at
ten million to one, exactly.
Ok, that’s looking at the long term where GPS is indeed accurate. That’s the
easy part on any GPSDO design.
I ran mine for about 8 weeks and it stays at
the desired ratio. I know this is not a perfect test because it could
have been running at zero hertz for 30 seconds and then 20MHz for 30
seconds but I assume the OCXO is better than that. The point is that once
you have the GPS working you DO have a pretty good 1Hz reference.
Well, not quite so fast. You just jumped over a massive amount of work that
normally gets done on a GPS. A unit that *was* swinging +/- 1x10^-8 every
second would pass your test. (which is not in any way to say that your design
actually does that). It would make a lousy GPSDO for most uses. You very much
*do* need to check the ADEV (or what ever) close in and tune your filter up to
match your parts.
Cost:
Motorola Oncore GPS $18
magnnetic patch antenna 6
OCXO (eBay) 19
Arduino, mini 3
PLL chip 2
TTL diver chip 1
Plug-in power cube 0
perf-board 1
Total cost of GPSDO $50
Just a side note - A *lot* of the $19 OCXO’s I have from eBay are in very poor
shape spec wise. Testing them before using them would be a very good idea.
Bob
Actually I do have A Thunderbolt. I place the 10MHz output of the above
unit and the TB on my dual channel scope and was able to see the phase of
the two 10MHz references was locked. I saw the phase drift over about an
hour but then it would pull back. But I made this very simple and it
could be better.
Actually I've added some features to it like a 2 line by 16 character LCD
display and some status LEDs. And I can log data to a computer via a USB
cable so it is easy to plot data and it is using my more expansive mast
mounted timing antenna.
The Arduino based design is OK for controlling an OCXO but I think it is
best used for controlling my Rubidium oscillator. The RB is so stable I
should only update the frequency control every few hours at most.
On Sat, Dec 13, 2014 at 7:21 PM, Jim Harman <j99har...@gmail.com> wrote:
On Sat, Dec 13, 2014 at 9:36 PM, Bob Camp <kb...@n1k.org> wrote:
The problem with “build it yourself” is that there is no way do know if
you got it right unless you have something to compare your design to. You
*will* make mistakes as you build one of these….
I think you will have the same problem with an off-the-shelf unit if you
don't have at least one reference for comparison. However speaking from
experience with Lars Walenius' Arduino-based design, I can say that it is
not hard to make a working system, even without another reference. Along
the way you will learn a tremendous amount about how these systems work,
plus a lot about Arduino programming.
Lars' design will run stand-alone, but if you want it can send very useful
logging data to a PC, much more informative than a "locked" led on a
commercial unit.
Total cost including processor, Adafruit GPS shield, and $25.00 ebay OCXO
is about $100.00
--
--Jim Harman
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--
Chris Albertson
Redondo Beach, California
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