I've designed a GPSDO, but how "good" is it?
Hi
Consider that your VCXO has a minimum tune range of 20 ppm. There are no typicals or or
max limits shown. Just to toss out a number, say it’s 30 ppm in some cases. They list linearity at 5%.
That’s likely per 55310, so it’s a box spec. Say that gets you to a 1.5:1 slope ratio. Your most sensitive
part of the curve now would equate to 45 ppm.
That would give you a 0.7 ppb LSB if the DAC is perfect. If it’s good to what many are (they are monotonic),
you have steps at or above your 1 ppb limit. That assumes the loop only steps 1 step at a time. With a FLL
at 100 seconds, that’s very unlikely.
Any time you “pop” the frequency by 1 ppb, you go out of your accuracy limit. That without the VCXO moving with
a standard deviation at 1 second of 1 ppb. That error would add on top of your steps.
One alternative - find an oscillator with a lot less EFC range. That part is designed to hold 4.6 ppm forever and to
guarantee lock to another source that is also at +/- 4.6 ppm forever. There are a few other bits and pieces involved so
it really needs to be > +/- 11 ppm EFC at ship.
In your case, an EFC that corrects the aging of the part (4.6 ppm) is plenty good enough.
Bob
On Aug 17, 2015, at 5:37 PM, Nick Sayer via time-nuts time-nuts@febo.com wrote:
On Aug 17, 2015, at 12:49 PM, Nick Sayer via time-nuts time-nuts@febo.com wrote:
On Aug 17, 2015, at 12:07 PM, Attila Kinali attila@kinali.ch wrote:
You really should read the wikipedia article on the PID loop and implement
a simple PI loop (no need for the D part). That's not more effort than what
you already did, but gives you better stability.
I’ve done PID before (for a reflow oven controller), but thought that the current code was easier to understand. I’m going to try the GPSDO simulator and see how it matches up. It’s entirely possible that an improvement could be made in the time-to-lock, but the steady state performance appears to my eyes to be as close to optimal as I could envision. But I’m new at this, so it’s entirely possible that I’m not looking at it correctly.
I thought some more, and in principle, I could use the 100 second sample error as the proportional and 1000 second cumulative error as the integral. What I wanted to insure with my hand-coded decision making was that the system was not completely insensitive to momentary excursions in the steady state, but that it didn’t overreact. I suppose that could just mean that Kp = 1 and Ki =~ 2 or 3.
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Nick wrote:
I believe I'm at or better than the stability I originally sought.
Part 1 of my question is whether that's actually true or whether my
naivety is presenting me with a delusion. Part 2 is whether the
price point at which I've arrived will support the level of
stability I've achieved, or am I delusional in thinking people would
or should pay what I'm asking for what I'm offering.
* * *
10^-9 was the target all along. Question number two that I came with
was whether or not it was reasonable to ask $175 for 10^-9.
From your description, I doubt that you have achieved 1e-9 from 1S
to 1000S (the range of primary interest for frequency and TI
counters) -- but I'll defer on that point until Tom reports on its performance.
As to question 2 (assuming that your GPSDO does exhibit stability
of 1e-9 from 1S to 1000S):
There may be people who would pay $150 or even $175, but in my view
that is only because they do not know how it performs and what other
GPSDOs are available and how they perform. For starters, consider these:
http://www.ebay.com/itm/171504585820
Jackson Labs LTE-Lite board-level GPSDO with TCXO ADEV <7e-11 at
tau >/= 10S. Brand new with warranty from an established GPSDO
manufacturer. $185
http://www.ebay.com/itm/400953748215
independent measurements at http://www.ke5fx.com/gpscomp.htm
GPSDO with OCXO, small production from hobbyist (BG7TBL). Published
Allan Deviation graph better than 3e-11 from 1S to 7kS; better than
5e-12 at 100S. Complete kit -- comes with PS and external antenna. $149
http://www.ebay.com/itm/181777773438
Trimble commercial board-level GPSDO card with OCXO; includes power
supply. $119
http://www.ebay.com/itm/301705767019
Nortel by Trimble NTBW50AA packaged GPSDO with OCXO; kit with antenna
and cables. $159
http://www.ebay.com/itm/221777430088
Lucent by Symmetricom Z3810AS GPSDO timing system; two disciplined OCXOs. $150
The OCXO units all have ADEVs in the low e-11s at 1S and generally
get better at longer tau. The Jackson Labs TCXO product is not far
behind, owing to the careful selection and testing of the TCXOs by
Jackson Labs. With the parts that you have chosen based on your
budget, you cannot expect to get near this level of performance even
if you improve the disciplining algorithm.
So, to put it bluntly: Even if you hit your design goal, you have a
nice learning project -- but not anything that justifies offering
them for sale.
Best regards,
Charles
Sent from my iPhone
On Aug 18, 2015, at 4:42 PM, Charles Steinmetz csteinmetz@yandex.com wrote:
Nick wrote:
I believe I'm at or better than the stability I originally sought. Part 1 of my question is whether that's actually true or whether my naivety is presenting me with a delusion. Part 2 is whether the price point at which I've arrived will support the level of stability I've achieved, or am I delusional in thinking people would or should pay what I'm asking for what I'm offering.
* * *
10^-9 was the target all along. Question number two that I came with was whether or not it was reasonable to ask $175 for 10^-9.
From your description, I doubt that you have achieved 1e-9 from 1S to 1000S (the range of primary interest for frequency and TI counters) -- but I'll defer on that point until Tom reports on its performance.
As to question 2 (assuming that your GPSDO does exhibit stability of 1e-9 from 1S to 1000S):
There may be people who would pay $150 or even $175, but in my view that is only because they do not know how it performs and what other GPSDOs are available and how they perform. For starters, consider these:
Well, that all puts the nail in the coffin, then. If better ones are cheaper, then I am wasting my time. I don't know how I missed them when I was searching for one a few weeks ago. All I found were manufactures not listing actual prices and eBay listings for used ones.
Thanks to everyone for the feedback - of all sorts.
Nick wrote:
Well, that all puts the nail in the coffin, then. If better ones are
cheaper, then I am wasting my time. I don't know how I missed them
when I was searching for one a few weeks ago. All I found were
manufactures not listing actual prices and eBay listings for used ones.
I did not mean to suggest that all of the GPSDOs I listed are
new. The first two are newly manufactured (although the second one
contains a surplus OCXO). The third and fourth are used surplus
units (I do not count that against them -- arguably the best
time-nuts GPSDO of all time is the Trimble Thunderbolt. TBolts were
readily available as surplus ten years ago, but are scarce and
expensive these days). The last one on the list is surplus, but the
units currently available were never commissioned -- they are brand
new in the original packing.
I wasn't trying to kill the project, just to give some
perspective. Depending on your primary interest (learning about
oscillator disciplining, or selling gizmos with short development
cycles), there is still much to be learned if you persevere -- and
the possibility exists that several dozen revisions down the road you
may have a gizmo that does justify being offered for sale. The
particular design issues related to GPS disciplining just do not lend
themselves to a quick learning and development curve.
Best regards,
Charles
On 08/16/2015 08:39 PM, Bob Camp wrote:
The “wear out” on a Rb is not the same as wear out on a
Cesium standard. In a Cs, you have a finite number if atoms
loaded in at the factory. When those atoms all travel down the
tube, it’s dead. In an Rb, the atoms don’t go anywhere. There is
no built in “end of life”. They mostly die from MTBF sorts of things
due to complexity and high temperatures, There are HP Rb’s
built many decades ago still running on their original cells.
Bob, that is fascinating information.
I will use my surplus Rubidium units more often now.