On Feb 11, 2017, at 5:52 PM, gkk gb <modjkl...@comcast.net> wrote:
Thanks Bob,
I should clarify the MTIE measurement extends 100000 seconds (the others are
less time). Is it a reasonable question to ask if GPS is needed? Or are there
other variables that are involved?
Good point about the temperature stability, I hadn't considered that. Can I
place in a temperature chamber to provide a better thermal environment, or does
that cause other issues (vibration from blowers, EMI noise, etc.)? Other ways
to mitigate temperature changes?
It seems a Rubidium is good after a timescale of 100 s. What do people do below 100 s to
characterize quartz oscillators. Do they simply try to find the most stable parts they
can afford and break the x-axis (tau) into two regions using difference references for
each? If so, are there generally accepted "gold" standards anyone can recommend
for crystal products with the best stability to use as a reference between 0.1 and 100
seconds, for example?
On February 11, 2017 at 6:29 AM Bob Camp <kb...@n1k.org> wrote:
Hi
Backing up a bit here.
On Feb 10, 2017, at 7:35 PM, gkk gb <modjkl...@comcast.net> wrote:
Hello experts, I need a Rubidium frequency reference for my company, and wonder
if I also need to GPS discipline it.
I characterize crystal-based OCXOs for ADEV, MTIE, and TDEV, and my longest
measurement time is 100,000 seconds (28 hours).
If your longest measurement is a 100,000 second ADEV, then your measurement
time will be out in the
1,000,000 to 10,000,000 second range. Is that really what you are doing?
If 100,000 seconds ADEV is your longest measurement, what is the shortest tau
you are interested in?
A Rb is not going to be much use for testing a good OCXO at shorter tau. Where
the crossover happens
depends a lot on the grade of OCXO you are working with. By the time you get to
1 second
most OCXO’s will be noticeably better than most Rb’s.
I'm looking at this graph from SRS for PRS10,
http://www.thinksrs.com/assets/instr/PRS10/PRS10diag2LG.gif
I would suggest that plot is probably not the best one to depend on for GPS
performance. In a GPSDO setting
the cut over points are all over the place depending on which design you look
at.
and thinking that as long as I calibrate a Rubidium source annually, there's no
need for a GPS (since it only appears to degrade stability). Is this true in
general, or is the graph misleading me because it may be true here, but not
always.
The big issue is going to be temperature stability. If you have a Rb that is
(say) 5x10^-10 over 0 to 50C, that is likely 1x10^-11 / C (or maybe more). A 2C
delta in
your lab as the HVAC cycles will give you a 2x10^-11 “hump” in your ADEV plot.
Also consider that if you want an “easy” measurement of the devices you are
testing, the reference source probably should be
5X better than what you expect out of the DUT. You probably will not have that
luxury in this case. That gets you into multiple
references and things like three corner hat testing.
So my question, is a GPS necessary to discipline a Rubidium standard to
characterize the best crystal oscillators for stability, or can I do without it
(and just calibrate the Rubidium annually to maintain accuracy) and actually
get better stability?
How many seconds out is a GPS generally needed to improve accuracy from a
Rubidium standard?
If you really are running 1,000,000 to 10,000,000 second long tests, you need
the GPS.
Lots of variables
Bob
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