Re: [time-nuts] Low-Cost Rubidium Performance
On 2/9/12 4:51 AM, WarrenS wrote:
Indeed,
ADEV is for random freq variation not easily measured by other means.
Temperature fluctuations do not cause random freq changes and the
temperature's effect should be removed if one wants accurate long term
ADEV numbers.
Why? Unless the unit is spec'd for use only at a constant temperature,
temperature variations are something it needs to deal with, and should
be included in any measurement of how good it is. In what way is
temperature variability special, that it shouldn't be included in ADEV
measurements, but all other contributors to variability should be?
Certainly, there may be a large non-random component to temperature
(diurnal, annual, in many environments), but there's also a significant
part which is random - should you somehow correct for one, but not the
other?
Shouldn't one expect the ADEV be better for a double oven OCXO than for
a bare crystal?
The only issue I see is it may make fair comparison difficult, unless
units are compared under identical conditions.
On 02/10/2012 02:31 AM, Bob Camp wrote:
Hi
To add another wrinkle to this.
Correcting ADEV for systematic errors and then not mentioning you did so strikes is something I find a bit problematic. If say you decide to take out 13th order drift, you should say you did so. The discussion of what to correct and how is older than ADEV. Since there is no "standard" set of corrections, one should be clear about what was done.
Agreed, but even if we know that system bandwidth affects low-tau
noise-forms, when did you last see the bandwidth given? Confidence
bounds is another issue.
But my main point is that you still look at the same damn plot rather
than looking at some other plot for the other effects. The point is to
separate them in order to clearer see each of them. For short-term,
noise dominates, and depending on where we are on the spectra the ADEV
or phase-noise plots serves us best. TIE curves is interesting, but MTIE
curves adds a certain aspect, drift analysis curves another aspect etc.
For jitter, separation of random noise and deterministic noise is am
important to properly engineer the system. MTIE is another of those.
I love my ADEVs, but as my suppliers know, it's not the only thing I ask
for in an oscillator.
How does my phase shift as the temperature cycles? ADEV won't tell me,
but I bet the guy in the next cubicle can enlighten you if needed.
This is all about understanding the limitations of the tools we have, so
we use them for what they do best. ADEV is a great tool, just don't over
use it. Whenever someone use their favorite tool (say ADEV, FFT or
Object Oriented programming) for everything and everything needs to be
answered by it, then the big warnings-bells goes off in the back of my
head. As great as it ever is, know the limits of what you use...
Cheers,
Magnus
Let me strengthen the argument below. Let's say you are building and
selling Rb units to the telcom industry and you've spec'd your product
for use in the range of -25C to +85C. Now you want to include an
ADEV plot in your sales literature. Should you place the unit in an
environmental controlled box while you measure it? I think you'd be
accused of fraud if you did not state that the plot is not
representative of the full temperature spec. I think in order to be
valid you always have to state the test conditions but you can use ANY
test condition, Place it outdoors, in a lab or what ever but just say
what you did and then your plot has meaning. I think the converse to
true, that without knowing the test conditions the plot is little
meaning.
Why? Unless the unit is spec'd for use only at a constant temperature,
temperature variations are something it needs to deal with, and should be
included in any measurement of how good it is. In what way is temperature
variability special, that it shouldn't be included in ADEV measurements, but
all other contributors to variability should be? Certainly, there may be a
large non-random component to temperature (diurnal, annual, in many
environments), but there's also a significant part which is random - should
you somehow correct for one, but not the other?
Shouldn't one expect the ADEV be better for a double oven OCXO than for a
bare crystal?
The only issue I see is it may make fair comparison difficult, unless units
are compared under identical conditions.
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--
Chris Albertson
Redondo Beach, California
Magnus wrote:
It's a huge difference what comes in the device and what environment
we put it in. A temperature dependence, is a systematic effect on the device.
To some degree we can control temperature, we can predict
temperature and deal with it. We can handle it as an engineering
concept and do steering loops etc. It's pretty systematic to me.
True, but while we can arrange to run the oscillator in a
constant-temperature environment to see how it performs without
temperature variations, we can never operate the oscillator without
any internal frequency variation, which would be necessary to fully
characterize its dynamic temperature dependence. The best we can do
is measure both temperature and frequency (the latter containing both
"internal" and "external" errors), and attempt to correlate the
temperature effects by curve fitting. This gets more and more
difficult as the device's response to temperature exhibits second,
third, and higher order effects (hysteresis, nonlinearity, etc.,
etc.). So the best we can hope to end up with is an educated guess,
and one person's guess will be different from the next person's (and
from his or her own next run). In practice, we are unlikely to even
attempt to correct for anything but first-order effects.
Think about recording an orchestra in a concert hall near a busy
street. You can make a semi-independent recording of the traffic
noise (just like measuring the temperature in our oscillator
example), but the complicated manner in which the traffic noise
enters the hall and reverberates inside it makes it impossible to
subtract the traffic noise from the original recording. What you end
up with has more to do with the choices you made in modeling the
noise ingress than it does with the traffic noise itself. That is
why any tinkering one does with the data must be exactingly
explained, and accompanied by the raw data so others can see for themselves.
Better to build the isotemp chamber and get clean data, if you want
to know how your oscillator behaves independent of temperature. Just
don't think that you can count on it for the same stability once you
take it out of the chamber.
Best regards,
Charles
Hi
Have I seen system bandwidth in published data? - pretty much never. Kudos
to Symmetricom for putting it up on the display on the 5125. The system
bandwidth debate goes back well into the 1960's. Pretty much every paper
Dave Allan ever presented up through the 1980's, he got a question about
bandwidth from the floor. I don't ever remember there being a definitive
reply back in that era.
Confidence bounds - same thing. How many points before you even display the
data? If I'm using 3 points and you are using 100 the error bars will be a
bit different. The reply to that question was generally - you really should
have 100 or more points. I doubt we would see much 100,000 second data if
people got rigorous on that....
By no means am I saying that ADEV shows all issues on all devices. We are in
agreement there. My point is that people should know what you did to your
data before you put it up on display. I have indeed seen people use a lot
higher order fits than a simple linear estimator.
Bob
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Magnus Danielson
Sent: Thursday, February 09, 2012 9:12 PM
To: time-nuts@febo.com
Subject: Re: [time-nuts] Low-Cost Rubidium Performance
On 02/10/2012 02:31 AM, Bob Camp wrote:
Hi
To add another wrinkle to this.
Correcting ADEV for systematic errors and then not mentioning you did so
strikes is something I find a bit problematic. If say you decide to take out
13th order drift, you should say you did so. The discussion of what to
correct and how is older than ADEV. Since there is no "standard" set of
corrections, one should be clear about what was done.
Agreed, but even if we know that system bandwidth affects low-tau
noise-forms, when did you last see the bandwidth given? Confidence
bounds is another issue.
But my main point is that you still look at the same damn plot rather
than looking at some other plot for the other effects. The point is to
separate them in order to clearer see each of them. For short-term,
noise dominates, and depending on where we are on the spectra the ADEV
or phase-noise plots serves us best. TIE curves is interesting, but MTIE
curves adds a certain aspect, drift analysis curves another aspect etc.
For jitter, separation of random noise and deterministic noise is am
important to properly engineer the system. MTIE is another of those.
I love my ADEVs, but as my suppliers know, it's not the only thing I ask
for in an oscillator.
How does my phase shift as the temperature cycles? ADEV won't tell me,
but I bet the guy in the next cubicle can enlighten you if needed.
This is all about understanding the limitations of the tools we have, so
we use them for what they do best. ADEV is a great tool, just don't over
use it. Whenever someone use their favorite tool (say ADEV, FFT or
Object Oriented programming) for everything and everything needs to be
answered by it, then the big warnings-bells goes off in the back of my
head. As great as it ever is, know the limits of what you use...
Cheers,
Magnus
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
Hi
I think we have enough data on current batch of FE-5680A rubidiums to guess
that the ADEV plots on it's data sheet could only have been done in a
constant temperature / environmental chamber...
That said, I see no reason it will not meet the actual spec limits over the
temperature range.
Bob
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Chris Albertson
Sent: Thursday, February 09, 2012 9:19 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Low-Cost Rubidium Performance
Let me strengthen the argument below. Let's say you are building and
selling Rb units to the telcom industry and you've spec'd your product
for use in the range of -25C to +85C. Now you want to include an
ADEV plot in your sales literature. Should you place the unit in an
environmental controlled box while you measure it? I think you'd be
accused of fraud if you did not state that the plot is not
representative of the full temperature spec. I think in order to be
valid you always have to state the test conditions but you can use ANY
test condition, Place it outdoors, in a lab or what ever but just say
what you did and then your plot has meaning. I think the converse to
true, that without knowing the test conditions the plot is little
meaning.
Why? Unless the unit is spec'd for use only at a constant temperature,
temperature variations are something it needs to deal with, and should be
included in any measurement of how good it is. In what way is temperature
variability special, that it shouldn't be included in ADEV measurements,
but
all other contributors to variability should be? Certainly, there may be a
large non-random component to temperature (diurnal, annual, in many
environments), but there's also a significant part which is random -
should
you somehow correct for one, but not the other?
Shouldn't one expect the ADEV be better for a double oven OCXO than for a
bare crystal?
The only issue I see is it may make fair comparison difficult, unless
units
are compared under identical conditions.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
--
Chris Albertson
Redondo Beach, California
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.