Rick,
On 12/14/2015 07:04 AM, Richard (Rick) Karlquist wrote:
On 12/13/2015 7:15 PM, Tom McDermott wrote:
It brings up a question: Is it possible to estimate the phase noise
of that
internal crystal from the ADEV measurements? There are a bunch of
papers that go the other way: from Phase Noise to Adev. Searching
brings up only one paper that goes from ADEV to Phase Noise but it's text
does not seem to be readily available. It apparently models the
oscillator
as a couple of well known error models.
-- Tom, N5EG]
In general, you cannot determine phase noise from ADEV,
even though you can determine ADEV from phase noise.
This is just a mathematical reality.
Mike Fischer (of HP) presented papers at the 1977 and
1978 that show conversions between PN and ADEV for
individual noise processes, where each process has a
specific slope of amplitude vs frequency. The only
time you can go from ADEV to PN is if you can isolate
a process.
1976 PTTI paper by Fischer:
http://tycho.usno.navy.mil/ptti/1976papers/Vol%2008_38.pdf
1977 PTTI paper by Chi (Fischer being adviser):
http://tycho.usno.navy.mil/ptti/1977papers/Vol%2009_34.pdf
1978 PTTI paper by Fischer:
http://tycho.usno.navy.mil/ptti/1978papers/Vol%2010_14.pdf
Things have happen since those papers, in many ways.
In the specific case of crystal oscillators, in general,
they follow a flicker noise of frequency process model
close to the carrier (within 100 Hz). You can often
assume that ADEV is dominated by this process and therefore
translate it to an equivalent PN. The way to tell if
ADEV is dominated by this process is that it will be
independent of tau, for tau of 0.1 sec or less.
On the IEEE UFFC site, there is a tutorial on crystal oscillator
design by Mike Driscoll (you don't have to be a member to
access it). I believe this covers this topic. You go
from ADEV to "frequency noise" and then to phase noise
http://www.ieee-uffc.org/frequency-control/learning/2003_IEEE_Tutorial.PDF
Other tutorials is at:
http://www.ieee-uffc.org/frequency-control/tutorials.asp
In particular, check out Francois Vernottes presentation:
http://www.ieee-uffc.org/frequency-control/learning/pdf/Vernotte-Varience_Measurements.pdf
He covers a lot of ground there, and in the end he also covers
curve-fitting to estimate noise levels.
You can "practice" this calculation on any crystal oscillator
that has published ADEV and phase noise. It is of course
extremely easy to screw it up :-) What I have found
is that most crystal oscillators seem to obey the flicker
model.
I have been able to measure flicker noise on crystals that
were not installed in an oscillator, and then install them
in an oscillator and the ADEV turned out to be what
was predictable from the phase noise. It really works!
Indeed. However, one has to be careful with ones methods, or one will
not estimate the noise-levels properly. Regardless if you use the
frequency method or time method, doing meterologically sound and
traceable measurements remains a difficult task indeed.
Cheers,
Magnus
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