Bruce
Before we go around again and discuses what my simple tester can and can not
do and why,
It would be helpful if you would take the time to better understand how it
works and why it works the way I have done it.
You really should try one yourself if you can't see why it works.
You are going to be surprised and embarrassed at how good it works.
Why you're at it, try the "swing test" with anything you have. Let me know
how that goes.
I'm not saying that may tester will match someone's Latest ever changing NEW
idea of what the "correct AVAR" should be,
After all it just Logs correct, integrated, Freq difference data of ANY
noise type
and does it without adding any dead time or aliasing all by uisng pretty
much using ANY ADC capability of over sampling at the tau Zero rate.
If one then uses the data log with something like the classic Stable 32 S/W
or Ulrich's Plotter,
it gives is the exact same results as other methods costing much much more,
over the whole tau range.
This is limited only be its reference oscillator (Same way that all others
are limited of course, Doen't get much better than that).
If that is not good enough for you, them you need to discuss the results
with Symmetricon and others that give the same answer as mine, not me.
If for some reason you want to set one up wrong so that it matches the
results of some other special instrument, I'd be glad to tell you how to
have it add back in the dead time or aliasing artifact problems or whatever
else you would like it to do wrong, that it presently does correctly.
ws
******************
Bruce wrote
As long as one is aware that your method (as implemented by you) doesn't
actually measure Allan variance, it may be useful for comparing the
relative stability some sources for small Tau (unfortunately the range
of Tau for which the method may produce useful results depends on the
phase noise characteristics of the sources being compared).
To measure AVAR the technique has to have the same response to all phase
noise spectral components as does AVAR.
Since you do not integrate/average the frequency measures the phase
noise response of the method is not identical to that used in
calculating AVAR.
This technique probably works best when white phase noise dominates the
phase noise spectral region of interest (usually for small Tau).
For those who can follow the theory, the following paper shows how the
above method is affected by aliasing etc:
http://hal.archives-ouvertes.fr/docs/00/37/63/05/PDF/alaa_p1_v4a.pdf
The paper also shows how the required integration (needed to actually
measure AVAR) can be approximated from the discrete sample sequence.
Alternatively one could avoid the numerical integration by replacing the
ADC with a zero deadtime (ie not a dual slope converter. A multislope
algorithm like that used in the 34401A (but not the 3458A) should work
as the signal is integrated continuously) integrating ADC. One
possibility is to use a VFC as NIST did when they used this technique
some decades ago.
Of course, the classical DMTD setup undersamples the phase noise
spectrum and thus may suffer from aliasing artifacts.
Such aliasing artifacts have no significant effect when the phase noise
spectrum is flat.
Bruce
*********************
WarrenS wrote:
For the Really cheap time nuts,
It sounds like Bert Kehren has done a great Job building a Dual Mixer
tester.
There are other simpler, less standard ways to get good data for Allan
Variance and small frequency differences.
My VERY simple $10.00 analog tight PLL Tester BB (Previously posted)
pretty much accomplishes the same goals as his,
and it can do 1e-13 in a second, and 1e-11 in 10ms (limited of course
by the single reference Oscillator used)
A simple test that most can do at home, and still challenges the best
high end testers out there is Tom's the swinging Oscillator test.
http://www.leapsecond.com/pages/10811-g/
(The results from my PLL tester is attached)
ws
******************
----- Original Message ----- From: <EWKehren at aol.com>
To: <time-nuts at febo.com>
Sent: Tuesday, May 11, 2010 7:02 AM
Subject: [time-nuts] Dual Mixer
The Dual Mixer project is nearing completion.
Let me refresh every ones memory as to my goals.
a) Total cost less than $ 200
b) 1 E-13 with a one second offset
c) use parts attainable by every one
d) easy to assemble only a few surface mount parts
e) a five channel counter that yields 1 E 15 resolution and interfaces
directly to a PC via RS232 or USB
f) A counter that also gives you instant frequency difference at the
sample rate, not only Allan Variance
g) Modular so one can use only the Dual Mixer
h) Modular so one can use multiple units to do simultaneous
comparison of
more than two oscillators.
i) Isolation between D/M and counter so that the counter can be
powered
by the PC USB port
I am happy to report that all goals have been accomplished, attached
is a
picture of the D/M, limitation of the file size does not allow me to
attach
an actual board picture, but if you contact me direct I will send
you one,
the final board is actually nicer since the first layout had to
accommodate
several variances.
The D/M part leans heavy on the original NIST unit with a few
substitutions
and recommendations from Bob Camp. Also beside Opto Couplers SN65LVDS1's
have been included for those that want to use other counting methods.
Selection of filter capacitors allow the use at other offset
frequencies such as
10 and 100 Hz. The D/M fits in a standard 74 X 111 X 20 mm Euro case
and
the counter can be stacked below or next to it using the Opto
Isolators as
the inter connect. The SYPD-1's fit right on the board but
connections are
included to use the HP 10514 A. As a matter of fact removing the HP
mixer
board from its housing fits it nicely on the board and every thing
is still
inside the housing.
The counter will handle 1 an 10 Hz offset with a 1 E 14 resolution at 10
Hz. Thanks to Richard Mc Corkle we have great drawings and code,
available to
every one.
Code, drawings, list of material and PC board layouts and its file,
will
be available to every one once the project is completed.
I need help in the following areas
a) help me create a nice set of drawings that are computer generated
something I am not able to do
b) create the computer program that takes the output of the counter
board
and allows Allan Variance plots, frequency difference and dual
temperature
readings and plots using RS232 and USB.
c) an independent test by a third party.
As I said previously, I am not getting in the business of supplying
parts
but will work with people that will help achieve the three points listed
above. Presently I have boards on order and will have two
uncommitted board
sets and probably also component kits.
Please contact me directly.
Again thank you Corby Dawson, Richard Mc Corkle and Bob Camp.
Bert Kehren Miami
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