And still another way of looking at the problem is:
One wants to know the Average freq over each of the 1 sec Tau sample
intervals,
and do it in such a way that there is no dead spots and no overlapping
information. (within the stated bandwidth)
Can be done with Phase (which tends to have limited resolution at low tau)
BUT Easy enough to do with a properly set up "Tight Phase-Lock Loop".
Now send that raw data to "PLOTTER" and tell it that it is Freq data and let
it do its thing.
NO phase need be involved at the raw data, and I don't think 'Plotter' is
first turning Freq Data into phase to get ADEV Freq data.
but that is a Ulrich question.
"However its easier just to use the sampled phases in the alternative
formula."
Are we off topic again?
For me, It is just as easy to have "Plotter" use either formula, It does not
complain about which is harder.
BTW, using a ADEV program I wrote in excel. When using Freq data, it was a
little easer than using Phase data.
Both give the same answer, If there is enough accurate Phase resolution, i.e
0.1 pS. for low taus faster than 1 sec.
"One cannot use EFC samples spaced at intervals of Tau directly ..."
Right, I hope all know that. Needs to be integrated (averaged) Freq over the
sampled Tau period,
NOT instantaneous freq at some random points along the way.
The NIST VtoF converter did that integration and so will a simple RC filter
with oversampling and a PC.
Still don't see where it needs to reconstruct the Phase evolution.
On the contrary the phase is used to reconstruct the Average Freq.
AND one of the BIG problems is that is very hard to do accurately if a Phase
TI is being used at Taus below 100 ms. (10Hz)
Just so things do not get too far off the original topic, here is a
reminder:
"I would appreciate any comments or observations on the SIMPLEST scheme
for making stability measurements at 1e-13 in one sec."
ws
**************
----- Original Message -----
From: "Bruce Griffiths" <bruce.griffi...@xtra.co.nz>
To: "Discussion of precise time and frequency measurement"
<time-nuts@febo.com>
Sent: Saturday, February 06, 2010 2:24 PM
Subject: Re: [time-nuts] ADEV vs MDEV
Another way of looking at the problem is:
One has to reconstruct the phase evolution with time by integrating the
instantaneous frequency.
Then if the resultant phase evolution is sampled every Tau seconds and the
first differences taken and divided by Tau the result is a sequence of
average frequency samples required by the AVAR formula.
However its easier just to use the sampled phases in the alternative
formula.
With the tight PLL method one has a sequence of frequency samples averaged
over an interval on the order of the inverse PLL loop bandwidth.
One then has to use these samples to reconstruct the phase evolution over
time.
One cannot use EFC samples spaced at intervals of Tau directly in the ADEV
formula which requires a sequence of frequency averages over an interval
of Tau.
If one ignores this requirement the resultant stability measure is not
ADEV.
Bruce
Bruce Griffiths wrote:
The tight PLL method doesn't directly produce the average frequency over
Tau.
As explained in (see snapshot of relevant section):
NIST special Publication 1065 Handbook of Frequency Stability Analysis
<http://tf.nist.gov/timefreq/general/pdf/2220.pdf>
the average frequency deviations for averaging time Tau are needed for
the calculation.
You need to sample at a sufficiently high rate to avoid aliasing and
average (ie integrate) the individual EFC samples.
If one uses phase measures then the fluctuations in the frequency
averages can easily and directly calculated from the difference between
the phase measured at time intervals separated by Tau.
Bruce
WarrenS wrote:
Bruce said:
Thus NIST and others quietly dropped this method several decades ago.
Could it be another reason?
I'll bet that was after they wanted to do better than 1e14 resolution
AND had unlimited amounts of time and Money,
Something most time Nuts are not blessed with. I Never said it was the
BEST way.
JUST given the goal, which was 1e13 in one second, there is not a
simpler and cheaper way to do it.
And nothing you said counter that point.
The frequency measures need to be integrated (either implicitly or
explicitly) to produce phase measures which can then be used to
calculate ADEV, MDEV etc.
Well ONE of us certainly has something backward.
To calculate ADEV, MDEV etc. YOU need Freq Differences.
The first thing that happens when phase is used is that it is turned
into Freq by taking the difference between each sample.
Integrated Freq data, which is what "Tight Phase-Lock Loop Method" gives
you directly (no Phase conversion needed),
Need not FIRST turned into Phase so that it can then be turned back into
Freq.
BUT in any case there is no difference in the noise, for a given
bandwidth, If you don't run out of digits and You have enough
resolution.
The "Tight Phase-Lock Loop Method" can EASY get sub pS resolution, which
is better than most other ways.
AND don't need filters and slue rate control and multistage limiters and
on & on to do it, an RC works fine to replace all the stuff.
ws
*****************
----- Original Message ----- From: "Bruce Griffiths"
<bruce.griffi...@xtra.co.nz>
To: "Discussion of precise time and frequency measurement"
<time-nuts@febo.com>
Sent: Saturday, February 06, 2010 12:11 PM
Subject: Re: [time-nuts] ADEV vs MDEV
Sounds good but you still haven't found its Achilles heel:
The frequency measures need to be integrated (either implicitly or
explicitly) to produce phase measures which can then be used to
calculate ADEV, MDEV etc.
The major problem is that integration amplifies the small errors that
are inevitably present.
In practice (except for very noisy sources) the technique isnt
particularly useful for Tau more than a few times the inverse PLL
bandwidth.
Thus NIST and others quietly dropped this method several decades ago.
This is alluded to in Steins recent paper availble on the Symmetricom
website:
*The Allan Variance – Challenges and Opportunities*
Bruce
WarrenS wrote:
Peat said:
I would appreciate any comments or observations on the topic of
apparatus with demonstrated stability measurements.
My motivation is to discover the SIMPLEST scheme for making stability
measurements at the 1E-13 in 1s performance level.
If you accept that the measurement is going to limited by the
Reference Osc,
for Low COST and SIMPLE, with the ability to measure ADEVs at that
level,
Can't beat a simple analog version of NIST's "Tight Phase-Lock Loop
Method of measuring Freq stability".
http://tf.nist.gov/phase/Properties/one.htm#oneone Fig 1.7
By replacing the "Voltage to freq converter, Freq counter& Printer
with a Radio shack type PC data logging DVM,
It can be up and running from scratch in under an Hr, with no high end
test equipment needed.
If you want performance that exceeds the best of most DMTD at low Tau
it takes a little more work
and a higher speed oversampling ADC data logger and a good offset
voltage.
I must add this is not a popular solution (Or a general Purpose one)
but
IF you know analog and have a GOOD osc with EFC to use for the
reference,
as far as I've been able to determine it is the BEST SIMPLE answer
that allows High performance.
Limited by My HP10811 Ref OSC, I'm getting better than 1e-12 in 0.1
sec (at 30 Hz Bandwidth)
Basic modified NIST Block Diag attached:
The NIST paper sums it up quite nicely:
'It is not difficult to achieve a sensitivity of a part in e14 per Hz
resolution
so one has excellent precision capabilities with this system.'
This does not address your other question of ADEV vs MDEV,
What I've described is just a simple way to get the Low cost, GOOD Raw
data.
What you then do with that Data is a different subject.
You can run the raw data thru one of the many ADEV programs out there,
'Plotter' being my choice.
Have fun
ws
*************
[time-nuts] ADEV vs MDEV
Pete Rawson peterawson at earthlink.net
Sat Feb 6 03:59:18 UTC 2010
Efforts are underway to develop a low cost DMTD apparatus with
demonstrated stability measurements of 1E-13 in 1s. It seems that
existing TI counters can reach this goal in 10s. (using MDEV estimate
or 100+s. using ADEV estimate). The question is; does the MDEV tool
provide an appropriate measure of stability in this time range, or is
the ADEV estimate a more correct answer?
The TI performance I'm referring to is the 20-25 ps, single shot TI,
typical for theHP5370A/B, the SR620 or the CNT81/91. I have data
from my CNT81showing MDEV< 1E-13 in 10s. and I believe the
other counters behave similarly.
I would appreciate any comments or observations on this topic.
My motivation is to discover the simplest scheme for making
stability measurements at this performance level; this is NOT
even close to the state-of-the-art, but can still be useful.
Pete Rawson
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