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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