[time-nuts] ADEV vs MDEV

Sat Feb 6 22:24:36 UTC 2010

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.griffiths at xtra.co.nz>
>> To: "Discussion of precise time and frequency measurement" 
>> <time-nuts at 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
>>>>
>>>>
>>>>