[ntp:questions] NTP phase lock loop inputs and outputs?

Unruh unruh-spam at physics.ubc.ca
Sun May 25 22:48:43 UTC 2008


"David L. Mills" <mills at udel.edu> writes:

>Bill,

>Ahem. The first point I made was that least-squares doesn't help the 
>frequency estimate. The next point you made is that least-squares 
>improves the phase estimate. The last point you made is that phase noise 

No. The point I tried to make was the least squares improved the FREQUENCY 
estimate by sqrt(n/6) for large n, where n is the number of points (assumed
equally spaced) at which the phase is measured. I am sorry that the way I
phrased it could have been misunderstood.


The phase is ALSO improved proportional to sqrt(n)
. 
This assumes uncorrelated phase errors dominate the error budget. 



>is not important. Our points have been made and further discussion would 
>be boring.

Except you misunderstood my point. It may still be boring to you. 


>Dave

>Unruh wrote:
>> "David L. Mills" <mills at udel.edu> writes:
>> 
>> 
>>>Bill,
>> 
>> 
>>>If you need only the frequency, least-squares doesn't help a lot; all 
>>>you need are the first and last points during the measurement interval. 
>> 
>> 
>> Well, no. If you have random phase noise, a least squares fit will improve
>> the above estimate by roughly sqrt(n/4) where n is the number of points.
>> That can be significant. It is certainly true that the end points have the
>> most weight ( which is why the factor of 1/4). Ie, if you have 64 points,
>> you are better by about a factor of 4 which is not insignificant. 
>> 
>> 
>>>The NIST LOCKCLOCK and nptd FLL disciplines compute the frequency 
>>>directly and exponentially average successive intervals. The NTP 
>>>discipline is in fact a hybrid PLL/FLL where the PLL dominates below the 
>>>Allan intercept and FLL above it and also when started without a 
>>>frequency file. The trick is to separate the phase component from the 
>>>frequency component, which requires some delicate computations. This 
>>>allows the frequency to be accurately computed as above, yet allows a 
>>>phase correction during the measurement interval.
>> 
>> 
>> He of course is not interested in phase corrections. 
>> 
>> 
>> 
>> 
>>>Dave
>> 
>> 
>>>Unruh wrote:
>>>
>>>>David Woolley <david at ex.djwhome.demon.co.uk.invalid> writes:
>>>>
>>>>
>>>>
>>>>>Unruh wrote:
>>>>
>>>>
>>>>>>I do not understand this. You seem to be measuring the offsets, not the
>>>>>>frequencies. The offset is irrelevant. What you want to do is to measure
>>>>
>>>>
>>>>>Measuring phase error to control frequency is pretty much THE standard 
>>>>>way of doing it in modern electronics.  It's called a phase locked loop 
>>>>
>>>>
>>>>Sure. In the case of ntp you want to have zero phase error. ntp reduces the
>>>>phase error slowly by changing the frequency. This has the advantage that
>>>>the frequency error also gets reduced (slowly). He wants to reduce the
>>>>frequency error only. He does not give a damn about the phase error
>>>>apparently. Thus you do NOT want to reduce the frequecy error by attacking
>>>>the phase error. That is a slow way of doing it. You want to estimate the
>>>>frequency error directly. Now in his case he is doing so by measuring the
>>>>phase, so you need at least two phase measurements to estimate the
>>>>frequency error. But you do NOT want to reduce the frequency error by
>>>>reducing the phase error-- far too slow. 
>>>>
>>>>One way of reducing the frequency error is to use the ntp procedure but
>>>>applied to the frequency. But you must feed in an estimate of the frequecy
>>>>error. Anothr way is the chrony technique. -- collect phase points, do a
>>>>least squares fit to find the frequency, and then use that information to
>>>>drive the frequecy to zero. To reuse past data, also correct the prior
>>>>phase measurements by the change in frequency.
>>>> (t_{i-j}-=(t_{i}-t_{i-j}) df
>>>>
>>>>
>>>>
>>>>>(PLL) and it is getting difficult to find any piece of electrnics that 
>>>>>doesn't include one these days.  E.g. the typical digitally tuned radio 
>>>>
>>>>
>>>>A PLL is a dirt simply thing to impliment electronically. A few resistors
>>>>and capacitors. It however is a very simply Markovian process. There is far
>>>>more information in the data than that, and digititally it is easy to
>>>>impliment far more complex feedback loops than that.
>>>>
>>>>
>>>>
>>>>
>>>>>or TV has a crystal oscillator, which is divided down to the channel 
>>>>>spacing or a sub-multiple, and a configurable divider on the local 
>>>>>oscillator divides that down to the same frequency.  The resulting two 
>>>>>signals are then phase locked, by measuring the phase error on each 
>>>>>cycle, low pass filtering it, and using it to control the local 
>>>>>oscillator frequency, resulting in their matching in frequency, and 
>>>>>having some constant phase error.
>>>>
>>>>
>>>>>>the offset twice, and ask if the difference is constant or not. Ie, th
>>>>>>eoffset does not correspond to being off by 5Hz. 
>>>>
>>>>
>>>>>ntpd only uses this method on a cold start, to get the initial coarse 
>>>>>calibration.  Typical electronic implementations don't use it at all, 
>>>>>but either do a frequency sweep or simply open up the low pass filter, 
>>>>>to get initial lock.
>>>>
>>>>
>>>>And? You are claiming that that is efficient or easy? I would claim the
>>>>latter. And his requirements are NOT ntp's requirements. He does not care
>>>>about the phase errors. He is onlyconcerned about the frequency errors.
>>>>driving the frequency errors to zero by driving the phase errors to zero is
>>>>not a very efficient technique-- unless of course you want the phase errors
>>>>to be zero( as ntp does, and he does not). 
>>>>
>>>>
>>>>
>>>>




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