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

Unruh unruh-spam at physics.ubc.ca
Tue May 20 16:29:53 UTC 2008


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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