[ntp:questions] Loop Filter Gains vs. Polling Interval

David L. Mills mills at udel.edu
Tue May 17 02:02:51 UTC 2011


Edward,

The loop time constant varies directly with the poll interval. See "How 
NTP Works" in the current documentation. Note that the default value and 
range are purposely optimized for public time servers in order to manage 
network overhead and are not appropriate for the most accurate LAN 
servers. For that the poll interval and thus loop time constant should 
be clamped below this, but not below 4 (16 s) for typical 100-Mb/s networks.

Dave

Mischanko, Edward T wrote:

>Dave, 
>
>How do I adjust the loop time constant so that it is shorter??
>
>  
>
>>>The integral component should be retained.  The effect of having too 
>>>long a loop time constant should be that the system fails to track
>>>      
>>>
>clock 
>  
>
>>>wander, so would be more vulnerable to temperature changes or
>>>      
>>>
>forgetting 
>  
>
>>>to disable power management of clock frequency.
>>>      
>>>
>
>-----Original Message-----
>On Behalf Of David Woolley
>Sent: Monday, May 16, 2011 2:01 AM
>To: questions at lists.ntp.org
>Subject: Re: [ntp:questions] Loop Filter Gains vs. Polling Interval
>
>Mischanko, Edward T wrote:
>  
>
>>Can anyone tell me, does the sensitivity for frequency adjustment
>>    
>>
>lessen
>  
>
>>as the polling interval increases?  I ask because I'm observing that
>>    
>>
>my
>
>They are linked, but I have a feeling that the loop time constant is not
>
>clamped, even though the poll interval is.  The aim is to always achieve
>
>a certain level of oversampling.  At the very least, the intent is to 
>vary the time constant and then choose a poll interval that is 
>appropriate for that.
>
>  
>
>>offset increases and the frequency adjustment decreases to the point I
>>fall out of sync at polling intervals above 256.  What am I doing
>>    
>>
>wrong?
>
>The integral component should be retained.  The effect of having too 
>long a loop time constant should be that the system fails to track clock
>
>wander, so would be more vulnerable to temperature changes or forgetting
>
>to disable power management of clock frequency.
>
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>  
>




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