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

David Woolley david at ex.djwhome.demon.co.uk.invalid
Mon May 19 08:05:45 UTC 2008


skillzero at gmail.com wrote:

> I was just giving an example for illustrative purposes, but this
> hardware can be up to 50 Hz off in normal conditions.

You shouldn't use your phase locked loop to correct this sort of 
1,000ppm error.  You should calibrate it out before you start.

Note that, if you only let ntpd use its PLL algorithms, its response to 
this sort of error is very poor.  With ntpd you would really need to 
ensure it does a frequency calibration, in which it waits about 15 
minutes without correcting frequency and looks at the total drift, then 
instantaneously applies a total correction for that drift.

Phase lock loops are good at tracking slow variations in remote 
frequency/compensating for slow variations in local frequency, but they 
are not good at getting initial lock, unless you sample very fast.  (A 
pure hardware PLL would sample at 44.1kHz, in this case, but you 
presumably have network jitter and packetisation effects that compromise 
your ability to do this.

Even with fast true phase locking at the signal frequency, PLLs that 
have to change frequency generally have a fast acquisition mode, before 
they enter the tracking mode.




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