[time-nuts] Phase noise and jitter

hasweb at has.org.nz hasweb at has.org.nz
Mon Oct 13 19:38:54 UTC 2008

Javier Serrano wrote:
> Dear nuts,
> I would like to know if there is a clear explanation somewhere with
> considerations on how to choose an upper frequency limit when integrating
> phase noise to find jitter. Let's say I'm interested in the raw jitter
> measurement which comes from integrating phase noise without applying any
> filter to it. For a given application, I can easily understand that I can
> define a lower integration limit if the time spans I'm interested in are
> shorter than some value. For example, we run a synchrotron with a 1.2 second
> cycle time. Phase noise in our clocks below say 0.1 Hz should be of no
> concern since it is "common mode" to all the triggers we define within any
> given cycle using counts of the clock we are characterizing (incidentally I
> am also interested in your comments on how a phase noise measurement would
> fare against Allan deviation in this frequency area). I have a bit more
> trouble with the upper frequency limit. Am I right in saying that the right
> answer in principle is to integrate to infinity but due to Physics the phase
> noise will at some offset fmax be so low that the contribution of
> integrating from fmax to infinity would be negligible? How can I then work
> out experimentally which is the value of this fmax? Maybe extrapolating the
> slope of the curve I measure using for example a low bandwidth PLL
> technique? Thanks for any insight.
> Cheers,
> Javier

You could do that but the resultant integrated phase noise will be  
quite large and you should take into account the finite bandwidth of  
the signal processing/distribution system.

In practice all amplifiers (buffers, isolation etc), comparators etc  
have a finite bandwidth which rolls of the response to input phase  
noise as well as limiting the devices own contributions to high offset  
phase noise. A typical high quality 10MHz ultra low phase noise  
distribution amplifier may only have a 3db bandpass  of to 1 - 20MHz  
and a corresponding 100MHz distribution amplifier may have a bandpass  
of perhaps 80-120MHz.

Typical high end instruments for measuring Allan deviation may only  
have a bandwidth of a few Hz.

Even logic devices have a finite gain bandwidth in the transition region.

The Allen deviation can be calculated from the phase noise if the  
phase noise is known for all frequencies of interest.


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