[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.
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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