Phase
Noise and Jitter
Phase noise and jitter are
equivalent ways of describing the stability of a signal source. Phase noise is a measurement in the
frequency domain while jitter is measured in the time domain.
Both measurements should give
equivalent and interchangeable results.
In practice they seldom do due to the way the measurements are conducted
and the instruments used.
Both measurements have
boundary conditions. For example in the
frequency domain the sideband phase noise is usually specified over some offset
frequency range such as 100Hz to 10MHz.
In the time domain the period over which the measurement is made is specified
such and an equivalent boundary condition would be from 100ms to 100ns.
Both of these measurements
are limited by the equipment available to make the measurement. The spectrum analyzer is used in the
frequency domain for making sideband noise measurements while a fast digital
oscilloscope is generally used in the time domain to make direct reading jitter
measurements. The spectrum analyzer measurement will be RMS dBc and will be
limited by the dynamic range and phase noise of the spectrum analyzer. The oscilloscope will usually be setup to
make peak-to-peak jitter measurements and will be limited by the trigger
stability, timebase stability, and resolution.
Jitter measurement using an
oscilloscope are especially prone to a number of measurement problems resulting
in an inaccurate reading of the frequency source’s jitter.
Jitter should be made on a
single input signal and is the measured jitter from on cycle to the next over
some period of time. For synthesizers
such as ours, user sometimes measure the jitter by triggering on the 10MHz
reference and measuring the peak-peak accumulated jitter of the high frequency
output. This is definitely not a
measurement of the source’s jitter but more a measurement of the oscilloscope’s
timebase and trigger jitter multiplied by the RF output frequency divided by
the timebase frequency.
We recommend measuring the
phase noise with a spectrum analyzer and then calculating the phase jitter
using any of the multiple on-line jitter calculators.
By the way, most of the
on-line phase noise to jitter calculators don’t agree with each other. The various reference notes that describe
how to calculate jitter from phase noise are misleading and obscure as
well.
We’ve developed our own MS
Excel based jitter calculator and believe it be accurate. None of the calculations are hidden so you
are invited to analyze our math and make recommendations. We invite your feedback.
Other links on phase noise to
jitter