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.

 

PN_TO_JITTER.xls

 

 

 

Other links on phase noise to jitter

Rakon

Raltron

Analog Devices MT-008

Analog Devices AN-741

JitterTime Calculator