Signal and Power Integrity Tutorial: Measuring Clock Jitter Sensitivity to Power Rail Noise, Pt. 2

Figure 1. 400 mVpp oscillation on the power trace
is due to 48 MHz clock noise.

In Part 1, we used a function generator to create a power source with a known perturbation. Seeing that the noise on the power rail and the clock period were synchronous when we observed both traces together using a WavePro HD oscilloscope, we knew that there was a clear relationship between the two to be further investigated. Now, we're ready to examine more closely how the clock jitter responds to voltage variations on the power rail.

Signal and Power Integrity Tutorial: Measuring Clock Jitter Sensitivity to Power Rail Noise, Part 1

Figure 1. Voltage variations on the power rail
shown in the same grid as the clock period track
(jitter track). These waveforms are the basis of
the clock jitter sensitivity measurement. The
inverse relationship between the jitter track and
the power trace shows that the clock
is sensitive to variations in rail voltage.

In a previous post, we described A Robust Method for Measuring Clock Jitter with Oscilloscopes as variation in a clock signal’s period. Clock jitter is characterized by the standard deviation (sdev) of the clock period measurement. The track function of the clock period sdev shows us the variations in jitter over time, synchronous with the waveform source. 

In this post and the next, we’ll show how to make use of the clock period track function to match jitter variations to possible sources of jitter, in particular to voltage variations on the clock power rail. The offset voltage of a function generator powers a clock signal source. By creating a known variation in the function generator output, we can match that to the resulting clock jitter to calculate the clock jitter sensitivity to rail voltage changes. A known clock jitter sensitivity value can help you predict how a design will respond to rail voltage changes.

Signal and Power Integrity Tutorial: A Robust Method for Measuring Clock Jitter with Oscilloscopes

Figure 1. Clock jitter measured as a variation
of clock signal absolute period.

Clock jitter is the variation of a clock signal’s frequency or period. Either measurement carries the same information, but the period measurement is a simple time interval measurement easily performed using a real-time oscilloscope. If we have a robust way of measuring clock jitter, we have the basis for measuring the clock signal’s sensitivity to other features in the environment that can affect the period. Voltage noise on the power rail is just one external force that can affect clock jitter, which we'll show you how to measure in a future post.

In this post, we’ll demonstrate a robust method for measuring clock jitter using an example from Dr. Eric Bogatin’s webinar, “The Impact of Power Rail Noise on Clock Jitter.”  

The clock in our examples is a 5-stage ring oscillator which generates a square wave signal between 10 and 66 MHz. The test instrument is a WavePro HD 12-bit, 4-Ch, 8 GHz, 20 GS/s, 5 Gpts oscilloscope with 60 fs sample clock jitter.

In the process, we make a series of oscilloscope sample clock tests and timebase adjustments as  consistency checks. While measuring jitter is less about absolute accuracy than about the relative precision of measuring the time interval from cycle to cycle, a fundamental part of that is ensuring the absolute accuracy of the oscilloscope’s timebase.

The Periodic Table of Oscilloscope Tools: Analyze (Part II)

Figure 1: The Analysis
tools in an oscilloscope
lend it debug power

Oscilloscopes are central to many engineering tasks, but perhaps to none more so than debugging. Something is going on with your design but you don't know what it is. However, armed with an oscilloscope with the sort of sophisticated analysis tools found in Teledyne LeCroy's instruments, even Mr. Jones can get to the bottom of the problem. Let's continue our survey of the Periodic Table of Oscilloscope Tools with more on analysis tools.

The History of Jitter

Figure 1: The story of jitter spans 45-baud telegraph
machines to 160-Gbaud optical fiber

Jitter is a signal-integrity gremlin that's been with us for a long time. In fact, it's been with us since before anyone really needed to care about it. But as time has worn on, our perception of jitter has certainly changed, and with it our approaches to diagnosing it, measuring it, and ultimately dispatching it. Here, we'll begin a traversal of the "jitter story," surveying where we've been, where we are, and where we may be going in our dealings with the phenomenon.

Back to Basics: Jitter

Figure 1: Jitter is short-term variation
of a signal with respect to its
ideal position in time

Anyone working in applications that involve digital data, clocks, and serial data in general will eventually bump up against issues concerning jitter. Jitter is a subject of keen interest to every strata of the electronics industry. Chip makers, board integrators, system integrators, you name it: Everybody wants, and needs, to come to terms with jitter. It impacts reliability, manufacturability, and cost at all levels. And, of course, it's of keen interest to purveyors of test instruments, including us here at Teledyne LeCroy. In this first post of a projected series on jitter, we'll look at some of the tools built into modern digital oscilloscopes for jitter measurement and analysis.