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.

How to Use Memory Properly

In a recent post, we addressed setting sample rate for serial data acquisition, but let’s look again at how time per division (time/div), memory length and sample rate all interact, and what you can do to optimize your use of oscilloscope capture memory when setting up your timebase.

Figure 1: Sample rate as a function of time/div for three different memory lengths.
Longer memory extends the range of time/div settings that support the highest sample rate.

Fast Fourier Transforms: Stickshift Edition

Figure 1: Shown at left is a 50-kHz input sine wave
with the FFT of the same signal at right

Perhaps you're old enough to remember when more cars had stickshifts. They're a little bit more work to drive than cars with automatic transmissions, but the experience can be much more rewarding. Oscilloscopes these days are like cars with both types of transmissions, and you can use either one for many tasks. One of those tasks is fast Fourier transforms (FFTs), and in this post we'll take you through driving an oscilloscope to perform an FFT with a stickshift.

Getting The Most Out Of Your Oscilloscope: Trigger Delay

Figure 1: Pre-triggering, or trigger delay, is a useful tool for
debugging applications

Triggering is one of the most basic, yet most useful, tools your oscilloscope offers you. Say you want to see what led up to, and/or what follows, a trigger condition. You're looking at an interesting waveform such as that shown in Figure 1. You have the trigger's delay position set at 10% and 90%.

Oscilloscope Basics: Controlling an Oscilloscope (Part II)

Figure 1: An example of a
touch screen-equipped
oscilloscope.

In a recent post, we discussed how to control a modern digital oscilloscope using the front-panel controls. That was a natural place to begin, given that it's the "traditional" means of controlling the instrument and the one that most seasoned users cut their teeth on. But there's more than one way to skin this cat these days. Many of today's oscilloscopes carry touch screens that do everything the front-panel controls can do, plus some things they cannot do.