Situational Awareness: Testing Oscilloscope Outer Limits

Fig 1. 40 ps signal measured full bandwidth on a
1 GHz oscilloscope shows visible over/undershoot.

Nothing is perfect. Every test instrument has its limits, and knowing the limits to your oscilloscope’s bandwidth in response to real-world signals helps to develop situational awareness when making measurements. This is especially true when testing signals that are at or very near the specified bandwidth limit of the instrument.

The measurements we’ll demonstrate were made on a WaveSurfer 4104HD, a 12-bit, 4-channel, 1 GHz bandwidth oscilloscope that samples at up to 5 GS/s.

Transmission Lines (Part V): Reverse-Engineering the DUT

Figure 1: Every DUT can be thought of as a Thevenin
voltage source with some internal resistance

There are always two primary elements of any test and/or measurement application: the oscilloscope and the device under test (DUT). Getting valid measurement results depends, first and foremost, on the oscilloscope's capabilities given the task at hand. It also depends on what we'll call "situational awareness," or the operator's understanding of the oscilloscope and of the characteristics of the DUT.

Transmission Lines (Part II): More on Bandwidth vs. Rise Time

Figure 1: In the frequency domain (right), a near-ideal
square wave displays predictable 1/f amplitude dropoff

We began this series about transmission lines by thinking about some pertinent principles and relationships that can help form our thinking about the topic. In particular, we'd covered the relationship between bandwidth and rise time and why we have this rule of thumb that says that bandwidth can be estimated using 0.35/10-90% rise time.

Transmission Lines (Part I): Introduction

Figure 1: All oscilloscopes
have a Cal output like the
one pictured here

Somewhere on the front panel of almost any oscilloscope is a "Cal" reference signal output (Figure 1). That signal is really intended for adjusting the capacitance compensation screw to calibrate a 10X high-impedance probe, but most of us know it simply as the Cal signal. Have you ever noticed that the Cal signal's rise time seems to be highly dependent on the length of the cable attached to it, and maybe even wondered why?

Probing Techniques and Tradeoffs (Part IV)

Figure 1: Applying bandwidth filters to a 2.5-GHz clock
signal clearly shows the effect of bandwidth on rise time

The topic of probe bandwidth is a broad and deep one. We began our discussion of bandwidth in an earlier post with some basic information about what bandwidth means and the importance of the -3 dB point. Next, we looked at a Fourier deconstruction of a square wave into its fundamental and the lower-order harmonics, and covered the importance of bandwidth in capturing enough harmonic content to understand the signal's overall shape.

Probing Techniques and Tradeoffs (Part II)

Figure 1: A snapshot of available probes from
Teledyne LeCroy

Our first post in this series concentrated on connectivity and various means by which one might apply an oscilloscope probe to a circuit or device under test. Now, we'll look at an "ideal" probe vs. a real-world probe, and then begin a discussion of probe specifications.

How Does Sampling Rate Affect ESD Pulse Measurements?

Figure 1: Characterization of an ESD pulse's rise time
depends largely on the oscilloscope's sampling rate

In continuing our look at ESD/EMC pulse measurements, it would be useful to consider how sampling rate figures into the equation. What sort of sampling rate makes sense to use for capturing an ESD pulse? The answer to that question depends primarily on your pulse's rise time.

Why IEEE's Pulse Definitions and ESD Pulses Don't Mix

Figure 1: The IEEE's pulse definitions, which don't fit
the bill for measuring ESD pulses

The IEEE's pulse definitions, found in the organization's Std 181-2011 that covers transitions, pulses, and related waveforms, set the bar for how pulse measurements are determined. These definitions, which are in the DNA of all oscilloscopes, are just the thing for measuring repetitive pulses such as clock signals but not so much for ESD/EMC measurement requirements. In this post, we'll discuss why that is and what you should do differently for measuring ESD pulses.

Oscilloscope Basics: Choosing an Oscilloscope

Figure 1: An oscilloscope such as
Teledyne LeCroy's HDO6054-MS
serves a very broad range of
applications

Choosing an oscilloscope might seem to be a challenging task, but it doesn't have to be. Rather, it's a more-or-less logical process based on your measurement needs. Having said that, if the application for the instrument is "general lab work," the decision can become trickier.

Oscilloscope Basics: Using The Display Graticule

Figure 1: The display graticule, the grid of intersecting lines
overlaying the signal display area, is the original
oscilloscope measurement tool

Today's digital oscilloscopes come packed with an abundance of measurement capabilities, all available at the touch of a button or two. Want to know the amplitude of a square wave? Easy. Want to know the standard deviation of that amplitude? Minimum/maximum or mean? All easily compiled for you over hundreds or thousands of acquisitions.