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You need to test, we're here to help.

19 June 2018

Rise-Time Degradation and ISI Jitter

Shown are the signals from two extreme bit patterns overlaid on top of each other with no interconnect in the channel
Figure 1: Shown are the signals from two extreme bit patterns
overlaid on top of each other with no interconnect in the channel
In discussing inter-symbol interference (ISI), the phenomenon in which information "leaks" from one bit to subsequent bits, we've identified a couple of root causes of ISI jitter. The first is reflection losses caused by impedance discontinuities, while the second is group delay dispersion, a consequence of the differing propagation speeds of different frequencies through a given material. We looked at these forms of distortion in both the time and frequency domains.

13 June 2018

Inter-Symbol Interference (or Leaky Bits)

Inter-symbol interference, or ISI jitter, is the result of information from one bit "leaking" to subsequent bits
Figure 1: Inter-symbol interference, or ISI jitter, is the result
of information from one bit "leaking" to subsequent bits
In reviewing the subject of debugging high-speed serial links, one important aspect of signal integrity we must touch on is inter-symbol interference (ISI). ISI is the phenomenon in which information from one bit "leaks" to some subsequent number of bits.

12 June 2018

How Much Transmission-Line Loss is Too Much?

This plot represents the differential insertion-loss profile for a 20" FR-4 microstrip trace
Figure 1: This plot represents the differential insertion-loss
profile for a 20" FR-4 microstrip trace
One of the fundamental facts of transmission lines is losses. Any effort to debug the performance of a high-speed serial data link begins there. But it begs an equally fundamental question: How much loss in a transmission line is too much? How do we quantify losses, and what is the connection between attenuation at the Nyquist frequency and the eye diagram? Is there a rule of thumb one might apply, some sort of rough estimate of how much loss might be too much for your channel to bear at a given data rate?

06 June 2018

A Look at Transmission-Line Losses

Using a 3D field solver to simulate a differential trace
Figure 1: Using a 3D
field solver to simulate
a differential trace
In surveying the subject of debugging high-speed serial data links, we've noted that there's no one cause for signal-integrity issues between transmitter and receiver, and there's certainly no one solution. But let's begin with the low-hanging fruit: electrical losses in the transmission line. We've previously done a series of posts on transmission lines (beginning here), but it's worth it to have a quick refresher.

05 June 2018

Introduction to Debugging High-Speed Serial Links

These images depict the degradation of serial data traffic as it makes its way from transmitter to receiver
Figure 1: These images depict the degradation of serial data
traffic as it makes its way from transmitter to receiver
In recent years, the data rates in serial links have increased exponentially across any number of standard protocols, including PCI Express, USB, and even SATA and SAS. With higher data rates comes more challenges for system designers, validation engineers, and test engineers with respect to signal integrity (SI). Some SI effects are much more prominent at higher data rates than they were for lower-speed versions of the same protocols. In this series of posts, we'll examine these SI effects, look at some methods of improving system performance, and discuss some SI analysis solutions as well as measurement considerations.

18 May 2018

Examples of IoT DDR Debug Scenarios

Using the oscilloscope's Track math function can help pin down timing anomalies
Figure 1: Using the oscilloscope's Track
math function can help pin down
timing anomalies
Our last post considered some broad aspects of debugging DDR memory on Internet of Things (IoT) devices, such as how chip interposers can help with probing access and the benefits of virtual probing software. Let's now take a look at some particular examples of problems with these memory chips and their controllers and see how debugging with an oscilloscope might be approached.

15 May 2018

Debugging DDR Memory on IoT Devices

Embedded systems such as IoT devices often require chip interposers to gain access to signal lines on DDR memory
Figure 1: Embedded systems such as IoT devices often require
chip interposers to gain access to signal lines on DDR memory
Internet of Things (IoT) devices are, at heart, just another embedded computing system, albeit one with an extremely well-defined function. As such, there's bound to be some amount of on-board data storage, and the storage medium of choice these days is typically double data-rate (DDR) memory. DDR memory transfers serial data on both the rising and falling edges of the clock signal, which is the characteristic from which it derives its name.

10 May 2018

Debugging Ethernet, SATA, and PCIe for IoT Devices

A generic IoT block diagram shows serial-data links in blue
Figure 1: A generic IoT block diagram
shows serial-data links in blue
In our ongoing review of debugging serial-data standards for Internet of Things (IoT) devices, let's now turn to three more popular protocols: Ethernet, SATA, and PCIe. Ethernet is found in computer networking applications, while the Serial Advanced Technology Attachment (SATA) connects host bus adapters to mass-storage devices. The Peripheral Component Interconnect Express (PCI Express or PCIe) handles communication between root complexes (motherboards) and expansion-card interfaces.

08 May 2018

Debugging CANbus For IoT Devices

Temperature data from thermocouples is shown both encoded and decoded on the oscilloscope's display
Figure 1: Temperature data from
thermocouples is shown both
encoded and decoded on the
oscilloscope's display
A myriad of serial-data standards come into play when we're discussing Internet of Things (IoT) devices. We've talked about I2C, SPI, and UART in a previous post. Yet another serial-bus standard that comes under the IoT umbrella is the Controller Area Network (or CANbus) standard. CANbus enables microcontrollers and peripheral devices to communicate with each other in applications without an intervening host computer. In the past, it's been typically used in automotive applications, but CANbus has found its way into a wider scope of applications of late.

04 May 2018

Debugging Low-Speed Serial Data on IoT Devices

Serial-data links handle traffic between ICs and peripheral devices in the IoT world
Figure 1: Serial-data links handle traffic between ICs
and peripheral devices in the IoT world
Our last post discussed the difficulties in acquiring the many sensor signals that may be input to a deeply embedded system such as an IoT device as well as a hardware solution to the problem. Another aspect of IoT debugging and validation is the low-speed serial data standards used to facilitate communication between ICs and between controllers and peripheral devices (Figure 1). To that end, let's take a look at three such low-speed standards: I2C, SPI, and UART.

02 May 2018

Acquiring and Characterizing IoT Sensor Signals

IoT devices use many sensors to collect data about their ambient environment
Figure 1: IoT devices use many sensors to collect data
about their ambient environment
If we recall our earlier post with its definition of what constitutes an Internet-of-Things (IoT) device, one of the main functions of such devices is to sense its environment and digitize the collected data. Often, an IoT device uses many sensors to collect information about its environment (Figure 1). Having the ability to capture and analyze signals from numerous sensors simultaneously is critical to ensure proper and optimal functionality of the IoT device's design.

30 April 2018

Investigating IoT Wireless Signals (Part II)

This screen capture depicts frequency demodulation and subsequent Manchester decoding of the bit stream
Figure 1: This screen capture depicts frequency demodulation
and subsequent Manchester decoding of the bit stream
Internet of Things (IoT) devices must communicate with their peers--other IoT devices--as well as with the host system that governs their activities. In our previous post, we examined how to perform amplitude and frequency demodulation of RF bursts, such as Bluetooth Low Energy (BLE) advertising bursts. We'll continue with other methods of analyzing RF signals.

23 April 2018

Investigating IoT Wireless Signals

Figure 1: Many IoT devices accept
wireless antennas using U.FL
connectors
Many IoT devices use wireless methods to communicate with other devices or with host systems. Just as with DC power-rail signals, probing of RF signals should be done with as little noise and as non-invasively as possible and with the best possible signal fidelity. Effective probing opens the door to Wi-Fi and Bluetooth signal analysis with RF demodulation, vector signal analysis, and spectrum analysis.

19 April 2018

IoT Digital Power Management and Power Integrity

The half-bridge output current from each DC-DC phase is known as the inductor current
Figure 1: The half-bridge output
current from each DC-DC phase
is known as the inductor current
An Internet of Things (IoT) device derives its power either from a 12-V DC supply or from a battery. In either case, power is fed to one or more power rails that operate at different voltages. These rails power the CPU and other functional blocks on the PC board. In this post, we'll take a look at how to examine an IoT's power supply for proper digital power management implementation and for power integrity.

16 April 2018

Anatomy of an IoT Device

IoTs include SOCs, DDR, DPM ICs, wireless, and MCUs
Figure 1: IoTs include SOCs, DDR,
DPM ICs, wireless, and MCUs
There's already more Internet of Things (IoT) devices deployed than there are humans on Earth. That gap will increase radically in coming years, and the explosion in IoT devices means a commensurate explosion in the need for debugging tools. So what's in an IoT device to debug, anyway?

04 April 2018

Debugging the IoT

Chances are you're already using the IoT in various ways
Figure 1: Chances are you're already
using the IoT in various ways
By now, we're all familiar with the phrase "Internet of Things" (IoT); some of you may be directly involved with that concept on some level as a designer/technologist. Here, we'll begin a series of posts on the IoT with some broad discussion of what it's all about, and then segue into how oscilloscopes and related hardware/software are among the best tools available for design and debug of IoT-related devices.

15 March 2018

An Example of Three-Phase Power Measurements

Screen capture of a 10-s acquisition of AC input and PWM output of a 480-V motor drive
Figure 1: Screen capture of a 10-s acquisition of AC input
and PWM output of a 480-V motor drive
To follow up on our last post on three-phase power calculations, and to wrap up this series of posts on the fundamentals of power, we'll walk through an example of a set of three-phase power measurements. We'll base our discussion on a single screen capture of measurements taken on a 480-V motor drive with 480-V AC input and 480-V maximum drive output. For this example, we used a Teledyne LeCroy Motor Drive Analyzer.

14 March 2018

Three-Phase Power Calculations

Three-phase power calculations entail summing of the individual phases's power calculations
Figure 1: Three-phase power calculations
entail summing of the individual
phases's power calculations
Until now, our discussions of power calculations have encompassed only single-phase systems with one voltage and one current. Now we'll turn to three-phase systems, which can be thought of as a collection of three single-phase systems. In general, three-phase power calculations are a simple summing of the individual phase power calculations and should be balanced across all three phases.

13 March 2018

Power Calculations for Distorted Waveforms

The sum of many sine waves, of varying amplitudes and frequencies, comprises the rough- looking square wave shown in red
Figure 1: The sum of many sine waves, of varying
amplitudes and frequencies, comprises the rough-
looking square wave shown in red
Our last post covered basic power calculations for pure sine waves, which are useful only up to a point in that pure sine waves are rather rare in the real world. Almost any real-world waveform carries some amount of distortion. Because distorted voltage and current waveforms comprise multiple frequencies, the relatively simple techniques used to measure power for pure, single-frequency sine waves no longer apply.

09 March 2018

Power Calculations for Pure Sine Waves

 For a purely resistive load, power = voltage * current, with both vectors in phase
Figure 1: For a purely resistive load,
power = voltage * current, with both vectors in phase
Wouldn't it be wonderful if every sine wave we encountered in the real world was pure, with no distortion? It sure would make life easier. Alas, it's pretty much never the case. But in reviewing sinusoidal power calculations, it's best that we begin with the simplest case: a single, pure sinusoidal line voltage and single, pure sinusoidal line current supplying a linear load.

Back to Basics: AC Sinusoidal Line Current

A single-phase AC current vector rotates at 50 or 60 Hz
Figure 1: A single-phase AC current
vector rotates at 50 or 60 Hz
We've reviewed the basics of AC line voltage in previous posts. Now we'll turn our attention to the other fundamental component of line power. Regardless of whether you call it "grid," "household," "line," "utility," or "mains," AC sinusoidal current is what flows from the power utility's lines into every home and business.

02 March 2018

More Basics of Three-Phase AC Sinusoidal Voltages

In the Wye three-phase connection, neutral is present but sometimes inaccessible
Figure 1: In the Wye three-phase
connection, neutral is present but
sometimes inaccessible
Our last post in this series on the essential principles of power covered the basics of three-phase voltages: their composition of three voltage vectors, how they're generated, how they're measured (line-line or line-neutral), and conversion of line-line values to line-neutral values. Here, we'll pick up the thread with more on three-phase AC voltages.

01 March 2018

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

Every DUT can be thought of as a Thevenin voltage source with some internal resistance
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.

27 February 2018

Transmission Lines (Part IV): More Essential Principles

The return current in a transmission line is as important as the signal current
Figure 1: The return current in a transmission line
is as important as the signal current
In our continuing survey of the topic of transmission lines, we'd begun in our last post to cover some essential principles that govern their behavior. We're not quite done with those, so in this post we'll discuss more principles that should be part of the foundation of how you think about interconnects.

26 February 2018

Transmission Lines (Part III): Essential Principles

All interconnects are transmission lines with a signal path and a return path (not ground)
Figure 1: All interconnects are transmission lines with a signal
path and a return path (not ground)
Now that we've covered some of the principles and assumptions that underlie transmission lines in two prior posts, we can now directly address the topic with some essential principles that you need to understand.

23 February 2018

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

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

20 February 2018

Transmission Lines (Part I): Introduction

All oscilloscopes have a Cal output like the one pictured here
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?

16 February 2018

Probing Techniques and Tradeoffs (Part XI): Non-Ideal Situations

VP@Rcvr builds a transmission-line model to virtually move less-than-ideal probing points
Figure 1: VP@Rcvr builds a transmission-line model
to virtually move less-than-ideal probing points
In using an oscilloscope to investigate transmission-line performance, we often encounter situations in which we don't have the luxury of probing at the ideal location. Fortunately, there are software tools that enable us to virtually move our probing point close to the receiver.

12 February 2018

Probing Techniques and Tradeoffs (Part X): More Best Practices

Chip clips; they're not just for snacks anymore
Figure 1: Chip clips;
they're not just for
snacks anymore
In probing circuits, as with most endeavors, there are some best practices you can use to enhance your chances of obtaining optimal measurements. We began exploring this concept in our last post, and we'll continue here with more best practices.

09 February 2018

Probing Techniques and Tradeoffs (Part IX): Best Practices

The typical manner of using a hands-free probe holder can cause issues
Figure 1: The typical manner
of using a hands-free probe
holder can cause issues
Having covered many of the theoretical aspects of probing signals, it's now useful to cover some best practices for high-speed active probing. We'll use some examples involving probing of DDR memory to illustrate what works best and what might not be a good idea from a practical standpoint.

08 February 2018

Probing Techniques and Tradeoffs (Part VIII): Gain/Attenuation vs. Noise

Noise comparison of a Teledyne LeCroy D1605 probe and a competing model
Figure 1: Noise comparison of a
Teledyne LeCroy D1605 probe and
a competing model
When discussing oscilloscope probes and dynamic range as we've been doing of late, we must also touch upon the associated topics of internal gain/attenuation and how that relates to noise.

06 February 2018

Probing Techniques and Tradeoffs (Part VII): More on Dynamic Range

Input offset range is how much differential offset a probe can apply to an input signal to bring it within its differential-mode output range
Figure 1: Input offset range is how much
differential offset a probe can apply to
an input signal to bring it within its
differential-mode output range
In our last post in this series, we'd begun discussing the third of three types of dynamic range as applied to probes, and that is input offset range. This is the maximum differential offset that a probe can apply to the input signal to bring it within the probe's differential-mode dynamic range.

05 February 2018

Getting The Most Out Of Your Oscilloscope: Physical-Layer Tools

Trigger dialog boxes will match the protocol of interest
Figure 1: Trigger dialog boxes will
match the protocol of interest
Debugging and validation of the physical layer of serial-data links is a preeminent oscilloscope application area these days. Today's real-time digital oscilloscopes have a wealth of tools to help you dig into any/all serial protocols and learn what's really going on electrically with your serial links.

02 February 2018

Getting The Most Out Of Your Oscilloscope: Math Functions

Parameter math functions provide a way to create custom parameters
Figure 1: Parameter math functions
provide a way to create custom
parameters
Parameter math functions are an important part of an oscilloscope's analysis capabilities. Using parameter math, you can create custom parameters based on simple arithmetic relationships between existing parameters. It allows you to add, subtract, multiply, divide, or rescale parameters (Figure 1).

01 February 2018

Getting The Most Out Of Your Oscilloscope: Sequence and History Modes

Sequence mode grabs rare triggered events from long captures and stores them in segments
Figure 1: Sequence mode grabs rare triggered events from
long captures and stores them in segments
When using an oscilloscope, there are bound to be instances in which you need to capture a large number of fast pulses in quick succession, or, conversely, a small number of events separated by long periods of time. Both are challenging for typical signal acquisition modes. But many Teledyne LeCroy oscilloscopes provide what's known as Sequence mode, which lets you capture these events while ignoring the long intervals between them.

Getting The Most Out Of Your Oscilloscope: WaveScan and XDEV Custom Parameters

Using WaveScan to search for rare glitch events
Figure 1: Using WaveScan to search
for rare glitch events
In earlier posts about how to maximize your oscilloscope's utility, we've discussed how to properly capture a waveform, making measurements, and extracting more meaningful information from those measurements that might be readily apparent. Now we'll look at how to correlate anomalous behavior from a waveform with other waveforms we may have captured.

31 January 2018

Getting The Most Out Of Your Oscilloscope: Tracks and Trends

The track math function shows how data changes over time
Figure 1: The track math function
shows how data changes over time
Our discussion of cursors and parameters leads us neatly into the topics of tracks and trends, which are both means of extending parameter measurement into a more analytical direction. We can do this using math functions that are part of your Teledyne LeCroy oscilloscope's toolset.

30 January 2018

Getting The Most Out Of Your Oscilloscope: Cursors and Parameters

Cursors (top) and parameter measurements (bottom) are both powerful tools in their own right
Figure 1: Cursors (top) and parameter
measurements (bottom) are both
powerful tools in their own right
Oscilloscopes give us a wealth of tools with which to view, measure, and analyze the performance of a circuit. Broadly speaking, two classes of such tools are cursors and parameter measurements. Taken on their own, both classes afford the user a great deal of capability. Put them together, though, and you can really start to gain deep insights into what's going on with a waveform.

29 January 2018

Getting The Most Out Of Your Oscilloscope: Documentation

A LabNotebook entry quickly and easily saves everything you need to later replicate your measurements
Figure l: A LabNotebook entry quickly and easily saves
everything you need to later replicate your measurements
If we really want to get the most out of our oscilloscopes, we'd be remiss in overlooking the documentation capabilities that modern instruments provide. It's so important to document our working sessions so they're consistent and repeatable. Oscilloscope setups can be quite intricate for a given measurement or procedure. Why not let the instrument help us keep track of such things?

Getting The Most Out Of Your Oscilloscope: Trigger Delay

Pre-triggering, or trigger delay, is a useful tool for debugging applications
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%.

26 January 2018

Getting The Most Out Of Your Oscilloscope: Navigation With MAUI

Oscilloscope UIs such as Teledyne LeCroy's MAUI provide tons of shortcuts and touch gestures
Figure 1: Oscilloscope UIs such as
Teledyne LeCroy's MAUI provide
tons of shortcuts and touch gestures
Today's real-time digital oscilloscopes are easier to use than their predecessors, and new models get easier all the time. Oscilloscopes often are equipped with touch screens and elaborate user interfaces that provide a plethora of shortcuts and smartphone-like touch gestures that make common operations extremely fast and easy.

Getting The Most Out Of Your Oscilloscope: Setup

Choosing a effective sample rate is key to seeing the finer details of a waveform
Figure 1: Choosing a effective sample rate is key
to seeing the finer details of a waveform
Today's real-time digital oscilloscopes are so packed with bells and whistles (or "features," if you prefer) that you can forget how to use many of them. In fact, you might not even realize some exist! But they're all there for a reason, and they're all useful, maybe even more so than you know. To that end, we'll take a tour of a typical Teledyne LeCroy oscilloscope's features and give you some pointers as to how, and when, you can best take advantage of them.

24 January 2018

Making On-Die Power-Rail Measurements (Part III)

This screen capture shows the idle-state conditions
Figure 1: This screen capture shows
the idle-state conditions
Having reviewed the test setup for on-die power-rail testing and our expectations of what the tests should show us, let's walk through the measurements and look over the results. We'll also note whether our results align with our expectations or are outside of those expectations.

Making On-Die Power-Rail Measurements (Part II)

When switching from low to high, PDN noise flows from the Vdd rail through to the Vss rail
Figure 1: When switching from low to
high, PDN noise flows from the Vdd
rail through to the Vss rail
Now that we're ready to begin some on-die power-rail measurements, it's a good idea to step back for a moment and anticipate what these measurements should show us. What actually happens on the die when CMOS gates are switching on and off? What should we expect to see on the on-die power rails? And what happens at the clock edges?

Making On-Die Power-Rail Measurements (Part I)

Our test setup for the on-die measurement examples
Figure 1: Our test setup for the
on-die measurement examples
Our exploration of on-die power-rail measurements has brought us to the point of demonstrating some examples of these measurements. As noted in the prior post, we will use an Atmel 328 microcontroller demo board, prepared with firmware to control it explicitly for our purposes, and with coaxial cable in place on the bottom of the board to serve as transmission lines for our signals of interest.

Measuring Shared On-Die Power Rails


This schematic represents the signal paths  in typical, general-purpose I/Os
Figure 1: This schematic represents the signal paths
in typical, general-purpose I/Os
If you're taking power-rail measurements on a semiconductor die that is appropriately instrumented with sense lines and a direct connection to the die's core PDN, you're in pretty good shape. But what if that's not the case? If the die's I/O power rails are shared by the core power rails, here's a way to get your core power-rail measurements anyway.

22 January 2018

Setting the Stage for On-Die Power-Rail Measurements

Measuring on-die Vdd rail noise requires a suitably instrumented die and package
Figure 1: Measuring on-die Vdd rail noise requires
a suitably instrumented die and package
Armed with a suitable oscilloscope and active voltage-rail probe, you're now ready to make some power-rail measurements on a semiconductor die. Of course, making measurements on a die is a little different than making measurements on a printed-circuit board. This is where careful design-for-test at the die level comes in, because the chip, its packaging, and the board on which it will be mounted must be instrumented so as to make the on-die measurements possible.

Power-Rail Noise: Small Signal, Big DC Offset

Your scope's vertical adjust has its limits
Figure 1: Your
scope's vertical
adjust has its
limits
We've been working through the various challenges in making power-rail noise measurements. One of those challenges is RF pickup that can often swamp the noise signal, and the way around that is to ensure a coaxial connection from the oscilloscope's input down to the power rail itself. We want a high signal-to-noise ratio (SNR), so we're better off with a 1X probe than with a 10X attenuating probe. We want high bandwidth, so we want a 50-Ω termination at the oscilloscope input.

19 January 2018

Bandwidth vs. Current Load in Power-Rail Measurements

Connecting a 6" length of coaxial cable between a low-impedance DUT and a 1-MΩ produces ringing artifacts on your signal acquisition
Figure 1: Connecting a 6" length of coaxial cable between
a low-impedance power rail and a 1-MΩ produces
reflections and ringing artifacts on your signal acquisition
Among the various challenges we've discussed in measuring noise on power rails are RF pickup and signal-to-noise ratio (SNR). Here's another: how do you achieve high bandwidth in your measurements while also minimizing current load on your DUT? Given that your DUT is a power rail, you really don't want to draw too much current from it. But these two measurement criteria are at loggerheads with each other. It's a quandary, and it has to do with the fundamental nature of signals on interconnects.

18 January 2018

How 10X Attenuating Probes Kill Signal-to-Noise Ratio

Figure 1: Signal waveforms captured using a 10X attenuating
probe (top) and a BNC probe (bottom) with tips open
We've begun discussing things that can derail (see what we did there?) your power-rail measurements, such as the deleterious effects of RF interference. In the same context, one should always be mindful of certain characteristics of oscilloscope probes; namely, the 10X attenuating probes that are often lying around on the testbench.