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ABCs of Probes
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Tektronix Probe Selector importance. Be sure that you understand the capabilities
and limitations of the measuring equipment that you're
With this on-line, interactive tool you can select by series, using. Also, before making any measurements, become
model number, or standards/application and fine tune thoroughly familiar with the system or circuitry that you will
your search with your specific testing requirements. be measuring. Review all documentation and schematics
The list of matching products will update with each click. for the system being measured, paying particular attention
Try it now at: www.tektronix.com/probes to the levels and locations of voltages in the circuit and
heeding any and all cautionary notations.
Tektronix Resources
Additionally, be sure to review the following safety
Our continually expanding library of technical briefs, precautions to avoid personal injury and to prevent
application notes and other resources will help ensure you damage to the measuring equipment or the systems to
get the most out of your probes and other equipment. which it is attached. For additional explanation of any
Simply contact your local Tektronix representative or visit of the following precautions, please refer to Explanation of
www.tektronix.com. Safety Precautions.
Observe All Terminal Ratings
Use Proper Grounding Procedures
Connect and Disconnect Probes Properly
Safety Summary Avoid Exposed Circuitry
When making measurements on electrical or electronic
Avoid RF Burns While Handling Probes
systems or circuitry, personal safety is of paramount
Do Not Operate Without Covers
Do Not Operate in Wet/Damp Conditions
Do Not Operate in an Explosive Atmosphere
Do Not Operate with Suspected Failures
Keep Probe Surfaces Clean and Dry
Do Not Immerse Probes in Liquids
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ABCs of Probes
Table of Contents
Probes - The Critical Link to Advanced Probing Techniques . . . . . . . . . . . . . 46 - 54
Measurement Quality . . . . . . . . . . . . . . . . . . . . . 4 - 13 Ground Lead Issues . . . . . . . . . . . . . . . . . . . . . . . . . . 46
What is a Probe? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Differential Measurements . . . . . . . . . . . . . . . . . . . . . . 50
The Ideal Probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Small Signal Measurements . . . . . . . . . . . . . . . . . . . . 53
The Realities of Probes . . . . . . . . . . . . . . . . . . . . . . . . . 7
Some Probing Tips . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Explanation of Safety Precautions . . . . . . . . . . 55 - 56
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Observe All Terminal Ratings . . . . . . . . . . . . . . . . . . . . 55
Different Probes for Different Needs . . . . . . . . . 14 - 25 Use Proper Grounding Procedures . . . . . . . . . . . . . . . . 55
Connect and Disconnect . . . . . . . . . . . . . . . . . . . . . . . 55
Why So Many Probes? . . . . . . . . . . . . . . . . . . . . . . . . 14
Avoid Exposed Circuitry . . . . . . . . . . . . . . . . . . . . . . . . 56
Different Probe Types and Their Benefits . . . . . . . . . . . 16
Avoid RF Burns While Handing Probes . . . . . . . . . . . . 56
Floating Measurements . . . . . . . . . . . . . . . . . . . . . . . . 22
Do Not Operate Without Covers . . . . . . . . . . . . . . . . . 56
Probe Accessories . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Do Not Operate in Wet/Damp Conditions . . . . . . . . . . 56
A Guide to Probe Selection . . . . . . . . . . . . . . . . 26 - 31 Do Not Operate in an Explosive Atmosphere . . . . . . . . 56
Do Not Operate with Suspected Failures . . . . . . . . . . 56
Choosing the Right Probe . . . . . . . . . . . . . . . . . . . . . . 26
Keep Probe Surfaces Clean and Dry . . . . . . . . . . . . . . 56
Understanding the Signal Source . . . . . . . . . . . . . . . . . 27
Do Not Immerse Probes in Liquids . . . . . . . . . . . . . . . 56
Understanding the Oscilloscope . . . . . . . . . . . . . . . . . .29
Selecting the Right Probe . . . . . . . . . . . . . . . . . . . . . . . 31 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 - 59
How Probes Affect Your Measurements . . . . . . 32 - 40
The Effect of Source Impedance . . . . . . . . . . . . . . . . . 32
Capacitive Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Bandwidth Considerations . . . . . . . . . . . . . . . . . . . . . 35
What To Do About Probing Effects . . . . . . . . . . . . . . . 40
Understanding Probe Specifications . . . . . . . . 41 - 45
Aberrations (universal) . . . . . . . . . . . . . . . . . . . . . . . . . 41
Amp-Second Product (current probes) . . . . . . . . . . . . . 41
Attenuation Factor (universal) . . . . . . . . . . . . . . . . . . . 42
Accuracy (universal) . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Bandwidth (universal) . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Capacitance (universal) . . . . . . . . . . . . . . . . . . . . . . . . 43
CMRR (differential probes) . . . . . . . . . . . . . . . . . . . . . . 43
CW Frequency Current Derating (current probes) . . . . 44
Decay Time Constant (current probes) . . . . . . . . . . . . . 44
Direct Current (current probes) . . . . . . . . . . . . . . . . . . 44
Insertion Impedance (current probes) . . . . . . . . . . . . . . 44
Input Capacitance (universal) . . . . . . . . . . . . . . . . . . . . 44
Input Resistance (universal) . . . . . . . . . . . . . . . . . . . . . 44
Maximum Input Current Rating (current probes) . . . . . 44
Maximum Peak Pulse Currnt Rating (current probes) . . 44
Maximum Voltage Rating (universal) . . . . . . . . . . . . . . 45
Propagation Delay (universal) . . . . . . . . . . . . . . . . . . . . 45
Rise Time (universal) . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Tangential Noise (active probes) . . . . . . . . . . . . . . . . . 45
Temperature Range (universal) . . . . . . . . . . . . . . . . . . . 45
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Figure 1.1. A probe is a device that makes a physical and electrical connection
between the oscilloscope and test point.
Precision Measurements Start at the In essence, the probe is the first link in the oscilloscope
measurement chain. And the strength of this measurement
Probe Tip chain relies as much on the probe as the oscilloscope.
Probes are vital to oscilloscope measurements. To under- Weaken that first link with an inadequate probe or poor
stand how vital, disconnect the probes from an oscilloscope probing methods, and the entire chain is weakened.
and try to make a measurement. It can't be done. There
In this and following sections, you'll learn what contributes to
has to be some kind of electrical connection, a probe of
the strengths and weaknesses of probes and how to select
some sort between the signal to be measured and the
the right probe for your application. You'll also learn some
oscilloscope's input channel.
important tips for using probes properly.
In addition to being vital to oscilloscope measurements,
probes are also critical to measurement quality. Connecting What Is a Probe?
a probe to a circuit can affect the operation of the circuit, As a first step, let's establish what an oscilloscope probe is.
and an oscilloscope can only display and measure the signal
Basically, a probe makes a physical and electrical connection
that the probe delivers to the oscilloscope input.
between a test point or signal source and an oscilloscope.
Thus, it is imperative that the probe have Depending on your measurement needs, this connection can
be made with something as simple as a length of wire or with
minimum impact on the probed circuit and
something as sophisticated as an active differential probe.
that it maintain adequate signal fidelity for
At this point, it's enough to say that an oscilloscope probe is
the desired measurements.
some sort of device or network that connects the signal
If the probe doesn't maintain signal fidelity, if it changes the source to the input of the oscilloscope. This is illustrated in
signal in any way or changes the way a circuit operates, Figure 1.1, where the probe is indicated as an undefined box
the oscilloscope sees a distorted version of the actual signal. in the measurement diagram.
The result can be wrong or misleading measurements.
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ABCs of Probes
Physically attaching the probe to the test point also establish-
es an electrical connection between the probe tip and
the oscilloscope input. For useable measurement results,
attaching the probe to a circuit must have minimum affect
on the way the circuit operates, and the signal at the probe
tip must be transmitted with adequate fidelity through the
probe head and cable to the oscilloscope's input.
These three issues