Get Test and Measurement Insights With the Right Oscilloscope Probe
Oscilloscopes are incredibly useful test and measurement instruments. Many people focus on an oscilloscope's features and functions but ignore the role of oscilloscope probes.
Oscilloscope probes aren't mere wires. They are the eyes and ears of oscilloscopes. A probe type is designed to cater to different signal types, power levels, and measurement needs. Whether troubleshooting a radio frequency transmitter or fine-tuning the timing on a high-speed digital logic circuit, there is an ideal probe designed for the task.
In this article, learn about oscilloscope probes and concepts like the probe attenuation factor, bandwidth, and loading effects that you must know to select the right probes for your projects.
What are oscilloscope probes?
Oscilloscopes enable engineers to visualize signal waveforms. They measure the amplitudes of voltage signals over time, display their waveform shapes, and calculate key characteristics like peak-to-peak voltages, frequencies, pulse widths, rise times, fall times, modulation, timings, and more. Many oscilloscopes even double up as spectrum analyzers by providing frequency-domain visualization of signals.
But how do oscilloscopes receive signals from test points in the first place? That happens via oscilloscope probes.
An oscilloscope probe is an essential accessory that:
- senses the target electrical characteristic on the device under test (DUT)
- optionally modifies the signal to make it compatible with an oscilloscope
- physically connects a test point to an oscilloscope's input channel
Think of scope probes as analogous to photographic lenses. Just as different lenses help cameras adapt to different lighting conditions, scope probes help an oscilloscope sense different electrical parameters with various characteristics. Probes are not simple wires; they contain passive, sometimes even active, components that enable oscilloscopes to sense:
- normal analog voltages
- low- and high-speed digital signals
- high-voltage signals
- high-frequency signals
- radio frequency (RF) signals
- currents
- power supply characteristics like noise and ripple
- optical signals
What are the different types of oscilloscope probes?
Fig 1. 10076C high-voltage passive probe
Probes can be classified along three complementary axes — purpose, construction, and measurement method.
Types based on the purpose
Based on what they sense, here are some common types of probes:
- voltage probes
- current probes
- high-voltage probes
- high-frequency probes
- power rail probes
- optical probes
- time-domain reflectometry and time-domain transmission probes
But there are other aspects on which probes are classified in other ways, as we'll see next.
Types based on construction — passive and active probes
Based on their construction and components, probes are classified as:
- Passive probes: They contain only components like resistors and capacitors that passively respond to input signals. They are simple by construction, but that limits their applicability to special applications.
- Active probes: They contain more complex circuits with active components (like transistors or operational amplifiers) to achieve better input impedance and other characteristics. To function correctly, they require additional power from the oscilloscope or batteries.
Some key differences between passive and active probes are depicted below.
Fig 2. Active probes vs. passive probes
Types based on measurement method
Fig 3. N2802A high-frequency differential probe
Based on the measurement method, probes are either single-ended or differential.
A "single-ended probe" is the simpler and more common of the two. It measures the voltage at a test point against the ground reference of the circuit. To get the reference, it has a short ground lead that's connected to the circuit ground. It's called single-ended because only the primary lead senses the actual signal.
In contrast, a "differential probe" consists of two sensing leads of similar lengths and construction, designed to filter out noise. Their probe tips are placed at two test points where the differential voltage is to be measured. The signals from the two tips go into a differential amplifier in the probe.
The amplifier subtracts the common mode noise that's equally affecting both signals and amplifies their difference. The probe's advantage is that its noise-removing design enables it to measure low-amplitude signals in the presence of comparable levels of noise. Neither lead needs to be grounded.
Essential concepts of oscilloscope probes
Some aspects of scope probes are essential to understand for selecting the right probe for an application. As mentioned before, probes are not just simple leads but contain passive or active circuitry. The concepts below are factors or outcomes stemming from their internal circuitry and working.
How do oscilloscope input capacitance, resistance, and impedance affect probes?
Fig 4. Scope and probe resistances and capacitances
When an oscilloscope is connected to the circuit under test, it acts like an additional load. To minimize these loading effects, each oscilloscope input channel typically contains a resistor and capacitor.
A high input resistance, of the order of 1-10 megaohms (MΩ) keeps the current drawn to a minimum. It's combined with a low input capacitance of 10-20 picofarads (pF) to ensure high impedance for high-frequency signals.
A probe's impedance must be compatible with this input impedance.
What is probe loading, and how does it affect signal measurements?
When a probe touches a circuit under test, it becomes a part of it and affects its working. Probe loading is the effect of the probe on the circuit being measured. Probe loading can result in inaccurate measurements because of two phenomena:
- Capacitive loading: Most probes have an inherent capacitance that acts like a parallel capacitor and low-pass filter at the test point. It adversely affects high-frequency input signals by either filtering out some of the content or slowing down rise times.
- Resistive or impedance loading: Probes also have an inherent resistance or impedance. At a test point, a probeacts like a voltage divider. It can reduce signal amplitude, especially if the impedance of the probe is comparable to the circuit's impedance.
Good probes are designed to minimize these effects through high input impedances and low capacitances. The main reason for using an active probe is to dynamically adjust its high input impedance and low capacitance for a particular application and prevalent signal frequencies.
Low probe loading ensures high signal integrity and accurate measurements.
What is the importance of probe bandwidth in oscilloscopes?
Fig 5. Measured amplitude is close to true amplitude until the bandwidth threshold.
In the context of oscilloscopes, bandwidth is the signal frequency range over which the measured amplitude is generally close to the true amplitude with no more than 3 decibels (dB) of attenuation.
At the bandwidth threshold, the signal has attenuated by 3 dB. Beyond this bandwidth frequency, the attenuation increases, and the amplitude either drops sharply (a flat frequency response) or rolls off gradually (a Gaussian response).
However, attenuation isn't the only factor to consider. A bandwidth of 100 megahertz (MHz) means the instrument can accurately measure a simple sinusoidal signal whose fundamental frequency is within 100MHz.
But a more complex waveform, like a square wave of a digital signal, consists of a fundamental frequency and several higher harmonic frequencies as shown below.
Fig 6. Decomposition of a square wave into harmonics. The bandwidth should be high enough for measuring the higher harmonic frequencies.
To accurately measure the waveform's shape, the bandwidth must be three to five times the maximum fundamental frequency to accommodate the harmonics.
So for example, if you want to view universal serial bus (USB) 3.0 digital signals, its fundamental frequency is around 2.5 gigahertz (GHz). So you'd need a higher bandwidth of around 7.5-12.5 GHz at the oscilloscope.
To avoid the probe bandwidth from becoming a bottleneck, always ensure that it is higher than the oscilloscope's bandwidth.
What is probe attenuation and its significance?
The attenuation ratio or factor is a fundamental characteristic of a probe. It's the ratio by which a probe reduces the input voltage before it reaches the oscilloscope's BNC or other connector.
A very common ratio is 10:1, which means the voltage is reduced to a tenth of its true value. A ratio of 1:1 means no attenuation and very high sensitivity, which is suitable for low-voltage signals. Some high-voltagepassive probes have high attenuation ratios, like 100:1. The oscilloscope scales up the readings by the inverse attenuation ratio before displaying graphs and values.
Attenuation ratios address the following aspects:
- Maximum voltage protection: They keep input voltages below the oscilloscope's maximum rated voltage.
- Improved bandwidth: Attenuated probes typically have better bandwidth performance. High-frequency signals are better transmitted through a high-impedance attenuated probe, allowing for more accurate measurements of fast edge transients and high-frequency components.
- Reduced loading: Attenuated probes usually have higher resistance like 10 MΩ for a 10:1 probe compared to a 1:1 probe with a resistance of 1 MΩ. Higher resistance reduces the loading effect on the circuit.
- Better signal-to-noise ratio: When using a 10:1 probe, any noise picked up by the probe is also attenuated, thus improving the quality of the signal.
What is probe compensation, and why is it necessary?
Fig 7. Probe under compensation, overcompensation, and correct compensation
Compensation is an adjustment that ensures the probe and oscilloscope input impedances are matched in a way that minimizes distortion of the measured signal across most frequencies.
Without proper compensation, the capacitive loading of the probe may filter out high-frequency components and distort the signal.
Proper compensation involves calibrating a variable capacitor in the probe to match the input capacitance of the oscilloscope. Most passive probes have a small adjustable screw on the probe body for this purpose.
The probe is connected to an oscilloscope channel that is generating a calibration signal, typically a square wave. The waveform is observed on the scope. An uncompensated probe results in a waveform with rounded (under-compensated) or exaggerated (over-compensated) corners instead of sharp transitions between high and low voltages.
How do you select the right oscilloscope probe for your application?
Fig 8. Use short leads to achieve higher probe bandwidth and lower parasitic components
Here are some considerations and tips to select the right probe for your application:
- Measure timings accurately: Accurate timing measurements require high bandwidths. Generally, active probes have much higher bandwidths (>500 MHz) than passive probes. But the exceptions to this are the low-impedance resistor-divider passive probes. They support very high bandwidths (GHz range) and are rugged and cheap. However, remember that the scope should have a low input impedance of 50 Ω to use them, and this results in high resistive loading, which heavily reduces the signal amplitude.
- Analyze high-speed signals: Active probes are better for high-speed signals because of their low capacitive loading and high bandwidths. However, for high-frequency signals, keep in mind that the parasitic components in the leads result in far lower connection bandwidth and degraded performance. To keep the connection bandwidth high, use very short leads.
- Consider probe loading effects at higher frequencies: Input impedance reduces as signal frequency increases. At low frequencies, the input impedance may start at the published 10 MΩ. However, as frequencies increase, impedance drops at different rates for passive and active probes. The drop is far more rapid for passive probes than for active probes. Since higher impedance is ideal for reducing loading effects, consider using active probes for high-frequency signals instead of passive probes.
- Measure probe loading: Before probing a circuit, connect your probe tip to a point on your circuit and then connect a second probe to the same point. Ideally, you should see no change in the signals. If you see a change, it is caused by the probe loading.
- Current measurement tips: Degauss your current probe to remove residual magnetism from the core and compensate for any DC offsets.
- Ensure calibration and compensation: Ensure that your oscilloscopes and probes are calibrated. After calibration, ensure that the probe compensation is set correctly, especially for passive probes.
For more tips, check out the following PDFs:
- Eight Hints for Better Scope Probing
- Choosing the Best Passive and ActiveOscilloscope Probes
- Tips for Making Low Current Measurements with an Oscilloscope and Current Probe
Are there specialized oscilloscope probes for high-frequency or high-voltage applications?
For high-voltage applications, devices like the 10076C 500 MHz oscilloscope probe sports high attenuation ratios of 100:1, high impedances of 66 MΩ, and high maximum voltages.
For high-frequency applications, like measuring digital signals on dense circuits, probes like the N2874A offer very low input capacitance of two pF and high bandwidth of 1.5 GHz.
Grab the best oscilloscope probe for your test-
In this overview, we explained key concepts of oscilloscope probes, listed the various types of probes out there, and gave tips on selecting the right probes.
Keysight oscilloscopes and probes are extensively used in critical aerospace and defense, semiconductor, consumer electronics, telecommunication, automotive, and data center applications. Use the Keysightoscilloscope probes selection guide to find the best probes and accessory kits.
Contact us to help you select the right scope probe for your application.