Types of oscilloscope probes

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R&S®Essentials | Digital oscilloscope fundamentals

Types of oscilloscope probes

With the wide variety of probes available today, it can be challenging to choose the one best suited to your application. In this article, we explore the four main probe types - passive, active, differential and current - and explain their characteristics. Read on to learn which probe you need for your measurement!

Passive probes

Passive probes are characterized by their simplicity and lack of active components. Operating without the need for external power, these probes come standard with most oscilloscopes and are often interchangeable between different models and manufacturers. Inexpensive and robust, passive probes are user-friendly, requiring no intricate configurations. Their ease of use makes them a go-to for basic voltage measurements - just connect the probe to the scope, attach the ground lead and you're ready to start probing.

R&S®RT-ZP10 passive probe

R&S®RT-ZP10 passive probe

Key facts

  • Bandwidth: up to 500 MHz
  • Dynamic range: up to 1000 V (RMS)
  • All-rounders for general purpose applications
  • Extensive accessories for optimal contacting

Before making measurements with a passive probe, it’s important to compensate. Probe compensation is a calibration process that fine-tunes the ratio of capacitances within both the probe and the oscilloscope input. Not compensating probes can result in measurement inaccuracies, impacting parameters such as amplitude and pulse shape. Therefore, it is crucial to perform probe compensation when using a probe-oscilloscope pair for the first time or when conducting critical measurements.

The compensation procedure is straightforward:

  • Connect the probe to the scope's built-in compensation signal and ground.
  • Adjust the compensation capacitor until the signal achieves a rectangular waveform.

This ensures that the probe accurately represents the input signal characteristics, allowing for precise and reliable measurements.

Probe compensation examples

Want to learn more? Then check out our dedicated articles on Understanding passive oscilloscope probes and Understanding probe compensation.

Active probes - single-ended FET probes

An active probe, as the name suggests, incorporates active or powered components in its probe tip. The active component is typically a field-effect transistor (FET). A standout advantage of active probes is minimal loading over a wide frequency range - thanks to the low input capacitance that translates into a high input impedance. This ensures accurate measurements without unduly affecting the observed circuit.

In addition, active probes offer the benefit of a high input offset. In other words, an active probe can handle signals that aren't centered around zero volts. This feature is handy when you're dealing with signals that might have a DC (direct current) component or a non-zero baseline.

An active probe usually has a proprietary connector, which allows an oscilloscope to automatically detect and calibrate the probe. Power can be provided through this specialized interface or supplied externally. Note that most active probes require a 50-ohm termination setting on the oscilloscope channel.

 R&S®RT-ZS20 single-ended FET probe

R&S®RT-ZS20 single-ended FET probe

Key facts

  • Bandwidth: 1 GHz to 6 GHz
  • High input impedance: 1 MΩ
  • Low input capacitance: less than 1 pF
  • Very low added noise
  • Integrated high precision DC voltmeter (not available for all models)

Differential scope probes

Differential probes are designed to measure the voltage difference between two points in a circuit. They feature two inputs that can be connected to various points in the circuit, with no requirement for a ground reference at either location. Using an internal differential amplifier, these probes generate an output voltage that reflects the difference between the selected measurement points, often scaled by a user-defined attenuation factor.

Schematic of differential scope probe

An important characteristic of differential probes is their immunity to "common mode" signals, which are signals that are simultaneously present at both measurement points. This makes them great for measuring low-level signals in noisy environments. They can also be used for single-ended measurements, which are accomplished by simply grounding one of the leads.

R&S®RT-ZD003 differential probe

R&S®RT-ZD003 differential probe

Key facts

  • Bandwidth: 200 MHz to 4.5 GHz
  • Input impedance: up to 1 MΩ
  • Low input capacitance: less than 1 pF
  • Very low added noise
  • Integrated high precision DC voltmeter (not available for all models)

Current probes

All the probes we’ve been talking about so far - passive, active and differential - produce a voltage at the oscilloscope input. This is because oscilloscopes measure voltage as a function of time. But what if you want to measure current? Well, you need a way to a way to create a voltage that corresponds to a current in a consistent and predictable way. In other words, you need to “convert” a measured voltage into a current value. For example, 1 V at the scope input could be used to indicate that the measured current is 1 A.

Current probes are a way to achieve this conversion. They work by capturing the electromagnetic field generated by the current flowing through the conductor and converting it into voltage using a known ratio of volts per amp. These probes are positioned or "clamped" around the current-carrying conductor, with an arrow marker indicating the direction of current flow.

Most current probes are active devices, which means that they need an external power source for operation. While all current probes can detect and measure AC, some are also capable of measuring DC currents. AC current measurements rely on a current transformer, while DC or very low frequency AC measurements use a Hall effect sensor.

Normal current probes typically cannot handle large currents. This is where high current probes come in. These probes are characterized by a special structure that allows them to measure high current with low resistance. They often use specialized sensors to measure the magnetic field generated by current flow. This enables non-contact measurement, which is essential when dealing with high currents. Additionally, high current probes usually offer higher accuracy and resolution compared to normal current probes. This is necessary for applications such as power electronics and energy systems, where small changes in current can have significant implications.

R&S®RT-ZC20B current probe

R&S®RT-ZC20B current probe

Key facts

  • Non-intrusive DC and AC measurements
  • Current range from mA to 2000 A
  • Up to 120 MHz bandwidth
  • Robust design and easy operation

Current probes are often used for power measurements involving both voltage and current. In some cases, issues such as time offsets or "skew" can arise due to variations in propagation times within the probe leads. This skew can potentially lead to inaccurate power results.

Deskew fixtures are used to tackle this problem. They serve as specialized tools to identify and compensate for skew. These fixtures generate time-aligned voltage and current pulses, measured simultaneously by attached current and voltage probes. If the test waveforms exhibit skew, the appropriate deskew value can be entered on the oscilloscope. This correction ensures that the current and voltage waveforms are brought back into phase, mitigating the impact of skew on subsequent measurements and maintaining the accuracy of power calculations.

Before and after deskewing a current probe

Summary

  • Passive probes are inexpensive, robust and user friendly. They come standard with most oscilloscopes and are great for basic voltage measurements. It is important to compensate passive probes before making measurements with them.
  • Active probes have active or powered components in their tips. Their standout advantage is minimal loading over a wide frequency range - meaning more accurate measurements. They also have a high input offset, which is useful when dealing with signals with a DC component or a non-zero baseline.
  • Differential probes measure the voltage difference between two points in a circuit. They are immune to “common mode” signals, which makes them great for measuring low-level signals in noisy environments.
  • Current probes convert a measured voltage to a current value and are often used for power measurements that involve both voltage and current. They may require deskew fixtures to compensate for time offsets or “skews” that can arise due to variations in propagation times within the probe leads.

Not sure which oscilloscope probe meets your measurement needs best? Our experts will help you.

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