Oscilloscope Probe Q&A, Part 2

Question: How to choose a suitable oscilloscope probe?

Figure 1 Using short leads achieves higher probe bandwidth and lower parasitic elements

Tips and precautions for selecting an oscilloscope probe

Accurate timing measurement：Accurate timing measurements require high bandwidth. Typically, the bandwidth of active probes (>500 MHz) is much higher than that of passive probes. However, low-impedance resistive-divider passive probes are an exception. They support very high bandwidth (GHz range), and they are rugged, durable, and inexpensive. However, remember that when using these probes, the oscilloscope input impedance should be 50 Ω, which results in a high-resistance load and thereby significantly reduces signal amplitude.

Analyzing high-speed signals：Active probes are better suited to high-speed signals because of their low capacitive loading and high bandwidth. However, for high-frequency signals, remember that parasitic elements in the leads can significantly reduce connection bandwidth and degrade performance. To maintain high connection bandwidth, use very short leads.

Consider probe loading effects at high frequencies：Input impedance decreases as signal frequency increases. At low frequencies, input impedance may start from the nominal 1 MΩ value. However, as frequency increases, the impedance of passive probes and active probes decreases at different rates. The impedance of passive probes decreases much faster than that of active probes. Since high impedance helps reduce loading effects, active probes are recommended over passive probes for high-frequency signals.

Measuring probe loading：Before probing the circuit, connect the probe tip to a point on the circuit, then connect a second probe to the same point. Ideally, the signal should not change. If the signal changes, it is caused by probe loading.

Current measurement techniques：Degauss the current probe to eliminate residual magnetism in the core and compensate for any DC offset.

Ensure calibration and compensation：Ensure the oscilloscope and probe are calibrated. After calibration, ensure the probe compensation is set correctly, especially for passive probes.

Question: Are there oscilloscope probes dedicated to high-frequency or high-voltage applications?

For high-voltage applications, such as 10076C 500 MHz oscilloscope probe, which has 100:1 high attenuation ratio,66 MΩ high impedance and high maximum voltage.

For high-frequency applications, such as measuring digital signals in dense circuits,N2874A and similar probes have 2 pF extremely low input capacitance and 1.5 GHz high bandwidth.

What is the rated current?

The rated current of a current probe is the maximum current value that the probe can accurately measure.

Current probes cover a wide rated current range, from a few milliamperes to several hundred amperes. It is important to select a probe with a rated current higher than the maximum current of the circuit under test to ensure measurement safety and accuracy.

What is a probe core?

Probe cores can be made from different materials. The core is the part of the probe that generates the magnetic field. Common probe core materials are ferrite and iron.

Ferrite is a ceramic material used for high-frequency applications. Its permeability is low, so magnetic fields do not easily penetrate it. This low permeability makes ferrite an ideal material for current probes because it prevents the magnetic field from saturating the probe coil.

Iron is a metal used for low-frequency applications. Its permeability is high, so it is less effective than ferrite at preventing magnetic saturation.

Figure 2 Ferrite-core coil with copper wire

What is magnetic saturation?

Magnetic saturation is the critical point at which a material can no longer be magnetized, and readings become inaccurate. Magnetic saturation occurs when the density of magnetic flux lines is too high and they interact with one another, preventing further increase in magnetization strength.

When the current flowing through a circuit is too large, a current probe may undergo magnetic saturation. At this point, the coil of the current probe is saturated by magnetic flux, and the probe can no longer measure current accurately.

What is a current clamp?

A current clamp is an electrical measurement device used to measure alternating current (AC) or direct current (DC). A current clamp can clamp around a conductor without contacting it. This non-invasive measurement method enables fast and safe current measurement.

Figure 3 A technician is using a current clamp to test the current on a circuit board

The probe in the figure isAC/DC High-sensitivity clamp-on current probe

A current clamp has two jaws that can open and close to clamp around a conductor. The jaws are connected to a current transformer to measure the current flowing through the conductor.

When using a BNC cable, clamp the current clamp around the conductor and connect the cable to the oscilloscope. The oscilloscope will measure the voltage induced in the conductor by the current flowing through it.

Question: What is the operating principle of a Rogowski coil?

A Rogowski coil operates based on Faraday's law, which states that the total EMF induced in a closed circuit is proportional to the time rate of change of the total magnetic flux linking the circuit. A Rogowski coil is similar to an AC current transformer, in which a voltage is induced in the secondary coil and is proportional to the current through the insulated conductor. The key difference is that a Rogowski coil has an air core, which is the opposite of a current transformer; the latter relies on a high-permeability steel core and a secondary winding for magnetic coupling. The air core provides lower insertion impedance, enabling faster signal response and a linear signal voltage. The air-core coil is placed around the current-carrying conductor in a toroidal arrangement, and the magnetic field produced by the AC current induces a voltage in the coil. An integrating circuit that integrates the output voltage signal can faithfully reconstruct the input current.

Figure 4 Rogowski coil schematic

Rogowski coil voltage proportional to the rate of change (derivative) of the current in the coil loop

How do you connect an oscilloscope probe?

First, select a probe that matches the expected signal voltage and frequency: passive probes are suitable for general-purpose measurements below several hundred megahertz. Active probes have higher bandwidth and input impedance and are ideal for low-level or high-speed signals.

Connect the probe's BNC connector to the oscilloscope input channel, then attach the probe tip to the test point. To maintain stability, use a probe hook or test-point adapter. For simple measurements or quick setup, some engineers use probes with alligator-clip ground leads for convenience. However, note that these longer leads may introduce noise and reduce signal fidelity at high frequencies.

What are the consequences of improper oscilloscope probe selection?

Using the wrong probe can cause measurement errors or overload the oscilloscope input. A passive probe may have insufficient bandwidth for high-speed digital signals, while an active probe may be unnecessarily high-performance (and expensive) for low-frequency measurements.

Make sure the probe matches the signal type, frequency, and voltage level. Also confirm that the probe attenuation setting matches the oscilloscope channel configuration;10:1 probes are common, but for low-voltage signals, one can also use 1:1 probes. Incorrect matching will result in insufficient or excessive measurement range, as well as waveform distortion.

Reprinted from Keysight Technologies