Oscilloscope Probe Q&A

To select the appropriate probe for a specific application, it is essential to understand certain aspects of oscilloscope probes. As previously stated, a probe is not a simple wire but contains passive or active circuitry. The following concepts are factors or results arising from their internal circuitry and operating principles.

Question: How do oscilloscope input capacitance, resistance, and impedance affect the probe?

Resistance and capacitance of the oscilloscope and probe | Effect of oscilloscope input capacitance, resistance, and impedance on the probe

Figure 1 Resistance and capacitance of the oscilloscope and probe

When the oscilloscope is connected to the circuit under test, it acts as an additional load. To minimize these loading effects, each oscilloscope input channel usually includes a resistor and a capacitor.

1-10 Megohm (MΩ) -order high input resistance can keep current at the lowest level. It combines with 10-20 picofarad-level (pF) low input capacitance to ensure high impedance for high-frequency signals.

The probe impedance must match this input impedance.

Question: What is probe loading? How does it affect signal measurement?

When a probe contacts the circuit under test, it becomes part of the circuit and affects its operation. Probe loading refers to the effect of the probe on the circuit under test.

Probe loading can cause inaccurate measurements for two reasons:

Capacitive loading: Most probes have inherent capacitance, which acts as a shunt capacitor and low-pass filter at the test point. It adversely affects high-frequency input signals by either filtering out some signal components or slowing the rise time.

Resistive or impedance loading: Probes also have inherent resistance or impedance. At the test point, the probe acts like a voltage divider. It reduces signal amplitude, especially when the probe impedance is comparable to the circuit impedance.

High-quality probes minimize these effects through high input impedance and low capacitance. The main reason for using an active probe is the ability to dynamically adjust its high input impedance and low capacitance according to the specific application and commonly used signal frequencies.

Low probe loading ensures high signal integrity and accurate measurement.

Question: What is the importance of probe bandwidth in an oscilloscope?

Figure 2 Before the bandwidth threshold, the measured amplitude is very close to the true amplitude

In an oscilloscope, bandwidth refers to the signal frequency range over which the measured amplitude is typically very close to the true amplitude, with attenuation not exceeding 3 decibels (dB)。

At the bandwidth threshold, the signal is attenuated by 3 dB. Beyond this bandwidth frequency, attenuation increases, and the amplitude either drops sharply (flat frequency response) or rolls off gradually (Gaussian response).

However, attenuation is not the only factor to consider.100 megahertz (MHz) of bandwidth means the instrument can accurately measure fundamental frequencies at 100 MHz simple sinusoidal signals within.

However, more complex waveforms, such as the square wave of a digital signal, consist of a fundamental frequency and several higher harmonics, as shown in the figure below.

Figure 3 Square-wave harmonic decomposition

The bandwidth should be sufficiently high to measure higher-harmonic frequencies.

To accurately measure the waveform shape, the bandwidth must be three to five times the maximum fundamental frequency to accommodate the harmonics.

For example, to view a Universal Serial Bus (USB) 3.0 digital signal whose fundamental frequency is approximately 2.5 gigahertz (GHz). Therefore, the oscilloscope requires higher bandwidth, approximately 7.5-12.5 GHz。

To prevent the probe bandwidth from becoming a bottleneck, always ensure that it is higher than the oscilloscope bandwidth.

Question: What is probe attenuation? What is the significance of probe attenuation?

What does probe attenuation mean?

The attenuation ratio or attenuation factor is a basic characteristic of the probe. It is the ratio by which the probe attenuates the input voltage before it reaches the oscilloscope's BNC or other connector.

10:1and1:1What are the differences between oscilloscope probes??

10:1 is a very common attenuation ratio, meaning the voltage is attenuated to one-tenth of its actual value.1:1 The attenuation ratio means no attenuation; sensitivity is very high, suitable for low-voltage signals. Some high-voltage passive probes have very high attenuation ratios, such as 100:1. The oscilloscope scales the reading by the reciprocal of the attenuation ratio before displaying the graph and numerical value.

The attenuation ratio of oscilloscope probes is mainly reflected in the following aspects:

Maximum voltage protection：They ensure the input voltage remains below the oscilloscope's maximum rated voltage.

Higher bandwidth：Attenuating probes generally have better bandwidth performance. High-frequency signals pass more easily through high-impedance attenuating probes, enabling more accurate measurement of fast-edge transients and high-frequency components.

Lower loading: Attenuating probes typically have higher resistance, for example10:1the probe resistance at the attenuation ratio is 10 MΩ, while1:1the probe resistance at the attenuation ratio is 1 MΩ. Higher resistance can reduce the loading effect on the circuit.

Higher signal-to-noise ratio: When using 10:1 a probe, any noise picked up by the probe is also attenuated, thereby improving signal quality.

Question: What is probe compensation? Why is probe compensation needed?

Figure 4 Oscilloscope probe undercompensation, overcompensation, and correct compensation

Oscilloscope probe compensation is an adjustment that ensures the probe and oscilloscope input impedances are matched, thereby minimizing distortion of the measured signal over most of the frequency range.

Without proper compensation, the capacitive loading of the probe may filter out high-frequency components and cause signal distortion.

How do you properly compensate a probe?

The correct compensation method involves calibrating the variable capacitor in the probe so that it matches the input capacitance of the oscilloscope. Most passive probes have a small adjustable screw on the probe body for this purpose.

Connect the probe to an oscilloscope channel that produces a calibration signal (usually a square wave). Observe the waveform on the oscilloscope. An uncompensated probe will cause the waveform to have rounded corners (undercompensation) or exaggerated corners (overcompensation), rather than sharp transitions between the high and low voltages.

Reprinted from Keysight Technologies