Oscilloscope Voltage Probe Types and Key Specifications

With the development of wide bandgap semiconductor devices, the switching speeds of power electronic devices are getting faster and operating voltages gradually increase, which increases the impact of voltage probe performance on transient voltage measurement results of power electronic devices. This paper analyzes the measurement principles of several commonly used oscilloscope voltage probes in laboratories. Based on circuit analysis models of voltage probes, the bandwidth Key factors such as >/rise time, parasitic inductance, and common-mode rejection ratio affect the results of high-frequency transient voltage measurement. Finally, an experimental platform is established, and the results verify the validity of the theoretical analysis

types of oscilloscope voltage probes and their main performance indicators

1.1 types of oscilloscope voltage probes

Oscilloscope voltage probes can be divided into passive and active probes based on whether power is needed. Passive probes are further divided into low-impedance and high-impedance probes based on input impedance. To reduce the load effect of input impedance on the signal, the commonly used passive probe in switching power supply signal measurement is the high-impedance probe; Active probes can be divided into single-ended probes and differential probes based on the type of signal being measured. Single-ended probes measure single-ended ground signals, differential probes are used to measure signals that are mutually referenced at both ends

Additionally, based on the voltage to be measured, voltage probes are further divided into high-voltage probes and low-voltage probes; According to bandwidth size, voltage probes are further divided into high-bandwidth probes and low-bandwidth probes. Figure 1 are several typical types of oscilloscope voltage probes, among which the optical isolated probe is a differential probe with a high common-mode rejection ratio

Figure 1 typical oscilloscope voltage probe

There are many types of oscilloscope voltage probes, which essentially reflect the complexity of the measurement environment and the diversity of user needs. This requires consideration from two aspects: On one hand, users can select the most suitable probe based on different measurement environments and functional requirements, which can improve measurement accuracy; On the other hand, the limited versatility of this specific probe will increase the user's selection cost, because if users do not thoroughly understand the probe's performance indicators or the characteristics of the signal to be tested, it will be difficult to make effective selection

1.2 main performance indicators of oscilloscope voltage probes

An ideal oscilloscope voltage probe should have the following main features: flexible connection methods, perfect signal fidelity, zero signal source load, comprehensive noise resistance, etc., but in practical applications, none of these can be absolutely achieved. For example, convenient connection requires the probe to handle different physical measurement environments; perfect signal fidelity requires zero attenuation, unlimited bandwidth, and linear phase; zero signal source load requires infinite input resistance; comprehensive noise resistance requires the probe to be completely isolated from external electromagnetic environments. Nevertheless, actual voltage probes can approach these requirements to the greatest extent, sufficient to meet most measurement needs

The performance of oscilloscope voltage probes is mainly measured by the following indicators:

(1) Maximum rated voltage, i.e., the voltage range the probe can measure

(2) attenuation coefficient, which is the multiple by which the probe reduces the input signal. Typical attenuation coefficients include1X, 10X, 50X, 500X etc. The higher the attenuation coefficient, the higher the probe's maximum rated voltage.

(3) input impedance. The input impedance of a probe can be divided into input resistance and input capacitance, input impedance decreases as frequency increases. At low frequencies, input impedance is mainly determined by input resistance, while at high frequencies, input impedance is mainly determined by input capacitance

(4) compensation range (passive probe), that is, the range of oscilloscope input capacitors that the probe can compensate. If the oscilloscope input capacitance is too large or too small, it will cause the probe and oscilloscope to fail to match perfectly

(5)bandwidth/ rise time. The bandwidth of the probe refers to the amplitude-frequency response of its output signal dropping from DC gain to -3dB frequency, probe rise time refers to the amplitude of the probe output signal when an ideal rectangular pulse signal is input. The amplitude of the probe output signal is measured by 10% to 90% time

(6) Propagation delay, i.e., the delay time between the output signal and the input signal. Propagation delay generally varies among different probes, which is an important reason for the measured multi-channel signal asynchrony

(7) common-mode rejection ratio (differential probe). Common Mode Rejection Ratio (Common-Mode Rejection Ratio, CMRR) (dB) is used to measure the ability of differential probes to suppress the common-mode component of the input signal, with the expression being

The performance indicators of different types of voltage probes generally show significant differences. Table 1 summarizes the basic characteristics and main performance indicators of typical oscilloscope voltage probes.

Table 1 Typical Characteristics of Common Oscilloscope Voltage Probes

Note: Data is sourced from mainstream probe manufacturers.

Reprinted from Tektronix