Passive (Single-Ended) Probe Attenuation Function Analysis

Passive probes, as a common accessory for oscilloscopes, are usually supplied as standard by the manufacturer when purchasing an oscilloscope. These passive probes consist of several parts such as the probe head, probe cable, compensation device or other signal conditioning network, and probe connectors. They do not use active components such as transistors or amplifiers and therefore require no additional power supply, making them easier to use and more cost effective.

Passive voltage probes offer a wide choice of attenuation coefficients to cope with different voltage ranges, with 10× passive voltage probes being the most commonly used due to their wide applicability. For applications where the signal amplitude is 1V peak-to-peak or lower, a 1× probe may be more appropriate. And in applications where low and medium amplitude signals are mixed (e.g., tens of millivolts to tens of volts), switchable 1×/10× probes offer greater convenience. However, it is worth noting that such switchable probes are essentially equivalent to integrating two probes with different characteristics within a single probe, which differ in characteristics such as attenuation coefficient, bandwidth, rise time, and impedance (R and C). As a result, these probes may not be able to achieve a perfect match with the oscilloscope's inputs, thus not achieving the optimal performance that a standard 10× probe can provide.

The attenuation function of passive probes is realized by internal resistors, a design that extends the voltage measurement range of the oscilloscope. When the internal resistor is used in conjunction with the oscilloscope's input resistor, a voltage divider is formed. A typical 10x probe, for example, has an internal 9MΩ resistor. When this probe is connected to an oscilloscope with a 1MΩ input impedance, it will produce a 10:1 attenuation ratio on the input channel of the oscilloscope. This means that the signal amplitude displayed on the oscilloscope will be one tenth of the actual measured signal amplitude. Therefore, when using this type of probe, we also need to adjust the attenuation ratio to the corresponding value (e.g. 10X) in the channel settings of the oscilloscope to ensure the accuracy of the measurement results.

In addition, the attenuation function not only enables us to measure signals beyond the voltage limiting range of the oscilloscope, but also brings other advantages. An attenuation circuit results in higher resistance (which is often beneficial) and lower capacitance, which is especially important for measurements of high-frequency signals. Higher resistance helps to reduce current flow, which reduces noise and interference, while lower capacitance helps to improve signal response speed and measurement accuracy