Types of oscilloscope probes

As a fault finding tool for electronics repair, oscilloscope permits users to view the waveforms on the different parts of a circuit. To achieve this, an oscilloscope probe is needed to connect the oscilloscope to the required points.

There are two basic types of oscilloscope probes — the active probes and the passive probes. Aside from being cheaper, passive probes are more preferred by oscilloscope users since they can also be used to take a vast variety of measurements.

Meanwhile, oscilloscope probes can also be classified in terms of the offered level of attenuation of the signal. Thus, oscilloscope probes are also classified into 100X, 10X, and 1X.

The 1X oscilloscope probe is the most basic type. Unlike most probes, it does not attenuate incoming voltage. It also features a length of coax connected to the probe itself and a connector, which is usually a BNC connector, to interface to the oscilloscope.

Suitable for low frequency applications, the 1X probes generally offer similar input impedance of the oscilloscope, typically one M ohm.

A 10X oscilloscope refers to a probe with an integrated attenuator that delivers an attenuation of 10. This allows the circuits’ impedance to be enhanced by a factor of 10. While the 10X probe is attenuating the signal, it can also reduce the signal entering the oscilloscope.

To deliver a 10:1 attenuation, the 10X oscilloscope probe utilizes a series resistor paired with one M ohm input impedance. This oscilloscope probe also compensates for existing frequency variations. Placing a small variable capacitor across the series resistor enables it to equalize the frequency response. This quick adjustment can be easily achieved by connecting the oscilloscope probe to the oscilloscope’s tiny square wave oscillator output.

Unlike 10X and 1X oscilloscope probes, 100X oscilloscope probes are rather rare. This type of oscilloscope probe is generally used when monitoring high voltages and when high level of attenuation is required. Since it is uncommon, the 100X is considered a specialized oscilloscope probe. When utilized in normal applications, it delivers a small amount of signal to the oscilloscope’s input that creates noise on the input amplifiers.

The choice of oscilloscope probe is often based on what is currently available on the lab. It has to be noted that, for most applications, the 10X probe generally provides the best result. Probes that can switch from 1X to 10X also offer a good alternative.

However, there are instances when 10X probes do not deliver all the performance required by some applications. To solve this problem, users are advised to integrate an active electronic circuit on the oscilloscope’s remote end.

Active oscilloscope probes also preserve the signal during the transition, offering specially designed integrated circuits. Although it features a fast rise time, a low signal level and high input impedance during sampling, active probes effectively maintains the integrity of the signal and provide better level of performance.