Understanding passive oscilloscope probes

Active vs. passive probes

Oscilloscope probes can be divided into two main categories: active probe and passive probes. Passive probes have no active components and therefore can operate without power from the scope. They are extremely common, and a set of passive probes is usually included with every oscilloscope. In fact, passive probes are often interchangeable between scopes, even with scopes from different manufacturers, although they will have different performance and other characteristics.

Passive probes are relatively inexpensive, especially compared to active proves, and they are also more rugged than active probes. Furthermore, passive probes are very easy to use: no complicated configuration, just connect the probe to the scope, attach the ground lead, and start probing.

Passive probe attenuation

1x probes

A 1x probe simply connects the scope to the test point without any additional attenuation. This means that 1x probes have a high sensitivity and can see small signals more easily. Therefore, a 1x probe is useful when the signal level is low, e.g. less than 1-volt peak to peak. 1x probes have a low bandwidth because the input capacitance of the scope acts as a low pass filter. In low frequency applications, this can be an advantage, e.g. when a probe is used to filter out high frequency noise on the measured low-frequency signal.

10x probes

Ten – X probes are the “standard” probes supplied with most scopes. They reduce signal amplitude by a factor of ten. Usually by incorporating a 9 megaohm resistor in the probe tip to act as a voltage divider. This added attenuation makes 10x probes good for high-voltage measurements. The added attenuation also increases the scope input impedance by a factor of 10, so 10x probes create less circuit loading than 1x probes. One additional advantage of 10x probes is that they have much wider bandwidth than 1x probes. The probe tip in a 10x probe contains capacitance that helps cancel out the scope’s inherent input capacitance. This is particularly important when measuring signals with high frequency components like square waves and pulsed signals.

1x probes

10x probes

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Automatic probe detection

How does the scope know if we’re using a 1x or 10x probe? In some cases, a scope can detect whether a probe is 1x or 10x and adjust its settings automatically. That is possible, because many 10x probes have a metal pin on the scope side, and if this contacts a sensing ring on the scope input, the scope knows a 10x probe is being used.

Switchable probes

Some probes can be used as either 1x or 10x probes, by moving a switch on the side. There are called switchable probes. When using a switchable probe, be sure to adjust the scope settings accordingly if the probe attenuation isn´t automatically detected.

10x probes with a metal pin

Switch located on the probe

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Probe compensation

Probe compensation is important whenever using probes, and especially in the case of 10x probes. Probe compensation is used to match the oscilloscope’s inherent input capacitance with the capacitance in the probe tip. It’s a good idea to compensate passive probes before use, since this will reduce amplitude and pulse shape inaccuracies in the measured signal.

Summary

Passive probes are the most common type of oscilloscope probe
Passive probes are inexpensive, rugged and easy to use in a wide variety of applications
Two most common attenuation factors are 1x and 10x
1x probes are good for small signals, but have limited bandwidth
10x probes are good for larger signals, have wider bandwidth, and load the circuit less than 1x probes
When using a probe that attenuates the input signal, it´s important to use the correct scope settings
There are some probes that can be switched between 1x and 10x
For good results it´s important to properly compensating probes

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