Knowledge about oscilloscope probes

Before understanding the structure of the probe, You need to first understand the structure of the input interface of the oscilloscope, because this is the place connecting the probe, the input interface circuit of the oscilloscope and the probe constitute our detection system.

Most of the input interfaces of the oscilloscope are in the BNC or BNC-compatible format. The input end of the oscilloscope has a matching resistance of 1 m ohm or 50 ohm. There are many kinds of oscilloscope probes, but the oscilloscope can only be matched with 1 m ohm or 50 ohm. Different Kinds of probes need different matching resistance forms.

From the perspective of voltage measurement, in order to have little impact on the tested circuit, the oscilloscope can use a high input impedance of 1 m ohm, but the bandwidth of the high impedance circuit is easily affected by the parasitic capacitance. Therefore, the input impedance of 1 m ohm is widely used with measurements below M bandwidth. Generally, a 50 ohm transmission line is used for higher frequency measurement. Therefore, the 50 ohm matching of the oscilloscope is mainly used for high frequency measurement.

Traditionally, most of the oscilloscope with less than MHz bandwidth on the market only has 1 m ohm input, because it is not used for high-frequency measurement; MHz ~ Most 1 GHz bandwidths of the oscilloscope have 1 m ohm and 50 ohm switching options, while taking into account high and low frequency measurement; 2 GHz or higher bandwidths of the oscilloscope is mainly used for high frequency measurement, so most of the only 50 ohm input. However, with the market demand, some 2 GHz and above scopes also provide 1 m ohm and 50 ohm input switching.

In a broad sense, the test cable also belongs to a probe, such as a BNC or SMA cable, which is both inexpensive and highly performing. However, BNC or SMA interfaces must be provided on the tested circuit when the cable connection is tested. Therefore, the applications are limited, mainly used for RF and microwave signal testing. A dedicated probe is required for testing digital or general-purpose signals.

The oscilloscope probe can be divided into passive and active probe based on whether the power supply is needed, according to the measurement signal type can be divided into voltage probe, current probe, Light Probe and so on. The so-called passive probe refers to the entire probe is composed of passive devices, including resistance, capacitance, cable, etc.; and the active probe generally has an internal amplifier, the amplifier needs power supply, so it is called an active probe.