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The basics of an oscilloscope

Oscilloscope probes

How to use an oscilloscope

Video of using an oscilloscope

Oscilloscope probes

- an overview or tutorial about the various types of oscilloscope probe that are available for use with oscilloscopes.

Oscilloscopes are widely used for test and repair of electronics equipment of all types. However it is necessary to have a method of connecting the input of the oscilloscope to the point on the equipment under test that needs monitoring.

To connect the scope to the point to be monitored it is necessary to use screened cable to prevent any pick-up of unwanted signals and in addition to this the inputs to most oscilloscopes use coaxial BNC connectors. While it is possible to use an odd length of coax cable with a BNC connector on one end and open wires with crocodile / alligator clips on the other, this is not ideal and purpose made oscilloscope probes provide a far more satisfactory solution.

Oscilloscope probes

Oscilloscope probes normally comprise a BNC connector, the coaxial cable (typically around a metre in length) and what may be termed the probe itself. This comprises a mechanical clip arrangement so that the probe can be attached to the appropriate test point, and an earth or ground clip to be attached to the appropriate ground point on the circuit under test.

Care should be taken when using oscilloscope probes as they can break. Although they are robustly manufactured, any electronics laboratory will consider oscilloscope probes almost as "life'd" items that can be disposed of after a while when they are broken. Unfortunately the fact that they are clipped on to leads of equipment puts a tremendous strain on the mechanical clip arrangement. This is ultimately the part which breaks.

1X and 10X oscilloscope probes

The 1X probes are suitable for many low frequency applications. They typically offer the same input impedance of the oscilloscope which is normally 1 M Ohm. However for applications where better accuracy is needed and as frequencies start to rise, other test probes are needed.

To enable better accuracy to be achieved higher levels of impedance are required. To achieve this attenuators are built into the end of the probe that connects with the circuit under test. The most common type of probe with a built in attenuator gives an attenuation of ten, and it is known as a 10X oscilloscope probe. The attenuation enables the impedance presented to the circuit under test to be increased by a factor of ten, and this enables more accurate measurements to be made.

As the 10X probe attenuates the signal by a factor of ten, the signal entering the scope itself will be reduced. This has to be taken into account. Some oscilloscopes automatically adjust the scales according to the probe present, although not all are able to do this. It is worth checking before making a reading.

Adjusting 10X oscilloscope probes

Other types of probe

Although they are not as common, 100X probes are also available. These oscilloscope probes tend to be used where very low levels of circuit loading are required, and where the high frequencies are present. The difficulty using the is the fact that the signal is attenuated by a factor of 100.

Although 10X probes are widely used because of their superior response, they are not able to provide all the performance that may be needed for some applications. By using active electronic circuits in the remote end of the oscilloscope probe it is possible to offer very high levels of performance. However active probes are very costly and are normally only used in specialised professional applications

Summary

Oscilloscope probes are an essential addition to any oscilloscope. In most case 10X passive scope probes may be used, but other types of test probe need to be considered dependent upon the applications envisaged.