Oscilloscope probes

- an overview of some of the main features of oscilloscope probes, or scope probes, and the points to note when choosing and using these test probes.

Oscilloscopes are an essential tool for fault finding for electronics development, repair or diagnostics work. The oscilloscope enables the waveforms on various parts of the circuit to be viewed in a graphical format. To enable the oscilloscope to connect to the required points, oscilloscope probes are used. Although it is possible to use a signal line and earth return connection to form a simple oscilloscope probe, this approach does not provide the optimum performance as both electrical and mechanical aspects need to be considered to meet the necessary requirements. Accordingly proper test probes are required.

Oscilloscope probes may be categorised, and they can fall into two main sections:

• Passive oscilloscope probes
• Active oscilloscope probes

Passive oscilloscope probes

The great majority of test scope probes used with oscilloscopes are the passive variety. They enable a wide range of measurements to be made, and cover most applications. In addition to this, passive test probes are far cheaper than active ones as would be expected.

The most basic form of oscilloscope probe, or scope probe, is what is often termed the 1X probe. It is so called because this type of scope probe does not attenuate the incoming voltage as many other probes do. It consists of a connector to interface to the oscilloscope (generally a BNC connector), and a length of coax which is connected to 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.

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.

10X oscilloscope probes also allow some compensation for frequency variations present. A small variable capacitor enables adjustment to be made. Most oscilloscopes have a small square wave oscillator output. By attaching the oscilloscope probe to this a quick adjustment can be made. As the square wave requires all the harmonics to be present in the correct proportions to provide a "square" wave, the probe can be quickly adjusted accordingly. If the leading edges of square wave, when viewed on the screen has rounded corners, then the high frequency response of the probe is low and an adjustment can be made. However if the leading edges have spikes and rise too high, falling back to the required level, then the high frequency response has been enhanced and this needs to be adjusted. Only when the square wave is truly square is the frequency response correct.

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.

Active oscilloscope probes

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.

Active oscilloscope probes use specially developed integrated circuits. By placing these chips right at the point at which the signal is probed, it enables the signal to be preserved during its transition from the point at which it is sampled to the input of the oscilloscope, in some instances using differential techniques. In this way the signal integrity it maintained, despite the fact that it may have a fast rise time, may have a low signal level, or require a high input impedance at the point at which it is sampled.

Although active probes are more expensive than their passive cousins, they offer a better level of performance that may be essential in some circumstances.

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.