Oscilloscope probes

- a summary or tutorial about the basics of oscilloscope probes, or scope probes, 1X, 10X, 100X and how to use and scope probe specifications.

This oscilloscope tutorial is split into several pages each of which address different aspects of oscilloscopes and their operation:

The oscilloscope, or scope for short, is 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 or scope probes are required.

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.

A whole variety of scope probes can be bought and used. Fortunately, there is a high degree of inter-changeability between scopes and scope probes. However it is necessary to know which types to use, and what the scope probe specifications may be when choosing the correct type to use for a given application.

Oscilloscope probes may be categorised, and they can fall into one of two main areas:

• 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.

Scope probes are generally classified according to the level of attenuation of the signal they provide. Types including 1X (giving a 1 : 1 attenuation ratio), 10X (giving a 10 : 1 attenuation ratio) and 100X (giving a 100 : 1 attenuation ratio) are available:

1. 1X scope probes   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.


2. 10X scope probes   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. In particular the level of capacitance seen by the circuit is reduced and this is reduces the high frequency loading of the circuit by the probe.
   
   As the 10X probe attenuates the signal by a factor of ten, this obviously means that the signal entering the scope itself is 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.
   
   The 10X scope probe uses a series resistor (9 M Ohms) to provide a 10 : 1 attenuation when it is used with the 1 M Ohm input impedance of the scope itself. A 1 M Ohm impedance is the standard impedance used for oscilloscope inputs and therefore this enables scope probes to be interchanged between oscilloscopes of different manufacturers.
   
   10X oscilloscope probes also allow some compensation for frequency variations present. A small variable capacitor placed across the series resistor to compensate for the small capacitance across the scope input terminals. The series variable capacitor in the scope probe enables adjustment to be made to equalise the frequency response. 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.
   
   Many modern scope probes adopt a slightly different approach, although the end result is the same. A laser-trimmed thick-film electronic circuit is sued in the head of the probe. This combines the 9 M Ohm series resistor with a fixed-value bypass capacitor. A small value adjustable capacitor is then placed in parallel with the input capacitance of the oscilloscope. It is this capacitor that is adjusted to provide the compensation.


3. 100X scope probes   Although they are not as common as the 1X and 10X scope probes, 100X probes and other values including 20X and 1000X are also available. These oscilloscope probes tend to be used very high voltages need to be monitored and a high degree of attenuation is required or if very low levels of loading are needed. These probes are not common and tend to be quite specialised. If they were used for normal applications, the 100X attenuation would result in very small signal levels being presented to the input of the oscilloscope and as a result, noise on the input amplifiers of the scope would tend to be visible.

Often the choice of scope probe that is used will depend on what is available, in the laboratory however for most applications a 10X probe is the best all round type of probe, and as a result, these are the most commonly found and purchased. Switchable probes that can switch between 1X and 10X may be another solution.

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. Not only is the input resistance very high, but more importantly the input capacitance is very low. With capacitance normally providing the limiting factor, the reduction in capacitance enables sensitive waveforms to be measured far more accurately.

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

Differential oscilloscope probes

In some instances it may be necessary to measure differential signals. Low level audio, disk drive signals and many more instances use differential signals and these need to be measured as such. One way of achieving this is to probe both lines of the differential signal using one probe each line as if there were two single ended signals, and then using the oscilloscope to add then differentially (i.e. subtract one from the other) to provide the difference.

Using two scope probes in this way can give rise to a number of problems. The main one is that single ended measurements of this nature do not give the required rejection of any common mode signals (i.e. Common Mode Rejection Ratio, CMMR) and additional noise is likely to be present. There may be a different cable length on each probe that may lead to a time differences and a slight skewing between the signals.

To overcome this a differential probe may be used. This uses a differential amplifier at the probing point to provide the required differential signal that is then passed along the scope probe lead to the oscilloscope itself. This approach provides a far higher level of performance.

In this way, differential scope probes are a form of specialised active probe.

Oscilloscope probe specifications

When buying or selection a scope probe for any application it is necessary to considered the main specifications. A number of the more important and common scope probe specifications are detailed below:

• Accuracy:   The accuracy of any oscilloscope probe is of great importance. Typically for voltage sensing probes the accuracy refers to the attenuation of the signal by the probe as in the case of a 10X probe. The accuracy will be dependent upon the accuracy of the series resistor in the probe, but it is also dependent upon the accuracy of the input resistance of the scope. Accordingly, the scope probe accuracy specification is only correct or applicable when the probe is being used with an oscilloscope having the assumed input resistance.
• Attenuation:   This is the ratio of the output signal to the input signal in terms of voltage. Common scope probe attention levels are 1 (i.e. no attenuation) and 10 although probes with attenuation levels of 100 are occasionally available for specialist applications. Sometimes probes may be switchable between 1X and 10X.
• Bandwidth:   The maximum bandwidth is the frequency at which the response falls by 3dB (i.e. -3dB) of the low frequency value. When choosing a scope probe, the value of the bandwidth should be well above that of the maximum frequencies anticipated, otherwise unexpected results may be seen. It is also worth choosing a scope probe with a bandwidth higher than the scope itself. In this way the full potential of the scope can be used. As a general rule, for accurate amplitude measurements, the bandwidth of the scope probe and also the oscilloscope should be five times greater than the frequency of the waveform being measured. This enables any harmonics of the fundamental signal to be adequately captured.
• Cable length:   It is necessary to consider the length of the cable. The longer the cable, often the lower the bandwidth. However it is still necessary to ensure that the length is sufficient to accommodate easy working. Common cable lengths are: 1.3m, 2m, and longer lengths for higher voltage probes where it may be necessary for greater distances between the equipment being tested and the scope and people using the scope.
• Common mode rejection ratio, CMMR:   This specification is applicable to differential probes. It is a measure of the ability of the probe to reject any signals that are common to both inputs.
• Compensation range:   The compensation range of the scope probe is the range of input capacitance of the oscilloscope that the probe can compensate, i.e. it can be used with oscilloscopes with input capacitance levels within a given range.
• IEC 1010:   This refers to the safety rating for the scope probe. Different categories are used for different types of equipment.
• Input capacitance:   This is the typical input capacitance of the probe. It will depend to some degree on the capacitance of the scope, and the adjustment of the probe capacitor, but typical values for the overall input capacitance can be given.
• Input resistance:   This is the system input resistance, i.e. the sum of any resistor in the probe(9 M Ohm for a 10X probe), plus the scope input resistance (typically 1 M Ohm)
• Input voltage:   A maximum input voltage is specified. This is the highest voltage that the probe should be connected to. It will include both DC and AC components and will effectively mean the peak instantaneous voltage that may be seen by the probe. While the voltages that can be tolerated by passive probes may extend into hundreds of volts, active probes may only tolerate maximum input voltages in the region of a few tens of volts.
• Readout detection:   When using some scope probes on particular scopes, it is possible that they detect the attenuation level and adapt the readout accordingly. If this feature is required, it is necessary to ensure that the scope and probe are compatible.
• Differential voltage :   This probe specification is applicable to differential probes. It is the voltage difference between the two points on the probe.
• Rise time:   This is the time required for the leading edge of a pulse to rise from 10% to 90% of its final value. For accurate pulse rise and fall time measurements on, the overall rise time of the system (scope and probe combined) should be three to five times faster than the fastest transition to be measured.
• Tip or head style:   Details of the scope probe tip or head style may also be given. Reference to the manufacturers specifications may be needed. Typically the clip will have a curved end that will clamp into the wire or test point.

Summary

Oscilloscope probes are an essential addition to any oscilloscope. In most cases 10X passive scope probes are the most widely used, although 1X probes are also in widespread use. They can be used for general purpose and many other applications where loading of the circuit must be kept to a minimum. On occasions other types of test probe ranging from high voltage to active probes. In all cases the scope probe specification must be considered to ensure it is adequate for the intended application.

Further pages from this tutorial
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