Oscilloscope Probe Specifications & Parameters

- there is no use in buying and using an expensive oscilloscope if the probes do not have a sufficiently high specification.

There are several key specifications and parameters that are used to define the operation of the oscilloscope probes.

It is important to ensure that the oscilloscope probe performance matches that of the oscilloscope used - there is no use in employing a high performance scope, of the probe specification does not enable its performance to be utilised.

An oscilloscope probe attached to an item of equipment being tested

Key scope 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 scope probe specification details 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 probe 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 manufacturer's specifications may be needed. Typically the clip will have a curved end that will clamp into the wire or test point.

These are some of the main general scope probe specifications used for general probes. Other specifications will be given for specialist types of probe, or where a ley parameter is of particular importance.

By Ian Poole

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