White paper #6 | Measurement speed of E-field probes.

How to define speed in E-Field Probes

Most E-field probe datasheets show a specification about speed. But in practice, speed is often defined in different ways, for example as measurement speed or sampling rate. As this could lead to confusion, this editorial explains the different expressions related to measurement speed and how to interpret these terms and specifications correctly.

Sample speed

The expression “sample speed” is normally referred to as the sampling speed of an AD converter. Normally this is the maximum achievable (internal) speed of a Field probe. Some manufactures take the sum of the sample speed of all individual axis (x + y +z-axis) multiplied by the number of frequency ranges of the probe (for example high and low band). Although this represents the maximum speed of the samples are taken inside the probe, it gives an unrealistic picture of the real measuring speed of a probe. Especially if one considers most probes use internal averaging to achieve stable low field strength measurements.

Sample speed (burst mode)

This refers to the same definition as above with the exception, the samples are stored to (internal) probe memory to be evaluated at a later moment in time. The total number of samples which can be taken, depends on the available internal memory.

This method is useful to evaluate transients and radar pulse signals. Due to the inherent non-continues mode, no real time measurements can be made. Apart from this, the time required for data transfer and time to achieve correlation required for field leveling, makes the overall measurement speed multiple factors slower compared to the sampling speed itself.

Measurement speed (streaming mode)

The measurement speed (streaming mode) is defined as the maximum number of isotropic and/ or X + Y+ Z-axis readings/sec the probe can measure. In contradiction to the sample speed, the measurement speed gives a good indication how fast the probe can actually measure the field strength. However, care should be taken while, in streaming mode, a probe can give multiple readings which are identical, due to the internal buffering in the probe interface or the probe driver. This also implies, when the EMC software requests a new field strength sample from the probe, one can get a result which is measured before the question is asked! This might lead to incorrect measurement conclusions and longer measurement time due to unstable leveling algorithms.

Measurement speed (triggered mode)

The measurement speed (trigger mode) is defined as the maximum speed at which the probe can take unique, accurate (corrected) isotropic field measurements, taken AFTER the probe is requested to give a new reading. In this mode, one is assured to get a measurement reading which is taken AFTER the field strength request is issued, leading to accurate test results.

It is obvious, measurement speed defined in triggered mode on paper will lead to lower speed values. However, in applications where field leveling or time correlation is required, this is the only specification to look for!

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