Measurement speed of E-field probes | White paper |

How to define speed in E-Field Probes

Most datasheets show a specification about measurement speed of E-field probes. But in practice, speed is often defined in different ways, for example as measurement speed or sampling rate. To avoid confusion, this editorial explains the different expressions of speed. Furthermore it explains 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). Afterwards the (x,y,z) sum is multiplied by the number of frequency ranges of the probe (high and low band). Although this represents the maximum speed of the samples are taken inside the probe, it gives an unrealistic picture. The real and usable measurement speed of E-field probes differs from the sample speed. Especially if one considers most probes use internal averaging to achieve stable low field strength measurements.

Measurement speed burst mode

Burst mode refers to the same definition as above with one exception. The taken samples are stored into the (internal) probe memory to be evaluated at a later moment in time. The available internal memory determine the total number of samples which can be taken.

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. Therefore it makes the overall measurement speed multiple factors slower compared to the sampling speed itself.

Measurement speed of electric field probes- streaming mode

Measurement speed in streaming mode is the maximum number of isotropic and/ or all -axis readings/sec the probe can measure. The speed of measurements in streaming mode give a good indication how fast the probe can actually measure. However, in streaming mode a probe can give multiple readings which are identical. This is due to the internal buffering in the probe interface or the probe driver. Therefore, when the EMC software requests a new field strength sample, the measured result can be before the original request! This might lead to incorrect measurement conclusions and longer measurement time due to unstable leveling algorithms.

Measurement speed of E-field probes – triggered mode

Measurement speed of E-field probes (trigger mode) is the maximum speed at which the probe can take unique, accurate and corrected isotropic field measurements. First, the probe gives the command to give a new reading, only after this an unique measurements are supplied. Triggered mode assures that measurements are taken AFTER the field strength request. This approach leads to the most accurate test results.

It is obvious, measurement speed defined in triggered mode on paper will lead to lower speed values. However, when field leveling or time correlation are applicable, this becomes maximally important. Even more so this is the only usable and relevant specification to look for!

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