By Jason Galluppi, Supervisor Systems Control Software, AR
In radiated immunity testing, it is common practice to utilize a radio frequency (RF) field probe to measure the electric field at the location of the unit under test (UUT). Most RF field probes are designed to measure the root mean square (RMS) of a continuous wave (CW) electric field over a broad range of frequencies and amplitudes.
RF field probes are also commonly used to measure modulated fields. In general, this is not a good practice if the RF field probe is diode based. Depending on the analog response time and the sample rate, measuring a modulated field with a diode based RF field probe will be unpredictable and could yield erroneous results.
In some modulated field measurements, such as those found in MIL-STD 461, calibration factors can be obtained to relate modulation amplitude to CW amplitude. These calibration factors will only be good for the specific amplitude and modulation scheme that was used to obtain them. In addition, this will only work well with a slow modulated field where the analog response time of the RF field probe is also slow. Unfortunately, most modern diode based RF field probes have a reasonably fast analog response time. This coupled with a slow sample rate will cause measurements to be unstable. In this case, averaging many readings over time can be used to obtain a stable reading.