RF immunity tests are typically performed in shielded chambers to prevent exposure of laboratory personnel to hazardous fields and to prevent malfunction of sensitive equipment. The conducted RF immunity test described in ISO 11452-4 utilizes a clamp-on current injection probe to induce RF current into the EUT harness at frequencies from 1 MHz to 400 MHz at levels ranging from tens to hundreds of milliamps. Those currents create fields near the test bench at levels high enough to effect operation of unshielded equipment. The radiated RF immunity test described in ISO/IEC 61000-4-21 utilizes a reverberant chamber containing a mechanical mode tuner which, when a sufficient number of tuner positions have been obtained at a given test frequency, produces a statistically uniform field within the useable volume of the chamber. The test frequency range is 300 MHz to 3 GHz with field strengths as high as 200 V/m (CW and AM) and 600 V/m (radar pulses).
Maintaining the integrity of the shielded chamber prohibits directly connecting measurement instrumentation to the test setup over conductive cabling. Inside the shielded chamber, RF fields couple to the cable, which then acts as a radiating antenna outside the chamber. To avert that problem, we use isolated connections using RF hardened fiber-optic transmitter and receiver sets. The converted signals exit the chamber though non-conductive fiber-optic cables routed through waveguides having a lower cutoff frequency above the frequency range of the test. The optical signals are converted back electrical form by the receiver, which is connected to the measurement instrumentation.
In Figure 1, the test setup (not shown) and RF hardened fiber optic transmitters are placed