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TESTING 1, 2

Part 4: Immunity—Not for Circuitry
by George Novacek

Start ı Interference Levels ı Let the Lightning Strike ı Ready To Go? ı Sources and PDF

Section 19 of the Environmental Conditions and Test Procedures for Airborne Equipment takes us into the black magic of design. Induced Signal Susceptibility deals with the equipmentıs ability to work when exposed to interfering signals from other equipment, power supplies, or electrical transients generated by an external source and coupled to the system through wiring and packaging. Testing involves exposing the equipment and its wiring to magnetic and electric fields of varying intensity.

The first test is performed at low frequencies (e.g., 400 Hz to 15 kHz). Here, the only line of defense against interfering signals is proper cabinet designing, wiring, shielding, and grounding. Electrical filtering is rarely practical, as these frequencies are too close (if not within) normal operating range.

By using symmetrical interfaces, you go a long way towards ensuring interference-free operation. Switching transients are represented by 600 Vp_-p with 50ı1000-ms bursts of pulses of 0.2ı10-ms repetition rate. This type of interference is handled quite efficiently by our High Intensity Radiated Field (HIRF) protection circuits.

SUSPECTIBILITY TO RADIO FREQUENCY

Radio Frequency Susceptibility, discussed in Section 20, divides the interfering signal, a modulated RF carrier, into two groups according to the frequency and the way it is injected into the equipment.

The frequency bands between 100 and 400 MHz overlap. The immunity levels have grown over the past decade with research into the existing environment and equipment failures.

HIRF susceptibility is a major concern with the high-power transmitters, radar, measuring equipment, and cell phones. Urban legend has it that a fighter aircraft onboard a carrier retracted its landing gear and dropped on its belly following a radar sweep. I doubt if itıs true, but the legend illustrates the importance of keeping susceptibility under control.

How do you protect against the effects of high-intensity radiated fields? By proper cabinet design, bonding, and cable shielding. Whatever is left gets filtered by our electronics. The old adage, "a good antenna is the best amplifier" can be paraphrased to "a well-designed cabinet and shielding scheme are the best interference filter." Letıs take a look at the levels of interference weıre up against.

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