Published October 1999

TESTING 1, 2

Part 4: Immunity—Not for Circuitry
by George Novacek

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

INTERFERENCE LEVELS

Most users of commercial product are happy when their product works when exposed to e-fields of 1–5 V/m. Designers can achieve this with multilayer circuit boards, SMT, careful layout, keeping leads short, bypassing, and rudimentary low-pass filtering on the interface lines. A shielded cabinet is not necessarily a prerequisite.

Industrial and less critical airborne designs are required to work at interference levels of 15–30 V/m. In addition to the design steps listed above, a full metal shield is needed around the equipment. The most critical part is the wiring between the connector and the circuit board. The board holds a low-pass filter for each line.

Even if the cabinet is properly bonded and the outside cable bundle is shielded, the wires between the connector and the filters act as antennas, radiating interference picked up externally and receiving interference generated internally. Obviously, these wires need to be shielded, and the shorter they are, the better.

The problem of the internal wire length can be solved by using a filter connector, where each filter pin is a low-pass filter. In theory, such an arrangement can provide acceptable performance all the way up to several hundred volts per meter. Unfortunately, filter connectors have limits.

One limit is the maximum working voltage of a distributed capacitor. Many pins can take no more than 50 VDC and are therefore unsuitable for work in higher level fields or where transient and lightning protection is needed—remember the 600-V spike? Pins with higher operating voltage and transient protection are available, but their cost is sky-high (in the hundreds), even for a highly specialized market.

When the susceptibility level hits 100 V/m or more, the best choice is the classic, dual-cavity design (see Photo 1). The cabinet is divided into dirty and clean cavities. The external signals are conditioned, transients clipped, and lightning surges arrested in the dirty cavity.

Photo 1—This design can withstand 400-V/m e-fields and Level 4 lightning hits. The dirty cavity on the left contains transient protection for the connector lines. The battery of low-pass, feed-through filters in the wall between the dirty and clean cavities protects the unit from electrical interference.

All lines going from the dirty cavity into the clean cavity feed through a low-pass filter (see Figure 1). Equipment built like this successfully works in e-fields up to hundreds of volts per meter.

Figure 1—This is an exploded view of the cabinet shown in Photo 1. Notice the two cavities are completely enclosed and isolated from the world and from each other. The connector in the back connects the module into a PCI-type bus in an avionic rack.

With extremely powerful fields of 1000 V/m and more, other methods, such as fiber optics, warrant a close look.

An important issue during susceptibility tests is the pass/fail criteria. In other words, you have to know how the equipment will respond to the interference. This answer depends substantially on the criticality of the system (see the criticality sidebar from Part 3 of this series). At no time should permanent damage result, although there are exceptions when damage is acceptable as long as the unit’s failure does not affect any interconnected equipment. But typically, damage is a no-no.

Critical equipment must show no functional upset and must keep operating without a hiccup. Essential equipment may detect signal corruption by HIRF, revert to a fail-safe mode, and later recover automatically or by a manual reset.

EMISSION—NOT ME!

Section 21, the Emission of Radio Frequency Energy, is susceptibility in reverse. In addition to not being affected by interference, we must also make sure that our system doesn’t generate interference that affects other equipment by radiation and/or conduction. The DO-160D defines the allowable radiated and conducted emission levels.

In producing a design that successfully protects our equipment from outside interference, we are well on our way to keeping home-grown interference from escaping. But, protection from outside interference is no guarantee, and it’s quite common to have an unpleasant surprise—usually thanks to an internally generated clock or its harmonic.

We may have to provide separate shielding for microprocessors and clock generators, making sure bus tracks are sandwiched between power and ground planes, review the bypassing scheme, and so on. Even though the offending circuit is inside the clean cavity and sealed shut like a sardine can, there could be leakage through unintended apertures, such as displays, improperly terminated shielding, ground loops, you name it.

To bring the severity of the allowable emission levels home, a 200-V/400-Hz thyristor switch optimized and individually adjusted for zero crossing through –55°C to 70°C operation exceeded the emission requirements by up to 40 dB in the 10-kHz to 1-MHz band.

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