|
 Part
4: Immunity—Not for Circuitry
by George
Novacek
Start
• Interference Levels
• Let the Lightning Strike
• Ready To Go? • 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 equipment’s
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 50–1000-ms
bursts of pulses of 0.2–10-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.
• signals between
10 kHz and 400 MHz are coupled through induction
into the interconnect cable bundles by toroid
transformers to measure conducted susceptibility
• signals between
100 MHz and 18 GHz, where the upper limit is sometimes
raised according to the equipment application,
are coupled through radiated RF fields to measure
radiated susceptibility
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 it’s
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." Let’s take a look at the levels of
interference we’re up against.
NEXT
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TESTING
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