50.13 CS115, conducted susceptibility, bulk cable injection, impulse excitation
50.13 (5.13) CS115, conducted susceptibility, bulk cable
injection, impulse excitation.
Applicability and limits: The
requirements are applicable to all electrical cables interfacing with EUT
enclosures. The basic concern is to protect equipment from fast rise and fall
time transients that may be present due to platform switching operations and
external transient environments such as lightning and electromagnetic pulse.
The requirement is intended to replace "chattering relay" type requirements
(RS06 in MIL-STD-461C) commonly used in procurements of equipment for aircraft
applications in the past. The chattering relay has been criticized as
unscientific and non-repeatable. The CS115
requirement has a defined waveform and a repeatable coupling mechanism.
The 2 nanosecond rise time is consistent with rise times possible for the
waveforms created by inductive devices interrupted by switching actions. The
30 nanosecond pulse width standardizes the energy in individual pulses. In
addition, it separates the rising and falling portions of the pulse so that
each may act independently. Also, each portion may affect different circuits.
The 5 ampere amplitude (500 volts across 100 ohm loop impedance calibration
fixture) covers most induced levels that have been observed during
system-level testing of aircraft to transient environments. The 30 Hz pulse
rate is specified to ensure that a sufficient number of pulses are applied to
provide confidence that the equipment will not be upset.
Many circuit interfaces are configured such that potential upset is
possible for only a small percentage of the total equipment operating time.
For example, a microprocessor may sequentially poll various ports for input
information. A particular port may continuously update information between
polling intervals. If the transient occurs at the time the port is accessed,
an upset condition may result. At other times, no effect may occur.
Possible tailoring by the procuring activity for contractual documents is
lowering or raising the required amplitude based on the expected transient
environments in the platform. Another option is to adjust the pulse width
based on a particular environment onboard a platform or for control of the
energy content of the pulse.
Test procedures: The excitation waveform from the
generator is a trapezoidal pulse. The actual waveform on the interconnecting
cable will be dependent on natural resonance conditions associated with the
cable and EUT interface circuit parameters.
A circuit diagram of the 50 ohm, charged line, pulse generator required by
is shown in Figure A-12. Its operation is essentially the same as impulse
generators used to calibrate measurement receivers except that the pulse width
is much longer. A direct current power supply is used to charge the
capacitance of an open-circuited 50 ohm coaxial line. The high voltage relay
is then switched to the output coaxial line to produce the pulse. The pulse
width is dependent upon the length of the charge line. The relay needs to have
bounce-free contact operation.
FIGURE A-12. Circuit
diagram of CS115 pulse generator.
The calibration fixture with terminations is a 50 ohm transmission line.
Since the injection probe is around the center conductor within the fixture, a
signal is being induced in the loop formed by the center conductor, the two 50
ohm loads, and the structure of the fixture to which the 50 ohm loads are
terminated. From a loop circuit standpoint, the two 50 ohm loads are in
series, providing a total loop impedance of 100 ohms. Because of the
transmission line configuration, inductance effects are minimized. Measurement
of induced current levels is performed by measuring a corresponding voltage
across one of the 50 ohm loads. Since the 50 ohm loads are in series for the
induced signal, the total drive voltage is actually two times that being
Paragraph 126.96.36.199b(3) of CS115
requires verification that the rise time, fall time, and pulse width portions
of the applied waveform are present in the observed waveform induced in the
calibration fixture. Figure A-13 shows a typical waveform that will be
present. Since the frequency response of injection probes falls off at lower
frequencies, the trapezoidal pulse supplied to the probe sags in the middle
portion of the pulse that is associated with the lower frequency content of
the applied signal. The relevant parameters of the waveform are noted. It is
critical that an injection probe be used with adequate response at higher
frequencies to produce the required rise time and fall time
FIGURE A-13. Typical CS115 calibration fixture
As also specified in CS114,
testing is required on both entire power cables and power cables with the
returns removed to evaluate common mode coupling to configurations which may
be present in different installations. In some installations, the power
returns are routed with the high side wiring. In other installations, power
returns are tied to system structure near the utilization equipment with
system structure being used as the power return path.
RS06 was previously included in MIL-STD-462. RS06 was a
formalization of the "chattering relay" test used widely throughout the
military aircraft industry. This test procedure improves on RS06. The
chattering relay has been found to be effective for determining upset
conditions of equipment. The basic concept was to electrically connect the
relay coil in series with a normally closed contact and allow the relay to
continuously interrupt itself. The wire between the coil and contact was used
to couple the transient onto EUT cables. The greatest concern with the
chattering relay is that it does not produce a repeatable waveform since an
arcing process is involved. The particular relay being used and the condition
of its contact and coil mechanics play a large role. CS115
retains the most important characteristic of the chattering relay which is the
fast rise time waveform and also has the important advantage of a consistent
The same calibration fixture used for CS114
can be used for this test procedure. An available design is shown in Figure A-9.