I realized earlier that my reviews have covered most of the MIL-STD-461 test methods in the past year, so I decided to prepare this to capture those items that have been omitted in prior articles. Since this is a series close out and the topics have little in common, I’ll take on each instead of using commonality to group multiple test methods.
CS115 – Impulse Excitation
The purpose of CS115 testing is to evaluate the ability of a device to tolerate electromagnetic energy from switching transients that could couple into the device wiring. Test parameters establish the waveform simulating the coupling and device performance determines the acceptance criteria. The device performance specification should establish a quantifiable pass/fail deviation from the standard operation of the device that is considered acceptable or not acceptable.
CS115 testing became a standard test call out with the release of MIL-STD-461D back in 1993, with applicability for aircraft and space systems but could be used for other systems when specified. This test uses bulk current injection to apply the transients replacing the “chattering relay” RS06 test specified in MIL-STD-462 Notice 6. This change established more stable test parameters, a big step in standardization. We note that the revision “D” also deleted the CS06 test in favor of CS115 being used on power lines. Many procurements continued to call out testing equivalent to CS06 after revision “D” release and a similar CS106 test was included in revision “F” but it was deleted by revision “G” citing that measurements demonstrated that CS115 achieved the cross-coupling from injecting the CS106 transient.
The applicability in the current standard, revision “G”, includes ground systems and ships when specified. Over the years as revisions have been released, CS115 testing has remained relatively unchanged.
CS115 Test Equipment
A coaxial charged line pulse generator is used to generate the specified waveform (see Figure 1) where an open circuit coaxial cable will acquire a charge and then discharge into the output circuit connected to a current probe when switched connecting the charged line. Switching is controlled by a pulse rate controller and relay. The waveform goal is seldom attained because of inductive coupling so expect to see something like the acceptable waveform Figure 1 left side. Note that waveform parameters are met during the calibration process and highly distorted when testing a cable.
A current injection probe rated for at least 500 MHz is used as the coupling transformer and a current monitoring probe is used during testing. The high-frequency performance is necessary to meet the required 2 nS transition time. The probe factor to b used for the data reduction should be the factor at 175 MHz to encompass the sine frequencies to create the pulse duration and transition time.
Drive cable specified for 50 Ω impedance, 0.5 dB or less insertion loss at 500MHz and a 2-meter length is called out. The cable length can affect the pulse duration so select a cable that is close to 2-meters. If the waveform calibrates properly, the drive cable length should be good but watch for excessive insertion loss.
Figure 1: CS115 Waveform (Top is Goal) (Bottom is Acceptable)
A calibration fixture forming a coaxial transmission line with 50 Ω impedance that supports placement of the current injection probe for the waveform calibration process.
An oscilloscope with the bandwidth necessary to display the transition time without altering the waveshape. The current monitoring probe and the calibration fixture depend on a measurement impedance of 50 Ω, so provide the correct impedance by oscilloscope selection or an external terminator but NOT both simultaneously.
Attenuators, coaxial loads, LISNs and other assorted hardware will likely be needed to accomplish the testing.
As with most MIL-STD-461G testing, a signal integrity check or calibration is specified to either check the equipment functionality/accuracy or to establish test level settings and waveform parameters. In the case of CS115, or calibration process verifies the waveform and pulse generator settings required to produce the test current.
Configure the test equipment as shown in Figure 2 using the components that will be used in the test. Pay attenuation to the cables selected to ensure that the cables used in calibration will support the probe placement in the test configuration. Note that in the figure I placed a 50 Ω termination as a reminder that this is required. The attenuator may be necessary and is to be used instead of a 10X oscilloscope probe to provide the 50 Ω termination.
Start the pulse generator in a repetitive mode at the 30 Hz pulse repetition rate and adjust the pulse amplitude to the test limit current level. Verify the waveform parameter for pulse duration, pulse repetition and transition times. If any of the parameters are not met, resolve the problem and complete the calibration. Leave the pulse repetition setting and record the pulse generator amplitude setting to use as the drive level during testing. Capture oscilloscope recordings/pictures of the waveform for use in the test report. Depending on the placement of the injection probe, a negative pulse may appear. If this occurs, you can turn the probe over or any pulse generators have a polarity selector to provide the pulse that appears as the standard indicates. The pulse polarity is not significant for the test but showing the pulse a positive going prevents reviewer questions.
Figure 2: CS115 Calibration
Once the calibration is complete, configure the test equipment as shown in Figure 3 with the current monitor probe located 5 cm from the Equipment Under Test (EUT) selected connector unless the connector metallic backshell is longer than 5 cm. If the backshell is 5 cm or longer, then place the probe at the point where the cable meets the backshell but not over the backshell. MIL-STD-461G states that each interface connector constitutes a cable, however, if the installation routes wiring from a single connector in different directions the coupling will not be equal, so the common mode test would not simulate a potential differential mode problem when installed.
Confirm that the EUT is operating properly, start the pulse generator and adjust the pulse generator amplitude to the setting established during the calibration. Monitor the EUT operation for susceptibility while applying the transients for one minute. Measure the applied current for the cable under test and record the waveform. The current may be higher or lower than the established test limit because of the characteristic impedance. So, record the current with the pulse generator t the calibrated setting, whatever the current flowing in the cable. The waveform will likely be distorted because the cable is subject to various reactive elements from the wiring terminations and physical position relative to the ground plane. During the test, the cable arrangement should be maintained in the standard configuration to minimize variations from parasitic reactance.
If susceptibility is noted, lower the amplitude using the susceptibility threshold determination method described in MIL-STD-461 section 4.
The attenuator noted in the calibration process is not in the test configuration figure because it is not likely to be required for the monitor probe. However, should the signal overdrive the oscilloscope, an attenuator is used instead of a 10X probe to maintain the 50 Ω termination.
Note that power cables are tested on the bundle, power with neutral and ground and phase leads without neutral or ground. Based on this a 3-phase power cable would test phases A-B-C simultaneously but not individually.
Figure 3: CS115 Test Configuration
CS109 – Structure Current
The purpose of CS109 testing is to evaluate the ability of a device to tolerate current flowing on the device chassis in the 60 Hz to 100 kHz frequency range. Chassis current will create a magnetic field that may radiate into the internal circuitry and affect device performance. This test is applicable to Navy procurements of equipment operating frequency of 100 kHz or less and a sensitivity of 1 µV or less. The test method procedure originated with the release on MIL-STD-462 Notice 4 in 1980 and has remained relatively unchanged to date. As with most susceptibility tests, the device performance specification should establish a quantifiable pass/fail deviation from the standard operation of the device as acceptable.
CS109 Test Equipment
A signal generator covering the test frequency range with an amplifier to produce the required current.
Isolation transformers to control the chassis grounding to a single point location associated with the chassis face subject to test.
A current probe to monitor the current flowing in the test circuit and a 0.5 Ω resistor to prevent a short circuit from overloading the signal source. A measurement receiver is used to monitor the current sensed by the probe. Remember that the measurement receiver should match the current probe 50 Ω impedance.
A coupling transformer with a 0.5 Ω secondary to match the resistor in the circuit.
First, you notice that I omitted the calibration process. Calibration is not used for this test method assuming that your current probe and measurement receiver are calibrated. You may recall that periodic calibration is not required for current probes in MIL-STD-461G, but during CS114 calibration the monitor probe is verified. You should use that method to check your probe to be used for this test.
Figure 4 shows the test configuration and this deserves some discussion. Notice that the figure points out a single point ground associated with the EUT chassis face where the test current is applied. The isolation transformer disconnects the ground associated with the power input circuit. In the figure, other connectors are not indicated but for most devices a connection is present. You may have to provide isolation for these connections too, especially if they present a ground to the system such as a shielded cable termination. It is best to obtain total chassis isolation and then attach the desired single point ground. This isolation is necessary to prevent alternate current paths that would divide the current, resulting in an under-test.
Keeping the test circuit wiring perpendicular to the face for at least 50 cm tends to reduce coupling of the field from the cable into the internal circuitry preventing a higher-level field from affecting the test.
Selecting the face for test is related to the installation. For example, if the device is mounted in an equipment rack, the chassis would be subject to rack chassis current on any face so all faces would be subject to test. Guidance in MIL-STD-461 is provided for selecting the faces subject to testing.
Figure 4: CS109 Test Configuration
Once the configuration is established, the EUT is verified to be operating properly and the signal is applied to the test circuit. At the start frequency, adjust the amplitude to produce the test current and then sweep the test frequency range while monitoring the EUT performance. The sweep speed must be the greater of the minimum specified in MIL-STD-461 or the EUT cycle time (the time it takes for the EUT to complete the process). Record the applied current and any indications of susceptibility for the test report.
RS105 – Transient Electromagnetic Field
The purpose of RS105 testing is to evaluate the ability of a device exposed to an external electromagnetic environment to tolerate an Electromagnetic Pulse (EMP) event. This test is associated with the EUT enclosure – the cabling should be protected by shielded conduit is this potential environment is applicable.
RS105 Test Equipment
Transverse electromagnetic (TEM) cell, parallel plate radiation system or equivalent to provide the radiated field between the plate where the test pulse voltage is applied.
Transient pulse generator with a single pulse generated and the capability to generate the greater than 50 kV voltage amplitude per meter of plate separation. A 2-meter plate separation would require at least 100 kV.
Oscilloscope that will store the pulse with enough bandwidth (700 MHz or more) to capture the waveform parameters. Sampling up to 1 gigasample per second is needed for the digitizing oscilloscope. A high voltage probe with at least 1 GHz bandwidth is necessary.
LISNs are placed in the power input circuit and terminal protective devices (TPDs) are used to protect the power distribution from the high-level transient generated for testing.
B-dot (magnetic field) or D-dot (electric field) sensors are used to sense and measure the time rate of change of the generated field.
Integrator to convert the sensor rate of change output into an integrated waveform from the sensor.
Figure 5 provides a generic test configuration based on a parallel plate radiation system. Configure the equipment as indicated in the figure but before installing the EUT place the sensor in the center of the vertical plane where the EUT front will be located. Connect the sensor to the oscilloscope second channel so the sensor signal and HV probe waveforms can be measured simultaneously.
Generate a pulse and adjust the generator to produce the required peak amplitude. Verify that the waveform parameters (i.e., amplitude, rise time pulse width) meet the requirements. Record the drive pulse waveform as measured by the oscilloscope. The amplitude must be at least 50 kV but can exceed that by 6 dB or up to 100 kV.
Repeat the calibration process with the sensor located at each of the four corners where the EUT front will be located making sure that the waveform parameters are met. If the amplitude is adjusted during any of the calibration points, then repeat the measurements at the previous points to obtain generator settings that satisfy all sensor points simultaneously.
Remember that the voltages have the potential to be lethal so pay close attention to all safety precautions and interference with other equipment in the laboratory is a risk to isolate or shield the test location.
Figure 5: RS105 Test Configuration
After the calibration is complete, remove the sensor and install the EUT in the parallel plate. Orient the cables normal to the field and shield the cables properly. Note that the test volume is bounded by ½ of the length and height of the plates.
Enable operation of the EUT and allow it to stabilize. Apply the pulse starting at 10% of the peak amplitude and increase in steps sizes of 2 or 3 until the peak amplitude is attained. Apply the required number of pulses at a rate of not more than one per minute while monitoring the EUT performance for susceptibility indications.
Repeat the test with the EUT in each of three orthogonal planes recording all the specified measurements for report entry.
The test equipment manufacturer literature and training will provide vital information on the equipment use and need for pressurized dry chambers for the electrodes. As mention earlier, lethal voltages are in use so maintain safety precautions.
As with all MIL-STD-461 tests, planning is an essential element and planning includes defining the hardware necessary to accomplish the testing. Make sure you are aware of reporting requirements, so you will capture the necessary information during test.
Contact me with questions relative to this review or any other subject matter related to EMC, Electrical Safety or Environmental testing or designing for compliance.