Introduction
This article discusses CS114, including the updates contained in MIL-STD-461 revision “G”, the current revision and the associated nuances that lead to test errors. The fundamental idea behind CS114 testing is to simulate currents induced into cabling from electromagnetic (EM) fields from high level emissions, intentional or unintentional. This test allows an evaluation in the laboratory where cable length and low frequency radiator systems prevent coupling from or generation of the high-level field. When dealing with CS114, the engineer needs to keep in mind that the test goal is induce current in proportion to an associated radiated field. At the lower end of the test frequency range, conduction via power cables from noisy power sources can be the coupling path but inductive reactance of the wiring tends to attenuate the conducted interference.
The limit curves consider cable resonance and current induction of approximately 1.5 mA per V/m of the radiated field at resonance. The lower frequency slope considers that coupling is proportional to frequency with a 20 dB/decade slope. The upper frequency slope is based on measured data.
This test method originated as a power line test using direct injection over the 50 kHz to 400 MHz frequency range by the CS02 test method. The CS02 method connected the signal source to the power line via a coupling capacitor. The coupling capacitor blocked the power line frequency while passing the interference signal with less than 5-ohms of RF impedance. The signal source provided an output power of 1-watt (7Vrms into a matched load). The test level applied 1 Vrms at the power terminals but if the signal source was unable to produce the test voltage the EUT was considered compliant if susceptibility was not demonstrated. During test the voltage appearing across the EUT terminals was measured and recorded which would support characterization of the circuit impedance.
In 1993, MIL-STD-461D introduced the CS114 test method effectively replacing the CS02 method with test frequency range of 10 khz to 400 MHz. This added signal line testing and placed a set of calibration curves correlated to an associated radiated field test level. The testing was accomplished by applying a pre-calibrated forward power but limiting the induced current to 6 dB above the highest calibration level for the applicable curve across the test frequency range. The test configuration was nearly the same as used today.
MIL-STD-461E became the standard in 1999 and instituted a change regarding the test current. The test current curve followed the calibration curve with a 6 dB higher level. This placed a focus on using the cable induced current instead of using the calibration curve derived from field levels and allowing the current to elevate if the impedance was low. The 6 dB difference is related to calibration being accomplish with a 100-ohm calibration circuit restricting the current by 6 dB for the 50-ohm drive system.
MI-STD-461F maintained the basic approach and exempted antenna ports from the requirement except for surface ships and submarines. Navy applications also added a power line common mode test over the 4 kHz to 1 MHz frequency range. This was implemented from an issue with solid-state power generation systems and became applicable to all systems that could be used for this power system.
MIL-STD-461G has returned to the revision “D” approach where the calibration level is the target and the test current can be allowed to go to 6 dB above the highest calibration level across the complete frequency range. This supports a better correlation to cable current from platform measurements of cable current and exposure to low level illumination. The low-level illumination and associated current will identify resonant frequencies for the actual cable and analysis of laboratory data or platform level current injection can be used to assess the risk at a full threat level. If the installed cable characteristic impedance is low, then the calibration drive level will produce a high current and a high impedance will limit the current.
Revision “G” released in 2015 is the current standard and will be used as the basis for the following discussion detailing the test procedures for CS114.
CS114 Drive Level Calibration
As with most MIL-STD-461G tests, we start with a calibration process, except in this case we are establishing the forward power drive level necessary to produce the calibration current in the test fixture. Figure 1 shows the primary step calibration configuration, where the signal drive system with monitoring and the injection probe to be used for test, are assembled. The secondary of the injection probe is placed in the calibration jig to allow the signal coupling to the cable center conductor. The secondary configuration establishes a closed loop for current flow via the terminator on one terminal and the measurement receiver on the other terminal. Note that the terminator and measurement receiver configuration places these two 50Ω loads in series establishing a 100Ω circuit impedance. The configuration includes the current monitor probe to be used during test with a terminator on the measurement port.
Once configured the signal source is adjusted to the start frequency and the amplitude elevated to produce the calibration current in the secondary circuit. The process continues through the test frequency range with adjustments to the amplitude to maintain the calibration current. The forward power is recorded throughout the test frequency range to obtain the drive levels that will be the target settings for testing although the cable under test current is an alternate limiting factor.
Figure 1: CS114 Calibration Check Drive Level Configuration
CS114 Monitor Probe Calibration
MIL-STD-461G brought into effect the second part of the CS114 calibration process with a signa integrity check of the monitor probe. Recall that the “G” revision removed a requirement for periodic calibration of passive devices. This monitor probe calibration effectively accomplishes a calibration of the current monitor probe by monitoring the current in the secondary circuit while producing the current that we just established for the drive levels.
Figure 2 shows the configuration where we have now placed a measurement receiver on the current monitor probe and a terminator in place on the initial location of the measurement receiver. The process has the signal source recreate the drive levels and the current monitor probe should measure the calibration current. A 3 dB tolerance is allowed and if that is not attained, the test equipment should be examined and the issue resolved prior to testing.
This may seem like a bit of overkill by checking the monitor probe with each test in place of an annual probe calibration. However, we needed to check the signal integrity anyway, and with automated tools, the complete check doesn’t take much longer. Note that the 3-second dwell per frequency step can be reduced to the settling time for measurement since we are not needing to dwell for a susceptibility indication during the test phase.
Figure 2: CS114 Monitor Probe Calibration Configuration
CS114 Conducted Susceptibility Testing
Now that the calibration process has been successfully completed we can move the probes to the selected cable for testing as shown in Figure 3. The current monitor probe is located 5-cm from the EUT and the injection probe is 5-cm from the current monitor probe. The signal source is adjusted to the start frequency and amplitude elevated until attaining the lesser of the calibration forward power or the test current.
Remember that the test current is 6 dB above the highest calibration current and that it covers the complete test frequency range.
The test frequency range is tested with specified frequency step sizes and a dwell time minimum of 3-seconds at each step. The dwell time may need to be increased if the EUT operation or response is longer. The test process is repeated for each cable to be tested.
Figure 3: CS114 Test Configuration
CS114 Odds & Ends
Current Flow: The test configuration establishes a specified cable arrangement that some feel may not be significant for a conducted test. That lack of significance is definitely invalid for this test. Figure 4 shows a sketch of a typical CS114 test layout. The cable is placed above the ground plane with distributed capacitance along the cable length and the cable wiring tends to be inductive. As the test frequency increases, the capacitance reactance impedance decreases and the inductive reactance impedance increases. This allows the loop current to circulate in smaller loops, as indicated, with increasing frequency. Failing to maintain the standard configuration or failing to control the configuration can induce major effects on the circuit current.
- Cable elevation changes the capacitance, and depending on the frequency and other parasitic parameters, the effects can influence the circuit current by more than 30 dB.
- Cable proximity to other cables affect cable-to-cable coupling via the distributed capacitance and can cause other cables to be the coupling path into the EUT and misleading the problem identification.
- Cable radiation from current flow can illuminate the other cables, support equipment, the EUT and produce interference to neighbors.
- Probe position can allow the current loop to circulate the interference without current entering the EUT during higher frequency tests.
- Support equipment becomes part of the circuit, especially at lower test frequencies, so isolation becomes a necessary part of the configuration.
Figure 4: CS114 Test Current Flow
Split Cable: MIL-STD-461G assumes that a cable is associated with a single interface connector. This establishes that CS114 cable bundle testing would be performed on the entire cable. If the installation calls for wiring from a connector to be routed in different directions, the common mode test may be inappropriate since the wiring would be exposed to different fields and could produce a differential current in the interface – just a thought for consideration.
Backshell Length: The cable backshell can influence the probe placement. Note that MIL-STD-461G indicates that backshells of 50 mm or less are part of the EUT chassis and if longer than 50 mm the backshell is part of the cable.
Core Wiring: CS114 testing calls for testing of the power cable bundle, testing the power wiring including neutral and ground and testing of the power phases (positive) without ground or neutral. We recognize that the use of shielded power cables is not permitted except for selected installations, but it has been a long practice to cut open the shielded cable to access the power core wiring for test. This is inappropriate. In paragraph A.4.3.8.6 of MIL-STD-461G we are reminded that the intent is to test cables as they are installed, and removing the shield was not the intention.
Susceptibility Threshold: If susceptibility is observed, a threshold of susceptibility is to be measured. The threshold is determined by reducing the interfering signal amplitude until the EUT does not show susceptibility and then lower an additional 6 dB to compensate for hysteresis effects then increase the signal amplitude until susceptibility is observed. This applied current amplitude in dBμA is the threshold. Until revision G was released the number of threshold measurements was not specified. Now the standard indicates that a minimum number of threshold measurement to identify the start and end frequencies of susceptibility be performed, as well as the worst-case measurement. I believe that more points should be used to help map the profile of the susceptibility such a 2 to 3 frequencies per octave of the frequency range where susceptibility was observed. Note that threshold measurements should be provided for each different susceptibility indication if applicable.
Note also that a susceptibility indication could be related to a radiated issue associated with the conducted testing. Current injection into the cable is simulating coupling of a radiated field. The current flowing in the cable will have the effect of producing a radiated field from the cable serving as an antenna. This radiating field is located adjacent to the EUT chassis and can be significant. This high-level field may penetrate the chassis or apertures and cause susceptibility. It may be beneficial to temporarily place a foil shield over the chassis and see if the threshold changes indicating that the problem may not be entering via the cable. This can also be used to examine other cables for coupling.
Summary
The CS testing is included with most qualification test programs and many hold a misconception that the configuration for conducted tests is not significant. I recall hearing an auditor tell the test personnel that cable placement didn’t matter for the conducted testing. As indicated in this review, we must overcome that philosophy and understand the influences affecting the test program. The testing can be very time consuming but can be automated with the realization that the automated tools accomplish the task properly. Monitoring the EUT performance continually is an integral part of the process and note that the simulation and monitoring equipment can easily be susceptible, so isolation in the test configuration design needs to be considered.
Hopefully you find this a worthwhile read and if you want to discuss or have questions, don’t hesitate to contact me.
