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Conducted susceptibility testing: a mil-std-461f cs114 tutorial.

RF signals appearing on wiring are common in virtually all installations of electronic equipment. Wiring acts as a receive antenna for radiated RF signals that induce currents in conductors; this may produce voltages in circuitry sufficient to impair operation.

Interfering signals are nearly always differential at the terminals of the victim circuitry; however, most field to cable coupling induces common-mode voltages. A common-mode to differential-mode conversion may occur due to unbalanced circuitry or parasitic elements that produce essentially unbalanced impedances at the victim terminal.

MIL-STD-461F CS114, a conducted susceptibility test applicable to power and signal conductors, is a simulation of RF signals inducing currents onto the interface cables. It is used in lieu of creating RF fields for the purpose of radiated susceptibility testing.

In an actual installation, cables may be fairly long, a significant fractional wavelength, and effectively receive an RF transmission such as in the case of a long power line cable running from a circuit breaker to a receptacle. In the test environment, the space may not permit installation and exposure of the long cables, and evaluation using a radiated RF field may severely under-test the risk. Accordingly, CS114 directly induces the current simulating the effects of exposure to radiated interference.

EMC engineers generally agree that the potential for interference exists. Tests over the years have shown many ill effects of the risk from conducted susceptibility, ranging from data loss to circuit destruction, and many issues between the extremes. The first time I saw uncommanded movement of large machines, the risk truly became obvious.

The Limits

Now let's discuss testing by MIL-STD-461F.

Primary Limit--Test Current

Five limits are presented in MIL-STD-461F, representing potential currents for various environments (Figure 1).

[FIGURE 1 OMITTED]

The limit curve appearing in MIL-STD-461F is the calibration limit. This is NOT the test current. The test current is the calibration limit plus 6 dB.

Secondary Limit--Power Limit

A secondary limit is defined by performing maximum forward power measurements using the calibration current as defined in MIL-STD-461F for the applicable test level. This standard brought a noted change to the limits by adding a lower frequency test range for power lines associated with U.S. Navy shipboard applications.

The Precalibration Test Process

* Select the applicable limit; if multiple limits are applicable, select the most stringent limit.

* Configure the test equipment as shown in Figure 2 with the injection probe on the center conductor of the test fixture. When placing the injection probe in the test fixture, check for loose fixture plates and ensure the center conductor is snug by applying slight pressure. The plates and conductor should not move.

[FIGURE 2 OMITTED]

* Adjust the signal generator to the start frequency and the amplitude to produce the calibration current in the fixture circuit. The receiver measures a voltage so the current is the measured voltage minus 34 dB (subtracting 34 dB is the same as dividing linear voltage by 50 [ohm]) to obtain the current in linear terms:

34 dB = 20 Log(50)

* Record the power amplifier output with the RF voltmeter. Scan the signal generator over the test frequency range, maintaining the forward power necessary to produce the calibration current. This scan will allow the current to vary and the forward power to change so a decision on acceptable tolerance is needed prior to the scan.

The decision will need to allow the current to stay above the minimum level and below a maximum current you determine. This allowance is not defined in the standard so a personal tolerance should be established.

The use of an automated drive system can easily set a very tight tolerance with set points for each step of the test frequency range. Manual testing should allow for more flexibility in the test levels.

The test system equipment including probes and amplifiers may need to be changed to cover the entire test frequency range. At defined frequency points where the hardware is changed, obtain the forward power measurements at the same frequency with both hardware sets. This ensures continuity across the transition.

Modulation is not used for the test signal for the calibration process. Also remember that the calibration current is used.

The Test Process

* Configure the EUT and test equipment as shown in Figure 3. The test calls for application of the interfering signal at the specified test current into the EUT. As a result, the current monitor probe is placed on the EUT side of the cable under test.

[FIGURE 3 OMITTED]

During the test, compliance is demonstrated when the precalibrated maximum forward power is applied, irrespective of the induced current. If the EUT loop impedance power is higher than the calibration loop impedance, the maximum forward power is the test level limiting parameter.

For this reason, the applied current is recorded and should satisfy the question: Is the applied current what is expected? The test engineer should be able to answer that question. If the answer is no, then examine the circuit and test instrumentation for proper connection and operation. In my experience, this review has identified an improper test configuration buried under connector backshells or cable shields that should not be in the configuration.

* Power on the EUT. While monitoring the EUT for indications of susceptibility, set the signal generator to the start frequency and then adjust the output amplitude until the lesser of the test current or the precalibrated forward power is reached. Apply the specified modulation and remain at the set frequency for the dwell period. Dwell period is the cycle time of the EUT but not less than 3 seconds. Assuming that the EUT is not susceptible, scan the test frequency range, maintaining the test current or precalibrated forward power at the scan rate specified in MIL-STD-461F.

* Document the test results presenting the applied current and frequencies at which the test was conducted. Provide any susceptibility indications and threshold measurements.

* Repeat the test for each cable identified for test. Testing is specified for each cable bundle including complete power cables, power cables excluding power returns and grounds, and power cables that include signal wires in the power cable bundle.

The power cable test that excludes signal wires, power returns, and ground indicates a single wire (phase) test or multiple wires (polyphase) grouped.

Threshold Measurements

If the EUT is affected by the susceptibility test signal, it is necessary to determine the threshold of susceptibility. MIL-STD-461F provides guidance on threshold measurements. A summary is provided:

* At the frequency of interference, lower the signal amplitude until the indication of susceptibility is not present.

* Reduce the signal an additional 6 dB; remember that 6 dB is half the current.

* Gradually increase the amplitude until the indication of susceptibility reoccurs. This is the threshold of susceptibility. Record the threshold level and the indication of susceptibility.

This process generally works, but if damage to the EUT is indicated, an analysis would be needed to determine the failure mode. As often happens, the EUT may be susceptible over a wide frequency range.

How many frequencies should be measured? The laboratory needs to establish an approach to make a sufficient number of measurements that envelope the frequency range where susceptibility occurs and identifies the lowest threshold level.

Consider an approach of measuring thresholds at the frequency where susceptibility is first noted, highest frequency, lowest threshold, and at least three frequency points per octave over the range of susceptibility. In addition, other points of inflection on the threshold curve may be needed. If multiple indications of susceptibility are present, then each indication needs to be addressed.

Odds and Ends

Modulation

MIL-STD-461F calls for testing with pulse modulation (PM) of 1 kHz with a 50% duty cycle. The on/off ratio of the modulation must be at least 40 dB (100:1).

Frequently, low-frequency signal generators do not support pulse modulation so amplitude modulation (AM) is used as an alternative. Using a square wave source for the modulating signal and setting the generator for 100% modulation, the AM produces the desired effects. But with it, a couple of undesired effects are present.

AM causes an increase in the signal amplitude during the crest of the modulating signal, and with 100% modulation, the addition is 6 dB or double the unmodulated voltage. This results in an over-test condition. Secondly, the over-test may place the RF amplifier into saturation. This over-drive of the amplifier could produce spurious signals and would definitely not be a 6-dB increase.

Radiation From Cables

Many test facilities perform CS114 testing in an open lab location, not in a shielded enclosure. The test stimulates cable currents from radiated signals and that the inverse occurs; that is, radiated signals are emitted from the cables from the induced currents associated with the test. The radiated signal levels can be significant and may easily exceed the FCC limits.

Cable radiation also can cause the test article to be susceptible from the fields that radiate energy into other parts of the system. Isolation can be used to confirm the true coupling mechanism affecting the EUT, but assuming that conducted is conducted may lead you away from addressing the true problem.

Shielded Power Cables

In some specific applications, shielded power cables should be used for testing if appropriate. CS 114 testing calls for testing the phase or positive leads separately such as without returns or grounds.

Most interpret that to mean a shielded power cable should be opened to get access to the phase leads for testing. That is not the case. In Appendix A Construction and Arrangement of EUT Cables, it is stated that it is not the intent to inject to the core wiring unless specifically directed by the procurement contract.

Conclusion

Hopefully, this summary of MIL-STD-461F CS 114 testing is a refresher to old-timers and an introduction to the new arrivals in the EMI/EMC testing discipline. This test lends itself to automating the test process, but it doesn't imply that a recorded precalibration should be played back indiscriminately. The current levels must be monitored during the test to avoid over-testing.

About the Author

Steven G. Ferguson is vice president of operations at Washington Laboratories. He has been working in the compliance test arena for more than 35 years at test laboratories and manufacturing companies designing products, developing procedures, and performing tests. Mr. Ferguson also presents a hands-on course in testing to MIL-STD-461 for multiple government and industrial clients. Washington Laboratories, 7560 Lindbergh Dr., Gaithersburg, MD 20879, 301-216-1500, e-mail: stevef@wll.com

by Steven G. Ferguson, Washington Laboratories
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Title Annotation:EMC TEST
Author:Ferguson, Steven G.
Publication:EE-Evaluation Engineering
Geographic Code:1USA
Date:Jun 1, 2009
Words:1745
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