The continuing evolution of MIL-STD-461: Version F.

In 2007, MIL-STD-461 turned 40 and Version F was released on Dec. 10. MIL-STD-461F includes changes as a part of normal evolution to update requirements and methods to more closely evaluate products for the changing electromagnetic environment.

The new method restores a test that was deleted by the release of MIL-STD-461D in 1993. Changes in the standard may be significant when qualifying products to the now current revision. In conjunction with the release of MIL-STD-461F, three Data Item Description (DID) documents were released.

Here is a clause-by-clause review of the standard where differences between the E and the F versions are notable.

Interchangeable Modular Equipment (4.2.7)

A new item in MIL-STD-461F requires qualification of assemblies when new line replaceable modules (LRMs) are incorporated into devices. The qualification can be by test or similarity assessment but requires approval of the procuring agency.

Construction and Arrangement of EUT Cables (4.3.8.6)

"Input power leads, returns, and wire grounds shall not be shielded." This requires breaking out the power leads that are part of a cable bundle shielded for the test (4.3.8.6.2).

In 1993, MIL-STD-461D definitively established the requirement to test with cables of the type used in the installation. This was a major move to force responsibility for EMI/EMC control into the hands of the integrator/supplier or whoever had providence over the interconnecting and power cables.

This revision removes the capability to shield power cables as an EMI control measure because, in most installations, shielded power cables are not the norm. An exception is discussed (A4.3.8.6) where filtered power is provided from another device: Shielding may be supported.

In addition, shielded cables for Navy surface ship applications may allow an unshielded section for radiated tests but not for conducted testing. The test configuration needs to be described in the test procedure and approved by the procuring agency. But basically for most applications, the power cables should be unshielded.

Computer Controlled Instrumentation (4.3.10.2)

Verification of software needs to be described in the test procedure. For commercial software, identification of the manufacturer, model, and revision must be provided. For locally developed software, the control and methodology must be indicated.

This presents a challenge for test-procedure developers who aren't part of the designated test laboratory. The laboratories should prepare the necessary documentation and make that available to procedure writers to support designation of the test laboratory.

Bandwidths (4.3.10.3.1)--Alternate Scanning Technique

"Multiple faster sweeps with the use of a maximum hold function may be used if the total scanning time is equal to or greater than the Minimum Measurement Time defined ...," referring to the measurement time found in Table II. This may give an impression that the test duration can be reduced, but clearly that is not the intent.

The discussion in the appendix indicates that the faster sweep allows for capture of low duty-cycle or intermittent signals. However, in section 4.3.10.3.3, the scanning rate must be adjusted to capture infrequent emissions.

To assure capture, the overall scan needs to be at every one-half resolution bandwidth over the EUT cycle. Realizing that signals such as frequency-hopping modulations would require an exceptionally long sweep for all of the energy to be quantified, don't hesitate to use the faster sweep with multiple max-hold capture to show the envelope of this type of signal.

Frequency Scanning (4.3.10.4.1)

The susceptibility sweep rate or step size has been increased for frequencies above 1 GHz, allowing a faster susceptibility test. This change of sweep rate or step size reduces the test time even with a long EUT cycle time since the cycle time affects the dwell period and not the step.

Thresholds of Susceptibility (4.3.10.4.3)

MIL-STD-461F adds a statement, "Susceptibilities and anomalies that are not in conformance with contractual requirements are not acceptable. However, all susceptibilities and anomalies observed during conduct of the test shall be documented." What is the implication?

It is not unusual to test hot--at levels above that required. And the possibility exists to observe an anomaly at the elevated test levels only to find that the anomaly is not present when the correct test level is applied to the EUT. This new statement requires documentation of all observed, albeit compliant, anomalies.

Emission and Susceptibility Requirements, Limits, and Test Procedures (5.3)

A new CS106, formerly identified as CS06 in MIL-STD-461C and dealing with voltage spikes on power input lines, is listed in Table IV. The applicability list of Table V includes a few changes in applications associated with some of the test methods such as CE101, CS109, CS115, CS116, and RS101.

CEIOI (5.4)

Applicability has been added to surface ships. In addition, the appendix provides some tailoring guidance for high current loads or certain wiring considerations. Suggestions are made for the use of a 5-[micro]H line impedance stabilization network (LISN) and limit adjustments with frequency range changes.

MIL-STD-461 has long supported tailoring for several revisions, but this represents one of the few specific suggestions for tailoring. The tailoring falls on the procuring party, but a test plan could recommend the tailoring for approval by the procuring party. Calibration verification with all test equipment including cables, probes, attenuators, amplifiers, and receivers is accomplished prior to test. If multiple limits for different power inputs are specified, use the most restrictive limit to demonstrate.

CE102 (5.5)

No changes in the requirements except the tailoring regarding Section 5.4 affects CE102.

CE106 (5.6)

Testing of both the receive and the standby mode is unchanged. The CE106 transmitter limit for the 2nd and 3rd harmonics was redefined to a level of -20 dBm (87 dB[micro]V) or 80 dB below the fundamental, whichever requires the least suppression. All other harmonic and spurious emissions must be suppressed by 80 dB.

Assuming a 100-W (50 dBm, 157 dB[micro]V) transmitter, the suppression would be 70 dB to achieve the -20 dBm level for the 2nd and 3rd harmonics and 80 dB for all other frequencies. Making this measurement requires a dynamic range sufficient to show compliance to the limit.

Other issues include determining the frequency span associated with the harmonic emissions. How is the transmitter output power verified since in-band testing is not required? How is the fundamental suppressed without sacrificing sensitivity at out-of-band frequencies? Is the power in the harmonics sufficient to cause nonlinearity in the detection system? How do you handle connection to a transmit port with a type N connector during testing up to 40 GHz?

These are some of the questions that need to be addressed prior to testing--typically during test-procedure development--so both the right equipment and test approach are in place to support the test. Be prepared for several hardware configurations and the associated calibration verification.

CSI0I (5.7)

While there are no changes in the requirements, don't forget the capacitors. The higher-frequency losses in the LISN without the capacitors are fairly dramatic and result in a significant undertest. Make all personnel aware of the potential for shock hazards from the isolated oscilloscope configuration.

CS103, CS104, CS105 (5.8, 5.9, 5.10)

There are no changes in the requirements because the testing has been a tailored requirement in the contract for a long time. This test normally calls for a lot of preparation to achieve the test method and limits. Often, the procedure is developed as part of the contract preparation, and if not, the procedure and limits are developed and then added to the contract through test-procedure approval.

CS106 (5.11)

This is a new requirement that brings power-line voltage transient testing back into the requirements for some applications. It restores CS06 testing from MIL-STD-461C, superseded in 1993, but with only one pulse duration.

The details are spelled out in the standard, but basically the 5-[micro]s pulse at 400 V is precalibrated into a noninductive 5-[ohm] resistor. That generator setting is used as the maximum applied if the 400-V pulse is not generated during the test with a lesser generator setting.

The waveform characteristics are very well defined, and some of the older spike generators are not adequate for the specification. Once the generator level is calibrated, the positive and negative transients are applied to all ungrounded power inputs between phases or between the phase/positive and the neutral/return.

The application between the chassis and phase/positive is not applicable. The test duration is 5 minutes with a 5- to 10-pulse/s repetition rate for each polarity. Unlike the old CS06, phase synchronization is not applicable. Again, watch for the shock hazard with the ungrounded oscilloscope.

CS114 (5.13)

The testing is basically the same with an additional common-mode test for power leads in the 4-kHz to 1-MHz frequency range for some applications. Here is a refresher on the process because often it is accomplished incorrectly:

* Precalibrate the applicable calibration curve levels to establish a maximum forward power level for the test-frequency range. The standard also reminds us to use the same hardware as used for the calibration.