A MIL-STD-461F Tutorial
Cables—the nemesis of compliance, the antennas no one wants—often are the culprits or unwanted stepchildren in EMC testing. Controlling conducted emissions is an inherent problem that requires planning in the design phase. Selecting the appropriate control measures, installing them in an effective manner, and assessing if the EMC design conflicts with an operational requirement are integral to the product compliance.
To mitigate these problems, it's important to understand the testing process and associated test elements that may contribute helpful information. And since MIL-STD-461 supports tailoring the test approach to meet the test goals, we will examine some things to consider when the mold needs to be adjusted.
CE101: Conducted Emissions, Power Leads, 30 Hz to 10 kHz
CE101 testing is performed on power leads and returns that obtain power from sources that the EUT might share with other users.
Calibration Verification
First, and most critically, calibration assures that your measurement equipment is working properly and verifies that the measurement system has sufficient sensitivity such as the capability to measure signals as much as 6 dB below the applicable limit.
Figure 1 shows the calibration verification configuration. The process involves the following steps:
• Determining the limit and calculating the current at 6 dB below the limit for the selected verification frequencies.
• Selecting a convenient resistor value (R) and calculating the voltage needed to produce the current through the resistor circuit loop.
• Adjusting the signal generator to the selected frequency and amplitude to produce the rms voltage measured across the resistor.
• Using the measurement system hardware including the current probe, cable, and spectrum analyzer that will be needed to measure the current flowing in the circuit loop. The measurement system software should apply the appropriate correction and conversion factors to reduce the data. The resulting measurement should be 6 dB below the limit ??3 dB.
• Repeating all applicable frequencies.
• Taking corrective action and repeating the process if the calibration is not within the required tolerance.
If you have multiple applicable limits, then perform the calibration verification using the most stringent limit. This will prove the accuracy of the measurement system and the capability to detect at the lowest required level.
Configuration for Test
The configuration for test requires unshielded wiring for power with a cable length of 2.5 meters. If the power is contained in a bundle with other wiring, the power lines are separated as close as possible to the EUT power connector. Route the cables 5 cm above the ground plane with 2 meters exposed along the ground plane and 10 cm from the front edge. The last 0.5 meter of the power line is routed to the line impedance stabilization networks (LISNs). Terminate the LISN measurement ports with 50 Ω.
Testing
• Place the current probe 5 cm from the LISN power connection. Use caution when connecting near live terminals.
• Operate the EUT.
• Sweep the test frequency using the measurement system equipment that performed the calibration verification. The measurement system should apply the applicable correction and conversion factors and provide a graph comparing the measurements to the limit.
• Repeat for each power line.
• If over-limit emissions are detected, an ambient test is needed to verify that the EUT is not the source. For ambient testing, the EUT is replaced with a resistive load that draws the same rated current as the EUT. All support equipment must be operating normally. If the ambient measurements are within 6 dB of the limit, correct the problem and repeat testing.
The ambient sweep is performed only if the EUT measurements are noncompliant. In other words, run the ambient tests if needed. To speed testing, limit the frequency range to the problem areas.
Capacitance on the EUT power input could raise the current at high frequencies. MIL-STD-461F suggests that the capacitance be considered as part of the ambient test circuit with the resistive load. On the surface, this seems contradictory to the definition of ambient conditions, but the impedance of the capacitor at high frequencies could affect current from outside sources. Also, be aware of potential resonances if unexpected ambient issues are observed.
How close to the rated current should the current drawn by the resistive load be for the ambient tests? As close as possible, however, this is a question of measurement tolerance. For example, if 2 dB is measurement tolerance, then the load can have a tolerance of 25%. This is not addressed in the specifications, so a good idea is to include this determination in the test plan.
If over-limit emissions are detected, for mitigation purposes it is necessary to determine whether the emissions are common or differential mode. This information could aid in selecting the most effective control measure.
To make this determination, place the current probe on the phase and neutral (positive and return) leads simultaneously. If the emissions levels drop by at least 6 dB, differential-mode emissions are indicated. Little to negligible change in emissions indicates common-mode currents.
High current loads may present an issue for CE102 testing with the standard LISNs. An option to extend the CE101 test-frequency range to 150 kHz and implement a 5-??H LISN is discussed in the MIL-STD-461F appendix. Also, elimination of the LISN altogether can be considered with appropriate tailoring and limit definition.
CE102: Conducted Emissions, Power Leads,
10 kHz to 10 MHz
CE102 testing is performed on power leads and returns that the EUT might share with other users.
Calibration Verification
Again, calibration verification is a check of the data-collection system. Figure 2 shows the calibration verification configuration. A transient limiter and attenuator may be necessary to protect the spectrum analyzer. It must be in place during the calibration verification. The process involves the following steps:
• Selecting the limit and calculating the voltage at 6 dB below the limit for the selected verification frequencies.
• Configuring the calibration verification equipment. The oscilloscope is needed to measure the applied voltage because at lower frequencies the LISN impedance is low and loads a 50-Ω signal source to a lower value than is displayed on the source.
• Adjusting the signal generator to the selected frequency and amplitude to produce the rms voltage measured by the oscilloscope for the specified frequencies. The oscilloscope must be removed after the lower frequency tests are completed.
• Using the measurement system hardware including the LISN, cable, limiter, attenuator, and spectrum analyzer to measure the voltage at the LISN measurement port. The measurement system software should apply the appropriate correction and conversion factors to reduce the data. The resulting measurement should be 6 dB below the limit ??3 dB.
• Repeating for all applicable frequencies.
• Taking corrective action and repeating the process if the calibration is not within the required tolerance.
If you have multiple applicable limits, then perform the calibration verification using the most stringent limit. This will prove the accuracy of the measurement system and the capability to detect at the lowest required level.
Configuration for Test
Use the same test configuration setup as listed for CE101 but replace the current probe with the LISN measurement port.
Testing
• Connect the measurement system to the LISN measurement port. Use caution when connecting near live terminals.
• Operate the EUT.
• Sweep the test frequency range using the measurement system equipment that performed the calibration verification. The measurement system should apply the applicable correction and conversion factors and provide a graph comparing the measurements to the limit.
• Repeat for each power line.
• If over-limit emissions are detected, an ambient test is needed to verify that the test configuration is not the source. For ambient testing, the EUT is replaced with a resistive load that draws the same rated current as the EUT. Repeat the test. If the ambient conditions are greater than 6 dB below the limit, correct the problem and repeat testing.
The ambient sweep is performed only if the EUT measurements are noncompliant. In other words, run the ambient tests if needed. To speed testing, limit the frequency range to the problem areas.
As with CE101, consider capacitance effects which potentially could be even more significant because of the CE102 test frequency range and the tolerance for the resistive load during ambient tests.
The LISN presents a standard for measurement that was developed to simulate long power cable installation. The 50-??H inductance of many types of power LISNs simulates this cable inductance.
For high current loads and installations that normally have short power distribution distances such as aircraft power, the 5-??H LISN is more appropriate. If the situation demands, plan for the alternative LISN and other changes including CE101 testing to 150 kHz and the associated limit.
Another alternative is the use of a voltage probe for high current circuits. The absence of LISNs in the measurement circuit presents uncontrolled power line impedance. The voltage probe also is used routinely for in situ measurements. Noise from other equipment will affect the measurements. The voltage probe requires calibration and the determination of appropriate correction factors.
CE106: Conducted Emissions, Antenna Terminal, 10 kHz to 40 GHz
CE106 testing is performed on the antenna terminals of receivers, transmitters, and amplifiers but is not applicable to equipment with permanently mounted antennas. It excludes the frequency range defined by the larger of the bandwidth of the transmitted signal or within ??5% of the fundamental frequency. The test frequency range is based on the EUT operating parameters with a start frequency of 10 kHz, 100 kHz, 1 MHz, or 10 MHz and an end frequency of the lesser of 20 times the highest generated or received frequency or 40 GHz.
The receiver limit is defined as 34 dB??V across the frequency range. The transmit limit for harmonics and spurious emissions is 80 dB below the fundamental frequency amplitude except for the 2nd and 3rd harmonics. The 2nd and 3rd harmonics are suppressed to a level of -20 dBm or 80 dB below the fundamental, whichever requires less suppression.
The transmit limit is based on the transmit power, so the limit is tailored with the overall amplitude and segments for the fundamental and the 2nd and 3rd harmonics. During test, the fundamental power output is verified to be equal to the allowed fundamental limit to assure operation at the full power level.
For example, Figure 3 shows a limit for a UHF transceiver with a transmit power of 10 W, based on a transmit frequency of 310 MHz. The UHF operating range is 225 MHz to 400 MHz. Given these operating frequencies, CE106 testing requires a tunable EUT to be tested within 5% of the low, mid, and high tuned frequencies. A separate limit may be required for each frequency because the output power may vary across the tunable band.
Calibration verification checks are required with frequencies specified for the low, middle, and high frequencies in the band. This requirement is necessary for each hardware configuration used for testing.
The calibration verification process involves the following:
• Determine the frequency and calibration level.
• Apply a signal at the calculated points.
• Collect and reduce the measurement data.
• Verify that the measurement meets the requirement with a maximum tolerance of ??3 dB.
Testing the receiver or standby modes of operation normally is straightforward: Connect the measurement system including the cable, pre-amplifier, and spectrum analyzer to the antenna port and sweep the test frequency range measuring the emissions. Compare the results to the limit after applying the correction factors.
Testing the transmit modes requires a means to prevent damage to the measurement system from the high-level transmit power. Figure 4 provides a potential transmit test configuration set for a UHF transmitter. The EUT output is connected to a directional coupler to terminate the transmitter into a dummy load and provide an attenuated signal to the measurement system.
The notch filter is tuned to the transmitter fundamental frequency and provides additional attenuation of the transmit fundamental without attenuating the out-of-band emissions. A pre-amp is included to compensate for the losses of the directional coupler and notch filter.
As indicated in Figure 4, the testing requires a significant hardware set for measurement. Depending on the frequency tuning range of the transmitter, multiple notch filters may be necessary. Also, the calibration verification is needed for each set of hardware.
Testing is not required at or ??5% of the transmitter fundamental frequency. However, since the notch filter is available and the losses are calibrated, the transmitter fundamental range can be measured as part of the test. The measurement at the transmit frequency should be equal to the limit.
CE106 transmit testing requires planning, particularly in specifying and procuring the necessary hardware. Filters, in particular, often are not off-the-shelf items, and if the necessary filters aren't readily available, the testing will be delayed while the planning and hardware configuration are arranged.
Conclusion
MIL-STD-461F supports tailoring a test approach to evaluate a product, make a sensible test, and get valid results. And the area addressing conducted emissions testing is no exception. There are specifics that must be applied, but the standard does allow judicious modifications when preparing a viable test procedure and properly performing it.
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: [email protected]