Increased EMP Threats Lead to Changing Testing Standards
Part 3: Subsystem testing standards and solutions
Previously, we discussed the various types of electromagnetic threats and introduced the overall standards prescribed to test against these attacks, including MIL-STD-464C. In this post, we continue the conversation by diving into MIL-STD-461G/RS105 relating to subsystems.
MIL-STD-461G provides testing and verification requirements specifically for subsystems, whereas MIL-STD-464C tests the entire system. An example of this would be 464C testing an entire aircraft, where 461G tests only the radio from the aircraft. This standard covers testing for conducted and radiated emissions and susceptibility. For the purposes of this discussion, focus will be placed solely on the sections pertaining to EMP testing.
While MIL-STD-2169 defines the classified HEMP test environment for systems, RS-105 of MIL-STD-461G provides an unclassified version of the E1 pulse. This allows test facilities and defense contractors a means of designing and testing their systems for HEMP hardness without the restrictions associated with the classified MIL-STD. While most of the test procedures covered in MIL-STD-461G are fairly straight forward in their implementation, RS-105 has a unique set of challenges associated with the radiated transient environment, the associated field interactions, and the diagnostics required to measure the pulse within the test volume. The following description will cover a typical RS-105 test, along with some of the ways in which APELC and others have overcome the technical challenges associated with the test.
This standard defines the characteristic waveform, shown in Figure 1, which is a double exponential pulse, characterized by a 1.8 – 2.8 ns rise time, a full width half maximum (FWHM) of 23 ns ± 5ns, and a peak electric field strength of 50 kV/m (+ 6dB/- 0dB). This pulse is then launched onto a transverse electromagnetic wave (TEM) structure that is resistively matched to reduce transmission-line reflections within the test volume.
Figure 1 Unclassified free-field EMP time domain waveform for the E1 pulse (IEC 61000-2-9 and MIL-STD-464C)
Often, test engineers who are first becoming familiar with this standard are thrown off by RS105-2, which shows a “parallel plate radiation system.” Due to the fact that most systems that are presently used for RS-105 testing are instead what is known as a “conical” TEM-line, and do not include a parallel-plate section, the test engineer may ask why RS-105 equipment for larger enclosed under test (EUT) sizes does not, in fact, have a parallel plate section at all. As illustrated in Figure 2, major bends throughout the structure create diffracted waves off the corners located near the input and near the load. The unwanted waves distort the primary signal, often pulling the system out of specification.
Figure 2. An illustration of unwanted waves generated by the suggested geometry.
The preferred antenna geometry is shown in Figure 3. This line was proposed by Giri and Baum and is now accepted as the standard. The benefit of this geometry is that the single corner is moved well away from the test volume. There are no diffracted waves caused ahead of the test volume, and the reflected waves from the load come very late time in the pulse and are reduced by the 1/r field propagation.
Figure 3. Conical antenna and wave propagation
The structure size is also defined in the MIL STD and is designed to minimize interactions between the structure and the test article. Generally, the test volume, or EUT volume can be described as LEUT x WEUT x HEUT, and the surrounding structure must have the following minimum dimensions:
Lstructure = 2 x LEUT
Wstructure = 2 x WEUT,
Hstructure = 3 x HEUT
In reality, the structure must be substantially longer, in order to maintain the pulse into the EUT volume and create the desired impedance. Antenna angles between 14 – 17 degrees are common.
By combining these design criteria with our core Marx generator technology, APELC is able to produce a pulse which meets the desired pulse specifications in the EUT volume for both time and frequency domain, as seen in Figure 4 and Figure 5.
Figure 4. A sample radiated waveform compared to the ideal waveform
Figure 5. A comparison of the spectral content between a measured pulse and the theoretical waveform.
Diagnostics and Test Procedure
APELC uses industry-standard asymptotic conical dipole (ACD) D-dot probes for the measurement of electric field in the EUT volume and voltage at the output of the generator. Because MIL-STD-461G/RS-105 calls for a reference voltage probe and a D-dot or B-dot sensor, APELC believes in reducing system complexity by only using a free-field D-dot probe in the EUT volume. B-dot probes and additional ground-plane sensors can be made available upon customer request.
The authors of the standard recognized that the EUT would affect the electric field measured by the EUT probe, and therefore outline a calibration procedure in which five points are measured where the face of the EUT would be during the test. The corresponding reference voltage waveform is simultaneously recorded so that the test engineer can use this as a reference to ensure that the pulse is still within specification once the EUT is placed inside of the volume and the free-field EUT probe is removed from the test volume. This is due to the fact that the voltage on the top plate that the reference probe is measuring is a good indicator of what field is being generated in the test volume, yet, the calibration measurement at the EUT face ensures that the incident waveform is truly meeting the specification in both time and amplitude. Figure 6 details a typical RS-105 test set-up.
Figure 6 Typical RS-105 test set-up
Because no direct, resistive voltage probe can measure both the high-voltage present at the generator output terminals and the fast rise-time of the pulse, a d-dot sensor must also be used for measurement of the reference voltage. A Prodyn Model ADS-110R ground-plane sensor (or similar) is typically utilized as the voltage reference probe for the simulator. The manufacturer provides a NIST traceable calibration for the sensor output based upon sensor equivalent area, internal impedance, and dD/dt. From this measurement, an electric field can be measured and converted to a voltage by accounting for the top-plate height above the ground-plane. Moreover, the measurement is taken just beyond the output of the Marx generator where the entirety of the pulse is still bounded on the plate, so that the measurement being made truly represents the voltage on the plate and not a radiated field. The 2-meter EUT APELC RS-105 system is shown in Figure 7. The d-dot electric field sensor can be seen in the EUT volume for the calibration measurement.
Figure 7 APELC 2-meter EUT RS-105 test system
The following APELC systems meet the RS-105 waveform requirement outlined in MIL-STD-461G, Test Procedure RS105:
- ½ M EUT TEST SYSTEM: https://apelc.com/emi-emc/1-2m-eut-test-system/
- 1 M EUT TEST SYSTEM: https://apelc.com/emi-emc/1m-eut-test-system/
- 2 M EUT TEST SYSTEM: https://apelc.com/emi-emc/2m-eut-test-system/
To learn more about APELC, our products and our applications, please visit www.apelc.com.