Radiated susceptibility.(EMC TEST)

Some EMC tests are about as exciting as watching grass grow. However, I must admit that today's computer control beats the previous procedures that included highly manual testing followed by lengthy and often very tedious rechecking, recording, and analyzing results.

Throughout the years, the electric field (EF) radiated susceptibility/immunity test remains the most fun EMC test to run. It's so easy to produce the required EF at any desired frequency that we can really concentrate on testing the EUT.

Performing the RS Test

Radiated susceptibility (RS) testing in its various forms is mandated by a number of EMC specifications including MIL-STD-461, MIL-STD-464, and the EU. Those of you who have read the article titled "EMC Failures Happen" in the December 2007 issue of EE-Evaluation Engineering know that passing an EMC test won't guarantee that the unit will be immune to EMC problems in its operational environment. But it helps.

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RS testing, illustrated in Figure 1, requires the EUT to be illuminated by a low-, medium-, or high-level EF. If it will fit, the EUT is placed in a shielded enclosure with a layout that represents its normal operational configuration as closely as possible. The setup includes placing it on a ground plane made from material representative of the actual platform.

A transmit antenna is placed in front of the EUT's most susceptible RF pickup area at the separation distance prescribed by the test specification, typically 1 meter for military and 3 meters for the EU. The EF is established at the specified frequencies using a signal generator and an RF power amplifier to drive the transmit antenna. It may require a suite of signal generators, power amplifiers, and antennas to cover the entire frequency range.

The test signal is modulated using a frequency and waveform that correspond to worst case. In the event these are unknown, use the modulation called out in the specification.

The frequency range for the test is slowly swept at the prescribed EF level or higher. This can be done by a computer or manually.

Table 1 indicates the MIL-STD-461F susceptibility scan speeds. At frequencies where the EUT is susceptible, the scanning is stopped, the EF is reduced to the susceptibility threshold level, the first level is recorded, and then the EF is reduced by at least 6 dB. Now the EF level is increased until the susceptibility condition reappears. This second level is compared with the first, and the lowest level is the susceptibility. Performing the test in this way avoids the problems of hysteresis in the measurement.

Most test labs execute susceptibility tests manually because of the randomness of susceptibility occurrences. It's difficult enough to establish the susceptibility criteria. But then these criteria must be monitored and provided as feedback to a computer to tell it to stop when susceptibility occurs, automatically adjust RF levels to 6 dB below the susceptibility point, and perform a retest.

Humans do a much better job of correlating what may appear as random, nonrelated incidents. It's not uncommon for an absolutely unexpected, unplanned susceptibility condition to occur.

RF Test Equipment

The RS test requires equipment to create the EF and monitor it just to ensure that the EF is there. Following is a list of the equipment that would be used to do a MIL-STD-461E/F RS test:

Transmitters to Create the EF

* LISNs: Used to standardize the power input RF impedance.

* RF Signal Generators: Any standard generator with modulation that covers the frequency range.

* Modulation Generator: Used with the standard generator to provide the required modulation.

* Power Amplifier (PA): An RF signal booster used with the standard generator because no generator has enough output to produce the required EF.

* Transmit Antennas: Produces the EF. The EF required in conjunction with the gain of the transmit antenna determines the size of the PA.

* Directional Coupler: Because of antenna impedance mismatch variations, the directional coupler is required to determine forward power when precalibrating the EF.

* Power Meter: Used with the directional coupler.

Receivers to Ensure the EUT Is Exposed to the Correct EF

* EF Sensors: 10 kHz to 1 GHz. Used at the EUT to determine incident EF strength. Sensors that cover the 1-GHz to 18-GHz range also are available.

* Receive Antennas: Used at the EUT to determine incident EF strength in place of EF sensors; 1-GHz to 10-GHz double ridge horns and 10-GHz to 40-GHz antennas as approved by the procuring activity.

* Attenuator: Used to protect the measurement receiver and reduce errors from antenna VSWR.

* Measurement Receiver: Used with the receive antennas. One or more may be required to cover a test frequency range; could also be a spectrum analyzer.

* Data Recording Device: Connects directly to receiver output or a computer used to control the receiver.

From an EMC perspective, there's nothing unusual about a test setup that uses signal generators, RF power amplifiers, and antennas until it's time to perform such a test. Then try to find that equipment. All the equipment is readily available except for large broadband RF power amplifiers and antennas.

Since the antenna performance determines the amplifier power requirement, it's necessary to know the worst-case antenna gain and how far we need to squirt the RF to size the amplifier. Figure 2 shows how to calculate the power amplifier requirements based on antenna characteristics.

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The same set of amplifiers can be used with a wide assortment of transmit antennas. And there is a wide assortment, each with very different characteristics. It would be great if one antenna could be used to generate the RF field across the entire frequency range, but dimensional restrictions and antenna Q limit the maximum antenna bandwidth to about a decade. Table 2 shows the most popular antenna types used in the different frequency ranges.

Generating high-amplitude EF strengths in the 10-kHz to 30-MHz frequency range is difficult because antenna dimensions are very small with respect to a half wavelength, making the antenna efficiency very poor. As a result, there are some antenna alternatives such as the GTEM cell and parallel plate/triline used when testing smaller EUTs. For larger EUTs, the size of the line limits the usable upper frequency.

There always has been a struggle regarding the sizes of the EUT, the shielded enclosure, and the antennas. To minimize distortion and antenna loading, when an antenna is used in a shielded enclosure, the ends should be kept away from the wall by at least 0.5 meter. For small EUTs, the size of the antenna determines the enclosure size.

In the early days of RFI/EMI/EMC testing, RS tests were performed by feeding the 100,000-[micro]V modulated output of a standard signal generator into a41-inch monopole, tuned dipole, or horn antenna. E-fields weren't monitored.

MIL-STD-826 (1964), the first attempt at a tri-service standard and the basis of a number of procedures in MIL-STD-461, changed all that. Then, RS field strengths were monitored by antennas placed to the side or behind the transmit antennas.

Now for MIL-STD-461 measurements, we've shrunk the antennas, grouped three together aligned along the X-Y-Z axes, added amplification to make up for their inefficiency, called them EF probes, and placed them on or in close proximity to the EUT. To minimize EF probe susceptibility and field distortion, most utilize fiber-optic interfaces.

The EU EMC tests use an alternative approach in which the EF is precalibrated. Figure 3 shows the EU 16-point EF uniformity requirements.

Defining Susceptibility

We want to determine if the EUT will operate properly in an adverse RF environment. This can be defined by duplication of previously measured RF environmental levels or compliance with an EMC specification. Susceptibility to radiated EM energy primarily is due to RF pickup on wires and cables and generally results in malfunctions or degradation of performance. The latter often can be tolerated, but malfunctions cannot.

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The problem of establishing pass/fail criteria for susceptibility is determining how much degradation is tolerable before we conclude that the EUT is not working properly. Beware of any specification that states that the characteristics of the EUT during the susceptibility tests cannot change from those measured in the laboratory sans RF.

Four characteristics greatly influence the susceptibility of the EUT: frequency, amplitude, spatial relationships, and timing (FAST). They often are used as a culling approach to analyze EMC problems.

Frequency

For an RF device, in-band susceptibility generally stems from the culprit frequency or its harmonics coupling at the victim's tuned frequency, harmonic, or IF. Out-of-band or non-RF device susceptibility generally results from the culprit frequency coupling into a circuit through an RF response window created by wire, cable, or parasitic resonances.

Cable resonance is one of the most often occurring problems so failures frequently occur in the 30-MHz to 300-MHz range. Because the response frequencies are unknown, the entire RF spectrum must be scanned during an RS test. Signals must be modulated to determine if the system is susceptible to audio rectification.

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Amplitude

The interfering signal adds to the EUT internal noise. If the amplitude of the interfering signal being coupled into the EUT is at the same level as the intended signal, most likely the system will malfunction. Consequently, the amplitude of the interfering source energy level determines the susceptibility.