Pre-Compliance EMI Testing

Passing Compliance Tests the First Time

ost electrical and electronic devices must be tested by third-party labs to ensure that they comply with the relevant conducted and radiated emissions standards. The failure rate in compliance tests is often high, requiring costly and time-consuming redesign. With pre-compliance testing of electromagnetic interference (EMI) as part of the design process, manufacturers can identify problems early in the product cycle. Pre-compliance testing makes it easier to modify the design and electromagnetic properties of a product and increases the probability of passing compliance tests the first time.

Devices must be tested to show that they comply with the requirements of various standards, such as CISPR or MIL-STD. These standards are specified by the responsible regulatory authority, such as the Commission of the European Union (EU) or the Federal Communications Commission (FCC) in the U.S. The required compliance tests must be passed before a device can be put on the market.

Compliance testing is usually performed by a certified third-party test lab or test house. They have specialized equipment, special facilities (such as anechoic chambers), and trained testing personnel, all of which make compliance testing expensive. Testing fees can reach thousands or even tens of thousands of dollars (U.S.) per attempt.

Unfortunately, failing compliance tests is a common occurrence. Depending on the type of testing and the standards involved, the failure rate can be in the range of 70 to 90 percent. If a single part of the test is failed, the entire test is considered unsuccessful, and the device manufacturer must schedule a new test. Any necessary product redesign or remediation must be performed before retesting, and this requires additional time and money.

EMC Testing Becomes Part of the Design Process

Formal compliance testing only yields “pass-fail” results and does not provide much insight into the causes of the failure. Pre-compliance testing, on the other hand, can be stopped at any time and the reasons for issues can be thoroughly analyzed, tested, and debugged.

Figure 1 illustrates the electromagnetic compatibility (EMC) testing process. EMI debugging and analysis should be incorporated into the design process itself. If initial measurements do not reveal any serious issues, the equipment under test (EUT) moves into pre-compliance testing. The pre-compliance tests should come as close as possible to the associated compliance tests. If an EUT fails any of these pre-compliance tests, it goes back to the design and debugging phase for modification. Once pre-compliance tests have been successfully passed, the EUT then moves to full compliance testing at a lab or test house. Successfully passing the required compliance tests results in formal certification, allowing the device to be marketed.

Figure 1: The EMC testing process (Source: Rohde & Schwarz)

Test Location and Site

Formal compliance tests require specific test environments and specific test setups. For assessing conducted EMI, the required equipment and environment are quite simple. In addition to the test instruments and accessories, the test engineer needs only a simple ground plane and a non-conductive table. Therefore, conducted pre-compliance tests are often almost identical to full compliance tests.

On the other hand, radiated EMI compliance testing generally requires a shielded chamber or a suitable open-air test site. Due to the size, cost, and complexity of configuring these types of facilities, most radiated pre-compliance tests cannot precisely duplicate the compliance test environment.

As a result, modifications are often made when performing radiated pre-compliance tests, such as adding margins to the measurement results. For example, a smaller chamber leads to higher emissions than in the final compliance test as the distance between the antenna and EUT is smaller. In this case, emission limits must be raised to take the stronger signals into account. Going from a typical compliance distance of ten meters to a typical pre-compliance distance of three meters, as shown in Figure 2, might require approximately 10 dB higher emission limits.

Test Instruments: EMI Receivers and Spectrum Analyzers

There are two main categories of test instruments used for pre-compliance testing. Spectrum analyzers and EMI receivers are most commonly used to measure emission limits, whereas oscilloscopes are primarily used for debugging and troubleshooting.

Figure 2: For radiated pre-compliance tests the distance between EUT and antenna is relevant for determining proper limits. (Source: Rohde & Schwarz)

Figure 3: EMI receivers and spectrum analyzers are typical test instruments for pre-compliance tests. (Source: Rohde & Schwarz)

EMI receivers and spectrum analyzers (Figure 3) are frequency-domain instruments. They measure and display power as a function of frequency. Frequency domain analysis is essential for EMI testing since conducted or radiated power levels are measured over a range of frequencies defined by a standard. Spectrum analyzers and EMI receivers use automated routines that step through or scan the frequency range of interest. This functionality is either a built-in feature of the instrument or implemented by software.

Limit Lines

A “passing” result occurs when all measured values fall below a defined power-versus-frequency limit line. These maximum power values can either be configured directly on or loaded into the test instrument.

Detector types

Detectors determine how measurements during an interval are combined into a single measurement point. In Figure 4, you see the measurement of a pulsed signal. The results were calculated for each signal interval using different detector types. The average detector simply yields the average value over each interval. The peak detector selects the maximum value in each interval. Quasi-peak detectors were originally developed to better indicate the subjective annoyance level experienced by a listener hearing impulsive interference to an AM radio station. Quasi-peak or CISPR detectors are now generally used to measure the interference of a signal using a type of charging and discharging behavior. The effect of different detector types is shown in Figure 4.

Measurements made with a peak detector are much faster than those made with a quasi-peak detector, usually by at least several orders of magnitude. Additionally, peak detector results are always higher than quasi-peak results. If an EUT passes pre-compliance testing using the faster peak detector, it will also pass the slower tests with a quasi-peak detector. For this reason, the peak detector is more common in pre-compliance testing and the quasi-peak detector is more common in compliance testing.

Figure 4: Common detector types (Source: Rohde & Schwarz)

Spectrograms

In addition, EMI pre-compliance tests often use spectrograms. A spectrogram is a plot of power versus frequency versus time. In order to display these three quantities in only two dimensions, signal power or intensity is mapped to the visible color spectrum, with red indicating maximum power and purple or violet indicating minimum power. The most recent measurements appear in the top line of the display and then “flow” downwards.

Spectrograms are useful because they show how signals change over time and over a range of frequencies. This enables easy identification of time-varying signal behavior such as drifting or frequency hopping. Spectrograms also make it easy to see small signals in the presence of larger signals. Most spectrum analyzers and EMI receivers have spectrograms as a standard feature, and spectrograms are also common for oscilloscopes when displaying frequency-domain information in so-called FFT (fast Fourier transform) mode.

Preselection

In EMI testing, the input signal is neither known nor controllable. Therefore, it is possible that out-of-band or “off-screen” signals could overload the test instrument’s first mixer and cause compression or distortion, leading to invalid or misleading measurement results.

Preselection protects the first mixer. It is implemented as a switchable bank of filters that allows an EMI receiver to select only the frequencies of interest. The particular filter is chosen automatically by the receiver based on the configured input frequency. Many EMI standards require that the “measuring instrument” have preselection, and this is why compliance testing is performed with EMI receivers rather than with spectrum analyzers. Many spectrum analyzers also have a feature called preselection, but this is usually a high-pass filtering based on YIG technology and not a switchable filter bank.

Time Domain Scan

The classic measuring method of EMI receivers is the stepped frequency scan with a small resolution bandwidth. It is a highly accurate but slow method, especially for applications with wide spectral ranges such as radiated emissions measurements.

Modern EMI receivers support time domain scans by splitting the measurement range into large spectrum blocks. The instrument digitizes and processes each of them by using FFT. Time domain scan provides a significant speed improvement over the stepped scan without sacrificing accuracy. Time domain scan has been approved for usage in most types of compliance testing and also can save significant time during pre-compliance testing.

Test Instruments: Oscilloscopes

Oscilloscopes are primarily time domain measurements. They are a valuable measuring tool for locating, debugging, or remediating sources of non-complying emissions. Many modern oscilloscopes also support frequency domain measurements. In addition, modern oscilloscopes generally have a wide bandwidth. Oscilloscopes can be used to examine both conducted and radiated signals.

One potential drawback of using oscilloscopes for pre-compliance testing is that they usually do not natively support limit lines, although limit lines and other EMI-related features can be implemented in external software.

Fast Fourier Transform (FFT)

Some oscilloscopes can be used to display and analyze frequency domain data by performing FFT on acquired time domain data. This is helpful for pre-compliance testing as they display time and frequency domain data simultaneously. Users can correlate events in one domain with events in another domain. This is extremely helpful when debugging EMI issues, especially if the oscilloscopes are equipped with a frequency domain trigger. This trigger occurs when a frequency mask or region is violated, as shown in Figure 5. Once the oscilloscope has been triggered by this frequency-domain event, the related time-domain event can be analyzed to determine the root cause of this violation.

Wide bandwidth and the ability to correlate time and frequency domain data make oscilloscopes very valuable for debugging issues discovered during pre-compliance testing. Features such as spectrograms and limit lines can be supported by all three instruments. EMI receivers additionally offer preselection and time domain scans. EMI receivers are used for full compliance testing and using them for pre-compliance tests leads to a closer correlation with compliance test results.

Accessories Used for Pre‑Compliance Testing

In addition, there are a number of different tools and accessories which are necessary for precompliance measurements.

LISN

A line impedance stabilization network (LISN) is used in conducted emissions testing. One of the main functions of a LISN is to provide a stable impedance on the AC mains line end of the EUT’s power cord. Since power outlet impedance can vary widely, a LISN ensures consistent, repeatable results regardless of where the test is conducted. In addition, it blocks any RF signals present on the AC mains from entering the EUT via the EUT’s power cord. This ensures that any measured emissions are coming from the EUT rather than being conducted in from the AC mains network.

Antennas

Radiated compliance testing is always done in the so-called far field, with the antenna placed several meters from the EUT. Because of the wide frequency ranges required by most radiated testing standards, typically 1 GHz or more, a broadband antenna or a combination of antennas is needed to efficiently cover the entire frequency range. Some common examples are log-periodic antennas or biconical antennas.

Figure 5: A frequency mask trigger can be used to help identify the cause of this violation in the time domain. (Source: Rohde & Schwarz)

Figure 6: Typical near-field probes used in pre-compliance testing (Source: Rohde &Schwarz)

The same types of antennas can be used in both compliance and pre-compliance tests but recall that the distances between the antenna and EUT are often shorter in pre-compliance testing, requiring modifications to the radiated limit lines.

However, with regard to troubleshooting or debugging the causes of emissions, these types of antennas are not appropriate. They are too large and too bulky to provide precise information about which part or component of the EUT is generating non-compliant emissions.

Near-Field Probes for EMI Debugging

Near-field probes are the appropriate tools for use in close physical proximity to the source of an emission. As a practical matter, the near field in EMI debugging is of the order of a few centimeters. Because of their small size and the ability to physically position them close to the source, near-field probes have high spatial resolution. They allow users to precisely locate the source of an emission, for example, a pin of a chip or a trace on a printed circuit board. On the other hand, near-field probes only support relative measurements. They can be used to find sources of emissions but cannot be used to measure accurate power levels for the purpose of verifying limits.

Software

Specialized software is commonly used in pre-compliance testing, most often for scripting or automating tests. The software communicates with or controls multiple instruments and accessories via a single user interface. It can also easily incorporate antenna factors, cable loss, etc. into the measurement results. It also collects and displays the measured data with advanced options, such as customized limit lines. This provides higher speed and better repeatability than manual operation, allowing rapid and accurate pre-compliance testing to be performed even by users who are relatively new to pre-compliance testing.

Summary

Pre-compliance testing saves time and money by discovering potential issues early in the design cycle. Using the proper tools and techniques during pre-compliance testing greatly increases the chance of passing full compliance tests the first time.

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