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.
Figure 1 illustrates the electromagnetic compatibility (EMC) testing process. EMI debugging and analysis should be incorporated into the design process itself.
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.
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.
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 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.
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.
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.
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.