EMI/EMC Testing of Automotive Electronic Systems: Extremely Near Field EMI Scanning Technology

Central topics:

Very near field EMI scanning technology EMI near field radiation characteristics

Solution:

Spread Spectrum Clock Generator (SSCG) Radiated Characteristics Scan Comparison of a 2nd Generation Half-Duplex SerDes (serializer/deserializer) System and a 3rd Generation Full-Duplex System

Automakers often use the latest consumer electronics systems to differentiate themselves from other manufacturers' cars. These systems must work properly under a variety of harsh conditions. The same requirements apply to power systems, safety systems, and other automotive control systems. Once a failure occurs, these systems can lead to more serious consequences.

Automotive electronic systems are particularly sensitive to electromagnetic radiation from chips and printed circuit boards provided by suppliers. Therefore, SAE (formerly the Society of Automotive Engineers) has defined test specifications and established requirements to meet electromagnetic compatibility (EMC) and electromagnetic interference (EMI), and has continuously improved them. Using very near-field EM scanning technology, the supplier's design team can measure and immediately display the spatial and spectral characteristics of radiation through a desktop system, avoiding problems in later more expensive module, system or vehicle-level testing.

This article discusses several examples that demonstrate the value of this testing. The first example is about the radiated characteristics of a “spread spectrum clock generator (SSCG)”, which was scanned in the “off” and “on” conditions. In the second example, the design team compared a second-generation half-duplex serial deserializer (serializer/deserializer) system with a third-generation full-duplex system. The results verified the next-generation capabilities and their benefits, which not only helped the customer reduce the time to market, but also had a positive impact on the customer.

Very Near Field EMI Scanning Technology

The fast magnetic very near-field measurement instrument can capture and display visual images of spectrum and real-time spatial scanning results. Chip manufacturers and PCB design engineers can scan any board and identify constant or time-based radiation sources in the frequency range of 50kHz to 4GHz. This scanning technology helps to quickly solve a wide range of electromagnetic design problems, including filtering, shielding, common mode, current distribution, interference immunity and broadband noise.

During the development of any new PCB, design engineers must find radiators or RF leakage outside the design and characterize and deal with them to pass compliance testing. Possible radiators include high-speed, high-power devices and devices with high density or high complexity. The scanning system displays the spatial radiation characteristics in the form of superimposed on the Gerber file, so testers can accurately find the source of all radiation problems. After taking appropriate corrective measures, the design engineer can retest the device and immediately quantify the effect of the corrected design.

The scanning system consists of a scanner, small adapter, a customer-supplied spectrum analyzer and a PC running the scanning system software. The desktop scanner includes 2,436 loops that generate 1,218 magnetic field probes spaced 7.5 mm apart, forming an electronic switch array and providing up to 3.75 mm resolution. The system operates from 50 kHz to 4 GHz and is enabled by an optional software key.

This allows users to test their designs themselves without having to rely on another department, test engineer, or time-consuming off-site testing. Engineers can even make changes to the design and test it quickly after diagnosing an intermittent fault. The test results can accurately verify the impact of the design change.

With the scanning system, board designers can test and resolve electromagnetic compatibility issues in advance, thereby avoiding unexpected compliance test results. The scanner's diagnostic capabilities can help design teams reduce radiation testing time by more than two orders of magnitude.

EMI Near-Field Radiation Characterization: SSCG Example

A large semiconductor manufacturer implemented SSCG functionality on the parallel bus of the deserializer. SSCG functionality reduces radiation by spreading the peak radiation energy over a wider frequency band. As shown in Figure 1 below, the frequency variation occurs around the nominal clock center frequency (center spread spectrum modulation), spreading the spectrum by plus or minus 1.0% (fdev). At the receiver parallel bus side, the output modulates the clock frequency and data spectrum over time at a modulation rate of kilohertz (fmod). The target customers for the custom SerDes chipsets are automotive manufacturers who require low EMI emissions from installed electronics.

Figure 1: Spread spectrum clocking functionality.

The company hopes to use convincing quantitative evidence to show automakers that the SSCG function can effectively reduce EMI radiation. To achieve this goal, the design team first placed the device under test (DUT) on its internal scanner with the SSCG function turned "off", powered it up, and then captured the radiation characteristics in the PC. For effective comparison, the same DUT was scanned with the SSCG function turned on.

The very near field scanning system completes the spatial and spectral scans and displays the following radiation characteristics. Note that the scan results are superimposed on the Gerber design files, so that the specific radiators in the device under test can be immediately identified by analyzing the results. Figure 2 shows the radiation characteristics of the device under test when the SSCG function is "off".

Figure 2: EMI radiation characteristics measured when the SSCG function is “off”.

Figure 3 shows the spatial and spectral (amplitude and frequency) characteristics of the radiation of the device under test when the SSCG function is "on". By comparison, it can be found that the radiation has been significantly reduced.

Figure 3: EMI radiation characteristics when the SSCG function is “on”.

After comparing the test results, the design team found that the EMI emissions were significantly reduced due to the use of the SSCG feature. Reducing EMI emissions is a major challenge for automotive electronics engineers. Every time the customer support team presents these results to their automotive customers, they are generally very interested. Any feature that reduces EMI (in this case, the SSCG feature) can reduce time to market, shielding, and cost.

EMI Near Field Emissions Characterization: Next Generation SerDes Example

This is a second example from the same semiconductor supplier, which developed a second generation chipset solution for point-to-point transmission via a SerDes. In the third generation chipset, the design team used a different technology and upgraded the transmission capabilities. They embedded a bidirectional control channel together with the high-speed serial link, thus achieving bidirectional transmission (full duplex).

To quantitatively compare the emissions characteristics of the half-duplex deserializer with the next generation full-duplex design, the design team once again used their in-house EMI very near field scanner. They placed the original half-duplex board on the scanner to take a baseline measurement. After powering up the device under test, they activated the scanner on the PC. (See Figure 4)

Figure 4: Test environment for EMI scans of half-duplex and full-duplex SerDes devices.

Using the same test setup, the design team replaced the baseline board with the next-generation full-duplex chipset board, while maintaining the same specifications for each characteristic. As mentioned above, it is important to note that the spatial scan is overlaid on each generated Gerber design file to help engineers identify any radiated sources present.

The spatial and spectral characteristics of the baseline (half-duplex) system are shown in Figure 5. Figure 6 shows the radiated scan results in full-duplex mode.

Figure 5: Baseline scan results: SerDes in half-duplex mode.

Figure 6: Radiated emissions: SerDes in full-duplex mode.

The design team carefully compared the results of the spatial scan and the spectrum scan. Many people might think that the radiated characteristics would show higher electromagnetic output due to the extended bidirectional transmission function. In fact, there is no spike signal in full-duplex mode and the peak radiation is basically similar, and its EMI characteristics are even slightly improved compared to the baseline (the spatial scan results show a darker blue). The test results prove that there is no significant change in the new chipset in full-duplex mode (see Figure 3), and the design team has achieved full-duplex functionality without taking any additional mitigation measures.

These tests were performed using the semiconductor company's in-house very near field scanning system. In just a few minutes, the results shown above were obtained. Because the radiated characteristics results clearly show its superior performance, the design does not need to take any additional mitigation measures.

In contrast, testing new designs in third-party test chambers requires engineers to travel to an off-site test site and spend most of the day. The use of the test chamber often needs to be arranged several weeks in advance, which will cause significant delays in the development process.

The very near field scanning solution will not replace the need to test the design in a test chamber. However, this instrument can achieve fast front-to-back consistency testing capabilities in a simple desktop system.

Compared to far-field measurements made in a test chamber, very near-field EMI characterization provides real-time feedback. In addition, these measurements correlate well with far-field measurements made in a test chamber. Therefore, very near-field instruments such as the EMxpert can reduce the number of similar tests that are performed in a test chamber. Overall, this can help design teams speed up their testing process and get consistent test results from test chamber tests more quickly. Summary

Automotive

engineers are constantly faced with the challenge of reducing EMI and ensuring the electromagnetic compatibility of all automotive electronic systems. This becomes increasingly difficult when new devices are introduced without adequate testing. When suppliers can convincingly demonstrate that new features can reduce EMI as shown in the two examples above, they can generate great interest from customers.

In both examples above, the results provided by the supplier show that the use of the SSCG feature can reduce EMI, while the radiated characteristics of the new generation of SerDes are unchanged. Therefore, very near-field EM scans can shorten the design cycle of each product without taking any additional measures and reduce costs for automotive manufacturers.

For suppliers, very near-field EMI scan technology can provide a very convincing spectral scan and intuitively overlay the spatial scan results on the Gerber design files. These features can help design engineers record and measure the EMI characteristics of new feature groups in their products. Design engineers can then quickly retest after implementing new mitigation measures or other design changes. As a result, supplier design teams also shorten their time to market, and highly convincing scan results can lead to faster adoption of solutions by automotive manufacturers.