Verify chip designs and reduce time to market with EMI scans

Automakers often use the latest consumer electronics systems to differentiate their cars from other manufacturers. 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

Fast magnetic very near-field measurement instruments can capture and display visual images of spectrum and real-time spatial scan results. Chip manufacturers and PCB design engineers can scan any circuit 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 identify radiators or RF leakage outside of the design, characterize them, and address them to pass conformance 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 identify the source of all radiation problems. After taking appropriate measures to solve the problem, 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 resolution up to 3.75 mm. 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 again soon after diagnosing an intermittent fault. The test results can accurately verify the impact of the design change.

With the help of scanning systems, PCB design engineers can pre-test and solve electromagnetic compatibility problems, thus avoiding unexpected compliance test results. The diagnostic capabilities of the scanner can help design teams reduce radiation testing time by more than two orders of magnitude.

EMI Near-Field Radiated Characteristics: SSCG Example

A major semiconductor manufacturer has implemented SSCG functionality on the parallel bus of the deserializer. The SSCG function 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), and the spectrum is spread by plus or minus 1.0% (fdev). On 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 of the customized serial deserializer chipset are automotive manufacturers who require low EMI radiation characteristics for the electronic equipment installed.

Figure 1: Spread spectrum clocking functionality.

The company hopes to demonstrate to automotive manufacturers with convincing quantitative evidence 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 a 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 electromagnetic radiation was significantly reduced due to the use of the SSCG function. The biggest challenge for automotive electronics engineers is to reduce EMI radiation. Every time the customer support team shows these results to automotive manufacturers, they generally show great interest. Any feature that reduces EMI (SSCG in this case) can shorten time to market, reduce shielding and cost expenditures.

EMI near-field radiation characteristics: a new generation of SerDes example

This is the 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 radiated performance of the half-duplex deserializer to the next-generation full-duplex design, the design team again used their in-house EMI very-near-field scanner. They placed the original half-duplex board on the scanner and took baseline measurements. 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 also maintaining the same specifications for each feature. 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 existing radiated sources.

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 spatial scan results with the spectrum scan results. Many people might think that the radiation characteristics would show higher electromagnetic output due to the extended bidirectional transmission function. In fact, compared with the baseline, there is no spike signal in full-duplex mode and the peak radiation is basically similar, and its EMI characteristics are even slightly improved (the spatial scan results show a darker blue). The test results prove that there is no obvious 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. The results shown above were obtained in just a few minutes. Because the radiated characterization results clearly demonstrate superior performance, no additional mitigation measures were required in the design.

In contrast, testing a new design in a third-party test chamber requires engineers to travel to an off-site test site and spend the better part of a day. Use of the test chamber often needs to be scheduled weeks in advance, which can introduce significant delays to the development process.

A very near field scanning solution will not replace the need to test designs in a test chamber. However, this instrument can enable fast pre- and post-compliance testing capabilities in a simple desktop system.

Compared to far-field measurements made in a test chamber, very near-field EMI characterization can provide real-time feedback. In addition, these measurements have a high correlation with far-field measurements made in the test chamber. Therefore, very near-field instruments such as EMxpert can reduce the number of similar tests performed in the test chamber. Overall, this can help design teams speed up the testing process and get consistent test results from test chamber testing faster.