Carmakers use the newest consumer electronic systems to differentiate their vehicles. For customers to have an experience that sells more cars, the systems must work properly under a variety of harsh conditions. The same requirements also apply to the powertrain, safety, and other vehicle control systems that have more serious consequences if a fault occurs.
Automotive electronic systems are particularly sensitive to the electromagnetic emission profiles of the chips and printed circuit boards supplied by vendors. As a result, SAE (formerly the Society of Automotive Engineers) has developed and continually refines specifications for testing and establishing conformance to electromagnetic compatibility (EMC) and electromagnetic interference (EMI) requirements. Very-near-field EM scanning technology allows vendor design teams to quantify and immediately display spatial and spectral emissions profiles with a tabletop system and avoid issues in later, more costly testing at the module, system, or complete vehicle level.
This article looks at a couple of examples that shows the value of this testing. The first example compares the emission profiles of a new chip feature, Spread Spectrum Clock Generation (SSCG), one scan with the feature “Off,” the other with SSCG turned “On.” In the second example, the design team compared a second-generation half-duplex SerDes (serializer/deserializer) system with a third-generation full-duplex design. The results verified next generation features, documented the benefits, accelerated customer time-to-market, and created a positive impact in customer documentation.
Very-near-field EMI scan technology
Fast magnetic very-near-field measurement instruments capture and display visual images of spectral and real-time spatial scan results. Chip makers and PCB designers can scan any board and identify both constant and time-based emission sources in the range of 50 kHz to 4 GHz. The scanning technology facilitates rapid resolution of a wide variety of electromagnetic design issues including filtering, shielding, common mode, current distributions, immunity, and broadband noise.
During any new PCB development process, design engineers must find, characterize, and address unintended radiators or RF leakage to pass compliance testing. Ideal candidates include devices designed for high speed, high power, or with high density or complexity. The scanning system displays the spatial emission profile as an overlay on the Gerber file, so the tester can pinpoint the sources of any emissions problem. After implementing any needed mitigation measures, the designer can retest the device and immediately quantify the effectiveness of the revised design.
The scanning system consists of a scanner, compact adaptor, and a customer-supplied spectrum analyzer and PC running the scanning system software. The bench top scanner includes 2,436 loops resulting in 1,218 H-field (magnetic) probes spaced every 7.5 mm, forming an electronically switched array, which effectively provides a 3.75 mm resolution. The system operates in the range of 50 kHz to 4 GHz, enabled with optional software keys.
As a result, the user can personally test the design without having to rely on another department, test engineer, or time-consuming off site testing. After diagnosing even an intermittent problem, the engineer can implement a design change and retest in minutes. The results provide concrete verification of the effectiveness (or not) of the design change.
The scanning system allows board designers to pre-test and resolve EMC and EMI problems, thus avoiding unexpected compliance test results. The scanner’s diagnostic capabilities allow design teams to reduce emission testing times by more than two orders of magnitude.
EMI near-field emission profiles: The SSCG example
A major semiconductor manufacturer implemented SSCG on the deserializer’s parallel bus. The SSCG function provides a means to reduce emissions by spreading the radiated peak energy over a wider frequency band. As shown in Figure 1 below, the frequency variation occurs around the nominal clock center frequency (center spread modulation). The spreading of the spectrum is plus or minus 1.0% (fdev). At the receiver parallel bus, the outputs modulate the clock frequency and data spectrum over time at a modulation rate in the kilohertz range (fmod). The specified SerDes chipset targets automotive manufacturers who have established requirements for low EMI emissions profiles for installed electronic devices.
Fig. 1. These diagrams show a spread spectrum clock function.
The company sought to generate compelling, quantified evidence to present to their automotive customers that the SSCG function reduces EMI emissions. To do so, they first placed the device under test (DUT) on their in-house scanner with the SSCG function turned “OFF,” applied power, and captured the emissions profile in a PC. Then to provide a valid comparison, they conducted the identical scan on the same DUT, but this time, turned the SSCG feature “ON.”
The very-near field scanning system generated and displayed the following emissions profiles for both spatial and spectral scans. Note that the scan results overlay the Gerber design files, so those analyzing the results can immediately determine specific emitters on the DUT. Figure 2 below shows the profile of the DUT with SSCG turned “OFF.”
Fig. 2. EMI emission profile was measured with SSCG “OFF.”
By comparison, note the marked reduction in emissions confirmed by both spatial and spectral (amplitude vs. frequency) profiles in Figure 3 below with SSCG turned “ON.”
Fig. 3. EMI emission profile with SSCG “ON” shows significant emissions reduction.
After comparing test results, the design team observed that the profiles quantified the dramatic reduction of electromagnetic radiation due to the SSCG feature. Automotive electronic engineers’ biggest challenge is reducing EMI. Whenever the customer support team demonstrated these results to their automotive customers, the universal response was enthusiastic. Any feature that reduces EMI, in this case, the SSCG feature, results in faster time-to-market, less shielding, and lower costs.