Radiated immunity testing involves hitting the Raspberry Pi hard with narrow-band EM radiation while checking (amongst many other things) that the device is still able to send Ethernet frames to a hub. The first time the team did this, the light on the hub stopped blinking: no frames were making it through. They did it again but still nothing changed. Finally, they discovered that the hub (which gave every appearance of being CE marked, so it should have been able to get through these tests itself) was being knocked out every time somebody pressed the button,” Upton explains.
The team finally solved the issue by placing the hub outside the field, and the Raspberry Pi got through its immunity tests with no problem at all.
This shows that it’s not just the design but the components also that need to be robust. Yu Yinsheng, general manager, Foryard Optoelectronics, informs that CE and RoHS are the most common certifications that their products are designed for.
“While we do the design, we need other electronic components which we don’t produce by ourselves. We need to make sure that these components, which were bought from other suppliers, are also CE- and RoHS-certified. Otherwise, the whole designed part would not be allowed in the CE market. Now we have chosen electronic component manufacturers that we trust to be our partners,” Yinsheng explains.
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In India, we recommend manufacturers, particularly embedded electronics services providers, to get some pre-evaluation done, as the failure rate, especially in EMC testing, is very high
— Kalyan Varma, head–India, TUV Rheinland
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Challenges due to electromagnetic interference
Manufacturers, particularly those in embedded electronic services space, are recommended to get some preevaluation done, as the failure rate, especially in EMC testing, is very high.
“The first-time clearance is mostly like 10-20 per cent, as every product will have a certain amount of finetuning to be done. With pre-evaluation, you can have a better chance of higher pass percentage,” suggests Varma.
Pete Lomas, Raspberry Pi trustee and director of engineering at Norcott Technologies, led the team that designed the Pi. During the development of Raspberry Pi, Lomas says, the team was concerned that it would be an “open to atmosphere” design without the benefit of screening case.
“We designed countermeasures into the PCB from the outset. One decision was to use a full ground plane across the whole PCB to make sure that the ‘loop areas’ for potentially radiating signals were as small as possible. Decoupling was also important, and working on the advice of Broadcom engineers we placed these really close to the SOC power pins, which in this case was on the underside of the PCB directly below the processor and hence complicated the PCB design.”
“High-speed data lines, e.g., HDMI, were wired as differential pairs with matched lengths and carefully matched impedance. With all this in place, we were able to achieve a Class-A pass directly without PCB modifications. We need to do some modifications to achieve the more stringent Class-B and this work is ongoing,” adds Lomas.
