Feature: Interface design


Figure 4: Murata’s lab setup designed to measure the effectiveness of a common mode choke on a USB interface T e high data transfer rates associated with the diff erential


interfaces generate high-frequency noise artefacts that are conducted and radiated from the equipment’s PCB and USB cables. Board layout is a serious consideration for any electronics engineer, with PCB tracks representing small yet impactful parasitic inductance and capacitance, which become particularly relevant at high frequencies.


Tracks between the USB IC and the USB socket are particularly


prone to radiating EMI. One aspect of an EMI mitigation strategy is Earth guard traces and routing USB traces away from sensitive components. Board layout engineers should also keep USB D+ and D- traces well away from clocks and timing circuits. To reduce the impact of intra-system and external EMI, placing common mode chokes on both, the transmit and receive paths of


Figure 5: Noise levels recorded across the three tests by the spectrum analyser 26 July/August 2025 www.electronicsworld.co.uk


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