EMI Suppression Filters (EMC and Noise Suppression)PoC system requirements for inductors and noise suppression (1)

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Power over coax, or PoC, is a power transmission method that sends power over signal cables, thereby eliminating the need for separate power cables. PoC is used in automobiles and industrial equipment. Automotive applications for PoC include cameras for ADAS or surround view, where it helps simplify wiring designs and allows for lighter wiring harnesses. In industrial equipment, PoC applications include cameras for visual inspections and the like. Large manufacturing lines can require very long cables, and PoC makes it possible to reduce the number of cables and simplify cable routing.

PoC is often used for SerDes interfaces, where the serializer and deserializer are connected by a coaxial cable. High frequency signal and DC power are superimposed on this coaxial cable. When this is the case, a Bias-T circuit is added to prevent high frequency signals from leaking into the power line and DC current from entering the deserializer. With Bias-T circuits, capacitors that pass high frequency signals while blocking DC current are used along with coils that pass DC current while blocking high frequency signals.
In this article, the inductors used for Bias-T circuits are referred to as Bias-T inductors.

The purpose of a Bias-T inductor is to block AC and allow DC to pass, so the inductor must have high-impedance characteristics. If the impedance is too low, AC signal components may leak into the power line and signal components transmitted over the coaxial cable may be attenuated.

We investigated the effect that Bias-T inductors have on the characteristic impedance of the signal line.
We connected a network analyzer to a board populated with components such as a SerDes IC and a Bias-T circuit, and we used the TDR method to measure the characteristic impedance.

The characteristic impedance of the transmission line varies due to factors such as the pattern design and positioning of components on the board. The transmission characteristics can be improved by suppressing this variation and smoothing it out. To maintain smoothness, the Bias-T inductor must have sufficiently high impedance. This is because the characteristic impedance of the transmission line drops if the impedance of the Bias-T inductor is low. The figure at right shows an extreme example in which the Bias-T inductor is replaced with a short chip. We confirmed that this causes the characteristic impedance to suddenly drop from 50 Ω toward 0 Ω.