Ensuring Communication Quality and Filtering for Power-over-Coax - App Note

With the increasing speed and sophistication of interfaces in cars, the PoC (Power over Coax) approach is gaining in popularity, as it allows superimposing signals and power in one coaxial cable, for example for LVDS based automotive camera systems. TDK has issued an application note on "Ensuring communication quality and filtering suitable for PoC (Power over Coax)"

A PoC filter consisting of one or more inductors and chip beads is used on the circuit side to separate the signal and the power supply current. This is important in terms of maintaining communication quality. The PoC filter therefore requires inductors that realize high impedance for ac components over a wide bandwidth range from low to high frequencies.

The PoC approach combines communication signals and power supply within a single coaxial cable. To keep the signal and power separate, a PoC filter that uses inductors and chip beads so that the signal does not pass through the power supply line is required.

If the impedance characteristic of the PoC filter inductors does not satisfy the system requirements, communication quality will be degraded, which may result in malfunction and safety problems.

To ensure the communication quality, the PoC must be resistant to noise. This in turns requires inductors with high impedance for ac components over the entire bandwidth range from low to high frequencies. The high impedance requirement applies to communication frequency bands ranging from several MHz to several hundred MHz (sometimes up to several GHz).

With a conventional multilayer inductor, it is necessary to connect multiple coils in series because high impedance can otherwise not be realized over a wide frequency range.

Conventional multilayer inductors also have a problem in that a higher number of windings is required to obtain high impedance, which results in increased dc resistance. Purpose-designed PoC filter inductors from TDK adopt the single layer coil structure. Consequently, parasitic capacitance is kept low and the resonant frequency is higher compared to inductors with a multilayer winding structure.

Constantly high impedance is therefore achieved over a wider frequency range. At the same time, the inductors are designed to realize extremely low dc resistance.

Compared to conventional filter configurations, the use of inductors specifically designed for PoC filters allows a reduction in the parts count, making it possible to achieve the same impedance while realizing a smaller mounting footprint.

In the typical automobile, cameras and a wide range of electronic devices are installed, and their electrical systems are interconnected by means of multiple cables. Because of space limitations, the cables must be grouped together in harnesses, and there is a possibility of communication errors occurring due to noise interference components from the engine, ECUs and other external sources being superimposed on the cables.

The so-called BCI (Bulk Current Injection) evaluation is an important aspect in ensuring immunity against EMI. Incorrect selection of inductors and chip beads may also have an adverse influence on communication quality and thereby reduce EMI immunity.

Summary

Using the inductor designed for PoC applications as described above brings the following advantages:

• The impedance values required for a PoC filter can be achieved with a low parts count.
• Impedance remains suitably high over a wide range, thereby ensuring good waveform quality and resistance to noise.
• The lower parts count for the PoC filter enables a mounting footprint reduction by about 37%. (According to TDK data)
• DC resistance of the PoC filter configuration can be about 50% lower than with conventional solutions. (According to TDK data)