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Evolution of Technology: EMI filters in car navigation and entertainment systems

Modern in-car electronics involves navigation systems such as Global Positioning System (GPS), Vehicle Information and Communications System (VICS) and Electronic Toll Collection (ETC). There may also be entertainment systems containing things like DVD players and HDDs. One problem common to all these systems is the noisy environment found in a car. EMI filters are of paramount importance in these situations.

Passive EMI suppression filters are used to absorb and eliminate high frequency noise, and come in three types: capacitive, inductive and capacitive-inductive.

Capacitive noise suppression involves connecting a bypass capacitor to ground from a noisy signal or power line. The impedance of the capacitor decreases with frequency, and since noise is a high frequency phenomenon, it is bypassed to ground. Three-terminal or thru-type structures function effectively at high frequencies, minimising the effect of any residual inductance. These work at frequencies exceeding 1GHz.

The impedance of an inductor increases with frequency, so the inductor is connected in series with the signal line to attenuate high frequency noise. Desirable characteristics of inductors used in EMI suppression filters are that they should not induce resonance in the circuit, which can distort the signal. The inductors used in Murata's EMI suppression filters are designed to present a high impedance at high frequencies.

Combined capacitive-inductive filters are typically higher performance than capacitive or resistive types. This type of filter can be used in highspeed signal circuits. When used in DC power circuits, these filters prevent resonance from occurring in peripheral circuits, making it possible to achieve significant noise suppression.

Common mode chokes are also available for suppression of common mode noise and Murata also has a range of connectors with built-in filters to ensure that filtering takes place very close to interconnects for maximum effectiveness.

Enclosure considerations

Enclosures for car navigation and entertainment systems should ideally be made out of metal, as plastic enclosures offer little in the way of shielding. Designers should also be careful to minimise the size of any openings or gaps in their enclosures, or alternatively cover the opening with a separate metal case.

Noise can be produced inside the enclosure by individual components and by the cables or connections that join them. The signals in and out of today's high-speed digital signal processors (DSPs) are high frequency and are extremely good generators of noise. Examples of noise countermeasures can be seen in Figure 1. EMI filters are used in the cable connections, in the high-speed signal lines, in the DC power lines and in the high-speed transmission line.

Cable connections

An EMI filter should be installed near the connector to suppress the noise coming through as well as the noise radiated from the cable. This can be done using chip ferrite beads and capacitor-type EMI filters. These components should exhibit high impedance at high frequencies and suppress noise effectively over a wide frequency band. Suitable ferrite beads would offer high impedance above 100MHz, peaking between 300 and 500MHz. Obviously, they should be able to withstand automotive temperatures, and have a low DC resistance. A rated current of around 200mA is adequate.

Clock and bus lines

Signal lines like clock and bus lines carry high-speed signals that emit high frequency radiated noise. These lines should also incorporate EMI filters in the form of chip ferrite beads, but a different frequency response is desirable in this situation. Filters used on data lines should exhibit noise suppression effects over narrower frequency bands, say 2-300MHz, and at higher frequencies, above 1GHz if necessary. The narrower suppression band is essential to avoid distortion or suppression at signal frequencies. Again, automotive temperature ranges and low DC resistance are important.

DC power lines

As with a signal line, transient currents cause noise in a DC power line. A chip ferrite bead plus a bypass capacitor should be installed. In this situation, designers should look for parts that have current ratings in the Amp rather than milliAmp range, and remember to check for any de-rating at high temperatures, as this is required with some components. Low DC resistance, ideally less than 0.1, is also critical to avoid voltage drop across the devices. A common-mode choke could also be used to curb common-mode noise.

High-speed transmission lines

In USB or IEEE1394 connections, a signal is transmitted differentially at high speed, and noise is radiated from the cable. The common-mode noise generated in other circuits is often greater than these signals. Noise must therefore be suppressed without distorting or destroying the signal itself. For signal lines, chip ferrite beads similar to those used on clock or bus lines would be suitable, provided they do not attenuate at signal frequencies. When choosing a chip ferrite bead for power or ground lines, designers should again consider the current rating and DC resistance of the parts. If a part cannot be found that does not distort the signal, an alternative approach is to use a common mode choke, which is designed to have high inductive reactance and low DC resistance. An appropriate choke will attenuate noise in the 200 to 400MHz band, without suppressing the signal waveform.

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