TE - White Papers

AEROSPACE, DEFENSE & MARINE /// WHITE PAPER PAGE 3 Shielding Twisted pair cabling and differential transmission are the first step in maintaining signal integrity and controlling noise. Twisted pairs work well against common mode noise. Since common-mode noise appears on both conductors simultaneously, causing the potential on both sides to change relative to ground, the fact that the noise on each conductor is 180 degrees out of phase means that the noise is effectively canceled. The next step is preventing the cable from picking up (or transmitting) radiated noise Shielding is the main way to control radiated EMI. Shielded cable both contains EMI generated by the cable and protects against radiated emissions from external sources. Cable shields are foil, braid, or a combination of foil and braid. The key to good shielding is to provide a low-impedance path to ground. The connector backshell and housing accomplish this goal. TE Connectivity (TE) offers a range of backshells to terminate and ground the cable's shield. Filtering The IFE system should be filtered to ensure that any noise coupled or generated by this system is not coupled onto the current systems operating for flight applications. Filtering works against differential mode noise. Unlike common-mode noise, differential noise affects each conductor differently—the noise is entirely in the signal transmission path. Twisted pairs have minimal effect in controlling differential noise. The AC and DC power planes for IFE must be filtered to prevent harmonic noise below 2.4 GHz or above 5 GHz from being coupled onto the power bus architecture. Poor filtering will allow noise to couple onto the aircraft's communication system. Power supplies are usually internally filtered. Power outlets for passengers can also be filtered to keep the power distribution system clean. Signal lines can also be filtered to maintain signal integrity. Filtering, however, is typically done as a last resort when the IFE system has less than stellar performance or interferes with other systems. Filter connectors are available with L, C, LC, and pi configurations to match the input and output impedances of the circuit. Capacitive and inductive values can be varied to create low-pass, notch, and high-pass filters with different frequency bands and attenuation values. Depending on your needs, you can get tolerance values of 5, 10, or 20 percent. Filtering can be performed at a system control unit or nodal points of interconnects throughout the aircraft. While first filter connectors incorporated tubular components, planar array filter connectors surpass tubular types in performance, ease of manufacture, durability and cost. TE has replaced its range of tubular connectors with planar versions. The array provides the required capacitance for each "through pin" and the inductance is provided by ferrite beads suitably positioned on either side of the array. Pi filter configurations require two planar arrays. The ground path for each capacitor is provided through the planar array and makes contact with the shell of the connector via a compliant grounding spring. Within limits, each filtered contact can have a different value of capacitance from its neighbor. One alternative—surface-mount components within the connector—provides lower costs, is more accommodating of changes in the capacitance footprint, and offers lower performance, especially in attenuating high frequencies. This technology is particularly suited to applications such as ground-based communications and industrial uses where high volume is present and conditions are expected to be less demanding. selecting the best filtering options begin with an EMI scan, which will identify the frequency and intensity of generated noise. Typically, TE engineers use the scan data to design a filter specifically for this specific noise environment. Most industry-standard aerospace connectors are available with filtering options. The receptacle connector is the most common and effective position to locate the filtering. A shielded receptacle offers the required low-impedance path to ground for optimum performance. Filtered receptacles also help in minimizing RF windows in the chassis due to the continuous ground plane inside.

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