Page 8 - Power & Data Connectivity Expand Electronic Capabilities eBook

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Matched impedance connection
Characteristic impedance is one of the most important features for good signal integrity (low signal distortion). As 100 Ohm differential characteristic impedance is very often used for transmission lines, the whole electrical chain — the cable, the cable connector, as well as the equipment connector — must be adapted to this value. If one of the components doesn’t fit this requirement, it causes reflections, which degrade the signal, especially at high frequency. This is particularly true with multi-pin rectangular and circular connectors like D-sub, Micro-D, or MIL-DTL-38999.
The characteristic impedance depends on the cable architecture. The conductor diameter, the dielectric material, and the dielectric thickness are among the most important properties of an impedance-controlled cable. The same properties are just as important for the connector to avoid impedance jumps.
» Impedance measurements in time domain reflectometry: important mismatching on the micro-D connector (on the left) compared to MicroMach matched connector (on the right).
Low signal losses or attenuation
Attenuation in an electrical system is the loss in the
amplitude or strength of a signal as it passes along its
length. As the signal passes through the conductor, part of
the signal is absorbed. Attenuation is the result of resistance
in the conductor and associated dielectric losses that are
proportional to the length and frequency. Attenuation can
be reduced by improving the dielectric properties of the
insulation and the conductor resistance. The easiest way
to reduce attenuation is to decrease conductor resistance
by increasing the size of the conductor. The size of the
contacts is also important even if the contact connection
does not represent a long length. To reduce the conductor
resistance, high conductivity copper can be used, but in
high frequencies the plating plays a key role because of
the skin effect (the higher the frequency, the more the
signal migrates to the periphery of the conductor). Another
relevant parameter to consider is the structure of the conductor. A single or multi-stranded conductor will not behave in the same way in terms of attenuation, and flexibility of the cable will also be impacted.
A good cable for high data rate applications can be produced only if these parameters are controlled. SpaceWire and Low Mass SpaceWire have been developed to meet these goals and their performance is continuously improving to serve evolving application needs.
» Example of SpaceWire cable with four shielded pairs, manufactured by Axon’ Cable