signal’s path, presented as the ratio of outgoing to incoming signal. With a high insertion loss, a signal can no longer be clearly identified by the receiver and therefore a limit value of -3 db is typically set. Insertion loss is made up of different components: coupling losses, dielectric losses, reflection losses, line losses, and radiation losses. Coupling losses occur at the contact point between the male and female connectors. Reliable contacting with appropriate tolerances when plugged in is crucial to reducing signal transmission losses. The ohmic contact resistance should also be kept as low as possible through a large contact area and a high contact force at the contact point. Foreign materials on the connector surface, such as particles produced by abrasion, can increase the contact resistance. High-quality connectors are provided with at least a superficial layer of gold to prevent this from occurring. Apart from the insertion loss, the return loss must also be considered. Return loss is the portion of the reflected signal in the inserted signal. Given a connector’s impedance profile, there is little room for a system designer to further influence the return loss. The best way to do this is with the rise time or the pinout. The rise time describes the
time in which the signal lies between two defined amplitude values (typically between 10% and 90%). The lower the rise time, the greater the bandwidth and the closer the impedance is to that of the rest of the system. Another critical source of signal interference is crosstalk, or the undesired influence of a differential signal by another signal on a different line. The capacitive and the inductive coupling can be considered separately. Near-end crosstalk (NEXT) occurs when the signal transmission of a pair interferes with the signal transmission of a parallel pair, which mainly occurs through induction. The higher the frequency, generally the more interference will affect the second pair. If the absolute value in decibels is high, there is a high level of crosstalk attenuation, so only a small influence can be measured in the disturbed pair. At a value of -20 dB, 1% of the signal is crosstalk. At a value of -40 db, on the other hand, only 0.01%. In contrast to near end crosstalk, far end crosstalk (FEXT) occurs over the entire length of a line. Interference with the signal from a neighboring pair is therefore measured at the end of the transmission link and is usually lower because the interfering signal is attenuated along the line.
Crosstalk can be reduced by layouts in the pinout by placing two potentially influencing contact pairs away from each other with ground contacts. A contact design with the shortest possible signal The effect of near and far crosstalk on signal transmission. Graphic: ept GmbH.
Dependence of the impedance on the rise time. Graphic: ept GmbH.
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