Basics of High Speed ​​Connection Technology

EP Basics: High Speed ​​Design Basics of High Speed ​​Connection Technology

By Martin Wimmers *

High-speed data transmission forms the foundation of IoT applications at the smart factory – and requires particularly powerful connection technology. How are cable connector assemblies designed for high data rates and long transmission distances?

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Basics of High Speed ​​Connection Technology
High-speed data transmission: The high bit rates and long transmission distances place special demands on the design of cable connector assemblies. The choice of material and the component geometry play a special role.

(Image: Fischer Connectors)

The quality and speed of a data connection depends on many factors, not least the performance of the sender and receiver, as defined, for example, in the USB 3.0 specifications. But the basic peculiarity of high-speed data connections lies in the maximum required frequency (f.Max), which is very high in relation to the transmission path.

It is precisely this fact that makes the design of high-speed connection technology so demanding. Due to the high signal frequency, special physical effects can occur at the connection point between the data transmitter and the connector, which must be taken into account when designing cable connector joints.

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More specifically, the data connection becomes more susceptible to insertion and reflection loss as well as near and far crossing with increasing bit rates, signal frequencies and increasing transmission distances. As a result, even the smallest deviations in the design of cable connector joints can affect signal integrity.

What affects reflection and insertion loss?

Relevant standards address certain parameters for cable and plug assembly and provide information for a successful high-speed design.

Loss of reflection is the main problem affecting the signal integrity of connectors. They are directly related to impedance: If the input impedance of the transmitter differs from the input impedance of the receiver, some of the input energy will be reflected towards the transmitter.

The part of the energy that is not reflected can be attenuated by metallic or dielectric influences in the connector. Because the capacitive and inductive properties of a plug – and thus its impedance – are determined by the size, arrangement and shape of the pins.

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In addition, the impedance can also change along the signal path. The impedance can be increased by creating more space between the individual pins. It is reduced by either increasing the capacitive component or reducing the inductive component, for example through thicker signal legs.

Loss of reflection and loss of insertion – what is it about?

While reflection loss is considered the Achilles heel of connectors, insertion loss is a problem for cable connections. The measurement of the input loss indicates the influence of the resistor on the signal transmission. As the signal frequencies increase, the resistance also increases, which is why high-speed data transmissions with their high signal frequencies are particularly affected.

The most common cause of unacceptable insertion loss is too long cable length. Improper connection of connectors can also cause large insertion losses. In addition, high ambient temperatures can affect the behavior of the dielectric materials.

For this reason, PVC is e.g. not the preferred material for cables to operate reliably at high temperatures. Due to its special molecular structure, the plastic PVC produces unfavorable electrical effects.

Field coupling can occur between channels in a cable, called either near-end crossing or far-end crossing, depending on where it originates. The interference signal received on the transmitter side (at the “near end”) is called near end crosstalk (NEXT).

latency, near end crosstalk and far end crosstalk

Far than crosstalk (FEXT) arrives as an interference signal at the receiver end. Since both the original signal and the interfering signal are attenuated by the line, the FEXT level is lower than the NEXT level.

In addition to the bit error rate and bandwidth, the latency, i.e. signal propagation time, one of the most important quality indicators for a high speed data connection. This is the time in milliseconds that a signal needs from the transmitter to the receiver.

The farther apart the transmitter and receiver are, the greater the latency. In addition, the material influences the speed of signal propagation. A common example is the speed advantage of fiber optic cables compared to traditional copper cables.

Choice of material: Conductivity is not the only thing that counts

In principle, high-conductivity materials favor signal transmission in a high-speed data connection. Not only the specific resistance of the material plays a role, but also the structure of its molecular lattice.

Due to their orderly crystal structure, the metals silver, copper and gold in particular are considered to be particularly good conductors. With high-speed connectors, however, the current is transmitted not only via the metallic surface of the pins, but also via the polarization of the dielectric.

The relative dielectric constant of the insulator (better known as permittivity) is therefore an important selection factor for the material. Fluoropolymers are considered good dielectrics, while PVC is not recommended as an insulating material, especially in high speed applications.

EMC: Cheap materials and geometries

Last but not least, the choice of material and the connector geometry have a great influence on electromagnetic compatibility (EMC). If an interference current is emitted to the environment, a magnetic field is formed which can affect the signal transmission. This can be remedied by shielding plates with several contacts on the plug, which divide the current.

In summary, it can be said: The higher the signal frequency and the longer the transmission distance, the more susceptible the connection becomes to unwanted effects such as loss of input, distortion, noise or crosstalk.

Particular attention must therefore be paid to the component geometry on the one hand and the material on the other when designing high-speed connection technology. But also classic topics in connection technology such as shielding against electromagnetic interference (EMI) and electromagnetic compatibility (EMC) must not be ignored.

* Martin Wimmers is the CEO of Fischer Connectors Germany in Zorneding.

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