High-speed data transmission - the backbone of IoT and Industry 4.0 - stands and falls with high-performance and reliable connection technology. This is because unfavorable phenomena such as noise, distortion and insertion loss can impair signal transmission, especially when transporting data over long distances. Each design must therefore be optimized mechanically, electrically and in terms of EMI and EMC shielding.

The quality and speed of a connection depend on many factors, not least the performance of the transmitter and receiver. It is not for nothing that the specifications of the transmitter and receiver make up a lion's share of the USB 3.0 specifications. As a general rule, higher data transfer speeds require higher signal frequencies. Due to these higher frequencies, special physical phenomena occur at the connection point between the data transmitter and the connector, which do not have to be taken into account in designs for lower transmission speeds.

A question of geometry

With high-speed data connections, the maximum required frequency (fmax) of a signal is very high in relation to the distance it has to travel. In addition, the current is transmitted from one side of the connector to the other not only via the metallic surface of the pins, but also via the polarization of the dielectric. Accordingly, when designing high-speed connection technology, engineers cannot limit themselves to the direct current conductivity of metallic surfaces alone, but must also consider the influence of the dielectric materials.

The main cause of signal loss in cables is insertion loss, i.e. the loss of energy during signal transmission via a cable connection. With connectors, on the other hand, reflection losses are the main concern: if the input impedance of the transmitter differs from the input impedance of the connector, part of the input energy is reflected in the direction of the transmitter. Part of the remaining energy is lost due to metallic or dielectric losses in the connector, the 'rest' reaches the other side, i.e. the receiver. Material and geometry have a direct effect on the impedance.

Specifications of the physical layer

The OSI reference model distinguishes between seven successive layers in data communication:

• Physical layer
• Data link layer
• Network layer
• Transport layer
• Session layer
• Presentation Layer
• Application layer

The overall speed of a communication system on the data transmission layer depends on its architecture and the specifications of the sender and receiver. The data link layer above the physical layer is responsible for detecting any bit errors and correcting them if necessary.

The measured TDR impedance response of a non-optimized connector (left) compared to that of an optimized connector (right).

© Fischer Connectors

Normally, a total bit error rate (BER) of 1e-12 is tolerated in the bit transmission layer. In order to achieve the required BER, standards specify certain parameters for cable and connector assembly that influence the quality of data transmission. These include, among others:

• Impedance curve: usually measured with a Vector Network Analyzer (VNA) or a Time Domain Reflectrometry device (TDR) and expressed as a ratio of V/I or E/H
• Power delay: latency of signal propagation
• Insertion loss and return loss: attenuation and mismatch
• Coupling inductance (IC): allows connectors to be modeled as both a source or sink of interference, expressed in Henry
• Crosstalk level (depending on the type of interference, near-end crosstalk, NEXT for short, or far-end crosstalk, FEXT for short): field coupling between the channels within a cable, depending on the signal and ground assignment of the channels
• Losses during mode conversion
• Shielding: interference by and from surrounding devices
• Integrated parameters: arise from the interaction of several parameters

Design tips

From this knowledge of the factors influencing the quality of high-speed data transmission, some tips can be derived for the successful design of connector-cable assemblies. For example, a material with high conductivity proves to be advantageous in contact design. The conductivity results from the specific resistance of the metal, but the crystalline structure of the metal (the more ordered, the better the conductivity) also plays a role here. Silver, copper and gold have particularly good conductivity.

The author: Martin Wimmers is Managing Director of Fischer Connectors in Zorneding.

© Fischer Connectors

The relative dielectric constant of the insulating material (nowadays better known as permittivity) should also not be ignored. Fluoropolymers, for example, are known for their good electrical and dielectric properties, whereas PVC is not recommended as an insulating material, especially in high-speed applications. The topic of EMC should always be at the forefront of the design, as it starts with the choice of material and questions of connector geometry such as the number of contact points. The aim here is to reduce the magnetic field created by the dissipation of the interference current in the environment as much as possible - for example by splitting the current flow using multiple contact shielding plates. Ground channels can act as a 'buffer' between the individual signal channels to reduce the influence of these on each other, especially crosstalk.