In our series on Cable Design, this installment will discuss selecting the right shield for your cable.

Introduction

The cable shield is a conductive material that surrounds wires inside a cable. The purpose of a shield is to prevent noise from disturbing the signals within the cable and its surrounding environment. There are two different types of shields within cables: an individual shield, around a single twisted pair to prevent crosstalk inside the cable, and an overall shield, around the entire cable bundle to prevent electromagnetic interference (EMI) from radiating into and out of the cable bundle.

Key Considerations

There are several parameters that should be considered when selecting a cable shield:

• Shielding Effectiveness measures the shield’s ability to prevent EMI. Can also be described as “percent coverage."
• Flexibility has a significant impact on the compliance of the overall cable.
• Flex Life determines the ability of a cable to withstand flexing throughout the life of a cable.
• Low Resistance ensures that shield can mitigate low frequency signals.

Shielding Options

The various shielding options, shown in Figure 1, are described below.

1. (a) Foil shields have excellent shielding effectiveness – up to 100% in many cases – and is very thin which can lead to excellent flexibility.  The downsides are the poor flex life and the high resistance.
2. (b) Braided shields have good flexibility, flex life and low resistance. However, they are not ideal for high frequency applications as it is impossible to achieve 100% shielding effectiveness. Typical braid coverage is between 80% and 95%.
3. (c) Foil and Braided shields provide the best of both worlds and is the default choice for high frequency applications.  It combines the shielding effectiveness of the foil shield with the flex life of the braid.
4. (d) Serve shields consist of wires wrapped in a spiral around the wires. Serve shields work great for low frequency applications. However, the spiral wrap creates a coil effect that causes issues for signals above about 1 MHz.

Importance of Cable-Connector Shield Termination

Good overall shielding effectiveness for a cable assembly is only achieved if the shield is properly terminated to the connector, on both ends of the harness. As a general rule, the overall shielding effectiveness will be dictated by the size of the largest opening throughout the assembly (the smaller, the better).  This large opening most often occurs at the cable-connector transition.  In order to address this, it is important to maintain a termination across the full 360° of the cable-connector mate.

Conclusion

Ensuring proper shielding is a critical part of nearly all cable assemblies. When designing a cable, there are several different options that should be considered. Understanding the options laid out in this article should help engineers design their cables to adequately address their needs. Contact Omnetics to learn more.