How to Select Cable Shielding for Electromagnetic Compatibility – Part 1

Part 1 – Introduction

You need to select a shielded cable for your equipment, but how do you know the shield you select will ensure electromagnetic compatibility? A properly specified shield is cost-effective, preserves the electromagnetic environment, and provides adequate protection for the circuits at each end of the cable.
In this four-part series, we outline a systematic process for selecting cable shields that meet your specific project requirements

Shields Compliment Filtering

Cable shielding is often necessary for high-speed digital data transmission and equipment operating in harsh electromagnetic environments. Shields are also essential for controlling radiation from cables in power conversion equipment, such as motor drives and power inverters.
EMI filtering can be helpful in controlling electromagnetic interference, but in many circumstances, it is not possible simultaneously satisfy EMI requirements and preserve signal integrity with filtering alone. Cable shields play a critical role in balancing electromagnetic compatibility and signal integrity.

Shields Provide Many Benefits

Cable shielding affects all four aspects of electromagnetic interference.

Conducted Emissions – When grounded at both ends, the cable shield provides a return path for common mode current. Net current on the cable bundle is usually much lower for a shielded cable than for an unshielded cable, resulting in lower common mode voltage between interconnected circuits and lower cable radiation.

Radiated Emissions – Cable shielding is the last line of defense between cable conductors and the environment. Rapidly changing voltage and current on the cable conductors produces radiated emissions. One of the biggest reasons for cable shields is compliance with radiated emissions requirements.

Conducted Susceptibility – Also known as conducted immunity, low-frequency electromagnetic fields and crosstalk coupling from nearby cables induce noise on unshielded cable conductors. Adding a shield introduces a barrier that reduces interference coupling.

Radiated Susceptibility – Cable shields are needed in many applications to control current and voltage induced on cable conductors by external radio frequency field sources, such as broadcast transmitters, radars, welding equipment, and portable transmitters.

Shield Selection Process

Selecting a suitable cable shield is a three-step process:

Determine how much shielding is needed
Determine the shielding characteristics of the candidate shield
Determine whether the shield provides needed shielding

Although fairly obvious, these steps may not be simple. This process is the subject of the three next articles in this four-part series

How Much Shielding Do You Need?

You want to select a shield that provides enough attenuation to get the job done, without over-designing and adding unnecessary cost and complexity. Proper selection requires knowledge about how much energy the cables emit. It also requires information about voltage and current levels on the conductors that cause problems for the cable-connected circuits.

Circuits connected to the cable conductors drive the need for shielding effectiveness. In essence, you need to know how much noise the circuits put onto the conductors and you need to know the susceptibility thresholds of those circuits.

Understand your EMI requirements. Every product is subject to EMI requirements and must pass EMI tests to be sold in the marketplace or delivered to the end customer. Knowing how your product performs for each requirement with unshielded cables tells you how much cable shielding is needed. Without this knowledge, you are only guessing.

One way to assess EMI performance is to test the equipment with unshielded cables, usually an expensive undertaking. Alternatively, you can analyze the system using software such as EMI Analyst. The software approach allows the design to be modeled relatively quickly, at a fraction of the testing cost. Plus, software analysis naturally lends itself to comparative assessments of various design options.
In Part 2 of this series, we look at how you can quickly calculate emissions and susceptibility characteristics of your design so that you can determine how much shielding is needed.

How to Calculate Shield Properties

Surprisingly, cable manufacturers are not very helpful when it comes to providing useful shielding information. Many datasheets provide only optical coverage and material specifications. For example, a shield might have 85% coverage with tin-plated copper braid. While optical coverage and material may be helpful for comparing one shield to another, it provides no information about how well the shield performs in your application.

Cable Shielding Effectiveness is another metric provided by some cable manufacturers. Most often, shielding effectiveness values are given for coaxial cables, which isn’t very helpful if you need to shield a cable other than coax.

Shielding effectiveness values are often used by engineers when comparing one cable shield to another. However, in practice, shielding effectiveness may be very different than the value specified by the manufacturer. When a manufacturer states that their shield provides 80 dB of shielding effectiveness, what does that mean?

Part 3 of this four-part series looks at how to assess the properties of a shield that differentiate one shield from another, shield transfer impedance and shield transfer admittance, and how to extract needed information from data sheets and physical shield construction.

How to Correctly Assess Shielding Effectiveness

Once you have selected a shielded cable or have selected a shield for your cable bundle, how do you determine whether it is sufficient? How do you know if it is over-designed?

In part 4 of this series, we look at how to assess shield performance for all four aspects of electromagnetic interference: (1) conducted emissions, (2) radiated emissions, (3) conducted susceptibility, and (4) radiated susceptibility.

We also look at how to calculate shielding effectiveness. Unless your product has circuits with 50-ohm source impedance and 50-ohm load impedance, the shielding effectiveness for your cable will be different than the manufacturer’s specification. Perhaps very different. You may also be surprised to learn that shielding effectiveness may vary widely between EMI requirements. Shielding effectiveness for conducted susceptibility, for example, may be much higher or lower than for radiated emissions

In Upcoming Posts

This post is the first of a four-part series that takes a look at cable shielding for electromagnetic compatibility. In the next three posts, we get into the details.

Part 1 – Introduction
Part 2 – How Much Shielding Do You Need?
Part 3 – How to Calculate Shield Properties
Part 4 – How to Correctly Assess Shielding Effectiveness