How to Select Cable Shielding for Electromagnetic Compatibility – Part 2
How Much Shielding Do You Need?
You need to select a cable shield that provides enough attenuation to get the job done, without over-designing and adding unnecessary cost and complexity. However, before you can choose a shield, you first need to know how much shielding is required.
Part 2 of this four-part series looks at how to determine adequate shielding effectiveness.
For electrical cables, the amount of shielding needed is usually driven by two factors:
1. Noise emissions – When radio frequency energy on cabling is too high, a cable shield can be an effective way to attenuate electromagnetic fields emitted from the conductors.
2. Noise immunity – Cable shields provide a barrier to protect against external fields. When shielded, noise induced on conductors does not disrupt or degrade the operation of the cable-connected circuitry.
In both cases, circuits at the ends of the cable determine how much shield attenuation is needed. Clearly, before you can select a shield you need to know how much noise the circuits put onto the conductors and you need to know how sensitive the circuits are to noise induced on the conductors.
Let’s look at the emissions and immunity cases separately.
Radio frequency current and voltage on signal and power lines that are unintentionally put there by circuitry connected to either end of the cable is called conducted emissions. A secondary consequence of conducted emissions is cable radiation, i.e. conducted emissions cause radiated emissions.
Every electronic device must control its conducted and radiated emissions so that it does not interfere with nearby equipment. To this end, each device must comply with specified emissions limits, which are a function equipment type and where it is used. For consumer electronics and industrial equipment, limits are specified by governmental regulatory agencies. For equipment used in automotive, military, and aviation applications, limits are specified by industry-specific regulations.
Every cable-connected circuit emits energy that produces some level of radiated emissions. If the energy cannot be filtered or otherwise controlled before it gets onto the cable conductors and the resultant radiated emissions are higher than the specified limit, the cable must be shielded.
Unshielded Radiated Emissions
How do you determine whether cable radiation is greater than the allowable limit?
You have two options. You can measure the radiated emissions, or you can calculate them.
If you have representative circuitry and cables, and an EMI test laboratory, measuring the radiated emissions is the surest way to determine whether you need a cable shield and how much attenuation it must provide.
Alternatively, you can calculate the cable radiated emissions using software such as EMI Analyst. Software analysis allows the design to be modeled relatively quickly, at a fraction of the cost of testing. Plus, software analysis naturally lends itself to comparative assessment of various design options.
For example, the graph below shows predicted radiated emissions produced when excessive digital circuit voltage is allowed to propagation on an unshielded cable conductor. In this example, the cable needs shielding to control radiation between 50 kHz and 250 MHz, the frequency range over which emissions exceed the limit.
Because the specified limit and calculated radiation are already in units of decibels (dBuV/m), required shielding effectiveness is simply the difference between the two curves. To obtain minimum required shielding effectiveness, at each frequency subtract the limit value from the radiation value. Values greater than zero indicate shielding is needed.
For this example, the highest shielding effectiveness required is 57 dB at 30 MHz.
Establishing shielding effectiveness for immunity is a similar process.
Electromagnetic fields in the environment induce current and voltage on signal and power lines. If the induced levels are greater than the sensitivity of the circuitry connected at either end of the cable, circuit performance may be adversely affected. If the induced levels are high enough, the circuit may be damaged.
For immunity, cable shielding must attenuate external field levels so that noise induced at the circuits is below the circuit’s susceptibility threshold.
Establishing the susceptibility threshold of a circuit is beyond the scope of this article. However, the threshold is always less than the normal operating levels of the circuit, may be frequency dependent, and is often a function of system operation.
Unshielded Radiated Immunity
How do you determine whether immunity test levels are greater than circuit sensitivity?
Again, you have the option to test or to perform an analysis. In nearly all cases, analysis is more practical and cost-effective. Immunity tests tend to be both time-consuming and expensive.
Low-frequency immunity requirements are usually specified as bulk current injection tests, high frequency as electromagnetic field exposure tests. Both methods induce current and voltage on the cable conductors.
The graph above shows circuit voltage induced by an electric field impinging on a wire over a ground plane that is terminated at each end with 50-ohm ideal resistors. Above 2 MHz, induced voltage is greater than the stated susceptibility threshold, so shielding is needed.
In this example, since the induced voltage and susceptibility threshold voltage are not in units of decibels, the needed shielding effectiveness is calculated by dividing the induced voltage by the susceptibility threshold at each frequency and then converting to decibels.
For example, at 50 MHz the cable needs 20 * log (1.3 volts / 10 mV) = 42.3 dB shielding effectiveness.
In Upcoming Posts
This article is the second of a four-part series that looks at cable shielding for electromagnetic compatibility. In the next article of this series, we explore why shielding effectiveness is not an intrinsic property of the cable shield, but instead is a byproduct of the circuit impedances and cabling configuration.
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