Signal Integrity vs EMC
What’s the Difference?

By William D. Kimmel, P.E.
and Daryl D. Gerke, P.E.
Kimmel Gerke Associates, Ltd.

Signal integrity (or SI) is getting increasing attention in the EMC community, as evidenced by the addition of the Signal Integrity Committee in the IEEE EMC Society several years ago.

The issue of signal integrity has become more pressing in recent years. In years gone by, signal integrity was primarily in the domain of high speed computers and telecom applications – most designers never ran into transmission line problems and only occasionally ran into crosstalk problems (at least at the circuit board level). Although the emphasis of SI has been with high speed circuits, we need to remember that low-level analog designers have been fighting their own version of SI for years.

But the high speed microprocessor is leading the charge into SI at all levels, most prominently with the personal computers running well into the GHz range. Not to be outdone, the low level op-amps are dipping into the nanovolt range, placing extreme limits on any kind of noise.

Why has SI been adopted into the IEEE EMC Society? Because there is a major overlap between SI and EMC design – we say that SI is really EMC at the local level. We usually find that boards well designed from an SI standpoint are pretty good from an EMC standpoint. To understand this issue, let’s do a side-by-side comparison of EMC and SI – the comparison is summarized in the table.

The focus of SI is local to the specific signal path, primarily the circuit board. The intent is to get a signal from driver to receiver without excess degradation. If you are dealing with digital signals, there will be an envelope between the minimum output signal and minimum allowed input signal, constituting your noise budget. Your task is to keep the signal from degrading to unacceptable levels along the way. The two primary causes of degradation are signal reflections and crosstalk, generally becoming important when the risetime exceeds the path’s propagation delay.

If you have only one signal of interest, you will only be dealing with transmission line effects. If you have more signals, crosstalk becomes an issue. You will be interested in ground and Vcc noise regardless of path length.

In many cases, the signal is confined to the circuit board. In other cases, the signal must travel from one board to another, along a header/connector path (as from mother- board to daughterboard) or along a data cable. This means impedance needs to be controlled over the entire path, from chip to trace to connector to cable and so on to the receiver on another board.

Transmission lines are not an issue with low frequency analog circuits, but there is little tolerance to ground and power bounce – even a microvolt of noise may be too high in medical and scientific applications. So common ground paths are a prime consideration.

The focus of EMC is the entire system – that could be a single circuit board (common in handheld devices) or it could be a complex set of interconnected subsystems (common in military equipment). We may be interested in the circuit boards, the power supply, the enclosure and cables.

SI generally deals with signals in the time domain. EMC is generally considered to be a frequency domain issue. Certainly, emissions and RF susceptibility are purely frequency domain, and even with the transient tests, design revolves around knowing the frequency content.

Objective

The objective of SI is to get signals clean enough to meet functional requirements, and involves both amplitude and timing margins, both of which tend to disappear in actual applications.

The objective of EMC is twofold: self-compatibility within the various elements in the system and to pass the EMC tests. The EMC tests are intended to enhance the probability of self-compatibility, and in many cases, particularly for manufacturers of equipment, this is your only obligation. Where systems are involved, notably military and vehicular applications, passing the module test is only the first step in EMC. Once you plug the equipment together, you need to find the vulnerable areas and fix them.

Key Concerns

With SI, the key concerns are signal reflections, crosstalk ground bounce and power decoupling.

With EMC, you are concerned with emissions and immunity (aka susceptibility) and self-compatibility (making sure it works). With EMC, you will encounter a much wider range of interference problems than with SI, including motor controls, relays, and other power loads. In modern electronics, you will often find noisy loads operating close to sensitive circuits, and these may or may not be adequately covered by a regulation or subsystem specification.

Solutions

SI solutions involve careful circuit board layout to minimize crosstalk and reflections, attention to timing.

EMC solutions also include careful circuit board layout, but also include attention to grounding and shielding of enclosure and cables, I/O filtering and transient protection.

Signal Levels

SI signal levels are in the millivolt and milliamp range for digital circuits. As supply voltage levels drop, the noise margins drop as well. Analog circuits may have voltage levels as low as a microvolt or less, and currents in the picoamp range. EMC levels cover a much wider range. For emissions, the levels will be in the microvolt and microamp range. These levels are much too low to detect with an oscilloscope set for logic levels.

For immunity, interference levels for transients are in the kilovolt range, as evidenced by the IEC surge and EFT, and currents are in the ampere range.

While EMC and SI needs go hand-in-hand, the levels are much different. So when you see an application note from your chip supplier that tells you to decouple for Vcc droop and EMI, they are really thinking SI. Decoupling that is satisfactory for SI is entirely inadequate for EMI.

Summary

We see that, while EMC and Signal Integrity design requirements overlap to a significant extent, there are still major differences in emphasis. In most digital applications, EMC requirements will be more demanding than those of signal integrity. Signal integrity in analog applications are usually more demanding than EMC requirements. But the good news is that the requirements are usually not in conflict – boards that are well designed from a signal integrity standpoint are well on their way to being good EMC boards. And, in fact, we think that signal integrity design will be enhanced if the designers would adopt some of the EMC design guidelines – edge rate control helps to quiet the board in addition to controlling emissions.