The Effect of Signal Integrity on Electromagnetic Compatibility
Signal integrity becomes more important in electronic design as circuit speeds increase. Faster data rates and shorter rise/fall times make it more challenging to transmit a signal from point A to point B.
Signal distortion and degradation simultaneously have adverse effects on electromagnetic compatibility. Circuit radiation and circuit susceptibility both increase as signal integrity decreases.
How Does SI Cause EMI?
Have you ever noticed that papers on signal integrity (SI) often mention that signal integrity causes electromagnetic interference (EMI)? Have you ever noticed, though, that they rarely say how?
Using EMI Analyst software, the connection between SI and EMI is easy to see. The example below provides a concrete illustration of how changing just one signal property, skew, affects cable radiation, a critical EMC characteristic for many systems.
Signal Integrity is Not EMC
Before we jump into the example, it is important to make a distinction.
Signal integrity and electromagnetic compatibility (EMC) are not the same things. SI and EMC are two entirely different disciplines. There are overlapping interactions between SI and EMC, but each is a specialty unto itself.
SI affects EMC
SI is concerned with the analog characteristics of digital circuits.
EMC is concerned with making sure that circuit operation does not produce excessive interference and that circuits are not susceptible to interference.
SI is essentiall a time domain subject. EMC, except for transient events, is primarily a frequency domain discipline.
EMC Affects SI
A design engineer could be proficient in SI but know little about EMC. However, his design decisions might have a significant effect on EMC. Likewise, an EMC engineer could know little about SI and still be proficient in most aspects of EMC. Both engineers would benefit from having some knowledge about the other’s field of expertise.
SI and EMC Common Ground
Signal integrity and EMC overlap. Poor signal integrity produces greater radiated emissions from circuit traces and signal wires. Simultaneously, a circuit with poor signal integrity has less margin than a circuit with good signal integrity. Add electromagnetic interference to the system, and the circuit with poor SI is more susceptible.
Effect of Signal Skew on Radiated Emissions
The example below illustrates one way that SI affects EMC. Cable radiation produced by a balanced differential LDVS signal is calculated for two SI conditions. First, the skew of the LVDS signal is zero; then it is changed to 200 picoseconds. The change in radiation levels at some frequencies is dramatic.
The example uses a standard radiated emissions test setup. An antenna placed a fixed distance from the equipment containing the circuitry measures the emitted field strength over a specified frequency range, in this case, 10 MHz to 2 GHz.
The graphs below show the effect of signal skew on radiated emissions for this circuit. Predicted cable radiation is indicated by the reddish-brown line. The radiation limit is the blueish-green line.
First, radiated emissions are computed for the case where the circuit has no signal skew. The positive and negative driver outputs transition at the same time and the rise and fall times are perfectly matched. Then, radiated emissions are calculated when the circuit has just 200 picoseconds of signal skew. The skew introduces about 70 mV common mode voltage that pulses very briefly each time the circuit changes state.
Although both analyses show that radiated emissions are below the limit, notice that signal skew causes the cable radiation to jump by 55 dB at 90 MHz. When the additive effects of common mode noise from other circuits in the system are present, radiated emissions could easily exceed the limit, resulting in an EMI test failure.
Signal skew can have a dramatic effect on cable radiation because it induces common mode voltage on the signal lines.
Signal skew is just one signal integrity property. Other SI properties, such as impedance mismatch, crosstalk, return loss, and propagation delay also affects EMC.
While SI is not the same thing as EMC, SI affects EMC. The quality of the digital signals carried on system electrical cabling has a direct effect on electromagnetic interference emissions and susceptibility.