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Refresher on Signal Integrity and EMC

Signal integrity and electromagnetic compliance or EMC are inextricably intertwined, you might say. Those two factors are constantly at the forefront of a PCB design regardless whether it’s a conventional or smaller wearable PCB.

The question often comes up – at what frequencies does signal integrity become an issue? Actually, the answer is simple. A signal integrity issue pops up when the signal begins to lose its integrity. It occurs when rise time decreases to the point where PCB parasitic inductances and capacitances begin to produce troublesome noise signals and transients.

Two things result; PCB functionality is adversely affected and the product becomes an unintentional transmitter that interferes with other devices in the immediate area.

Like we said above, signal integrity and EMC are intimately correlated. The higher the levels of signal integrity, the more immune the product is to electromagnetic interference. Different techniques that are most effective to get EMC at the circuit and PCB level are also effective means of improving signal integrity and vice versa.

Signal integrity problems stem from a number of causes. Included are ground bounce, crosstalk, non-monotonic edges, skin depth, undershoot, overshoot, parasitic, stub length, gaps in planes, and rise-time degradation are all words or phrases related to signal integrity, among others. These issues are classified as: reflection and transmission line, cross talk, power distribution, and electromagnetic interference (EMI).

Let’s take for example reflection and transmission line. Unequal impedance of the source output, line, and receiver or load causes impedance mismatch. This mismatch means the transmitted signal is not fully absorbed within the receiver and the excess energy is reflected back to the transmitter. As shown in Fig. 1, at high data rates, this can cause signal overshoot, undershoot, and ringing, as well as stair step waveforms, which produce signal errors.

fig1-digital-signal

This is where an experienced PCB designer plays a big role to isolate these problematic areas ahead of time to increase signal integrity assurances.