It's a wonder anything ever works: signal integrity would be so much simpler without those pesky interconnects ruining our signals.

SIGNAL INTEGRITY RELATES to all the effects that cause signals at the output terminals of drivers to be imperfect when received at the input terminals of receivers. The terminals can be internal gates on the same chip with connections on the chip, or they can be between drivers on different chips with the signals traveling through packages, circuit boards, connectors, cables and backplanes. Ultimately, signal integrity is about how our gates' beautiful, pristine signals are screwed up by interconnects.

There are two general classes of signal integrity problems. The first class relates to what affects timing and the synchronization of signals, such as the jitter between data and clocks. The second class of problems is about noise--the analog distortions on the digital signals that screw up the correctly read sequence of highs and lows. An example of ringing, a common type of distortion of digital signals, is shown in FIGURE 1.

[FIGURE 1 OMITTED]

In order to be received as a high or low bit, the received voltage must be within a voltage range. The output voltage swings have a range as well. These two ranges have a gap, called the noise margin, that is the acceptable amount of distortion allowed for the output signal before a bit is falsely read by the input receiver.

It is impossible to eliminate noise. All we can do is try to keep it within the noise margin, which is typically about 10 to 15% of the signal voltage swing.

All these various sources of noise must add up to less than the noise margin, or the product may fail due to a bit error. When you consider all the different effects lumped under the term signal integrity, it seems like there is a neverending list of problems to keep track of: ground bounce, ringing, crosstalk, switching noise, rail collapse, delta I noise, capacitive coupling, reflections, discontinuities, impedance miss matches, lossy lines, terminations, branches, deterministic jitter, stubs, decoupling capacitors, loop inductance, via inductance, controlled impedance, time delay, skin depth, return current, layer transitions, differential impedance, collapse of the eye, common signal termination, differential signal termination, overshoot, intersymbol interference, ring back, etc.

All of these effects can be lumped into three general families of signal integrity problems. The first is signal quality problems on one net. This is all about impedance discontinuities. Whenever the instantaneous impedance a signal sees changes, there will be a reflection of the signal back to the driver and a distortion of what is transmitted. Minimizing signal quality problems is all about controlling the instantaneous impedance the signal sees.

The second family of signal integrity problems involves crosstalk between two or more nets. This type is due to excessive capacitive or inductance coupling. In uniform transmission lines, the capacitive and inductive coupling are of comparable magnitude and both must be controlled. When the return path is no longer a uniform plane, the inductive noise dominates, and this is known as switching noise. When the same return path is shared by multiple signals, the noise in the return conductor is known as ground bounce. Minimizing crosstalk is about keeping the capacitive and inductive coupling below a target value.

The third family of signal integrity problems is about the power distribution system (PDS). If the voltage rails to the chips drop more than about 5%, the chip may not run fast enough. Any impedance in the path from the regulator to the pads on the chip will cause voltage drops as the current draw from the chip changes. Minimizing rail collapse is about keeping the impedance in the PDS below a target value.

Somehow, we have to divvy up each of these sources of noise in a budget so that the probability of the total noise exceeding the noise margin is low enough to result in an acceptable product. And this must take place across the full range of manufacturing variation of the chips, packages, circuit board, connectors and operating temperatures, while meeting lower and lower cost targets.

The fact that 200 million working PCs and 500 million cell phone handsets were shipped last year is a tribute to the engineering skill that went into these leading edge products. The next time you meet a signal integrity engineer, you may want to say thanks. PCD&M

DR. ERIC BOGATIN is the CTO of IDI, and president of Bogatin Enterprises. He can be contacted at [e-mail].