Article first published in EDN Magazine , June 4, 1998
Signal integrity is no longer a "nice-to-know" subject. It has become essential to the proper operation of every high-speed digital product. Without due consideration of basic signal-integrity issues (ringing, crosstalk, ground bounce, and power supply noise), typical high-speed products can fail to operate on the bench or, worse yet, become flaky or unreliable in the field. To address these concerns, many leading-edge companies are now doing one or more of the following:
If you believe your company should bring in some signal-integrity experts and tools, you might wonder , Where should I put them? How should I best integrate signal-integrity functions within my company?
There are three basic choices. Some companies assign responsibility for signal-integrity issues to their pcb-layout department. Others assign this responsibility to their digital-circuit designers. The most aggressive engineering managers are choosing to constitute an independent department of signal integrity. The primary advantages of a standalone department are that it can be given a clear mission, and its performance can be tracked.
In a mature signal-integrity department expect to find about one signal-integrity specialist for every five digital-circuit designers.
What sort of a mission do you give to a department of signal integrity? Try this: Maximize the performance and minimize the cost of interconnection technology used in high-speed digital designs . This mission is easy to state and very valuable if implemented correctly.
What about the strategy of a signal-integrity department? That is a matter of considerable debate. I believe that the signal-integrity department should initially act in the role of a consultant to the rest of the design organization ”offering alternatives, evaluating solutions, but never mandating compliance. Once the signal-integrity department has established basic credibility by showing that it can deliver useful results on a pilot-program basis, it is ready to insert itself into the mainstream development process. The key to success is to ensure that the individuals working on signal-integrity issues are viewed as helpful, not harmful , and that they are truly well-informed as opposed to merely pedantic.
A signal-integrity specialist should know that most signal-integrity problems are easily observed . A good simulation or a good laboratory demonstration can usually put to rest any question about the efficacy of a particular solution. In this area signal-integrity specialists enjoy a natural advantage over their EMC counterparts.
How should a signal-integrity department be organized? The group should be led by a strong manager who can sell the program to other parts of the company. In its early stages, the signal-integrity department must influence the company's design, layout, and manufacturing processes without any form of direct control over them. This feat is accomplished through the development of personal relationships within the company and by selling the advantages of new approaches to the affected departments. Good managers know how to do that.
In a mature signal-integrity department, where full ringing and crosstalk analyses are run on each pcb, expect to find about one signal-integrity specialist for every five digital-circuit designers. A large department might have individuals who specialize in model-building, chip-level packaging, connectors, and so forth.
Signal integrity is a rapidly growing field. There is no one right way to build your department. The most important thing is to hire intelligent , self-motivated people with a healthy interest in properly balancing your signal-integrity, EMC, and manufacturing-cost objectives. Train them well, constantly keep on the lookout for new tools, and tear apart plenty of other people's products to see what the competition is doing. The payoff is easy to understand: better system-level performance, a more reliable product, and an overall reduction in cost. Who could ask for more?
Transmission Line Parameters
Pcb (printed-circuit board) Traces
Generic Building-Cabling Standards
100-Ohm Balanced Twisted-Pair Cabling
150-Ohm STP-A Cabling
Time-Domain Simulation Tools and Methods
Points to Remember
Appendix A. Building a Signal Integrity Department
Appendix B. Calculation of Loss Slope
Appendix C. Two-Port Analysis
Appendix D. Accuracy of Pi Model
Appendix E. erf( )