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Design007-Jan2019

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JANUARY 2019 I DESIGN007 MAGAZINE 47 cancel. Conversely, common-mode radiation from two coupled conductors is identical; it does not cancel, but reinforces. Unfortunately, differential-mode propagation can be converted to common mode by parasitic capacitance or any imbalance caused by signal skew, rise-fall time mismatch, or asymmetry in the channel. Also, dangling via stubs distort signals passing through an interconnect and also decrease the usable bandwidth of the signal. This is due to the via stub acting as an antenna, which has a resonant frequency determined by the quarter wavelength of the structure. The conventional solution to this problem is to back-drill (or control depth drill) the vias to bore out the via stub barrel, so that the via stubs are reduced in length if not completely removed. The Zeroth Rule In Part 1 of this series, I mentioned that there is always room for one more rule at the top, and this rule may be the foundation of earlier rules. The most important rule is to scrutinize the design proactively as it unfolds, so you have the ultimate control over the outcome. Do not totally rely on the tools. High-speed PCB design is not just a process. As we all know, things can and do go horribly wrong at times. You cannot teach people to be good PCB designers. I have presented many training courses over the years, and unfortunately, individuals, although very capable, just don't have what it takes. A successful high-speed PCB designer must: • Communicate well with others within the team and the industry • Appreciate the challenge and have good problem-solving skills • Be creative and thorough, and pride yourself in the quality of your work • Understand and implement IPC design standards and project-specific requirements • Have a thorough knowledge of your EDA tools and PCB fabrication, assembly, and testing processes • Understand transmission-line signal propagation, controlled impedance, and recognize where signal currents flow and how coupling occurs at high frequencies • Keep an eye on the ball during the entire design process, catching any small issues before they become a major problem But what really makes an exceptional PCB designer? Ability must align with enthusiasm! Key Points • Simulation should be done during the design process as part of standard practice • The eye diagram is a common indicator of the quality of a signal in high-speed digital transmission lines • Any signal integrity concern will create issues downstream with ringing causing excessive crosstalk leading to electromagnetic radiation • It is best to compare the skew between signals at the receiver to ensure the flight times are correct • Crosstalk can be coupled trace-to-trace on the same layer or broadside coupled by traces on adjacent layers • Traces routed in parallel and broadside cause greater amounts of crosstalk than those routed side-by-side due to the greater coupling area • To reduce crosstalk, critical trace segments should be spaced by three times the trace thickness where possible; alternatively, the trace to plane height can be reduced in the stackup if real estate is limited • It is best to design the PCB layout with electromagnetic compatibility in mind • Proactive designers recognize that an EMC problem should be addressed at the board level where the radiation emanates • Routing critical signals between the planes can reduce the EMI by more than 10 dB • Differential-mode propagation can be converted to common mode by parasitic capacitance or any imbalance caused by signal skew, rise-fall time mismatch, or asymmetry in the channel • Dangling via stubs distort signals passing through an interconnect and also decrease the usable bandwidth of the signal

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