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Adventures in High Speed PCB Design - Part 1

October 4, 2016 By: Chris Mesibov

As an electronics designer since 1987, I have encountered many recurring themes during projects. Board designers, and certainly board leads, contend with the typical pressures of shrinking schedules, expanding scope and “beyond control” issues. Project managers are always willing to roll down schedule constraints from on high and assert “perfect” schedules that ignore the “beyond control” issues such as inaccurate component specifications, performance and delivery problems. This post is aimed toward the Hardware Engineer Board Lead, and is told in a pseudo-story context so as to touch upon the many difficult situations in which HW engineers might find themselves. The story proposes how simulation can provide relief and solutions to common problems confronting the electronics designer throughout the development process. But before the story begins, it is important to identify three phases of a project where simulation can help solve problems and mitigate risk:

1. Pre layout simulation: If time is on your side prior to the critical design review, do as much signal integrity simulation as possible on all your critical signals. The results of these simulations should be used to instruct the PCB designer as to the routing constraints and other critical PCB geometries and properties that need to be maintained.

2. Layout and Post-layout simulation: Parts of the layout deemed critical should be extracted from the PCB CAD tool and simulated. Specifically, before the artwork is sent to the fabricator, this level of verification could catch mistakes due to geometries not managed by constraint configuration:
       a. Power and ground plane problems can be caught by power integrity simulation (PI).
       b. Ultra-high frequency transmission lines, long transmission lines, noise sensitive signal lines and interfaces where cross talk is a concern, all should be analyzed with signal integrity simulation (SI).

3. Testing stage simulation: During the prototype testing/debug phase, signal probing may present signal waveforms not anticipated during the design analysis (i.e. clock non-monotonicity). Executing a circuit simulation using device models can reveal the actual signal seen at the die as opposed to at the pin.

The project begins – architecture, and design

The provided requirements drive the architecture of the system and the board, which is your responsibility. The management team has demanded that your schedule is to be reduced, affording you little time to get your design and documents completed prior to a preliminary design review, and even less time to perform preliminary simulations on critical interfaces. Examples of simulation that could be performed at this point in the design process would be as follows:

Specialty circuits - Verify complex analog control functions.

  • Power control circuits - i.e. buck regulators, constant current drivers.
  • Critical interface transmission lines – i.e. high speed serial interfaces, multi-load, special termination situations.

Throughout this first phase, your team is gathered to participate in various preliminary and critical design reviews. The Stakeholders participating in these design reviews are FPGA designers, SMEs, software engineers, EMI/EMC compliance engineers, manufacturing engineers and component engineers. 

At the conclusion of the design reviews, your complicated board design starts a lengthy printed circuit board (PCB) layout period, during which you also must prepare a test plan and other required documentation. In our next installment of this story I plan to address competing factors in complex board development and how simulation can reduce design risk.