Original Date: 08/20/2001
Revision Date: 12/14/2006
Best Practice : Signal Integrity
Lockheed Martin Naval Electronics & Surveillance Systems-Surface Systems utilizes Hyperlynx simulation techniques to surface signal integrity problems before they occur. The company has realized a significant cost and schedule savings on new designs and solved signal integrity issues with old designs.
In the past, Lockheed Martin Naval Electronics & Surveillance Systems-Surface Systems (NE&SS-SS) had no formal practice dealing with signal integrity issues. Only guidelines and formulas from past experiences existed, with minimal or no evaluation in the laboratory environment. Problems were fixed as they occurred on the test floor or system, resulting in board and nest rework; design changes; and additional cycles through layout, fabrication, and test. Today, Lockheed Martin NE&SS-SS uses Hyperlynx simulation techniques to determine signal integrity problems before they occur.
Signal integrity begins with early evaluations as part of the design process. Simulations of signal integrity problems are determined prior to drafting efforts for board layout. Transmission line effects, impedance discontinuities, and crosstalk effects are found early in the design process. Drafting starts with place-and-route constraints being well defined. Using Boards, printed circuit board (PCB) designs are evaluated after drafting, but before fabrication to eliminate transmission line, crosstalk, and electromagnetic interference (EMI) problems. This approach points out corrections which will be required on the backplane or other modules that interact with the module being simulated.
To test the new approach, Lockheed Martin NE&SS-SS employed signal integrity simulations to evaluate new designs, and the results were used to perform board layouts and backplane design configurations. After the company developed and built these designs, actual measurements were taken to compare the signal integrity with simulated results. These comparisons showed excellent correlation of simulation to actual signal effects found. The design resulted in minimal overshoot, undershoot, reflecting, and ringing. This process was performed on multiple 72-bit, 66-MHz buses as well as 33-MHz and 66-MHz clocks and signals. For clock signals at devices across all boards and the backplane, measured skews were typically less than one nanosecond. The new clock distribution approach was also evaluated and used. The backplane operated at 33-MHz for 11 interconnected modules. Pre-layout simulations of modules and the backplane were performed along with post layout simulations. No crosstalk problems occurred on the modules or in the nest and, to date, no signal integrity problems have been found.
Using the Hyperlynx simulation on old designs to fix problems was the next test. An existing design was evaluated for signal integrity and many problem areas were detected. Predetermined problems of ringing of unterminated backplane interfaces caused system test failures. Coupling onto other buses caused false data transitions. Failures were intermittent which surfaced late in the integration phase. After modeling with Hyperlynx, the company captured detailed printed wiring board (PWB), wire, and circuit characteristics from the interface modules and the backplane. Simulations correlated closely with measurements in the test, and provided a basis for termination methods and values. Timing analysis was performed at critical interfaces using simulated propagation delays for modules and backplanes. Module setup and hold times were projected at the connector for system debug support. Crosstalk was also evaluated during artwork revisions, and critical signals shown to be at risk were re-routed. Stack up of multi-layered boards were modified to reduce impedance mismatch between layers and reduced the crosstalk levels. Risks of rework, after layout test and integration, were minimized with the ability to evaluate complex modules; backplanes; stack up; modules in the backplane environment; and multiple module operations and interconnects.
The signal integrity process enabled Lockheed Martin NE&SS-SS to shorten the module and backplane design verification time frame. No additional schedule or costs occurred during or after integration and test. The approach also resulted in less risk associated with signal integrity in end-item systems. The company realized a first-time success on five module types and backplane interconnect.
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