Speaker
Description
Summary
Increasing clock-speeds and decreasing signal rise- and fall-times means that PCB
traces on typical high-speed designs can no longer be considered as perfect point-
to-point connections. Typical signal-integrity effects can lead to crosstalk,
reflection and power-distribution noise problems that can cause false signal
switching. These problems are exacerbated with the introduction of vias or other
discontinuities.
It becomes necessary to provide more accurate models of interconnects and
associated discontinuities on a typical PCB. Electromagnetic field solvers can be
used to provide very accurate representations of interconnections but they are time-
and compute-intensive to obtain so can only be used to characterise relatively
small regions of the design. Using such a method for a full board simulation is
impractical. To overcome this problem, the electro-magnetic tool derived model is
typically extracted and used in a conventional circuit simulator to analyse signal-
integrity effects more fully.
Two examples will be discussed.
The Alice Silicon Pixel Detector design calls for an optimum low-mass cable which
minimizes crosstalk between a pair of 1.6Gbps signals and the adjacent traces.
The HFSS electro-magnetic field solver from Ansoft was used to analyse this cable
and provide such a design. The extracted corresponding model was subsequently used
to simulate in HSpice a full transmission channel incorporating a custom designed
LVDS transceiver.
The second case involves the study of the transmission of 1.6Gbps signals through
via discontinuities within a 18 layer PCB stack-up. A prototype board had already
been designed for the read-out chain of the LHCb experiment using the Cadence SI-
tools. These programs can give sufficiently accurate results for reasonably
complex designs but in this case, it was thought that the complexity of the via
interconnects warranted deeper study. Electromagnetic field solvers were again used
for further analysis.
In conclusion, these two case examples show that reliable high-speed PCB digital
design needs to take into account all design aspects that will affect signal
propagation. These can be examined using commercially available programs to provide
an efficient and practical way to study signal integrity effects.
