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HEWLETT-PACKARD
Fort Collins, CO
Technology Solutions
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HFSS for high-end servers
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Hewlett-Packard (HP) is a technology solutions provider to consumers, businesses, and institutions globally. The company's offerings span IT infrastructure, global services, business and home computing, and imaging and printing.
THE NEED
To increase electrical and mechanical reliability in a high-end sever through the use of tapered lines
THE SOLUTION
HFSS™
In high-end servers, the routing density in printed circuit boards (PCBs) is increasing, leading to a direct increase in the thickness of these boards. For mechanical considerations, thinner PCBs are preferred. However, in an ideal 50-ohm environment, which is sought for most high-speed signaling approaches, this directly reduces the width of the stripline conductors. This, in turn, increases the overall DC and frequency-dependent conductor losses.
For significant trace lengths (e.g., 20 inches or 50 cm), this can severely reduce the available voltage margin. To satisfy both electrical and mechanical constraints, a tool more common in microwave devices can be used: The taper line. Using tapered traces, narrower lines in a thinner board tailored to 50 ohms can be flared to wider lines of lower impedance. If the transformer is appropriately chosen, the reduction in total insertion loss outweighs the effect of return loss, making a tapered line a useful tool in high-speed systems.
At a recent Ansoft HFSS Users Workshop in San Jose, California, Dr. Karl Bois, an RF/Signal Integrity Engineer at HP, conducted a feasibility study using HFSS, showing that striplines with 75- to 100-micron-wide lines and 50-ohm characteristic impedance can be flared to double their initial width to produce a much better overall throughput. In addition to discussing his findings, Bois presented general guidelines outlining the overall applicability of these tapered lines as a function of total line length, bandwidth, and overall system losses.
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