摘要：
I recently encountered yet another pc-board-trace specification written in the frequency domain. Why do people do that?
This particular specification calls for a worst-case trace loss of –3 dB at 11.2 GHz. The trace goes inside a piece of test equipment that works at 3.2 Gbps.
The specified frequency, 11.2 GHz, is much higher than the system data rate. You may infer from the specification that the system designer wants an extremely clean channel with rise and fall times far shorter than the data-bit interval.
Unfortunately, the specification may imply a clean channel, but it doesn't specify one. All it says is "worst-case trace loss of –3 dB at 11.2 GHz."
A number of systems that meet this overly stringent specification fail to deliver good performance at 3.2 Gbps.
For example, a 10th-order, Butterworth brick-wall lowpass filter with a –3-dB cutoff residing precisely at 11.2 GHz satisfies the specification to a tee, but its time-domain response rings prodigiously.
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