Tom Bob , Signal Integrity

CHAPTER 1



Tom and Bob

As part of the book, I will be covering the conversation between Tom and Bob. Tom is a novice PCB designer willing to learn about Signal Integrity. He does not think much before asking a question. Some times he poses intelligent question. Sometime he irritates Bob with his question. But Bob has patience.

Bob is expert but not so much expert in the Signal Integrity, trying to answer Tom’s questions.

Tom – I was reading this rise time and knee frequency stuff. It says that the rise time is a significant thing and determines the highest frequency content. Does it mean that if I have a 1 KHz square wave that has a rise time of 100 ps, it should be treated as a high frequency signal with spectral frequency of or 3.5 GHz?

Bob – Ideally, yes, this 1 KHz signal should be treated as a 3.5 GHz signal at least from some of the SI angle, e.g., EMI. But Tom tell me why will anyone design a circuit with 1 KHz frequency with such a fast rise edge? Does it make sense? A very fast rising edge makes sense only when you want to achieve high data rate. A fast rising edge for high data rate reduces Inter Symbol Interference (ISI). But there is no use of 100 ps rise time on a 1 KHz clock signal. It will be source of unnecessary EMI. Fast rise time and high clock rate usually go hand in hand. A 100 ps rising edge will usually be observed in clock rates between 500 MHz to 1 GHz signals. And if you do see 100 ps rise time signal for a 1 KHz clock signal, use a capacitor to slow it down.


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