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From the above:

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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.

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Seems like that needs a bit more qualification... more like, "while not too many applications require super-fast edges but have low repetition rates, sometimes this will occur..."

3.5 GHz is the 3.5 millionth harmonic of 1 KHz. So the power spectral density up there will be zilch.

But a fast edge can ring at any frequency, and that could, for instance, mis-clock a flipflop. So just because a signal is 1 KHz doesn't mean it's "low frequency" and safe to use in a digital system.

John

John Larkin a écrit :

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Let's push it a bit further...

350MHz is still 350Kth harmonic of 1kHz and there's still almost nothing there. So we can safely ignore this part of the spectrum...

... and, once provided the signal fundamental frequency is low enough we can design a rather band limited channel while still achieving fast rise time.

Cute!

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Thanks,
Fred.

I think that's (sort of) what he's trying to say... it's just that there's plenty of application for fast pulse generators with lot repetition rates (e.g., TDR!), where you do need to treat the interconnects as "high speed."

Even better. Since you have now determined that we no longer need the range from 350MHz to 3.5GHz we could auction that off and issue revenue anticipation bonds to plug some pressing holes in the budget. Since the money will have been spent by the time the auction happens we could already look at whether the 35kth harmonic is good enough so we can pay the interest :-)

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SCNR, Joerg

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