[SI-LIST] Re: Placement of Decoupling Caps
- From: "Ingraham, Andrew" <Andrew.Ingraham@xxxxxx>
- To: "Signal Integrity" <si-list@xxxxxxxxxxxxx>
- Date: Thu, 1 Aug 2002 23:09:41 -0400
The quarter wavelength thing matters to you if you have a need to work
in the frequency domain. Say, if you're handling RF or video signals,
or maybe clocks. Or if you know that your digital signals contain
significant energy up to some frequency (based on their risetimes); but
that's a harder thing to get a handle on, because there will always be
some energy at yet higher frequencies.
A line can not be considered electrically short (as a function of
frequency) when it approaches or exceeds a quarter wavelength long.
The significance of a quarter wavelength is that it can significantly
transform impedances. For example, a transmission line that is a
quarter wavelength long and has a short circuit on the end of it, looks
like an open circuit at the other end (at the frequency at which you
said it is a quarter wavelength long, and at all odd harmonics of it).
Conceivably, if a capacitor (which might be said to look like a short
circuit) is attached using long traces that are a quarter wavelength
long at some useful frequency, the net effect is as if nothing was there
at all, at that frequency. High impedance; no current. Thus, you would
need to keep the lengths shorter than a fraction of a quarter
wavelength, to be sure that the capacitor looks like an energy filter at
the device to which it is attached. The full story is more complicated
than this, because the capacitor itself may be going through its own
resonances at such frequencies, unless the traces are exceedingly long.
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