"D. C. Sessions" wrote:
> Doug Yanagawa wrote:
> > Pat Zabinski wrote:
> > > One other point to note: as we increase the transmission line
> > > length, the RMS power goes up as well (as expected). However,
> > > this trend continues to a certain point, then the power actually
> > > reduces with increased line length. Can someone explain why
> > > the RMS power would be reduced with increased length? We're only
> > > seeing a small percentage change (~10-20%), but it's got
> > > me curious.
> > When the transmission line gets long relative to the switching
> > frequency, the driver is not completely charging and discharging the
> > net. Hmmm, maybe we could solve the worlds energy problems. But the
> > theoretical power mins might have nasty harmonics associated with them.
> All of the RMS-vs-peak etc. stuff has been fun, but mostly irrelevant.
> Assuming a reasonably stiff power supply (safe bet) and no shunt termination
> (iffy) each rising transition will induce a delta-V equal to the supply
> voltage on some amount of capacitance (the falling transitions return the
> charge to ground.) The line capacitance is very nearly the trace capacitance
> plus load devices up to prop delays half of the line high time; for most
> systems this is a safe assumption. If the system actually has ballistic
> signals -- the UI is less than the round-trip delay -- then running
> without shunt terminations is REALLY bold.
> SO! In the absense of highly structured activity, there will be a rising
> edge approximately once in four UIs, each of which will charge the line to
> Vdd. Power is thus (Vdd)(Vdd)(Cline+Cload)/UI -- good old CFV**2
> Clocks, of course, are the quintessential example of "highly structured
> If the line *is* shunt terminated, the power consumption becomes largely
> independent of frequency and is the statistically weighted sum of the
> power in the high state and the low state.
> D. C. Sessions
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