> > 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 email@example.com
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