Re: [SI-LIST] : coming up with average power estimates for buffers

Dennis Tomlinson ([email protected])
Mon, 02 Aug 1999 18:14:58 -0500

Pat Zabinski wrote:

[snipped RMS/Average power text]

> 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.

I believe your seeing the resonant effects of the line as it
approaches 1/2 wavelength (one bit time for your 1 0 1 0 1, etc.
pattern). If your excitation waveform consists of odd harmonics
only, and if "shoot through", package parasitics, and all other
anomalies are ignored, your steady state driver power should approach
0 as your line length approaches 1/2 wavelength. When purely random
data is considered, your driver power will increase as line
length increases - reaching flat power dissipation as the line
reaches, and then exceeds a one way delay of 1/2 bit time.

This reminds me of (tongue fully in cheek), a scheme put forth by
a cohort of mine for distributing clock in digital systems
without suffering the ill effects of propagation delay. Simply drive
a 1/2 wavelength line at one extreme end with a sinusoid. In the steady
state, at any instant in time, all points on the line would be of
the same polarity - but with differing amplitudes. At any point along
the line where clock is needed, attach a high-Z, wide dynamic range,
zero crossing detector (ZCD) at the end of a zero length stub. If all
ZCDs have equal propagation delay, then all will derive an identically
time matched replica of the clock. An added bonus is the steady state
driver power dissipation, which would be 0;-)

Time to get back to real products,


> Thanks,
> Pat Zabinski
> --
> Pat Zabinski ph: 507-284-5936
> Mayo Foundation fx: 507-284-9171
> 200 First Street SW [email protected]
> Rochester, MN 55905

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