From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Mon Nov 20 2000 - 10:18:25 PST
I don't know precisely how it might cause duty cycle distortion either ...
however, your analysis is too narrow because it doesn't include the
unavoidable effects of the driver and receiver.
For example, if the driven waveforms have some anomalies; like wiggles or
bends somewhat away from the 50% points, which might be caused by the
nonlinear behavior of the driver; or asymmetrical edges on the true and
complement signals; then unequal delays might shift the signal crossings to
points you are better off avoiding.
The transmission line does not create even order harmonics, but the receiver
can, when it slices the differential input and squares it up. If the
differential input level at the signal crossing point decreases enough, it
could influence the (imperfect) receiver to have asymmetrical delays. If
the delay mismatch is bad enough that the differential signal at the
receiver falls to zero for a short (non-zero) time, then all bets are off.
Very small offsets could lead to significant time differences between
leading and trailing edges.
> You said that mismatches cause duty cycle distortion. I don't understand
> the causal relationship. My reasoning goes as follows:
> Mismatches and reflections occur in transmission channels which are linear
> time invariant. A property of linear systems is no additional frequency
> components are created (intermodulation does not occur). For example, if
> a perfect square wave goes through a linear channel, only odd order
> harmonics go in and come out. Now duty cycle distortion implies even
> order harmonics are produced by a transmission channel with a perfect
> square wave input. But this is a contradiction since mismatches are a
> linear process which do not produce additional frequency components.
> The linear transmission channel affects positive and negative edges the
> same. These channels are characterized by an impulse response.
> That what it looks like to me. What do you think?
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