From: Charles Hill (email@example.com)
Date: Mon Nov 20 2000 - 09:33:06 PST
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?
At 07:42 AM 11/20/00 -0800, Larry Miller wrote:
>As others have or will point out, mismatches cause loss of input level.
>They also cause duty cycle distortion (DCD), in my opinion. The practical
>result of this is that EMI is increased because the electromagnetic fields
>of the P and N signals cannot scrinch down and cancel.
>Remembering that LVDS was originally designed as a cable link technology,
>you might take some guidelines from similar speed wire standards.
>For example, 100BASE-TX (which is actually 125 MHz, close enough to 155)
>the allowable DCD is 1/16 of a Unit Interval (0.5 ns out of 8 ns).
>Obviously, this gets tougher as you increase in frequency, but you
>probably ought to try for less than 50 ps.
>-----Original Message----- From: Khanh Le
>[SMTP:firstname.lastname@example.org] Sent: Monday, November 20, 2000 4:55
>AM To: email@example.com Subject: [SI-LIST] : matched
>I am new to this forum and have an issue with the PCB layout of
>differential pair signals such LVPECL, LVDS and CML. Are there any rule
>of thumb for matched delay ? e.g. what should be the maximum difference
>in trace length between the p and n of the differential pair, where the
>signal is still considered to be within the safe margin. I have signals
>running at 155MHz, 622MHz, 1.25GHz and 2.5GHz. Thank you all. Khanh
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