From: Ray Anderson ([email protected])
Date: Mon Nov 20 2000 - 10:10:10 PST
Maybe I read it wrong, but I thought his comment regarding 'mismatches'
was related to mismatches in delay (skew) rather than impedance mismatches.
Skew can cause duty cycle distortion of the received signal.
> 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?
> Chuck Hill
> 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.
> >My $.02
> >Larry Miller
> >-----Original Message----- From: Khanh Le
> >[SMTP:[email protected]] Sent: Monday, November 20, 2000 4:55
> >AM To: [email protected] 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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