RE: [SI-LIST] : implementing delay on board

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From: Saeen Malik (saeens@hotmail.com)
Date: Wed Apr 11 2001 - 17:47:40 PDT


Thanks a lot Doug,

Is there a way to estimate ISI (a rule of thumb perhaps)??

>From: "Burns, Douglas" <Douglas.Burns@compaq.com>
>Reply-To: "Burns, Douglas" <Douglas.Burns@compaq.com>
>To: Saeen Malik <saeens@hotmail.com>
>CC: si-list@silab.eng.sun.com
>Subject: RE: [SI-LIST] : implementing delay on board
>Date: Wed, 11 Apr 2001 12:42:54 -0500
>
>What you are seeing is ISI (Intersymbol interference). Your input is
>125MHz/50% duty cycle. This means you drive high for 4ns/low for 4ns. A
>10 Inch line (@165ps/inch) will have a round trip time of 3.3ns.
>A 25 inch line has a round trip time of 8.25 ns. At 10 inches, the line
>settles before you drive the next edge (assuming driver = Zo). As you
>increase the line length, the reflected energy gets back to the driver
>after the Nth edge is driven. Classic case of switching a line faster
>than it's settling time.
>
>
>Doug
>-----Original Message-----
>From: Saeen Malik [mailto:saeens@hotmail.com]
>Sent: Wednesday, April 11, 2001 1:09 PM
>To: si-list@silab.eng.sun.com
>Subject: [SI-LIST] : implementing delay on board
>
>
>Hello SI gurus,
>
>I want to delay some clocks relative to data on my board to take care
>of setup and hold time requirements of the receiver. I am using
>Hspice and the topology is
>
>Driver ---- Transmission line ---- Receiver
>
>The source impedence of the driver is pretty close to the impedence of
>the transmission line, therefore, no series termination is required.
>(as per my simulations using Hspice). The risetime of the driver is
>approximately 600 ps (0-100%). The propdelay of the transmission line
>comes out to be 165 ps/inch.
>
>I am using Hspice version 2000.2 and am using ideal transmission line.
>(T elements). My understanding was, if the length of the transmission
>line is longer than twice the rise time, any extra delay I add should
>appear as pure delay (neglecting any degradation in risetime).
>But, when I do a spice run on trace lengths from 10 inch to 25 inch
>and measure the the delay between waveforms of different lengths,
>the results do not appear uniform. For example, I expect to see
>the delay between a 15 inch tline and 16inch tline to be 165ps, but it
>is not so. Sometimes it is more and sometimes it is less (across the
>length sweep 10-25inch). What could be the reason for this????
>
>One more thing, the signal has a 50% duty cycle and I am measuring the
>delay after 10 cycles (just to let the initial transitions die out).
>The frequency is 125MHz.
>
>
>
>Any inputs would be really appreciated...
>
>Saeen
>
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