From: Burns, Douglas ([email protected])
Date: Wed Apr 11 2001 - 10:42:54 PDT
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:[email protected]]
Sent: Wednesday, April 11, 2001 1:09 PM
To: [email protected]
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
_________________________________________________________________
Get your FREE download of MSN Explorer at http://explorer.msn.com
**** To unsubscribe from si-list or si-list-digest: send e-mail to
[email protected]. In the BODY of message put: UNSUBSCRIBE
si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP.
si-list archives are accessible at http://www.qsl.net/wb6tpu
****
**** To unsubscribe from si-list or si-list-digest: send e-mail to
[email protected]. In the BODY of message put: UNSUBSCRIBE
si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP.
si-list archives are accessible at http://www.qsl.net/wb6tpu
****
This archive was generated by hypermail 2b29 : Thu Jun 21 2001 - 10:11:31 PDT