At this point, I'm assuming it's not an error in the simulator (since I'm reproducing the same phenomena that Jason saw). I would love to have anyone recreate the simulation and, if you don't see the rise-time degradation, we can go through the exercise of changing parameters one at a time to discern the error. Here's my Hspice deck, to eliminate ALL ambiguity: Simulation of 60 to 35ohm Tlines .AC DEC 201 30K 6G .TRAN 50p 4n .option INGOLD=1 VPulseGen1 N_Vsrc GND Pulse (0 0.5 100p 32p 32p 10n 20n) AC= 2 Rsrc N_Vsrc N_TDRout R=60 T_Launch N_TDRout GND N_1 GND Z0= 60 td= 250p T_Lo N_1 GND N_2 GND Z0= 35 td= 250p T_Term N_2 GND N_Term GND Z0= 50 td= 250p Rterm N_Term GND R= 50 .print AC S21 = PAR('(v(N_term)/V(N_TDRout))') .END Thanks in advance for your help... Jeff Loyer (253) 371-8093 -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Loyer, Jeff W Sent: Thursday, January 31, 2002 1:31 PM To: 'mary@xxxxxxxxxxxx'; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: rise time performance Here's the circuit: Vsrc -> 60ohmR -> 60ohmTline -> 35ohmTline -> 35ohmR Risetime between 35ohmTline and 35ohmR is longer than between 60ohmR and 60ohmTline. All components are ideal. Jeff Loyer (253) 371-8093 -----Original Message----- From: Mary [mailto:mary@xxxxxxxxxxxx] Sent: Thursday, January 31, 2002 11:01 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: rise time performance You didn't specify the type of receiver. If your receiver was an open-circuit, your rise-time was limited by the capacitance of the trace (apparently about 5 pF for the 35-ohm trace and nearly 9 pF for the 15-ohm trace). If your receiver was matched, your risetime may have been limited by the inductance of the driver or receiver connections if these were included in your simulation. -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Loyer, Jeff W Sent: Thursday, January 31, 2002 12:10 PM To: 'jleung@xxxxxxxxxxxxxxx'; si Subject: [SI-LIST] Re: rise time performance Well, I kept waiting for someone else to add enlightenment, but I guess everyone else is busy reminiscing about the good 'ol days when college made REAL engineers :-) In the S.I. classes I've attended, I have never heard of this phenomena. I duplicated your results (risetime increasing by merely going through an impedance variation). In retrospect, it might have been predicted (RF folks have been making cool filters out of structures on PCBs for years), but I don't think it's "intuitively obvious to the casual observer". I'm surprised an RF person didn't respond to the question with a clear explanation (hint, hint). Meanwhile, you might want to look at the same simulation in the frequency domain (I did). That same impedance discontinuity has clearly different effects, dependent on frequency. Jeff Loyer -----Original Message----- From: Jason D Leung [mailto:jleung@xxxxxxxxxxxxxxx] Sent: Monday, January 28, 2002 6:08 AM To: si Subject: [SI-LIST] Re: rise time performance Hi all, I have found something interesting while I was doing some simulation and would like to see if there is anyone out there also see this. I have found that using different impedance for a transmission line, I can obtain different rise time at the receiver. Let say I have a simple net with a driver -> 60 ohms tline-> 35 ohm stripline(this is the part I am going to change for the experiment)-> receiver and the rise time for the driver is 0.25ns. For a 35 ohm stripline I can achieve a rise time of 0.4 ns at the output, and if we replace the 35 ohm stripline with a 85 ohm stripline I can achieve a rise time of 0.3 ns at the output. Moreover for a 15 ohm stripline I can achieve a rise time of 0.68 ns at the output. For my limited knowledge I can understand if I have a difference in impedance along the net, I am going to get different overshoot/undershoot since we have a different reflection coefficient, but I am having some probelms in understanding why we can achieve a different rise time with different impedance. Is there any kind soul out there, that can explain this phenomenon to me? thanks in advance Jason Leung ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu