Hi Guys, When I try using different impedance in the transmission line, I actually have something in my mind. I am trying to use different impedance to represent the short stub or long stub via.Since we know that for a long stub via we have a smaller L, so the impedance is smaller and for a short stub via the L is bigger ,and the impedance is bigger. So for a 50 ohms transmission line, a short stub via is better match than a long stub via. Moreover, I am using a transmission line model to represent the via. My observation is when I have a short stub via, the rise time degradation is smaller, so what I have for the rise time at the receiver side is closer to what I have in the driver side. thanks Jason "Loyer, Jeff W" wrote: > 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 > ------------------------------------------------------------------ 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