[SI-LIST] Re: rise time performance

  • From: "Loyer, Jeff W" <jeff.w.loyer@xxxxxxxxx>
  • To: "'Signal Integrity Mailing List'" <si-list@xxxxxxxxxxxxx>
  • Date: Thu, 31 Jan 2002 12:43:25 -0800

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





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