[SI-LIST] Re: rise time performance
- From: Steve Corey <steve@xxxxxxxxxxxxxx>
- To: "'Signal Integrity Mailing List'" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 01 Feb 2002 11:34:35 -0800
Most of the statements below are true. First, as predicted by Fred and
simulated by Jeff, if all circuit elements have infinite bandwidth,
risetimes will be preserved throughout the circuit, except for the
numerical loss that Jeff observed. One can only hope that the simulator
properly controlled its timestep internally, and the risetime difference
just had to do with the spacing between the displayed points. But as
Jeff has said, don't assume anything.
Looking back to the original post, the problem was stated with a driver
and receiver at either end of the net. This being the case, several
postings about the output and input impedances of the driver and
receiver explain the variation in risetime seen. It takes a certain
amount of charge to charge a capacitance, so the faster it is able to
run in, the more quickly the capacitance is charged.
Also worth noting is that these buffer impedances, depending on the
buffer design and the accuracy of the simulation models, are nonlinear.
That is, instead of being constant values, their R's, C's, and even
L's (once we hit 40 Gbit?) can vary with voltage and current. So, while
viewing the them as linear capacitors lends good insight to the question
at hand, it's likely only accurate to first order. This is the beauty
of nonlinear simulators and nonlinear modeling gurus...
-- Steve
-------------------------------------------
Steven D. Corey, Ph.D.
Time Domain Analysis Systems, Inc.
"The Interconnect Modeling Company."
http://www.tdasystems.com
email: steve@xxxxxxxxxxxxxx
phone: (503) 246-2272
fax: (503) 246-2282
-------------------------------------------
Fred Balistreri wrote:
> Bravo! We are now in agreement.
>
> Best Regards,
>
> "Loyer, Jeff W" wrote:
>
>>Aha and oops.
>>
>>Take the spice deck below and change the
>>.TRAN 50p 4n
>>
>>to
>>.TRAN .5p 4n
>>
>>and the rise-time degradation goes away. The degradation was an artifact of
>>an improper simulation. This agrees more with my understanding of ideal
>>transmission line models - that they cannot change a risetime. But, I'm
>>always learning, so will leave open the possibility that I'm wrong (I also
>>might have slept through the lecture on transmission line effects on
>>risetime).
>>
>>If someone on this esteemed list can send me an example of ideal
>>transmission lines changing the risetime (without adding capacitors or
>>inductors), I'd love to see it.
>>
>>P.S.
>>Note the key word "assume" in the message below. That should have been a
>>red flag that I was about to eat my words!
>>
>>Jeff Loyer
>>
>>-----Original Message-----
>>From: Loyer, Jeff W [mailto:jeff.w.loyer@xxxxxxxxx]
>>Sent: Thursday, January 31, 2002 12:43 PM
>>To: 'Signal Integrity Mailing List'
>>Subject: [SI-LIST] Re: rise time performance
>>
>>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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