[SI-LIST] Re: rise time performance
- From: Fred Balistreri <fred@xxxxxxxxxxxxx>
- To: jeff.w.loyer@xxxxxxxxx
- Date: Fri, 01 Feb 2002 08:34:38 -0800
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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--
Fred Balistreri
fred@xxxxxxxxxxxxx
http://www.apsimtech.com
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