[SI-LIST] Re: rise time performance
- From: Fred Balistreri <fred@xxxxxxxxxxxxx>
- To: arpad.muranyi@xxxxxxxxx
- Date: Fri, 01 Feb 2002 08:16:38 -0800
There was no capacitor in the original posting. The case was
ideal, meaning voltage source, lossless transmission lines and
resistance. Under those conditions there will be no tr/tf
degradation or change except that relection and the final voltage
levels may be different. I am not saying that the ideal case is
in any way realistic but it is theoriticaly correct. Storage
elements and/or copper losses changes the situation. Therefore
I disagree with your rc circuit statements.
Best Regards,
"Muranyi, Arpad" wrote:
>
> Since I received a couple of private messages disputing
> what I stated, I feel an HSPICE example may be helpful
> to illustrate what I said (see below). The waveform at
> nodes RCV1 and RCV2 are identical (until the reflection
> starts kicking in). Changing the resistor and impedance
> values from 30 to 50 to 70 Ohms will change the edge rate
> seen at the capacitor (which represents the input of the
> receiver).
>
> Arpad Muranyi
> Intel Corporation
>
> RC circuit
> **********************************************************************
> .TRAN 1.0ps 10.0ns SWEEP Rval LIN 3 30 70
> .OPTIONS POST=1 POST_VERSION=9007
> .param Rval=50
> **********************************************************************
> Vpls1 Pls1 0 PULSE (0.0V 1.0V 3.4ns 1.0ps 1.0ps 10.00ns 20.0ns)
> Vpls2 Pls2 0 PULSE (0.0V 0.5V 1.0ns 1.0ps 1.0ps 10.00ns 20.0ns)
> **********************************************************************
> R1 Pls1 Rcv1 R=Rval
> C1 Rcv1 0 C=10.0pF
> *
> X1 Pls2 Rcv2 Zd_lineW LENGTH=12 Zo=Rval DELAY=0.2
> C2 Rcv2 0 C=10.0pF
> **********************************************************************
> **********************************************************************
> **********************************************************************
> *** ***
> *** Intel SPICE Models ***
> *** ***
> **********************************************************************
> **********************************************************************
> * Filename: Zd_lineW.INC *
> * Version: 1.1 *
> * Call As: Xxxx in out Zd_lineW *
> * + LENGTH=x Zo=x DELAY=x *
> * *
> * Notes: Zd_lineW is a single conductor microstrip PCB trace model*
> * with an assumed SPICE node 0 as the ground return. *
> * The input parameters are the impedance of the trace and *
> * the propagation delay of the wave. *
> * *
> * LENGTH = length of trace (in inches) *
> * Zo = trace impedance (in Ohms) *
> * DELAY = propagation delay (in ns/in) *
> * *
> * Usage of this subcircuit assumes HSPICE 97.2.1 or higher. *
> **********************************************************************
> .SUBCKT Zd_lineW in1 out1
> + LENGTH= 1 Zo= 50 DELAY= 0.200
> *
> W in1 0 out1 0
> + N= 1 Umodel= Wzd_line L= '0.0254*LENGTH'
> *
> .MODEL Wzd_line
> + U LEVEL= 3 PLEV= 1 ELEV= 2 LLEV= 0 NL= 1
> + l11= '(DELAY*1e-9*Zo)/0.0254'
> + cr1= '(DELAY*1e-9)/(Zo*0.0254)'
> + r11= 0
> + rrr= 0
> .ENDS
> **********************************************************************
> * END OF FILE: Zd_lineW.INC *
> **********************************************************************
> .END
> **********************************************************************
> -----Original Message-----
> From: Muranyi, Arpad
> Sent: Thursday, January 31, 2002 4:53 PM
> To: si
> Subject: RE: [SI-LIST] Re: rise time performance
>
> Guys,
>
> It is much simpler than you may think. Imagine that your
> driver is an ideal switch, and the transmission line is
> a Thevenin resistor connecting the driver with the load
> capacitor. What do you get? A simple RC circuit. Guess
> what happens when you change the value of the Thevenin
> resistor? The RC constant changes as well as the slope
> of the (exponential) curve. The same thing happens with
> your original circuit. No need for any RF explanation...
>
> Arpad Muranyi
> Intel Coropraiton
> ==========================================================
>
> -----Original Message-----
> From: Loyer, Jeff W [mailto:jeff.w.loyer@xxxxxxxxx]
> Sent: Thursday, January 31, 2002 10:10 AM
> 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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