[SI-LIST] Re: rise time performance
- From: "Muranyi, Arpad" <arpad.muranyi@xxxxxxxxx>
- To: "'si'" <si-list@xxxxxxxxxxxxx>
- Date: Thu, 31 Jan 2002 22:54:29 -0800
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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