IBIS-USERS - EIA IBIS Open Forum Users Activity: RE: [IBIS-Users

From: <rrwolff@micron.com>
Date: Wed Jan 12 2005 - 07:52:17 PST

Lee,

The reflection coefficient is a measure of mismatch between the
impedance of the transmission line and the impedance of the load. It
indicates how much energy reflects off the load back down the
transmission line and with what phase shift. The reflection coefficient
can vary from -1 to +1 and indicates a phase shift from 0 to 180
degrees.

The equation for reflection coefficient (RC) is:
RC=(Zl-Zo)/(Zl+Zo), where Zl is the impedance of the load and Zo is the
impedance of the transmission line. Let's look at three cases where Zo
is 50 ohms in each case.

Case 1: The load is an open circuit, so Zl=infinity
RC=(infinity-50)/(infinity+50) = 1
The reflected voltage (Vr) would be forward voltage (Vf)*RC = Vcc*1 =
Vcc
The load voltage (Vload) is Vf+Vr = Vcc+Vcc = 2*Vcc

Case 2: The load is perfectly matched, so Zl=50
RC=(50-50)/(50+50) = 0
Vr = Vf*RC = Vcc*0 = 0
Vload = Vf+Vr = Vcc+0 = Vcc

Case 3: The load is a short (very, very low impedance), so Zl=0
RC=(0-50)/(0+50) = -1
Vr = Vf*RC = Vcc*-1 = -Vcc
Vload = Vf+Vr = Vcc+(-Vcc) = 0

You can see that in case 3, -Vcc is reflected towards the driver. I
hope this helps.

Randy Wolff
Micron Technology

-----Original Message-----
From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On
Behalf Of lee yang
Sent: Tuesday, January 11, 2005 6:08 PM
To: arpad.muranyi@intel.com; ibis-users@eda.org
Subject: RE: [IBIS-Users] IV Curves

Hi all,
Thanks for the answers, i appreciate them very much.
I am sorry to say that i am poor in SI, i couldn't get how the
reflection
for an open circuit transmission with coefficient=1 cause the voltage at

driver site become -vcc (the reflection is the multiplication of voltage

(vcc) and coefficient (+1) which end up with +vcc)? I am sorry but could

someone shed some light to get rid of my confusion?
Much appreciated.

-Lee

>From: "Muranyi, Arpad" <arpad.muranyi@intel.com>
>To: <ibis-users@eda.org>
>Subject: RE: [IBIS-Users] IV Curves
>Date: Mon, 10 Jan 2005 08:58:27 -0800
>
>Just a couple of refinements to the answers I have seen
>so far.
>
>1) Actually none of the IV curves are GND relative any more,
>this is just a terminology that is left over from the first
>version of the IBIS specification.
>
>Note that all four IV curves now have an associated [*** Reference]
>keyword, which defines the voltage for the origin of the IV curve.
>Therefore a pulldown IV curve for an RS232 driver could be referenced
>to -12 V, for example, which is not GND relative by any means...
>It is true, in most cases we reference the pulldown and GND clamp IV
>curves to 0 V, so it is easy to forget about this capability.
>
>2) Lynne said it right, but I would like to add, that even if you
>have a perfect open at the end of the T-line, the 2Vcc or -Vcc
>doubling will only happen if your driver is superconductive,
>i.e. very low impedance. This is because the T-line voltage
>doubling doubles the incident step, which is determined by the
>voltage division based on the T-line impedance, and the driver
>impedance. So the 2Vcc and -Vcc voltages are kind of an absolute
>maximum condition which will most likely never happen.
>
>Arpad Muranyi
>Intel Coproation
>====================================================================
>
>
>-----Original Message-----
>From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On
Behalf
>Of lgreen
>Sent: Monday, January 10, 2005 8:43 AM
>To: 'lee yang'; ibis-users@eda.org
>Subject: RE: [IBIS-Users] IV Curves
>
>Hello, Lee,
>Please see answers inserted.
>- Lynne
>
>-----Original Message-----
>From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On
Behalf
>Of lee yang
>Sent: Sunday, January 09, 2005 5:58 PM
>To: ibis-users@eda.org
>Subject: [IBIS-Users] IV Curves
>
>Hi IBIS experts:
>
>I have a few questions regarding IBIS data:
>1) What is the reason of ground clamp and pulldown data is ground
relative
>while power clamp and pullup data is vcc relative?
>-- This was an arbitrary decision made in the IBIS 1.0 specification.
>Note that GND-and Pulldown tables currents shift relative to ground
>(and currents usually go through 0A at GND), while Power- and Pullup
tables
>shift when Vcc changes (and currents usually go through 0A at Vcc).
>
>2) The IV curves data is covered from -vcc to 2vcc, in what condition
the
>voltage would become -vcc? (eg1: output is driving vcc and the receiver
end
>is being shorted, the reflection coefficient is -1, then the maximum
>negative voltage is 0V (vcc + (-1*vcc)=0); eg2: output is driving 0v
and
>the
>
>reciever end is being shorted, the reflection coefficient is -1 as
well,
>would it be any reflection if the output is driving 0V with reciever
end
>being shorted to ground?)
>-- A load with a small capacitance has a reflection coefficient close
to
>+1.
>For an open circuit termination, the reflection coefficient is +1.
>A rising waveform into an "open circuit" load can cause 2*Vcc at the
>driver,
>and a falling reflection (starting at Vcc) can cause -Vcc at the
driver.
>
>
>Hope you all could shed some light here.
>Thanks.
>
>Yours Faithfully,
>LY
>
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