RE: [IBIS-Users] IV Curves

Hi Randy Wolff,
Thanks a lot for your detail and useful explaination, i appreciate it very
much. Now, i have a much better idea about the reflection.
I have a further question:
In case 3 below, although the reflected voltage is -vcc, the total voltage
in driver site is 0V. Do you have any example that shows the voltage at
driver site is at -vcc? (which is cover by IBIS voltage range.)
Many thanks.

Appreciated,
Lee

>From: rrwolff@micron.com
>To: <changly_80@hotmail.com>
>CC: <ibis-users@eda.org>
>Subject: RE: [IBIS-Users] IV Curves
>Date: Wed, 12 Jan 2005 08:52:17 -0700
>
>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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Received on Wed Jan 12 16:08:12 2005