Re: [IBIS-Users] Rising/Falling Waveform generation


Subject: Re: [IBIS-Users] Rising/Falling Waveform generation
From: Scott McMorrow (scott@teraspeed.com)
Date: Thu Jun 27 2002 - 09:52:11 PDT


John,

My guess is there is something wrong with your simulation deck setup.
 Rfixture and Vfixture is simple load placed on the output of your
buffer during VT table extraction. It is no different than setting up
your spice deck by hand, attaching a pulse source to the input of the
buffer and a resistive load to the output of the buffer and simulating.
 If your buffer is capable of driving a 50 ohm load, then there should
not be a problem with spice2ibis. If there is, it would be generally a
setup or connectivity problem.

An Rfixture of 100K would be useless to an ibis simulator, since the
simulator will attempt to scale the rising and falling waveforms to the
actual load impedance of 50 to 75 ohms in system. This scaling would
cause significant waveform errors.

My suggestion is to setup the buffer independently of spice2ibis and
verify that it actually is capable of driving a standard load. If it
is, then you have an issue with the connections in the decks used for
ibis extraction.

Regards,

Scott

John Phillippe wrote:

>Dear Everyone,
> I can't seem to generate decent rising and falling waveforms in IBIS.
>After looking at the generated spice files, I found that the problem was
>with the Rfixture and Vfixture. I found that if I didn't use a test
>fixture of any kind, then the waveforms came out correct. However I had
>to manually generate them. So I went back and tried monkeying with the
>Rfixture values, and found that a 100K resistor seems to allow IBIS to
>generate a good waveform.
> I've never used an IBIS model in a simulation environment before, and
>so I guess my question is regarding the Rfixture, and what it is used
>for. Could I manually generate my edges and then specify an Rfixture of
>0 ohms in the ibis file? Is having an Rfixture of 100k going to cause
>any problems for the end users?
>
>
>

-- 
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com

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