Subject: RE: [IBIS-Users] damped oscillations in V-T curve
From: erik.van.der.ven@philips.com
Date: Fri Jun 27 2003 - 06:13:20 PDT
__________________
Hello Herbert,
Thanks for your reaction. Iam using HSPICE2002.2.2. Indeed when I use 50
Ohm load (equal to Rfix) everything is OK. With 70 ohm load this peaking
occurs. I tried to use two wave forms in the IBIS model, one with Rfix=50
and one with Rfix=70. But SPICE gave an waring that only one curve is
used, I don't know why. Could this method help to get better results? I
don't know what a simulator exactly does with two waveform tables.
Are there more users who can comment on this issue?
With kind regards,
Erik.
Ir. Erik van der Ven
Room DB1032
Business Line Networking Infrastructure
Philips Semiconductors BV
Gerstweg 2
6534 AE Nijmegen
The Netherlands
Phone: +31-24-3534334
herbert_lage@agilent.com
Sent by:
owner-ibis-users@eda.org
06/26/03 12:23 PM
To: Erik van der Ven/NYM/SC/PHILIPS@EMEA2
ibis-users@eda.org
cc:
Subject: RE: [IBIS-Users] damped oscillations in V-T curve
Classification:
Hi Eric,
I am not sure whether this will address your issue, but I noticed a while
back that HSpice's B-element when used for PECL buffers simulates best
when the actual load (in a system) does not deviate too much from the load
that was used to record the waveforms. This was certainly true for older
versions (2001.2), but may have been rectified in newer releases. It has
probably to do with the way HSpice interpolates between the "high" and
"low" state when switching PECL buffers. This is only an issue for PECL
buffers and not that much for other buffer types.
Another issue with buffer simulations is that HSpice does not like
"overclocking", that means a Vt transition needs to be finished before the
buffer is switched again.
As an aside, I tend to use "effective" loads so that eg. ac-coupled PECL
buffers are well behaved in the intended application. Please see example
below. This has worked well so far.
I hope this gives you some ideas where to look,
Best Regards,
Herbert.
| **************************************************************
|
| The V_fixture and R_fixture values for the waveform tables
| are equivalent values which produce the right loadline for
| a PECL output with a 130 Ohm dc-bias resistor and an ac-coupled
| (50+15) Ohm load. The bit pattern is assumed to be balanced so
| that the ac-coupling capacitor is charged to (Vol + Voh)/2.
|
|
| 15 Ohms 50 Ohms
| | |
| PECL-Output -----------/\/\/\----| |----/\/\/\------
| | | | |
| | |
| \ |
| / |
| \ 130 Ohms |
| / |
| | |
| | |
| ----- -----
| --- ---
| - -
|
| **************************************************************
|
[Falling Waveform]
|
R_fixture = 43.33
V_fixture = 1.300
V_fixture_min = 1.147
V_fixture_max = 1.487
|
| time V(typ) V(min) V(max)
|
0.000E+00s 2.316E+00V 1.985E+00V 2.633E+00V
1.000E-11s 2.316E+00V 1.985E+00V 2.633E+00V
2.000E-11s 2.316E+00V 1.986E+00V 2.633E+00V
3.000E-11s 2.316E+00V 1.986E+00V 2.633E+00V
. . . .
. . . .
. . . .
-----Original Message-----
From: erik.van.der.ven@philips.com [mailto:erik.van.der.ven@philips.com]
Sent: 26 June 2003 11:44
To: ibis-users@eda.org
Subject: [IBIS-Users] damped oscillations in V-T curve
Hello,
In my V-T curve I see a damped oscillation in the falling waveform (diff
PECL).
When I do a HSPICE simulation with different load resistances I observe
that the amplitude of the first peak increases dramatically while this is
not the case in a simulation of the original buffer in Spectre.
What should I do to model this behaviour correctly. And what causes this
increase in amplitude?
With kind regards,
Ir. Erik van der Ven
Room DB1032
Business Line Networking Infrastructure
Philips Semiconductors BV
Gerstweg 2
6534 AE Nijmegen
The Netherlands
Phone: +31-24-3534334
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