Subject: RE: [IBIS-Users] damped oscillations in V-T curve
From: herbert_lage@agilent.com
Date: Thu Jun 26 2003 - 03:23:32 PDT
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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