Thierauf Design & Consulting: Signal Integrity Design, Analysis & Training

If you are looking for a version of SPICE to perform some basic circuit analysis, including elementary signal integrity analysis, you might consider using one of the free versions provided by the various major semiconductor vendors.

As I describe at the end of this note, just be careful how you use the transmission line models, and you’re limited in the types of I/O models you’ll be able to simulate because these simulators don’t support IBIS or encrypted transistor models. You’ll need to use a “full blown” version of SPICE to perform professional quality SI simulations.

Arguably the best known of the free Windows versions of SPICE is LTSPICE, which Linear Technology has made available for some time now. LTSPICE IV can be downloaded from the Linear Tech website here:

The download includes models for a large selection of LTs product line, but models for a nice collection of transistors and diodes often used in switching power supply applications are also included. Models for other devices (such as transistors or diodes not included in the original library) can be easily added by the user.

Texas Instruments offers TINA (from Design Soft). It, too, is free and runs under Windows. It can be down loaded from the TI website:

TINA includes models of many TI analog components, and also for various transistors and diodes. Macro models for a number of logic devices and some analog components are provided, too. For instance, they have a macro for the 555 timer.

Analog Devices has just announced a new version of National Instruments Multisim for the evaluation of ADI’s components. The download is free, runs under Windows, but is limited to 50 components. The download license agreement precludes student use. Download the ADI version of Multisim from the ADI site:

This version of Multisim includes models for many components in ADI’s portfolio, and some transistors and diodes, too.

LTSPICE and Multisim include single conductor lossless and single frequency lossy transmission line models. These TLINES are useful in predicting the severity of reflections, and the effects of terminations. However, since the TLINEs are single conductor, crosstalk can’t be measured, but because the model includes the TLINE reference terminals, coupling over a noisy ground plane can be simulated.

The lossy TLINES include series resistance and (shunt) conductance terms, which are used to account for conductor and dielectric losses. But, because the values don’t change with frequency, these lossy TLINEs are only good for modeling losses at the one specific frequency for which the resistance and conductance are valid. So you shouldn’t expect these TLINEs to properly account for losses in pulse circuits, but they give good results when a single frequency sinusoid is transmitted.

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