Q: What are the functions of the SPICE Model Files ( MOS.s BIP.s, DIO.s, JFET.s, GAAS.s)?


A: SPICE Model Files contain lists of parameter names, their default values, limits, units, and attributes for all supported SPICE models. For example, the MOS.s file contains information relevant to the supported MOSFET models associated with all referenced circuit simulators. The User can load the information associated with a selected model into the UTMOST Parameters Screen by using the Spice Model File option in the Files Menu of UTMOST.

Adding a new parameter or modifying an existing parameter in the Spice Model File is possible. In order to add a new parameter the following steps must be followed:

  1. Open the Common Control Screen and toggle the Edit Names option to "Enabled".
  2. Open the Parameters Screen and increase the Number of Parameters by one. This will add a new row to the end of the parameter list.
  3. Type the new name of the parameter and enter the default value. Modify the associated attributes if required.
  4. Store the updated model into the selected SPICE model file.

It should be noted that if the user changes the SPICE model file name (i.e. MOS_new.s) and selects the Store Model option then only the currently active model is stored to this file. Information relevant to any other model must to loaded from the original MOS.s file which contains information about all supported models. The original SPICE model files which are supplied with UTMOST and can always be retrieved from the installation directory.


Q: How can I ensure that the RS and RD parasitic resistance parameters track one another during an optimization?

A: Any model parameters can be made dependent on each other during an UTMOST optimization. For example, parameter RD can be made dependent on RS, and only RS can be selected for optimization. The optimizer will use RS for optimization but RD will track the value of RS with a selected ratio which is also user definable.

Follow these steps to make RD dependent on RS for optimization:

  1. Open the Parameters Screen and select RS for optimization.
  2. Press the Attributes button. This will open the Parameter Attributes Screen.
  3. Enter the row number of the parameter RS in the "Depends On" column associated with the RD parameter. Enter 1 in the "Ratio" column (Figure 1).
  4. Start the optimization. RS will be optimized and RD will follow the value of RS exactly.

Figure 1

Figure 1

Q: How can I interrupt a measurement, simulation or optimization in UTMOST?

A: The escape (Esc) key on any keyboard can be used to perform an interruption. An UTMOST user can abort a measurement by simply pressing the Esc key. If the measurements were being stored in an UTMOST data log file then all measurements, except for the measurement which was in progress at the time of the interruption, will still be stored in this file.

An UTMOST optimization can also be interrupted in a similar manner. An UTMOST user may desire to interrupt an optimization if, for example, the number of optimization iterations for a particular optimization stage was set to a high value and the optimization converged to an acceptable result after few iterations. The optimization can be stopped by pressing the Esc key and the results of the last iteration will be stored in the optimized column of the parameters screen.