sch-rnd simulation (SPICE) tutorial

This tutorial goes through the topic of Circuit Simulation with sch-rnd. More specifically: lumped element simulation using SPICE (various implementations). Since sch-rnd itself does not contain any simulation code, all the methods described here rely on external SPICE simulation software.

As of writing (2023), there are a bunch of proprietary SPICE implementations and a few open source alternatives. This tutorial is focusing on the open source implementations. There are three main open source alternatives:

• ngspice: derived from the traditional Berkely SPICE, upgraded with additional extras (such as mixed analog-digital and behavioral simulation by merging xspice and CIDER).
• gnucap: independent implementation; may run faster than ngspice; no mixed analog/digital support in sch-rnd. Not recommended because of various problems in compatibility and stability.
• xyce: should work but is not tested with sch-rnd at the moment.

At the moment sch-rnd recommends using ngspice because that implementation has the most complete support and is the most well-tested with sch-rnd.

Two levels of simulation

Sch-rnd is designed to allow single schematics to be used for different workflows, e.g. both spice simulation and PCB layout. There are various (sometimes optional) abstractions over spice to make this possible.

The low level of simulation are:

• The raw spice approach; this assumes the user speaks spice and wants to run the simulator by hand, outside of sch-rnd (e.g. from a shell or Makefile). When using this method, the schematics will likely end up with spice commands and parameters that depend on a spice implementation. That is, "this sheet is for ngspice" or "this sheet is for gnucap".
• The more generic simulation approach, which introduces a higher abstraction layer over spice. The advantages are that the user does not need to know as much about spice, the same sheet can be used with different spice implementations and sch-rnd can coordinate executing the simulation and can provide a GUI for visualizing the results. It is possible to keep a plot window open, make changes on the sheet (e.g. tune resistor values) and rerun the simulation getting the plot updated with a single click. Simulator execution and GUI are optional, the abstraction itself can be used from command line too. The drawback is that the user needs to learn an abstraction that is specific to sch-rnd, while some minimal spice knowledge (for the models) is still required.

For new users the recommendation is: unless you already have a lot of experience with spice, it is better to go for the simulation approach.

The user manual contains a section on simulation.

Tutorial

The tutorial is a system of smallish examples. Each new example adds something and assumes previous examples are understood. The recommended way of reading the tutorial is to pick one of the raw simulator or the high level sim target and click through the examples for that column.

Note: the raw examples are always written for ngspice with considerations for other spice implementations mentioned at the end of each example. The high level simulation works only with ngspice at the moment.

title	raw ngspice	raw gnucap	high level sim	concepts
01_dc	yes	yes	yes	sch-rnd: sheet setup sim: dc operating point
04_passive_tr	yes	yes	yes	sim: transient (time domain) analysis, voltage source (pulsed)
06_passive_ac	yes	yes	yes	sim: ac (frequency domain) analysis
10_bjt_amp_tr	yes	yes	yes	sch-rnd: spice model, spice pinout sim: time domain sim and SINE() source
12_bjt_amp_ac	yes	yes	yes	sch-rnd: spice pinout with portmap sim: ac analysis
16_opamp_dc	yes	error	yes	sim: dc sweep
18_opamp_ac	yes	error	yes	sch-rnd: 2-slot opamp
22_custom_sym	yes	yes	yes	sch-rnd: symbol inline model card, spice/prefix
30_mixed	yes	N/S	TODO	sch-rnd: ADC/DAC bridging from terminal attributes sim: mixed analog+digital

Legend:

• error: supported in theory, but the simulation fails with an error
• N/S: not supported
• TODO: sch-rnd code missing or example not yet developed