++In this simulation we are going to calculate the DC operating point of a +simple voltage divider. This is also an introduction to setting up a +sheet for circuit simulation and setting up the high level simulation +infrastructure. + +
+The single-sheet schematic contains the voltage divider with all networks +named. There are no extra symbols for the simulation, the circuit is in its +original form, as designed for the PCB workflow. +
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+In SPICE simulation there are different analysis options available: these +are different simulation actions or operations or modes. Basically each +analysis is a different algorithm to run on the circuit. The simplest +analysis is called the "DC operating point analysis". +
+In the op point analysis, the simulator will apply all sources, assume +all inductors are shorted and assume all capacitors are open and then calculate +the voltage for all nodes. (Node is the spice terminology for an equipotential +electrical network.) This simulation is not time or frequency dependent, and +represents a single DC operating point once the circuit has already stabilized. +
+In our example, this means 5V would be applied at CN1 then out1 and +out2 are calculated. Since our voltage sources are ideal and capable of supplying +any amount of current, the voltage on the in network will be 5V; but the +voltages on out1 and out2 will depend on the resistor values of R1, R2 and R3. + + +
+Draw the schematics as usual; make sure there is a gnd network, spice won't +work without that. Ideally, use the stock gnd symbol for that. Make sure +all resistors have a unique name and a value. Spice understands the normal +SI suffixes such as k for kilo, but as it is generally not case sensitive, +m and M are both milli, so you will need to write meg to get mega. +
+Your symbols also need to have the proper spice pin numbers. First switch +your view from the default pcb to sim_ngspice: there's a button for this on +the top right part of the main window, on the left of the help/support button. +This pops up a view selector; click sim_ngspice twice and the selector will close +and the view will change. The new view will show pin numbers as seen by the +spice target. For plain resistors, pin ordering does not matter, but it is +important for polarized parts like diodes, transistors, and sources. The +stock library has spice pin numbers set up and should work without +modification. Later chapters of this tutorial will explain how to deal with +spice pin numbers in symbols. +
+Using the sim_ngspice view CN1 and CN2 are crossed out with red. This means +these symbols are not exported: spice can not simulate connectors. This happens +because the attribute spice/omit is set to yes on these symbols. The stock +library has spice/omit set on mechanical symbols. +
+The message log also has an error message saying that modifications could +not be applied to the sim setup because there is no active simulation setup. +This is normal when sim_ngspice is manually selected, which is the rare case. +Normally a simulation setup is activated from the simulation setup dialog and +that configures everything properly. + +
+High level simulation needs to store a lot of simualtion-specific configuration: +different simulation setups, different output (e.g. plot) options. Since +many real life designs use multiple sheets, these can not be stored on the sheet +level but has to be stored on the project level. Thus the high level sim +feature always operates on the project level and always stores configuration +in the project file, project.lht. This is true even on the single sheet case. +
+The dialog box for the high levle sim feature is accessible through the +File menu, Project... submenu, Circuit simulation submenu. This opens the +Simulation selector dialog, presenting a set of different simulation +setups (or sim setups for short). Different sim setups will simulate +
+A typical use case is to have a sim setup for evaluating the board in the +simplest DC case and a few other sim setups exploring how the circuit works +with different AC stimulus. Another typical use case is to take a complex +circuit apart and simulate different stages separately - this can be done +without adding any extra symbols or other clutter in the drawing, only by +high level sim configuration. +
+In this example, and in most of the examples, there would be only one +simulation, to keep things simple. Click on that one item on the list to +select it and click the Open... button. (This sim setup was once created +by clicking the New... button, which asked for a name for the new sim +setup, created it blank, and then opened the same sim setup dialog described +below.) +
+This opens the simulation setup dialog for the selected sim setup. +
+TODO: screenshot of the sim setup dialog +
+The sim setup dialog has 3 tabs and a control block on the bottom. Each +tab is configuring an important aspect of the given simulation setup +and is going to be described in detail below. + +
+The first tab defines the circuit that is going to be simulated: +
+This is equvalent to the process of physical testing of a circuit: +once the basic circuit is built as documented by the schematics, optionally +a part of it is isolated for testing (this is called test benching in sch-rnd) +and external stimulus, probes and sometimes artifical connections or loading +resistors and alike are added for the measurement (this is the modifications +part). +
+Select the only item in the modifications tableand click the Edit button. This +opens the modification editor that shows: +
+Or in short: this is the 5V power supply connected to CN1. +
+The advantage of having this as a modification instead of drawing it as a +voltage source symbol on the sheet is the less visual clutter on the drawing, +especially when not dealing with simulation. It is also more natural to say +"connect 5V to CN1 for this test" than to draw symbols of non-existing parts. +
+This advantage pays off even more when there are multiple different simulation +setups each having different sources installed at random parts of the circuit. +Without modifications, all sources of all simulation setups would need to be +added to the drawing (or on separate "test bench sheets"). + +
+The second tab, output config, defines the spice analysis to execute and +how the results are presented. A single simulation setup can define multiple +output, in which case each simulation is ran on the given setup, in the order +they are specified, and are presented in the same order. This is useful to +get multiple printouts or plots of the same or different analyses. +
+TODO: screenshot +TODO: describe widgets + +
+The third tab presents simulation results. Before the simulation is first +ran, this tab is empty. Click on the 'run' button in the bottom control block +of the dialog box; the 'run' button will activate the sim setup (selects +the sim_ngspice view and sets all sim related configuration) and run +the simulation (ngspice) in the background and fill in the third tab once +the simulation finishes. +
+In this simple example the output will contain: +
+CN2-3 = 2.38095238e+00 +CN2-2 = 1.19047619e+00 ++the voltages measured on pin 3 and 2 of the connector CN2, as requested in +the output config. +