Plotting Simulation Results

In this topic:

Overview

SIMetrix provides two methods of creating plots of simulated results.

The first approach is to fix voltage or current probes to the schematic before or during a run. SIMetrix will then generate graphs of the selected voltages and/or currents automatically. The probes have a wide range of options which allow you to specify - for example - how the graphs are organised and when and how often they are updated.

The second approach is to randomly probe the circuit after the run is complete. (You can also do this during a run by pausing first). With this approach, the graph will be created as you point the probe but will not be updated on a new run.

You do not need to make any decisions on how you wish to probe your circuit before starting the run. You can enter a circuit without any fixed probes, run it, then randomly probe afterwards. Alternatively, you can place - say - a single fixed probe on an obvious point of interest, then randomly probe to investigate the detailed behaviour of your circuit.

Fixed schematic probes are limited to single ended voltages and currents and differential voltages. The random probing method allows you to plot anything you like including device power, FFTs, arbitrary expressions of simulation results and X-Y plots such as Nyquist diagrams. It is possible to set up fixed probes to plot arbitrary expressions of signals but this requires manually entering the underlying simulator command, the .GRAPH control. There is no direct schematic support for this. For details of the .GRAPH control see the "Command Reference" chapter of the Simulator Reference Manual.

Fixed Probes

There are several types of fixed probe. Three of the commonly used probes are:

  1. Voltage. Plots the voltage on a net.
  2. Current. Plots the current in a device pin.
  3. Differential voltage. Plots the voltage difference between two points.
They are simply schematic symbols with special properties. When you place a fixed probe on the schematic, the voltage or current at the point where you place the probe will be plotted each time you run the simulation. The probes have a wide range of options which can be set by double clicking it. These options are covered in detail in section Fixed Probes.

There are more fixed probes available in addition to those described above. See Fixed Probes for details.

Fixed Voltage Probes

You can place these on a schematic with the single hot key 'B' or with one of the menus:

  • Probe > Place Fixed Voltage Probe...
  • Place > Probe > Voltage Probe
  • schematic popup Probe Voltage

Hint

If you place the probe immediately on an existing schematic wire, SIMetrix will try and deduce a meaningful name related to what it is connected to. If you place the probe at an empty location, its name will be a default (e.g. PROBE1-NODE) which won't be meaningful and you will probably wish to subsequently edit it.

Fixed Current Probes

You can place these on a schematic with the single hot key 'U' or with one of the menus Probe > Place Fixed Current Probe... Place > Probe > Current Probe schematic popup Probe Current...

Current probes must be placed directly over a part pin. They will have no function if they are not and a warning message will be displayed.

Fixed Differential Voltage Probes

These can be placed using one of the menus
  • Probe > Place Fixed Diff. Voltage Probe...
  • Place > Probe > Differential Voltage Probe

Random Probes

Most of the entries in the schematic's Probe menu are for random probing. You can probe, voltage, current, differential voltage, device power, dB, phase, Nyquist diagrams and much more. You can also plot arbitrary expressions of any circuit signal and plot signals from earlier simulation runs. Just a few of the possibilities to get you started are explained below. For a full reference see Random Probes.

Random Voltage Probing

  1. Select the schematic menu item Probe > Voltage...
  2. Using the mouse, place the cursor over the point on the circuit you wish to plot.
  3. Press the left mouse button. A graph of the voltage at that point will be created. The new curve will be added to any existing graph if the X-axis has the same units. Otherwise, a new graph sheet will be created.

Random Voltage Probing - On New Graph Sheet

  1. Select the schematic menu item Probe > Voltage (New Graph Sheet)...
  2. Using the mouse, place the cursor over the point on the circuit you wish to plot.
  3. Press the left mouse button. A graph of the voltage at that point will be created. A new graph sheet will be created for it unconditionally.

Random Current Probing

  1. Select the schematic menu item Probe > Current...
  2. Using the mouse, place the cursor at the device pin whose current you wish to plot.
  3. Press the left mouse button. A graph of the current at that point will be created. The new curve will be added to any existing graph if the X-axis has the same units. Otherwise, a new graph sheet will be created.

Random Current Probing - On New Graph Sheet

  1. Select the schematic menu item Probe > Current in Device Pin (New Graph Sheet)...
  2. Using the mouse, place the cursor at the device pin whose current you wish to plot.
  3. Press the left mouse button. A graph of the current at that point will be created. A new graph sheet will be created for it unconditionally.

Probing dB and Phase for AC Analysis

In AC analysis you will probably want to plot signals in dB and you may also want to plot the phase of a signal.

  1. Select the schematic menu item Probe AC/Noise > db - Voltage... for dB or Probe AC/Noise > Phase - Voltage....
  2. Using the mouse, place the cursor over the point on the circuit you wish to plot.
  3. Press the left mouse button. The new curve will be added to any existing graph if the X-axis has the same units. Otherwise, a new graph sheet will be created.

Probing dB and Phase for AC Analysis - On New Graph Sheet

  1. Create an empty graph sheet by pressing F10 or selecting menu Probe > New Graph Sheet
  2. Proceed as in above section.

Differential Voltage Probing

The schematic menu Probe > Voltage Differential... allows you to plot the voltage difference between two points. When you select this menu click on the schematic twice. The first is the signal node and the second the reference node.

Advanced Probing

The menu Probe > More Probe Functions... provides many more probing functions selectable from a tree structured list. More advanced plotting can be achieved with the menu Probe > Add Curve.... This opens a dialog box allowing you to enter any expression and which also provides a range of options on how you wish the graph to be plotted.