How can I find the name of a specific simulation example
(e.g., BJT inverter?)
Go through the ppt (or pdf) file containing that example. The
title of the slide is the file name for the example
described by the slide. Click on the "open
project" icon, and the GUI will show several directories
with names that are self-explanatory. For example, the
BJT inverter example would be in "bjt_circuits." Select
this directory and then the file name you have found for
the BJT inverter example (bjt_inverter.sqproj).
Can the cursor position be displayed in the graph window?
Yes. Single-click on the point for which you would like
to know the co-ordinates. The co-ordinates will appear near
the cursor.
Is zooming possible for the plots?
Yes. Mark the area of the plot that you want to expand,
by holding the left mouse button down. When you release
the mouse button, the GUI will show you the expanded plot.
To get back to the previous plot, right-click.
If two elements, A and B, are placed such that one of the
nodes (say, node X) of element A coincides with a node (say,
node Y) of element B, is a connection automatically implied?
NO. A connection is not implied; however, it can be
forced by editing the properties of the two elements and
assigning the same node name for node X of element A and node Y
of element B.
It is best to avoid the above situation since it can
lead to errors if the user forgets to force the connection.
(On the other hand, when a wire is used to connect two
nodes, a connection is automatically forced by the GUI,
by giving the same node name to the two (or more) element
nodes that are connected by that wire.)
How does one get screenshots of schematic diagrams and plots?
From the drop-down menu for "view", select snapshot
and then the desired format such as jpg, png, etc.
If you are in the circuit editor window, the circuit
schematic is saved; if you are in the graph window,
the current plot is saved.
The PNG option is a good one (in terms of resolution and
file size) both for the schematic and the waveforms.
If you click on the snapshot icon on the menu bar,
the schematic or plot is saved to the
clipboard and can be imported into, for example,
a word document, by cntrl v.
Is it possible to get the spectrum (Fourier analysis) of an output
variable?
Yes. In transient or SSW analysis, prepare an output block
first. Check the Fourier box, specify out_tstart
and out_tend
(the difference between the two should be one time period),
select the number of frequency components (NFourier).
After execution, go to Graph menu, and select the output
file you have used for Fourier analysis. Select "time" (to be
interpreted as the index of the frequency component) as the
x-axis and the variable of interest as the y-axis. The plot
will show the spectrum of that variable.
Is it possible to plot a multiple of an output variable?
Yes. It is possible to plot a*x+b where x is an output
variable. One simply needs to change the name of the output
variable from x to a*x or a*x+b or x+b (a and b being real
numbers). An example is given in rc1.sqproj in
the directory electronics_gnrl.
While selecting an output variable by clicking on an element,
a list of names appears. Can any of them be selected as an output
variable?
Generally, the answer is no. It is best to look up the
documentation for that element and figure out which variables
are made available as output variables.
How can one get help for a given element?
There are two ways of doing this.
Transient simulation: which method is best for circuits involving undamped (or slightly damped) oscillations?
The trapezoidal method should be used in these cases because the other methods (Backward Euler or Gear2) introduce some artificial damping.
Examples of slightly damped oscillations: RLC circuits, free acceleration of induction motor.
Note that there are many oscillatory problems in which the oscillations are "forced" and are not affected by damping. For example, a MOS ring oscillator, a Wien bridge oscillator, a 555 timer oscillator, a Schmitt trigger oscillator, etc. In these circuits, the "energy" of the oscillations is provided by a DC voltage source, and the results would not depend significantly on the method used for transient simulation.
How does one select the method parameters?
A detailed description of the various method parameters is given in the manual. For a quick start, it is best to find a "similar" example (in terms of elements and time scales), and copy/paste the solve blocks from there. It is likely that the parameters used there would work well for the new project as well.
For example, if one wants to simulate a Wilson current mirror circuit, the basic current mirror project file would be useful. Similarly, for a boost converter, the parameters set for a buck converter project would generally work well.
What is meant by "singular matrix" error? What are the typical reasons for it to occur?
A circuit simulator solves a matrix problem A x=b, where the matrix A is derived from the circuit equations. If the problem is nonlinear, the matrix is the same as the Jacobian matrix. If this matrix turns out to be singular, the simulator cannot proceed further because no solution can be found. This is the "singular matrix" error.
Some typical reasons for the singular matrix error are
Some commercial packages offer a large "parts library." What is meant by this term? When is it useful?
"Parts library" is a library of models of devices/ICs which are commercially available, e.g., transistors, Op Amps, JFETs.
A "part" may be (a) a basic device model (such as a BJT) in the library with its parameters suitably tweaked to match the characteristics of that part, e.g., parameters such as beta, early voltage, etc. may be set so that the simulated device characteristics closely represent the measured data for the part, such as 2N2222, (b) a subcircuit of components with parameters adjusted so as to reproduce the behaviour of the concerned part. The subcircuit may be a realistic representation of the part (i.e., it may include all BJTs/resistors/diodes that actually go into that chip), or it may be a simpler "behavioral" model which mimics the performance of that part but does not correspond device-by-device to that part. Many of the Op Amp models available in commercial packages or in the public domain are behavioral models.
A parts library is useful when the designer has specific specs in mind and is looking for a "final" solution, including which parts should be used for the job.
A parts library is also useful when one is designing a circuit which would be translated directly to a PCB (using a suitable tool provided by the vendor).
For teaching electronics courses, where the emphasis is more on phenomena and circuit operation rather than the fourth decimal place, the basic device models provided by the simulator are often sufficient, and an elaborate parts library is not required. In some cases, it may be a good idea to set the parameter values of the basic device using public-domain information about a specific part, e.g., 2N2222.
Switched-capacitor circuits: (1) Why are the elements enclosed in boxes? (2) Why are some of the nodes left hanging?
In computing the AC response of a Switched-Cap (SC) circuit, equivalent circuits of groups of elements are required. For this reason, it is necessary to keep these groups as such (and not break them up). The symbols for SC elements have been made up with the above consideration.
In the frequency domain, each node needs to be represented by two nodes: an even node and an odd node. For transient simulation, the odd nodes can be ignored, which is achieved by connecting some dummy elements to the odd nodes within the element templates. For AC simulation, both even and odd sub-circuits need to be considered.
In the circuit schematic, the even (odd) nodes of an element need to be conneced to the even (odd) nodes of other elements. If all of these connections are shown explicitly, the circuit diagram is bound to look messy and confusing. Therefore, the connections for the odd nodes are not shown explicitly. Instead, the odd nodes are connected by editing the element properties and assigning the same node name to the odd nodes of neighbouring elements. In short, although the odd nodes may appear to be hanging in the circuit schematics, they are actually connected to other odd nodes in the circuit.
Further information may be found in project/element documents.