xschem/xschem_library/analyses

Visual simulation setup library

This library makes it possible to set up simulations in a way similar to that in Qucs. Each analysis is represented by a symbol. Symbols (e.g. op.sym, ac.sym, ...) have attributes that specify the analysis parameters. The instances of these symbols are netlisted as commands in the control block of VACASK/Ngspice. The ordering is specified by the order attribute. If order is not set, 0 is assumed. The symbols (blocks) are netlisted in the ascending order based on the order atttribute.

Each block displays only those attributes that are specified (i.e. the string length is>0). Netlisting is triggered by the command_block.sym block. In order for the netlister to generate the control block you have to place it in the schematic exactly once. In order for netlisting and symbol rendering to work you have to source the analyses.init.tcl script at Xschem startup by adding the following line to xschemrc.

append postinit_commands {
  foreach i $pathlist {
    if {![catch {source $i/lib_init.tcl} retval]} {
      puts "Sourced library init file $i/lib_init.tcl"
    } 
  }
}

This will source all lib_init.tcl files in all libraries.

An example that demonstrates the capabilities of this library is in file demo.sch.

Setting up sweeps

A sweep is defined with an instance of the sweep.sym block. Each sweep has a name and a tag attribute. The tag is the name of the variable that will hold the swept values in the simulator's output file (applies only to VACASK). The name of the sweep is the one you have to refer to in an analysis to make sweep that particular analysis. To sweep an analysis set its sweep parameter to the name of the sweep that applies to that analysis. Each sweep.sym instance has a sweep parameter. This parameter makes it possible to chain sweeps and form multidimensional sweeps.

For Ngspice sweeps are supported only for the operating point analysis where they are limited to a maximum of 2 dimensions.

Available analyses

The following analyses are supported

Symbol	Analysis	VACASK	Ngspice
op.sym	operating point	op	op
dc1d.sym	1D operating point sweep	1D sweep+op	dc (1D)
dcinc.sym	small signal dc	dcinc	-
dcxf.sym	small signal dc transfer function	dcxf	tf
ac.sym	small signal ac	ac	ac
acxf.sym	small signal ac transfer function	acxf	-
noise.sym	small signal noise	noise	noise
tran.sym	transient	tran	tran
hb.sym	harmonc balance	hb	-

Verbatim blocks

To put verbatim text in a control block the verbatim.sym block can be used. The simulator attribute specifies the simulator for which the block will be netlisted. If the attribute is not specified the block is netlisted for all simulators. The contents of the block are specified by the verbatim attribute.

Sweeps and quoting

If you want to sweep an analysis, set its sweep attribute to the name of the sweep instance (sweep.sym). Nested sweeps are specified by setting the sweep attribute of a sweep instance. Ngspice has only limited sweeping support (maximal sweep depth is 2, sweeps are allowed only in operating point analysis, you cannot sweep arbitrary parameters).

All attribute values must be specified in double quotes. Some attributes values must include quotes (must be quoted). When quoting attributes the quotes must be escaped with a backslash (\"). The name and the sweep attribute should not be quoted. All other string attributes must be quoted. The ic and nodeset attributes (VACASK netlists) should be quoted if they represent the name of the store with stored operating point solution. When explicitly specifying nodesets and initial conditions these attributes should not be quoted.

Specifying outputs

For VACASK netlists differential outputs (dcxf, acxf, and noise analyses) are specified with the outp and outn attributes. These attributes are strings and must be quoted. If the output is a current, specify its name (i.e. vname.flow(br)) under outp and leave outn empty. In both cases outp and outn values must be quoted.

For Ngspice netlists differential outputs can be specified either in the same way as for VACASK. An alternative is to specify the output descriptor in SPICE3 style with outp (v(a), v(a,b), or i(vname)) while leaving outn empty. In this case do not quote the value of outp.

Postprocessing scripts

Postprocessing scripts can be invoked with instances of the the postproc.sym block. The tool attribute specifies the quoted external program to invoke. The file attribute specifies the quoted name of the file that will be passed to the tool. If tool is not quoted and its value is PYTHON the python3 interpreter is invoked under Linux and python.exe is invoked under Windows.

Ngspice netlisting notes

Nodesets and initial conditions specified with the nodeset and ic attributes are ignored. Nodesets and initial conditions must be specified with ˙.nodeset˙ and ˙.ic˙ directives in a verbatim block.

A dcxf block runs the TF analysis. The value of the in attribute specifies the quoted name of the input source. This attribute is ignored in VACASK netlists because VACASK computes the transfer functions from all independent sources to the given output.

For the noise block the number of points per summary can be set with the ptssum attribute. This attribute is ignored in VACASK netlists.

Credits

The analyses library was created by Arpad Buermen. Send your comments to: arpad[dot]buermen[at]fe.uni-lj.si