.SYMBOL Record

The SYMBOL record requests a symbolic analysis of the current circuit. The SYMBOL record can be used in one of two forms:

.SYMBOL

requests a symbolic analysis based on the values of the circuit elements. The resulting transfer function is in terms of powers of the Laplace variable S and constant coefficients.

.SYMBOL T1 T2 ...

requests a symbolic analysis wherein those elements named on the SYMBOL record (T1, T2, ...) are referred to by name. All other circuit elements are included by using their values. The resulting transfer function is in terms of powers of the Laplace variable S and coefficients that are functions of the named variables (T1, T2, ...).

The output of the symbolic analysis is the S domain transfer function between the first specified input source and the first specified output.

Note that symbolic analysis operates best on normalized networks. Any coefficient found whose magnitude is less than 1.e-4 is discarded. Consider the following example:

...

.symbol

.ckt v ein 1 0 1.0

r r1 1 2 1.k

c c1 2 gnd 1.u

vm eout 2 gnd

.end

.go

...

The transfer function for this RC low pass filter is:

$T(S)=\frac{1/r1}{[S*c1+(1/r1)]}$,

which, if no scaling is applied, becomes:

$T(S)=\frac{1.e-3}{[S*1.e-6+1.e-3]}$.

Since the magnitude of the coefficient of S in the denominator is less than 1.e-4, it will be discarded and the reported (incorrect) transfer function will be:

$T(S)=\frac{1.e-3}{1.e-3}$.

If the example above is frequency scaled by 1.e-3, using an FSCALE record, the capacitor becomes equal to 1.e-3 and the numerical transfer function is reported (correctly) as:

$T(S)=\frac{1.e-3}{[S*1.e-3+1.e-3]}$

with a note that a frequency scaling of 1.e-3 had been applied.

For best results when using symbolic analysis, use a combination of frequency and impedance scaling to scale the network to component values near unity. The output file will document the scale factors used to facilitate scaling of the resulting transfer function.

When the second form of the SYMBOL analysis is used the values of the selected circuit elements are not used and are irrelevant.