NAME_TABLE:

C_Function_Name: cm_s_xfer

Spice_Model_Name: s_xfer

Description: "s-domain transfer function"

PORT_TABLE:

Port Name: in out

Description: "input" "output"

Direction: in out

Default_Type: v v

Allowed_Types: [v,vd,i,id] [v,vd,i,id]

Vector: no no

Vector_Bounds: - -

Null_Allowed: no no

PARAMETER_TABLE:

Parameter_Name: in_offset gain

Description: "input offset" "gain"

Data_Type: real real

Default_Value: 0.0 1.0

Limits: - -

Vector: no no

Vector_Bounds: - -

Null_Allowed: yes yes

PARAMETER_TABLE:

Parameter_Name: num_coeff

Description: "numerator polynomial coefficients"

Data_Type: real

Default_Value: -

Limits: -

Vector: yes

Vector_Bounds: [1 -]

Null_Allowed: no

PARAMETER_TABLE:

Parameter_Name: den_coeff

Description: "denominator polynomial coefficients"

Data_Type: real

Default_Value: -

Limits: -

Vector: yes

Vector_Bounds: [1 -]

Null_Allowed: no

PARAMETER_TABLE:

Parameter_Name: int_ic

Description: "integrator stage initial conditions"

Data_Type: real

Default_Value: 0.0

Limits: -

Vector: yes

Vector_Bounds: den_coeff

Null_Allowed: yes

PARAMETER_TABLE:

Parameter_Name: denormalized_freq

Description: "denorm. corner freq.(radians) for 1 rad/s coeffs"

Data_Type: real

Default_Value: 1.0

Limits: -

Vector: no

Vector_Bounds: -

Null_Allowed: yes

• Description:
  The s-domain transfer function is a single input, single output transfer function in the Laplace transform variable `s' that allows for flexible modulation of the frequency domain characteristics of a signal. Ac and transient simulations are supported. The code model may be configured to produce an arbitrary s-domain transfer function with the following restrictions:

1. The degree of the numerator polynomial cannot exceed that

of the denominator polynomial in the variable "s".

2. The coefficients for a polynomial must be stated

explicitly. That is, if a coefficient is zero, it must be

included as an input to the num coeff or den coeff vector.

The order of the coefficient parameters is from that associated with the highest-powered term decreasing to that of the lowest. Thus, for the coefficient parameters specified below, the equation in `s' is shown:

.model filter s_xfer(gain=0.139713

+ num_coeff=[1.0 0.0 0.7464102]

+ den_coeff=[1.0 0.998942 0.001170077]

+ int_ic=[0 0])

It specifies a transfer function of the form

(N\left( s \right) = 0.139713 \cdot \frac{s^{2} + 0.7464102}{s^{2} + 0.998942s + 0.00117077})

The s-domain transfer function includes gain and in_offset (input offset) parameters to allow for tailoring of the required signal. There are no limits on the internal signal values or on the output value of the s-domain transfer function, so you are cautioned to specify gain and coefficient values that will not cause the model to produce excessively large values. In AC analysis, the value returned is equal to the real and imaginary components of the total s-domain transfer function at each frequency of interest.

The denormalized_freq term allows you to specify coefficients for a normalized filter (i.e. one in which the frequency of interest is 1 rad/s). Once these coefficients are included, specifying the denormalized frequency value `shifts' the corner frequency to the actual one of interest. As an example, the following transfer function describes a Chebyshev low-pass filter with a corner (pass-band) frequency of 1 rad/s:

(N\left( s \right) = 0.139713 \cdot \frac{1.0}{s^{2} + 1.09773s + 1.10251})

In order to define an s_xfer model for the above, but with the corner frequency equal to 1500 rad/s (9425 Hz), the following instance and model lines would be needed:

a12 node1 node2 cheby1

.model cheby1 s_xfer(num_coeff=[1] den_coeff=[1 1.09773 1.10251]

+ int_ic=[0 0] denormalized_freq=1500)

In the above, you add the normalized coefficients and scale the filter through the use of the denormalized freq parameter. Similar results could have been achieved by performing the denormalization prior to specification of the coefficients, and setting denormalized freq to the value 1.0 (or not specifying the frequency, as the default is 1.0 rad/s) Note in the above that frequencies are always specified as radians/second.

Truncation error checking is included in the s-domain transfer block. This should provide for more accurate simulations, since the model will inherently request smaller time increments between simulation points if truncation errors would otherwise be excessive.

The int_ic parameter is an array that must be of size one less as the array of values specified for the den_coeff parameter. Even if a 0 start value is required, you have to add the specific int_ic vector to the set of coefficients (see the examples above and below).

Example SPICE Usage:

a14 9 22 cheby_LP_3kHz

.

.

.model cheby_LP_3kHz s_xfer(in_offset=0.0 gain=1.0 int_ic=[0 0]

+ num_coeff=[1.0]

+ den_coeff=[1.0 1.42562 1.51620])