# HP ADS 1.5 User-defined Models User Manual

Brand: HP, Pages: 216, PDF Size: 1.33 MB

### Page 111 from 216

Writing SDD Equations 5-11

Implicit Cases

The two SDD equations

F[1,0] = f1(_v1, _i1)

F[1,0] = f2(_v1, _i1)

and

F[1,0] = f1(_v1, i1) + f2(_v1, _i1)

are equivalent and implement

In the case of an implicit representation, if there is only one expression for a port or,

equivalently, more than one expression for a port but all the expressions use the same

weighting function, do not use a weighting function other than 0. To see this, assume

that in the previous example the weighting function is not weighting function

number 0 but is the user-defined function H(w). Then in the frequency domain, the

implicit equation becomes

which is equivalent to

Here, upper-case letters are used to indicate frequency-domain values, and this

assumes that the weighting function does not evaluate to zero at a frequency of

interest.

You would want to use a weighting function other than 0 with an implicit

representation when two or more expressions are used for a port and different

weighting functions are used by the expressions. For example, the SDD equations in

this example:

F[1,0] = f1(_v1,_i1)

F[1,1] = f2(_v1, _i1)

implement

f1v1i1(,)f2v1i1(,) +0.=

Hw ()F1V1w ()I1w () , ()Hw ()fV1w ()I1w () , () +0=

F1V1w ()I1w () , ()F2V1w ()I1w () , () +0=

f

1v1i1(,)d

dt------f2v1i1(,) +0.=

### Page 112 from 216

5-12 Writing SDD Equations

Custom Modeling with Symbolically-Defined Devices

Using an SDD to Generate Noise

An SDD can generate noise only for AC and S-parameter simulations. If you want to

add 1/f noise to a current source, consider using a standard current noise source and

set its value with an equation so it is a function of frequency:

In = 1e-12 + 1e-6/(freq+1)

In the denominator, the 1 is added so that the equation is not divided by zero when

freq=0.

Summary

• The SDD is an n-port device.

•For port n, the voltage is denoted

_vn. The current is denoted _in. Positive

current flows into the terminal marked

+.

•The explicit representation is useful for voltage-controlled nonlinearities:

•The implicit representation is useful for the general nonlinearity:

•Weighting functions are used to give a frequency weighting to a spectrum.

Weighting function number 0 corresponds to no (that is, unity) weighting.

Weighting function number 1 corresponds to jw and is used to implement a time

derivative.

• SDD equations can reference the current flowing voltage sources or current

probes in the same network.

• When more than one expression is given for a port, each expression is

evaluated, converted into a spectrum, and weighted separately from the others.

The resulting spectra are added together to get the final spectrum.

• An SDD can generate noise only for AC and S-parameter simulations.

ifv() =

fiv(, ) 0=

### Page 113 from 216

Adding an SDD to a Schematic 5-13

Adding an SDD to a Schematic

SDDs can be added to a schematic in the same way as other components are added

and connected to a circuit. This section describes the mechanics of adding an SDD

component to a schematic and defining it.

To add an SDD:

1. From the Component Palette List, choose

Eqn-based Nonlinear.

2. Select the SDD with the desired number of ports, add it to the schematic, and

return to select mode.

3. Double-click the SDD symbol to edit the component.

4. The equations that define the SDD are entered as parameters in the Select

Parameters list. The left side of an equation identifies the type of equation, the

port it is applied to, and the weighting function

.

Select the equation you want to edit. (Note the buttons below the list to add, cut,

and paste equations as necessary.)

5. Under Parameter Entry Mode, specify the type of equation:

implicit, or explicit.

For more information on the types of equations, refer to the section “Defining

Constitutive Relationships with Equations” on page 5-4.

I[1,0] = _v1/50

function identifier

implicit equation

port 1

weighting function 0

### Page 114 from 216

5-14 Adding an SDD to a Schematic

Custom Modeling with Symbolically-Defined Devices

6. In the Port field, enter the number of the port that you want the equation to

apply to.

7. In the Weight field, enter the weighting function that you want to use.

Predefined weighting functions are 0 (the equation is multiplied by 1) and 1

(the equation is multiplied by jw). For more information on weighting functions,

refer to the section “Weighting Functions” on page 5-8. For information on the

procedure for adding a different weighting function to an SDD, refer to the

section “Defining a Weighting Function” on page 5-16.

8. In the Formula field, enter the equation. For long equations, click

More for a

larger entry area.

9. Click

Apply to update the equation.

10. Add and edit other equations for other ports as desired.

11. Click

OK to accept the changes and dismiss the dialog box.

### Page 115 from 216

Adding an SDD to a Schematic 5-15

Defining a Controlling Current

The equations for an SDD can be written in terms of the current flowing in another

device. For example, you can use the current flowing through a voltage source as part

of an SDD equation. You can specify only the current through devices that are either

voltage sources or current probes as control currents, and they must be in the same

network as the SDD. To specify a current as a control current, you enter the instance

name of the device in the C[] parameter of the SDD. For example, to use the current

flowing through a voltage source called SRC1, you would set the current parameter

C[1] to SRC1. The SDD equations use the variable _c1 to refer to this current.

To define a controlling current:

1. Double-click the SDD component to open the Edit Component dialog box.

2. Select C[1]= in the Select Parameters list.

3. Choose String and Reference as the parameter entry mode; File based should

not be used. In the C[Repeated] field, type the instance name of the device.

An example of a parameter definition is shown here.

4. To add another controlling current, select C[1] and click

Add. The parameter

C[2] appears in the parameter list. You can define this parameter for another

current.

5. Click

Apply to update the SDD definition.

6. To use the controlling current in an equation, type _cn in your SDD equation,

for example,

_v2 + _v1*_c1.

7. Click

OK to accept the changes and dismiss the dialog box.

C[1] = Vdds

instancenamecontrol current

parameter