# HP ADS 1.5 User-defined Models User Manual

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

### Page 116 from 216

5-16 Adding an SDD to a Schematic

Custom Modeling with Symbolically-Defined Devices

Defining a Weighting Function

A weighting function is a frequency-dependent expression that is used to scale the

spectrum of a port current. Weighting functions are evaluated in the frequency

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

(the equation is multiplied by jw). You can define your own weighting functions.

To define a weighting function:

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

2. Select any equation in the Select Parameters list.

3. Click

Add. The new equation is automatically selected.

4. From the Parameter Entry Mode list, choose

Weighting. Note that an H appears

on the left side of the equation to denote it is a weighting function.

5. In the Weight field, enter a value greater than 1. Each weighting function must

have a unique value.

6. In the Formula field, enter the weighting function.

7. Click

Apply to update the SDD definition.

8. Click

OK to accept the changes and dismiss the dialog box.

### Page 117 from 216

SDD Examples 5-17

SDD Examples

This section offers the following detailed examples that show how to use

symbolically-defined devices to define a wide range of nonlinear circuit components.

The examples include:

•“Nonlinear Resistor” on page 5-18

•“Ideal Amplifier Block” on page 5-20

•“Ideal Mixer” on page 5-23

•“Nonlinear Capacitors” on page 5-25

•“Full Model Diode, with Capacitance and Resistance” on page 5-28

•“Nonlinear Inductors” on page 5-32

•“Controlling Current, Instantaneous Power” on page 5-34

•“Gummel-Poon BJT” on page 5-36

You can find these examples in the software under the Examples directory in this

location:

Tutorials/SDD_Examples_prj/networks

### Page 118 from 216

5-18 SDD Examples

Custom Modeling with Symbolically-Defined Devices

Nonlinear Resistor

This section describes how to use SDDs nonlinear resistors with a cubic nonlinearity

example. This example is under the Examples directory in the following location:

Tutorials/SDD_Examples_prj/networks/Cubic.dsn

The nonlinear two-terminal resistor with constitutive relationship

exhibits a negative resistance for small v, and is widely used in the study of

oscillation theory. This two-terminal device can be modeled using a one-port SDD,

shown below. Since this is a voltage-controlled resistor, the SDD is defined using an

explicit equation.

With this setup, note the following points:

• This constitutive relationship specifies the current of port 1, and it is written as

a function of the voltage at port 1.

• The Weight field is set to 0 to indicate that the weighting function is identically

1.

Results of DC and harmonic balance simulations on this component are shown in

Figure 5-2.

iv()v33v – ¤. =

### Page 119 from 216

SDD Examples 5-19

Figure 5-2. Simulation Results For the Nonlinear Cubic Resistor

The data displays show:

• A DC plot of current versus voltage showing the cubic nature of the resistor.

• The spectrum of the resistor current when a 1MHz, 3 V sinusoidal waveform is

applied across the resistor. Note that the fundamental and the third harmonic

are the only non-zero terms.

• Current versus time with the same waveform applied at the input.

### Page 120 from 216

5-20 SDD Examples

Custom Modeling with Symbolically-Defined Devices

Ideal Amplifier Block

This example is under the Examples directory in the following location:

Tutorials/SDD_Examples_prj/networks/NonlinearAmp.dsn

A simple large-signal model for the gain of an ideal amplifier block can be expressed

as

where:

•v

i is the input voltage

•v

o is the output voltage

•V

s is the power supply voltage

•A is the gain in the linear region

This relationship has the characteristics that the gain is A for small v

i, and that vo

saturates at ±V

s, as shown in Figure 5-4 (a).

The amplifier is a two-port device, so one more equation is required to specify the

constitutive relationship. In the case of Figure 5-3 (a), where the ideal amplifier has

infinite input resistance and zero output resistance, you could use the above equation

and the equation to define the constitutive relationship.

To model the amplifier as shown in Figure 5-3 (b), with finite input resistance R

i and

non-zero output resistance R

o, the equations will be different. The SDD in this

example is based on this model.

Figure 5-3. Equivalent Circuit Model for an Ideal Saturating Amplifier

voVsAviVs¤ () tanh =

ii0 =