# HP ADS 1.5 User-defined Models User Manual

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

### Page 171 from 216

The Frequency-Domain Defined Device 6-21

Defining Input and Output Impedances

With the SDD, it is very straight-forward to include the input and output resistances

in the basic equations. For example, i[1]=_v1/50 simply defines a 50 ohm input

resistance. This is not as simple with the FDD, since each equation only defines the

relationship for a single output spectrum. Thus, i[1,1,0] = _sv(1,1,0)/50

defines a 50

ohm input resistance, but only for the fundamental spectral envelope. The input

resistance for the other spectral components is still infinite, which is the equivalent of

being undefined. This becomes more problematic at the output. It is possible to define

the output current and output resistance for a single spectral envelope, but to leave

the other spectral envelopes undefined. This may create an ill-defined circuit,

creating a singular matrix error due to an undefined voltage at certain spectral

frequencies. These problems are best circumvented by using actual resistors external

to the FDD. Of course, if the resistance for certain spectral envelopes is different from

this external value, that difference can be included in the defining spectral port

equations.

Compatibility with Different Simulation Modes

The FDD is not fully compatible with all the different circuit analysis modes of

Advanced Design System. Since DC, AC, transient, and convolution analyses only

define the baseband variables, any use of non-baseband spectral envelopes (harmonic

indices not equal to 0) are ignored in these analyses and the voltages and currents for

these spectral frequencies are set to 0. Similarly, DC, AC, and harmonic balance

analyses are steady-state analyses and time is always equal to 0, so any time-varying

functions are evaluated at time=0 and accessing delayed voltages is the same as

accessing the present voltage. The concept of generating time trigger events, of

course, is valid only in transient, convolution, and Circuit Envelope modes of

operation.

### Page 172 from 216

6-22 The Frequency-Domain Defined Device

Custom Modeling with Frequency-Domain Defined Devices

Components Based on the FDD

A variety of circuit components in Advanced Design System are based on the FDD.

Some of these components are:

• Tuned modulators and demodulators

• Phase lock loop components

• Counter, time, and waveform statistics probes

•Sampler

Many of these models operate on a few (often just one) of the input spectral

frequencies, and in turn output just one, or a few, different spectral frequencies. This

is consistent with the desired, or measured, primary frequency-domain behavior, and

simulations can be performed quite efficiently since all operations are done directly in

the frequency domain.

In cases where a model must include second and third-order interactions with other

spectral frequency components, and the underlying nonlinearity is an algebraic

function of the time-domain voltages and currents, the FDD may become too tedious

to generate all of the frequency-domain equations that define the multiple

interactions, and a broadband model (which can be developed using the SDD) may be

the preferred model.

The FDD spectral models, in general, will not function with AC and transient

analyses. These limitations are noted where the components are documented in the

Circuit Components manuals.

### Page 173 from 216

Adding an FDD to a Schematic 6-23

Adding an FDD to a Schematic

FDDs 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 FDD

component to a schematic and defining it.

To add an FDD:

1. From the Component Palette List, choose

Eqn-based Nonlinear.

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

return to select mode.

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

4. FDD parameters are entered in the Select Parameters list. The parameter is on

the left side of the equation. It identifies the type of parameter, the port it is

applied to, and, where appropriate, the harmonic index

.

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

and paste parameters as necessary.)

5. Under Parameter Entry Mode, specify the type of parameter to be defined:

current, voltage, frequency, trigger, or clock enable. Instructions for defining each

type of parameter follow.

6. Once a parameter is defined, click

Apply to update.

7. Add and edit other parameters as desired.

8. Click

OK to accept the changes and dismiss the dialog box.

I[1,1] = Vout/R

function identifier

current equation

port 1

harmonic index

### Page 174 from 216

6-24 Adding an FDD to a Schematic

Custom Modeling with Frequency-Domain Defined Devices

Defining Current and Voltage Equations

Current and voltage equations are the two basic types of equations for defining

constitutive relationships between the port voltages and port currents. For more

information about these equations, refer to the section “Defining Constitutive

Relationships with Equations” on page 6-8.

To define current or voltage equations:

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

2. By default, a current equation appears in the Select Parameters list. Select this

equation.

3. From the Parameter Entry Mode list, choose either

Current or Vo l t a g e. For

current equations, an I appears on the left side of the equation; for voltage

equations, a V is displayed.

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

apply to.

5. In the Harmonic indices field, enter the harmonic index that the equation

applies to, either an absolute index, or a locally-defined carrier frequency, in

which case the first index must be negative.

6. In the Formula field, enter the expression that defines the current or voltage.

7. Click

Apply to update the equation.

8. To add another equation, click

Add and repeat steps 3-7.

9. Click

OK to accept the changes and dismiss the dialog box.

### Page 175 from 216

Adding an FDD to a Schematic 6-25

Defining Frequency Parameters

The freq parameter can be used to define one or more carriers for an FDD. For more

information about the freq parameter, refer to the section “Specifying Carriers with

the Freq Parameter” on page 6-10.

To define a frequency parameter:

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

2. Select any parameter in the Select Parameters list.

3. Click

Add. The new parameter is automatically selected.

4. From the Parameter Entry Mode list, choose

Frequency. The left side of the

equation is changed to

Freq[n], where n is an index indicating that it is the nth

frequency parameter defined for the FDD.

5. In the Index field, enter the index that identifies the frequency.

NoteThis index is used only to specify which frequency parameter to use when

more than one envelope is specified for an FDD. It does not specify a frequency

offset.

6. In the Formula field, enter the expression that defines the frequency.

7. Click

Apply to update the parameter.

8. Click

OK to accept the changes and dismiss the dialog box.