Ports in Momentum

Ports enable energy to flow into and out of a circuit. Energy is applied to a circuit as part of the simulation process. A circuit solved using Momentum can have, at the minimum, one port.
Ports are defined in a two-step process. First, ports are added to a circuit when the circuit is drawn. Then, in Momentum, you specify the type of port in order to tailor the port to your circuit. This facilitates the simulation process.
This chapter begins with suggestions to keep in mind when adding ports to a circuit that will be simulated using Momentum. The remainder of this chapter describes the various port types in Momentum and gives instructions on how to specify a port type.

Adding a Port to a Circuit

You can add a port to a circuit either from the Schematic window or a Layout window. For instructions on how to add a port to a circuit, refer to Adding a Port to a Circuit. The procedures include considerations for adding ports to a circuit that will be simulated using Momentum.

Considerations

Keep the following points in mind when adding ports to circuits to be simulated using Momentum:

• The components or shapes that ports are connected to must be on layout layers that are mapped to metallization layers that are defined as strips or slots. Ports cannot be directly connected to vias. For information on how to define strips and slots, refer to Defining Metallization Layers.

• Make sure that ports on edges are positioned so that the arrow is outside of the object, pointing inwards, and at a straight angle.
• Make sure that the port and the object you are connecting it to are on the same layout layer. For convenience, you can set the entry layer to this layer; the Entry Layer listbox is on the Layout tool bar.
  
• A port must be applied to an object. If a port is applied in open space so that is not connected to an object, Momentum will automatically snap the port to the edge of the closest object. This will not be apparent from the layout, however, because the position of the port will not change.
• If the Layout resolution is changed after adding ports that are snapped to edges, you must delete the ports and add them again. The resolution change makes it unclear to which edges the ports are snapped, causing errors in mesh calculations.

  Note Do not use the ground port component ( Insert > Ground ) in circuits that will be simulated using Momentum. Ground components placed in layout are not recognized by Momentum. Either add ground planes to the substrate or use the ground reference ports that are described later in this chapter.

Ground port component toolbar button:

Port Calibration

In Momentum, there are two different lumped sources used in the calibration process to feed the circuits with calibration lines: a grounded source and a floating source.
The grounded source works well for low frequencies, however at higher frequencies, when the port-ground distance becomes electrically large, this source provides less accurate results due to unwanted substrate coupling in the calibration process.
The floating source works well at higher frequencies (unwanted substrate coupling is reduced), however it fails at low frequencies because the capacitive internal impedance of the source blocks the flow of the low frequency currents.
The default behavior in Momentum is an automatic switching between these two sources, depending on the frequency range of the simulation.
The source type can be explicitly controlled using the following environment variable:

MOM3D_USE_SOURCETYPE=0 (grounded, low frequency calibration source)

MOM3D_USE_SOURCETYPE=1 (floating, high frequency calibration source)

This variable can be set in either of these locations:

$HPEESOF_DIR/config/momentum.cfg

$HOME/hpeesof/config/momentum.cfg

Determining the Port Type to Use

There are five port types in Momentum. The purpose of ports is to inject energy into a circuit and to allow energy to flow into and out of a circuit. The different port types enable you to tailor the ports in your circuit according to your type of circuit and its function in the circuit. In general, you should select the port type that best matches the intended application of your layout.
The table below gives a brief description of each port type. You can use a combination of port types in your circuit, although you should note that port types have limitations on where they can be applied. Only the Single port type can be applied to objects that are on either strip or slot metallization layers. Only the Internal and Ground Reference types can be applied to ports that are on the surface of an object, the remaining types can be applied to ports that are connected to an edge of an object.

Note Strips and slots refer to metallization layers. For more information on these layers, refer to Defining Metallization Layers.

If you elect not to assign port types, any port in the circuit will be treated as a Single port type during simulation.

Note All port types can be defined in both of Momentum's simulation modes (microwave and RF). However, de-embedding (reference offset) is not available in RF mode.

Port Type	Description	*Port Connected to ***	Object on
Single (default)	The port is calibrated to remove any mode mismatch at the port boundary. Single ports on slot layers have polarity. Unless a port is given another type, it will be treated as a single port.	Edge of object	Strip or slot layer
Internal	The port is not calibrated. It is useful for making a connection with lumped elements or for representing other connections in the circuit.	Edge or surface of object	Strip layer
Differential	Two ports with opposite polarity. The port pair is simulated as a single port.	Edge of object	Strip layer
Coplanar	Two ports with opposite polarity. The port pair is simulated as a single port.	Edge of object	Slot layer
Common mode	Two or more ports excited with the same absolute potential and the same polarity. The ports are simulated as a single port.	Edge of object	Strip layer
Ground reference	Use an explicit ground for a single (strip), internal, or common mode port.Implicit ground is made available through the closest infinite metal, when no explicit ground port is present.	Edge or surface of object	Strip layer

Additional details about each port type and how to define them are given in the following sections.

Defining a Single Port

Single is the default port type. It has the following properties:

• It is connected to an object that is on either a strip or slot metallization layer.
• It can be applied only to the edge of an object.
• The port is external and calibrated. The port is excited using a calibration process that removes any undesired reactive effects of the port excitations (mode mismatch) at the port boundary. This is performed by extending the port boundary with a half-wavelength calibration (transmission) line. The frequency wavelength selected during the mesh or simulation process is used to calculate the length of the calibration line. For more information about the calibration process, refer to Calibration and De-embedding of the S-parameters.

• The port boundary can be moved into or away from the geometry by specifying a reference offset. S-parameters will be calculated as if the port were at this position. For more information, refer to Applying Reference Offsets.

• When two or more single ports are on the same reference plane, coupling effects caused by parasitics affects the S-parameters. The calibration process groups the ports so that any coupling in the calibration arms is included in the S-parameter solution. For more information, refer to Allowing for Coupling Effects.

• If the port is connected to an object on a strip layer, the substrate definition must include at least one infinite metal layer: a top cover, ground plane, or a slot layer, or a ground reference must be used in addition to the port. For more information on ground references, refer to Defining a Ground Reference.

• If the port is connected to an object that is on a slot layer, the port has polarity.

  Hint It is not necessary to open the Port Editor dialog box to assign this port type. Any port without a port type specified is assumed to be a single port.

  
  To define a single port type:

1. Choose Momentum > Port Editor .
2. Select the port that you want to assign this type to.
3. In the Port Editor dialog box, under Port Type, select Single .
4. Enter the components of the port impedance in the Real and Imaginary fields, and specify the units.
5. You can shift the port boundary, also referred to as the port reference plane. Shifting the boundary enables a type of de-embedding process that effectively adds or subtracts electrical length from the circuit, based on the characteristic impedance and propagation characteristic of the port. Enter the offset in the Reference Offset field, and select the units. A positive value moves the port boundary into the circuit, a negative value moves the port boundary away from the circuit.
6. Click Apply to add the definition to the port.

Avoiding Overlap

Be aware that when using single ports, the calibration arm applied to a port may be long enough to overlap another element in the circuit. In this case, the port will be changed to an internal port type, and no calibration will be performed on it. If this occurs, a message will be displayed during simulation in the Status window indicating the change. For information about internal ports, refer to Defining an Internal Port.

Applying Reference Offsets

Reference offsets enable you to reposition single port types in a layout and thereby adjust electrical lengths in a layout, without changing the actual drawing. S-parameters are returned as if the ports were placed at the position of the reference offset.

Why Use Reference Offsets?

The need to adjust the position of ports in a layout is analogous to the need to eliminate the effect of probes when measuring hardware prototypes. When hardware prototypes are measured, probes are connected to the input and output leads of the Device Under Test (DUT). These probes feed energy to the DUT, and measure the response of the circuit. Unfortunately, the measured response characterizes the entire setup, that is, the DUT plus the probes. This is an unwanted effect. The final measurements should reflect the characteristics of the DUT alone. The characteristics of the probes are well known, so measurement labs can mathematically eliminate the effects of the probes, and present the correct measurements of the DUT.

There are significant resemblances between this hardware measurement process and the way Momentum operates. In the case of Momentum, the probes are replaced by ports, which, during simulation, will feed energy to the circuit and measure its response. The Momentum port feeding scheme also has its own, unwanted effect: low-order mode mismatch at the port's boundary, although this is eliminated by the calibration process. However, in order for this calibration process to work well, it is necessary that the fundamental mode is characterized accurately. This can only be accomplished when the distance between the port boundary and the first discontinuity is sufficiently large, that is, there exists a feedline that is long enough to provide this distance.

As a basic example, consider a linewidth that varies abruptly in some part of your circuit, as shown in the example below.

Strictly speaking, all you need is to characterize is the variation of the step-in-width itself, as shown below.

As mentioned previously, it takes a little distance for the fundamental mode to settle, which means that this "short" structure might not yield the accuracy that you expect from an Momentum simulation. In this case, allow for some feed line length:

Now the simulation will yield accurate results, but the results will also contain the extra line lengths. To remedy this, use reference offsets. Although the circuit has been calculated with the long lines, reference offset shifting allows you to produce the S-parameters as if the short structure had been simulated instead:

The effect of the extra feed lines is mathematically eliminated from the S-parameter solution.

This process of adding or subtracting line length is generally referred to as de-embedding . This is the basic process:

During the solution process, the impedance and propagation constant has been calculated for the ports, based on their physical location in the circuit. When you know the impedance, propagation constant, and the distance of de-embedding, you can cancel out the extra lengths of line from the S-parameter results, by compensating for the loss and phase shifts of those lines. The net result is a set of S-parameters, calculated as if the extra line lengths were not there.

De-embedding Considerations

It is possible to de-embed right up to the discontinuity itself. However, make sure that you do not shift the reference offset beyond the first discontinuity. This would yield incorrect simulation results, as there is another linewidth beyond that discontinuity, which means that there is another set of impedance and propagation values that applies there.

Note You can de-embed away from the circuit, by placing reference offsets beyond the edges of the layout. This enables you to simulate the effect of a long feed line that was not drawn in the simulated structure.

Allowing for Coupling Effects

If you have two or more single ports that lie on the same reference plane, the calibration process will take into account the coupling caused by parasitics that naturally occurs between these ports. This yields simulation results that more accurately reflect the behavior of an actual circuit.

The following figure helps illustrate which ports will be grouped in order for the calibration process to account for coupling among the ports. In this setup, only the first two ports will be grouped, since the third port is an internal port type and the fourth port is on a different reference plane. Note that even though the second port has a reference offset assigned to it, for this process they are considered to be on the same plane and their reference offsets will be made equal.

If you do not want the ports to be grouped, you must add metal to the edge of the object that one of the ports is connected to. The ports will no longer be on the same plane, and will not be considered part of the same group.

Defining an Internal Port

Internal ports enable you to apply a port to the surface of an object in your design. By using internal ports, all of the physical connections in a circuit can be represented, so your simulation can take into account all of the EM coupling effects that will occur among ports in the circuit. These coupling effects caused by parasitics are included in your simulation results because internal ports are not calibrated. You should avoid geometries that allow coupling between single and internal ports to prevent incorrect S-parameters.

An example of where an internal port is useful is to simulate a bond wire on the surface on an object. Another example of where an internal port is necessary is a circuit that consists of transmission lines that connect to a device, such as a transistor or a chip capacitor, but this device is not part of the circuit that you are simulating. An internal port can be placed at the connection point, so even though the device is not part of the circuit you are simulating, the coupling effects that occur among the ports and around the device will be included in your simulation.

Internal ports are often used in conjunction with ground references. For more information, refer to Defining_a_Ground_Reference_and_to_Simulating with Internal Ports and Ground References.

An internal port type has the following properties:

• It can be applied to the interior of a circuit by applying it to the surface of an object.
• It can be applied to the edge of an object.
• It can be applied to objects that are on strip layers only.
• The orientation of the port is not considered if it is on the surface of an object. (For a description of port orientation, refer to Adding a Port to a Layout.)
• No calibration is performed on the port.

Because no calibration is performed on the port, the results will not be as accurate as with a single port. However, the difference in accuracy is small.

To define an internal port:

1. Choose Momentum > Port Editor .
2. Click the port that you want to assign this type to to select it.
3. In the Port Editor dialog box, under Port Type, select Internal .
4. Click Apply .

Defining a Differential Port

Differential ports should be used in situations where an electric field is likely to build up between two ports (odd modes propagate). This can occur when:

• The two ports are close together
• There is no ground plane in the circuit or the ground plane is relatively far away
• One port behaves (to a degree) like a ground to the other port, and polarity between the ports is developed.
• The ports are connected to objects that are on strip metallization layers.

The electric field that builds up between the two ports will have an effect on the circuit that should be taken into account during a simulation. To do this, use differential ports.

Differential ports have the following properties:

• They can be applied to objects on strip layers only.
• They are assigned in pairs, and each pair is assigned a single port number.
• Each of the two ports is excited with the same absolute potential, but with the opposite polarity. The voltages are opposite (180 degrees out of phase). The currents are equal but opposite in direction when the ports are on two symmetrical lines, and the current direction is approximated for other configurations.
• The two ports must be on the same reference plane.
  

  Note Port numbers for differential ports are treated in the following manner: On the layout, you will continue to see the port numbers (instance names) that were assigned to each port when they were added to the layout. Use the Momentum Port Editor dialog box to identify which pair of ports will be treated as a differential port. When Momentum simulates designs containing non-consecutive port numbers, the ports are remapped to consecutive numbers in the resulting data file. The lowest port number is remapped to 1, and remaining numbers are remapped in consecutive order. The port numbers are not changed in the design itself . A message in the Status window announces the change, and lists the mappings. For example, if you are simulating a design with ports numbered 1 and 3, the following status message informs you of the changes: Layout has non-consecutive port numbers. Output files will have consecutive port numbers. layout port -> output port 1 -> 1 3 -> 2 Also, when you view results, you will see S-parameters for the differential port numbers. In the example above, the layout would show p1, p2, p3, p4. The S-parameter results will be for combinations of the original P1 and P3 only.

  To define a differential port:

1. Choose Momentum > Port Editor .
2. Select the port that you want to assign this type to. Note the port number.
3. In the Port Editor dialog box, under Port Type, select Differential .
4. Under Polarity, make sure that Normal is selected.
5. Click Apply .
6. Select the second port.
7. In the Port Editor dialog box, under Port Type, select Differential .
8. Under Polarity, select Reversed .
9. Under Associate with port number, enter the number of the previously-selected port.
10. Click Apply .
11. Repeat these steps for other differential port pairs in the circuit.
12. Click OK to dismiss the dialog box.

Defining a Coplanar Port

This type of port is used specifically for coplanar waveguide (CPW) circuits. It is similar to a differential port, but coplanar ports are applied to objects on slot layers (that is, where slots are used in the design). Coplanar ports should be used in situations where an electric field is likely to build up between two ports. This can occur when:

• The two ports are close together
• Polarity between the ports develops
• The ports are connected to objects that are on slot metallization layers

The electric field that builds up between the two ports will have an effect on the circuit that should be taken into account during a simulation. To do this, use coplanar ports.

Coplanar ports have the following properties:

• They can be applied to objects on slot layers only.
• They are assigned in pairs.
• Each of the two ports is excited with the same absolute potential, but with the opposite polarity. The voltages are opposite (180 degrees out of phase). The currents are equal but opposite in direction when the ports are on two symmetrical lines, and the current direction is approximated for other configurations.
• The two ports must be on the same reference plane.

The illustration below shows ports pairs applied to a layout of coplanar waveguide slots. The coplanar port type is applied to each pair of ports in the design. The arrows in this illustration indicate the polarity assigned to each port and the direction of the voltages over the slots (arrows indicate the direction of the voltages).

Note Port numbers for coplanar ports are treated in the following manner: On the layout, you will continue to see the port numbers (instance names) that were assigned to each port when they were added to the layout. Use the Momentum Port Editor dialog box to identify which pair of ports will be treated as a coplanar port. When Momentum simulates designs containing non-consecutive port numbers, the ports are remapped to consecutive numbers in the resulting data file. The lowest port number is remapped to 1, and remaining numbers are remapped in consecutive order. The port numbers are not changed in the design itself . A message in the Status window announces the change, and lists the mappings. For example, if you are simulating a design with ports numbered 1 and 3, the following status message informs you of the changes: Layout has non-consecutive port numbers. Output files will have consecutive port numbers. layout port -> output port 1 -> 1 3 -> 2 Also, when you view results, you will see S-parameters for the coplanar port numbers. In the example above, the layout would show p1, p2, p3, p4. The S-parameter results will be for combinations of the original P1 and P3 only.

Be careful when assigning polarity to coplanar ports. An incorrect choice of polarity can change the phase of transmission type S-parameters by 180 degrees. To verify polarity, zoom in on a coplanar port. You will notice two sets of arrows applied to the port. One appears when you add the port component to the circuit. The second will appear after the mesh is computed. It indicates the direction of the voltage over the slot.

To define coplanar ports:

Note Coplanar ports can be applied to objects on slot layers only.

1. Choose Momentum > Port Editor .
2. Select the port that you want to assign this type to. Note the port number.
3. In the Port Editor dialog box, under Port Type, select Coplanar .
4. Under Polarity, make sure that Normal is selected.
5. Click Apply .
6. Select the second port.
7. In the Port Editor dialog box, under Port Type, select Coplanar .
8. Under Polarity, select Reversed .
9. Under Associate with port number, enter the number of the previously-selected port.
10. Click Apply .
11. Repeat these steps for other differential port pairs in the circuit.
12. Click OK to dismiss the dialog box.

Defining a Common Mode Port

Use common mode ports in designs where the polarity of fields is the same among two or more ports (even modes propagate). The associated ports are excited with the same absolute potential and are given the same port number.
Common mode ports have the following properties:

• They can be applied to objects on strip layers only
• A ground plane or other infinite metal (such as a cover) is required as part of the design
• Two or more ports can be associated
• Associated ports are excited with the same absolute potential (and same polarity)
• The ports must be on the same reference plane
  

  Note Port numbers for common ports are treated in the following manner: On the layout, you will continue to see the port numbers (instance names) that were assigned to each port when they were added to the layout. Use the Momentum Port Editor dialog box to identify which group of ports will be treated as a common port. Also, when you view results, you will see S-parameters for the common port numbers. In the example above, the layout would show p1, p2, p3. The S-parameter results will be for combinations of P1 only.

  To define a common port:

1. Choose Momentum > Port Editor .
2. Select the port that you want to assign this type to. Note the port number.
3. In the Port Editor dialog box, under Port Type, select Common Mode .
4. Click Apply .
5. Select the second port.
6. In the Port Editor dialog box, under Port Type, select Common Mode .
7. Under Associate with port number, enter the number of the port that you selected first.
   Make sure that the value in the Associate with port number field is the same for additional ports. For example, if you were associating three ports and the first port was assigned as port 1, for the second and third port, the value entered into the Associate with port number field would be 1. (For the first port you choose, no value is entered in this field.)
8. Click Apply .
9. Repeat these steps for other common mode ports in the circuit.
10. Click OK to dismiss the dialog box.

Defining a Ground Reference

Ground references enable you to add explicit ground references to a circuit, which may be necessary if no implicit grounds exist in your design.
Implicit ground is the potential at infinity, and it is made available to the circuit through the closest infinite metal layer of the substrate. Implicit grounds are used with internal ports and with single ports that are connected to objects on strip metallization layers.
There are instances where the distance between a port and its implicit ground is too large electrically, or there are no infinite metal layers defined in the substrate. In these cases, you need to add explicit ground references to ensure accurate simulation results. For more information on using ground references, refer to Simulating with Internal Ports and Ground References.

You can apply ground references to the surfaces of object. The object must be on strip metallization layers.

Note Multiple ground reference ports can be associated with the same port. To be associated with a single port, the ground reference port should be a port attached to an edge of an object in the same reference plane as the single port.

To add a ground reference:

1. Choose Momentum > Port Editor .
2. Select the port that you want to assign as the ground reference.
3. In the Port Editor dialog box, under Port Type, select Ground Reference .
4. Under Associate with port number , enter the number of the single or internal port that you want to associate with this ground reference. Make sure that the distance between the port and ground reference is electrically small.
5. Click Apply .

The Momentum Port Editor

Once the substrate and dielectric layers have been defined, the Port Editor ( Momentum > Port Editor... ) can be used to change the Momemtum specific characteristics of ports in a layout. You edit the port in the same manner as when you entered the initial definition.

Note Be aware that it is possible to edit port properties by selecting the port and choosing Edit > Properties from the Layout menu bar, however, this method is not recommended .

For more information on setting port definitions see the appropriate section:
Defining a Single Port

Defining an Internal Port

Defining a Differential Port

Defining a Coplanar Port

Defining a Common Mode Port

For ADS 2006A several new features have been added to the Momentum Port Editor:

• Embedded information about Port Types
• Group Editing of Port properties

Embedded Information About Port Types

To access the Port Editor select a port in your layout and choose Momentum > Port Editor from the ADS Layout toolbar. This opens the the Port Editor tool shown below.

The following table provides specific information about the Port Editors Info section.

Port Selection	Port Info
No ports selected	No ports selected in the layout.
On STRIP layer
Single Mode	Single Mode strip port
Interna	Internal strip port
Differential Mode	Differential Mode strip port
Common Mode	Common Mode strip port
Ground Reference	Ground Reference for accociated
On SLOT layer
Single Mode	Single Mode slot port
Coplanar Mode	Common Mode strip port

Group Editing of Port Properties

When starting the Momentum Port Editor, a graphic handler is installed to control the selection of ports in the layout. The graphic handler changes the mouse tracking style into a window-wide cross cursor.

Prior to the ADS 2006A release, the graphic handler was only activated when a Single Click event was performed to select a port using the mouse in the layout window. This port would remain active for editing until a new port was selected, or the Port Editor dialog was closed. It should be noted that only one port could be selected at a time.

In ADS 2006A multiple ports can be selected for editing simultaneously by using the following graphic handler options:

• Shift + Click
• Ctrl +Click
• Press and Drag
• Shift + Press and Drag
• Ctrl + Press and Drag
  The Shift + Click and the Ctrl + Click events are similar to a Single Click event, except that preveously selected ports are NOT deselected. This enables you to select multiple ports by repeating the mouse click event while keeping the Ctrl or Shift button pressed when selecting a new port.

Using Press and Drag enables you to select multiple ports with one mouse operation. The mouse tracking style is modified from a window-wide cross cursor into a rubber-banding rectangle during the Press and Drag operation.

After a Press and Drag operation, all ports which are located inside the rubber-banding rectangle will be selected for editing the properties. When a new Press and Drag operation is started, all preveously selected ports are automatically deselected.

The Shift + Press and Drag and Ctrl + Press and Drag event are similar to a single Press and Drag , except that preveously selected ports are NOT deselected. Hence, multiple ports can be selected by repeating the mouse Press and Drag event while keeping the Ctrl or Shift button pressed.

Feedback is given to the user in the Port Editor dialog about the multi-port selection.

When the user selects multiple ports for group editing, the Port properties of the selected port with the highest port number are displayed in the dialog. The user can than edit these port properties prior to applying them to all selected ports. When user selects the Apply button, the properties defined in the dialog are applied to all selected ports.

Group editing of ports works for all Port Types, except for:

• Differential Ports on strip layers
• Coplanar Ports on slot layers
  Because each of the pins of a Differential or Coplanar port require a different polarity, these port types need to be defined individually for each port. When the user tries to apply a Differential Mode or Coplanar Mode port type to multiple selected ports,you will receive a warning informing you that this is an invalid operation. The correct use model for these port types is displayed in the Warning dialog.
  
  Group editing of ports only works for ports which have the same process layer type (STRIP or SLOT). If you simultaneously select ports on STRIP and SLOT layers, you will only be able to group edit ports that have the same process layer type as the selected port with the highest number.
  
  If you attempt to simultaneously select port on STRIP and SLOT layers, a Warning dialog is displayed informing you that you can only select ports with a compatible process layer type.
  
  Any port with an incompatible process layer type is automatically deselected.
  

Remapping Port Numbers

Some designs contain non-consecutive port numbers. This results in simulation data files that are difficult to use. When Momentum simulates designs containing non-consecutive port numbers, the ports are remapped to consecutive numbers in the resulting data file. The lowest port number is remapped to 1, and remaining numbers are remapped in consecutive order. The port numbers are not changed in the design itself . A message in the Status window announces the change, and lists the mappings.

For example, if you are simulating a design with ports numbered 37 and 101, the following status message informs you of the changes:

Layout has non-consecutive port numbers.

Output files will have consecutive port numbers.

layout port -> output port

37 -> 1

101 -> 2

Port number remapping is done only for sampled and AFS CITIfiles and their corresponding S-parameter datasets. It is not done for Visualization and far field files. The remapping is done at the CITIfile level, and propagates to the dataset file. After remapping, all datasets are in sync with the new port numbering.