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Design Notes

Exposed Ground Node

This project shows how exposing the ground node works when S parameter data sets are used.  This can be very useful, in that the local ground of the dataset can be explicitly accessed.  Care must be taken that the results be interpreted correctly.  The closely related concept of exposing balanced ports is also explained.  These issues are discussed at the end  of these notes.  The examples use two port datasets, but the algorithm works for any number of ports. 

Overview

When S parameters are used in a schematic, they are inserted as a sub-circuit.  (The sub-circuit menu is obtained by selecting a schematic, and clicking Draw > Add Subcircuit.)  The designer has three options for grounding types: normal, explicit ground node, and balanced ports.  Explicit ground node exposes the ground node, so that it can be accessed in the schematic.  This is explained in the schematics in the folder: Exposed Ground Node.  The option balanced ports adds a local, "ground" port to each port of the S parameter file.  The schematics in the folder Balanced_Ports show how this works.

An exposed ground node can be viewed as a local ground for the S parameter file.  It is important to understand that the same ground is used for all ports in the S parameter file.  This implies that physically, the structure is electrically small, or has a very good (perfect) internal grounding system connecting the ports.  Normally, exposing the ground node is used for transistor data, where a common ground node in the measurement is being exposed.

Balanced ports extend the exposed ground node concept by creating a local ground node for each port.  This can be understood conceptually as attaching an ideal 1:1 transformer to each port, and using the exterior coil to create a local ground reference.  Details are given below.

It is possible to misuse this concept and obtain physically meaningless results.  The last section of these notes explains the pitfalls of the method if improperly used.

We now look in more detail at exposed ground nodes and balanced ports.

Exposed Ground Node Schematics

These schematics are all in the folder Exposed Ground Node.

Exposed Node

This schematic uses the dataset "FHX35LG", which is 2 port S parameter data for a transistor.  The dataset is imported as a subcircuit, and the ground node is exposed. PORT_1 is the gate of the transistor, PORT_2 is the drain, and port 3 is the source. PORT_3 exists in the schematic because the ground node was exposed.  In the actual measurement, the source was a common ground.

With Transformers

This schematic shows conceptually how exposing the ground node works.  Ideal transformers are attached to each port of the S parameter file.  The coil connected to the S parameter file is grounded to global ground, node 0.  The other coil, which is exposed to the outside world,, is not grounded.  Rather, all ports are referenced to PORT_3, which becomes the exposed "ground".  Notice that all ports are referenced to the same exposed ground port.  The method works for any number of ports, although the option in the S parameter import menu only works for 2 port circuits.

Balanced_Ports Schematics

These schematics are all in the folder Balanced_Ports.

Balanced_Ports

This schematic uses the data file "interconnect_data", which is a 2 port S parameter file representing an interconnect measurement.  The option for balanced ports was selected when the S parameter file was inserted as a subcircuit.  Four ports are created.  PORT_3 is the local "ground" for PORT_1, and PORT_4 is the local "ground" for PORT_2.  Notice connectors "Local_ground_1" and "Local_ground_2" are used to reduce the confusion caused by wires.

Balanced_With_Transformers

This schematic shows conceptually how balanced ports work.  The philosophy is the same as for the exposed ground node case, but each port has it's own local reference by adding a second port on the external side of each transformer.  Each port therefore has its own ground reference.  Dangers of misinterpreting the results are discussed below.

Measurements

Ground Node

This graph shows the S parameters for the exposed ground node schematics.  The measurements are identical using the transformer method or selecting expose the ground node on creation of the subcircuit.  Some measurements have been disabled to avoid a cluttered graph.

Balanced

This graph shows the S parameters for the balanced ports schematics.  The measurements are identical using the transformer method or selecting balanced ports on creation of the subcircuit.  Some measurements have been disabled to avoid a cluttered graph.

Proper and Improper Usage of Exposed Ground Nodes and Balanced_Ports

Exposing the ground is a perfectly valid concept when properly used, but, unfortunately, it is often misapplied.  As an example, assume the user starts with a two-port S-parameter file.  By exposing the ground, a three-port S-parameter file is obtained, with the third port being the "local ground" return. Remember, it has been assumed the local grounds of the ports are electrically the same; i.e., they are connected to each other by a very, very good ground return. The exposed node connects to that ground return.  By doing this, for example, the engineer can DC bias the ground return.  Typically, this procedure is used for transistor S-parameters. Transistors have three ports, but when measuring a transistor with a network analyzer, one port is grounded, and a two-port S-parameter file results.  By exposing the ground node of the S-parameter file, the designer can attach elements to the previously grounded port, for example, an inductance to the common source node. This concept can also be used where the transistor is housed in a package and the global circuit ground is attached to the "transistor package's ground."   The key point is that this method works because the local grounds of the ports are essentially the same, and are attached to each other by a perfect grounding structure.      

A commonly-made mistake is to assume that imperfect ground properties can be observed by looking at the exposed node, for example, the loss of the ground plane.  The exposed ground approximation assumes the ports’ local grounds are the same; however, imperfect ground properties would make the ports' local grounds different voltages.  There is even greater confusion with multi-port S parameter files, where differential ports, or local grounds, are requested.  For example, a two-port S-parameter file will now have four ports, with Port 3 corresponding to Port 1's "ground," and Port 4 corresponding to Port 2's "ground." This can be a useful tool, but unfortunately is misunderstood by many designers.  They incorrectly assume they are looking at the "local ground" of the port.  For example, they think they can measure the resistance of the original lossy ground by placing an Ohmmeter across the two new "ground" ports. They can't. The original S-parameter file did not have this information (it assumed the local grounds were at the same voltage!). These ports were created by the mathematical operation of  adding transformers.  A math trick cannot recover lost physics, no matter how hard the designer tries.

Matrix Method for Exposing the Ground

Mathematically, one can expose the ground node of and S parameter file in the following way:

• Convert the S parameter file to a Y parameter file, which is easier to manipulate.  (Y parameter files don't have to worry about port impedances.)

• A new row and column are added to the original Y parameter file.  The original file was of size NXN.  The new one is size (N+1)X(N+1).  The NXN matrix is unchanged.

            - Last Column (N+1 column): Each entry is the negative of the sum of the other Y parameters in the row; i.e., when finished the whole row adds to zero.

            - Last Row (N+1 row):  Each entry is the negative of the sum of all the original elements in that column.  The sum of any column therefore ends up zero.  Note - this means the (N+1,N+1) element of the matrix is the sum of all the elements of the original matrix.

• Convert the new matrix back to an S parameter matrix.  You need to know the port impedance of the exposed ground node to do this.

Matrix Method for Balanced_Ports

The original NXN matrix is turned into a 2NX2N matrix, with the "local" ground of port i being port i+N.

• Convert the original matrix to a Y matrix of size NXN.

• Make the new Y matrix - by mirroring the original Y matrix 3 times.  Think of the original Y matrix as block called Yold.

            The new matrix is:   [{Yold,-Yold},{-Yold,Yold}] and is of size 2NX2N.

• Convert back to S parameters.  You need the port impedances of the "local" grounds.

Schematic - Exposed_Node

Schematic - With_Transformers

Schematic - Balanced_With_Transformers

Schematic - Balanced_Ports