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Spice Example 1: 

Coaxial Crosstalk

This example walks through the steps neccessary to create a spice simulation using gSpiceUI vSACAMOS. It is not intended to be a tutorial on Spice. It is assumed that the neccessary steps have been taken to install Ngspice, and the components gschem and gnetlist of the gEDA project.

Schematic cross-section of the example of two coaxial wires in free space. This is a bundle of 2 coaxial cables (example 2 details the creation of a coaxial cable):

Cable Model Specification: 
inner conductor radius = 4.2e-4
shield radius = 1.47e-3
outer dielectric radius 2.5e-3
inner wire conductivity = 5.0e7
shield conductor thickness = 0.0
shield conductivity = 5e7

Frequency dependent material parameters, 
Inner dielectric permittivity model:
normalisation constant = 1e7
a order = 0 
a constants = 2.3 
b order = 0 
b constants = 1.0

Outer dielectric permittivity model:
normalisation constant = 1e7
a order = 0
a constants = 2.3 
b order = 0 
b constants = 1.0

Transfer Impedance definition:
R = 0.02 
L = 6.8e-10

The bundle is made up from 2 coaxial cable as defined above, placed at x = -0.01, y = 0.0 and x = 0.01, y = o.o respectively. 

A spice model in then created for a 1m length of cable.

Finally the Spice models are exported to the local symbol directory of the Nspice install.

Start gSpiceUI vSACAMOS

1. Launch the gspiceui.exe application.
2. the SACAMOS tool bar menu item sets out the sequence of operations neccessary to perform a Spice simulation. For the purpose of this example the first entry 'Import MOD' can be skipped as this was done in the above step.
2. From the 'SACAMOS' menu chose gschem to launch the scematic capture tool gschem.

Build the 2 Wire Bundle

1. Select 'gschem' to launch the schematic capture tool.
2 Draw the schematic representing the crosstalk problem where one cable is driven by a voltage source Vs and all other cables are teminated 50 ohms.
3. There is no need to define the parameters of the voltage sourec Vs as thes are defined in the GUI.
4. save the schematic. e.g twin_coax_demo
5. Select 'gnetlist' and browse to the schematic file just saved. The schematic will be loaded and a netlist generated.

Running the Spice Simualtion

From the file menu of the GUI
1. Select 'Open' and browse to the same folder the schematgic was save in, change the file filter to '*.net' and select the .net file corresponsind the schematic that was saved i.e The GUI will be populated with information from the .net file
2. Set up the simulation paremeters via the 'AC' tab:
Start frequency = kHz
Stop frequency = 00 MHz
Steps in total = 1000
Parameters = voltage
Complex parts = Magnitude
Signal source = Vs = 1v
3. Select a compent to monitor e.g. the near end load.
4. click the cog icon to start the simulation

Viewing the Results    

Once the simulation has completed the results will be tabulated under the NG-Spice tab on the GUI.

To plot these using gnuplot:
1. Select 'view results' from the SACAMOS drop down menu.
2. The results will be present in gnuplot, From here the plot can be interacted with and saved.