5.5 The Graph Menu

The Graph menu allows you to graph and to visualize all input and output functions of the Simulator. These include source waveforms, time and frequency domain responses and scattering parameters. It also allows you to set the attributes of the graphs and of the ports.
Figure 1-5: The Graph Menu.

The functions of the Graph menu are Source Waveform, Probe Responses, Johns Matrix, Probe 1, Probe 2, Probe 3, S11, S21, Graph Display Attributes, and Port Attributes. These functions operate as follows:

Source Waveform Source Waveform displays the source function selected under the Source Waveform menu.
Probe Responses  Probe Responses displays simultaneously the time domain and frequency domain responses at all probes (up to three). 
Johns Matrix Johns Matrix displays the Johns Matrices (impulse responses) assigned to dispersive Johns Walls in the structure. 
Probe 1 Probe 1 displays the time domain response V(t) or the frequency response V(f) (spectrum) at Probe 1 in Magnitude or Phase.
Probe 2 Probe 2 displays the time domain response V(t) or the frequency response V(f) (spectrum) at Probe 2 in Magnitude or Phase.
Probe 3 Probe 3 displays the time domain response V(t) or the frequency response V(f) (spectrum) at Probe 3 in Magnitude or Phase.
S11 S11 displays the Magnitude or Phase of the complex reflection coefficient S11 at the Input Port.
S21 S21 displays the Magnitude or Phase of the complex transmission coefficient from the Input Port to the Output Port.

Note To compute Scattering parameters, you must generate a reference signal identical to the incident wave at the Input Port. A reference structure must therefore be implemented that has the same geometry and excitation as the Input Port, and the probes must be in the same position with respect to the source. The S-parameters are computed as follows: 

, 
where the voltages are the Fourier transforms of the impulse responses at the probes assigned to the Input and Reference Ports. 

, 
where the voltages are the Fourier transforms of the impulse responses picked up at the probes assigned to the Output and Reference ports, and the impedances are the reference impedances (characteristic impedances) of the Input and Output; Ports. Reference and Input Ports must have identical geometry, impedance, mode of propagation, and excitation.
Graph Display Attributes Graph title, name, and scale of the axes can be edited in the dialog box. During a simulation, the scale of the axes adjusts dynamically to the signal, but the user can change the graph display attributes once the simulation is stopped. The display can be smooth or discretized (Bar Graph off or on). S-parameters can be displayed in dB or in absolute values.
Port Attributes  Any of the three probes (Probe 1 to 3) can be assigned to the reference, input or output port. To verify the number of a probe, proceed as follows:
  1. In the View menu, select Draw,
  2. In the Draw Menu, select Select Element,
  3. Point to a probe and click the left mouse button,
  4. Read the number of the probe in the status bar.

Input and reference ports must always have the same width, mode and characteristic impedance. However, the output port can have a different geometry, impedance and/or propagation mode. TEM or TE_10 modes may be selected. The width of the ports must be specified as well. The relative dielectric constant selected by the program is that of the Computation Region in which the probe is located. In the special case where all ports have the same characteristics, the port widths and modes selected are immaterial as long as they are the same for the input and output port.
 

Warning In the case of TE10 mode propagation in either the input or the output port, or both, S21 should be computed only at frequencies above the highest dominant cutoff frequency in the structure since singularities at cutoff frequencies can cause unreliable numerical results.