6.2.14                    FD-Probing

 

QuickWave offers another post-processing, called FD-Probing. The FD-Probing post-processing can be invoked for two types of objects and depending on that delivers separate results:

 

Note that a single FD-Probing check in the post-processing configuration dialogue activates a separate post-processing at each point source, point probe, or contour, for which FD‑Probing option has been checked.

 

There are four main applications of this type of post-processing, which will be discussed in the following sub-sections.

 

6.2.14.1   Analysis of Eigenvalue Problems

FD-Probing post-processing enables analysis of eigenvalue problems (resonant frequencies and field distribution of consecutive modes in arbitrarily-shaped and filled resonators). In this case a point source must be placed inside the resonator. It will serve as auxiliary excitation injecting energy into the structure through finite output impedance. We can watch the results of FD-Probing calculations performed on the current flowing between the source and the resonator. With pulse excitation, minima of this current indicate resonant frequencies. The background theory of this approach has been explained in publications [14] [26] and its practical application is discussed on User Guide 3D: Dielectric resonator example.

The FD-Probing post-processing can be also used for analysis of eigenvalue problems for periodic structures as shown in User Guide 3D: Eigenmodes in photonic crystals (PhC) example.

 

6.2.14.2   Field Integration along an arbitrary path

 

In many practical cases, we may wish to observe integral quantities, such as a line integral of the electric field along a pre-defined integration path. If the field is potential, then this integral has the physical meaning of voltage. Knowing the voltage between two metal objects allows predicting the hazards of arcing and electric breakdown.

The FD-Probing post-processing comes across such needs and enables extraction of currents and voltages by integrating H-fields along a virtual loop surrounding a conductor and E-fields along a virtual line connecting two conductors. At the end, the Fourier transform of these integrals is performed.

This information may be helpful in the analysis of quasi-TEM circuits for verifying the frequency range of their TEM behaviour. Also, lumped element models of parts of 3D structures can be extracted. Please note that for TEM lines, the integration of both electric and magnetic fields over specific contours can be also used for extracting the characteristic impedance of the transmission line, calculated as a ratio of the integration results.

 

The path of integration may be defined by the user with UDO objects from the contours library. Three of the available UDOs allow setting the path composed of 1, 3 or 7 segments (spanned between 2, 4 or 8 points, respectively). Another UDO facilitates reading the number of points and their coordinates from a text file. The points may be arbitrarily located in space but they will be snapped to the nearest mesh point (cell vertex), similarly as in the case of point sources. Each segment will be decomposed into a Manhattan-style route along cell edges.

 

For the examples of application of this option, refer to User Guide 3D: A parallel-plate transmission line and User Guide 3D: Contours E,H and combination of them.

 

6.2.14.3   Embedding Impedance for Lumped Elements

FD-Probing post-processing enables analysis of embedding impedance for lumped impedance elements. Typically such an analysis would be performed for lumped impedance elements placed between two metal elements (wires, plates or blocks). These metal elements should be separated by one FDTD cell and a Point Source/Probe port should replace the lumped impedance element. A simpler option is also available: the source can be placed directly on the wire (without manually making a one-cell gap), and the software will cut the gap automatically, provided that the Point Source/Probe port impedance is different than +INF. The Point Source/Probe port of +INF impedance placed on the wire will not add any excitation or loading to the circuit, only mark the point where current flowing in the wire will be calculated by H-field loop integration. In the port settings dialogue, the Exciting field for this source should be selected as the E-field component along the replaced element (perpendicular to the metal surfaces). The FD-Probing calculations will be performed on the current flowing between the source and the circuit, and on the port voltage (selected E-field integrated along the FDTD cell containing the Point Source/Probe port).

In the Results window with FD-Probing post-processing results we will see:

-        the Fourier transform of the current and voltage (amplitudes and phases),

-        the embedding impedance (Z=U/I) and admittance (Y=I/U) versus frequency (absolute value and phase),

-        the port reflection coefficient (absolute value and phase),

-        real and imaginary parts of impedance,

-        real and imaginary parts of admittance,

-        inductance defined as L=Im(Z)/(2pf),

-        capacitance defined as C=Im(Y)/(2pf),

-        E-field at point Ep,

-        H-field at point Hp.

Note that the calculated embedding impedance includes in parallel the capacitance of the FDTD cell into which the Point Source/Probe port has been inserted. This happens because the current considered by FD-Probing comprises two components: current flowing into the metal embedding the circuit, and current flowing into the cell capacitance. If the latter component should not be considered in a particular application, the S-Parameters post-processing should be selected for impedance calculation instead of FD-Probing.

The reflection coefficient is calculated with respect to the internal resistance of the Point Source/Probe port or 50 W, if R=+INF or R=0. It is denoted by a symbol S_number, where “number” is the value of R.

The practical application of this feature is shown for User Guide 3D: Wire antenna example.

 

6.2.14.4   Currents Induced in Wires

FD-Probing post-processing enables analysis of currents induced in wires. In this case a point probe should be placed on the wire. In the port settings dialogue, the Exciting field for this point should be selected as the E-field component along the wire, and the point probe impedance should be set to R[ohm]=+INF (the point will in fact act as a probe). Excitation Waveform will be ignored. The FD-Probing calculations will be performed on the current flowing in the wire (H-field integrated around the point probe). The application of the FD‑Probing post-processing for such purpose is shown and discussed based on the User Guide 3D: Wire antenna example.