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Is 'Field Solver' the name of a particular product?
In general, we indicate with the term 'field solver' any piece of software suited for solution of the Maxwell equations. In some software, Finite Element Analysis are used to model and to solve the electromagnetic problem.
One practical use of a Field Solver is to calculate the complex interactions of magnetic fields radiated from rapidly changing signals in PCB traces in close proximity and the crosstalk and reflections produced. A program that uses a Field Solver is Hyperlynx. This is a very good program if you are involved in high speed PCB design. It can calculate effective impedance of traces and crosstalk and reflections in those traces.
In general, we indicate with the term 'field solver' any piece of software suited for solution of the Maxwell equations like FastHenry and FastCap based on algorithms which consent the reduction of arbitrary geometries (preferibly tri- but also bi-bimensional), even complex, in lumped circuit elements (R, L, C) to them electromagnetically equivalent. In the case of FastCap and FastHenry, this equivalence is valid up to a certain frequency of the harmonic content of the signals involved, where the physical dimension of the conductive structures are comparable to the wavelenght; this limitation is usually referred to with the 'quasistatic' term, in contrast to the 'full-wave' approach.
However, a big deal of problems can be treated in this approximation, even real transmission lines, if we model geometrically small impedance discontinuites (like angles) with lumped elements and long, straight paths with distributed models whose parameters are calcuated from a 3D model stretched along one axis. Moreover, the quasistatic approximation not only allows to extract R, L and C parameters in a shorter time than full-wave solvers, but also to get a smaller and simpler SPICE-like model which ease the subsequent circuit simulations.
What is FastHenry?
In simple words, FastHenry is a program capable to compute the frequency-dependant self and mutual inductances, as well as the resistances, of a generic tridimensional conductive structure, in the magnetoquasistatic approximation.
The input data, describing the geometry and the frequencies of interest, must be provided in a file. This file specifies every conductor as a sequence of rectilinear segments connected between nodes. Every segment has a finite conductivity and the shape of a parallelepiped, whose height and width can be assigned. A node is a point in the 3D space. The section of a segment can be divided, if required, into an arbitrary number of parallel filaments (that is, parallelepipeds with smaller cross section than the original one), the whole of which constitutes the segment itself; it is then assumed that every filament carries a uniform current.
In this way is possible to model the high-frequency effects on the segments. As a matter of fact, when the frequency increases, the current is no longer uniformly distributed along the cross section of a conductor. Hovever, in limited regions of the section, the current can be reasonably approximated as uniform. Therefore, being able to specify an arbitrary discretization of the volume of the conductors, the accuracy of the results is affected accordingly and in general is better as the discretization is refined.
The results are provided in form of a Maxwell impedance matrix Z=R+jL, where the bold letters represent matrices. Then, the results can be converted in equivalent, SPICE-like, lumped elements circuit models with an utility provided with FastHenry, MakeLCircuit. The network thus obtained, however, is valid only for a single frequency. Alternatively, is possible to generate directly with FastHenry a SPICE-like circuit capable to model frequency-dependant inductances and resistances. This latter opportunity is very powerful because allows to see how signals in the time domain are degradated by the different response of the conductors at the various frequencies.
As a matter of fact, the classic approach, from which FastHenry differs, is the following: once the frequency response is calculated in the frequency domain, then a FFT or a Laplace transform is used to obtain a behavioral description in the time domain. Next, a convolution (or similar technique) is applied to get the temporal response of the circuit. The conversion from the frequency domain to the time domain is necessary, as the characterization of the lines in the frequency domain is in term of irrational functions of the frequency variable. Therefore, a generic SPICE-like simulator, being based on the numerical integration of ordinary differential equations, deriving from a lumped elements circuit whose parameters are characterized by rational functions in the frequency domain, in not capable to handle elements characterized by irrational functions.
However, is possible to find rational approximations of these functions, at least in a fixed range of frequencies; so we can operate only in the time domain. Following a similar approach, FastHenry is capable to generate Reduced Order Models (ROM) for the system valid, in function of the chosen order, up to a certain maximum frequency.
What is FastCap?
FastCap is a software for computing the self and mutual capacitances of a conductive tridimensional structure sunk in a dielectric, possibly discontinuosus, mean.
The input file specifies the discretizaion of the conductor surfaces and of the discontinuities in form of triangular or quadrangular panels in the 3D space. Since a constant charge density is associated to every panel, the panel dimensions are a key factor to obtain accurate results. Note, by the way, that the FastHenry input file format is not compatible with the FastCap one at all.
The results are provided as a Maxwell capacitance matrix. No way is currently implemented to automatically generate an equivalent circuit.
What is ConvertHenry?
The FastHenry input file format is not compatible with the FastCap one. Therefore, obtaining both the inductance and the capacitance values associated to a certain structure is in general compex, because two different geometric descriptions are needed.
To simplify this step, a program has been developed for the data conversion from FastHenry to FastCap, named ConvertHenry. Substantially, ConvertHenry reads the input file for FastHenry and generates a certain number of FastCap input files. More specifically, a different file is generated for every terminal port defined in the FastHenry input file (that is, for every group of conductors whose self and mutual inductance values one was interested in), plus an additional file containing the list of the generated files and some other information, like the dielectric constant values and the geometric offsets. This file is the main one needed by FastCap.
Every conductor file contains the description of the related FastHenry segments in terms of quadrangular panels. The discretization degree of the segments is specified when ConvertHenry is launched; moreover, the discretization of every face is automatically refined near the segment edges, as the charge will concentrate there because of the edge effect.
ConvertHenry also provides an option for substitution of any segment with a structure whose description, in FastCap format, is contained in a different file (an example of the use of this option could be for instance to substitute the segments representing the balls of a Ball Grid Array package with better spherical approximations).
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