FDTD modeling and validation of EM survey tools

Abstract

measured S parameters at each bias point, the intrinsic Y parameters at each bias point can be obtained [5]. The seven intrinsic elements can thus be uniquely determined by the four complex de-embedded intrinsic Y parameters [4, 5]. RESULTS AND DISCUSSIONSA uniform-doped MESFET with doping density of 2.9 ϫ 10 17 /cm 3 was studied here. The coplanar device has gate width of 6ϫ50 m and gate length of 1 m. S parameters were obtained through on-wafer measurements. Figure 2 illustrates the normalized circuit elements as a function of frequencies when V gs ϭ Ϫ 0.5V and V ds ϭ 8 V. The extracted intrinsic elements have no frequency dependence as illustrated in Figure 2. This proves the validity of extraction method and equivalent circuit topology. Radio-frequency transconductance gm and rf channel conductance gds as a function of V ds with V gs as a parameter are illustrated in Figure 3. Once gm and gds at all bias points are extracted, rf I-V can be calculated from an integral of the bias-dependent rf transconductance gm and rf channel conductance gds in the V gs and V ds voltage plane. The S parameters were measured at 0.1 V per step for V ds less than 2 V to improve rf I-V accuracy. Figure 4 illustrates the measured static I-V curves and the calculated rf I-V curves. As expected, the rf I-V curves differ from static I-V curves. Power saturation and gain compression caused by knee voltage and maximum channel current can thus be accurately modeled through the obtained rf I-V curve. SUMMARYIn conclusion, an approach to uniquely determine each circuit element from measured S parameters is used to obtain rf I-V curve for uniform-doped MESFET. This resulting rf I-V curve is very useful to evaluate and model device rf performance. INTRODUCTIONFinite-difference-time-domain (FDTD) methods, which are widely used for electromagnetic simulations in many applications, are now often used as an effective simulation technique for geophysical applications [1][2][3][4][5]. In many geophysical applications, the source is a current loop that is very small compared with the wavelength. This Letter describes details of the FDTD source model that are needed for accurate analysis of realistic current loops used in cross-borehole EM surveys for highly conducting ore bodies in resistive hosts. FDTD SIMULATION OF CURRENT LOOPSTransmitters and receivers used in cross-borehole geophysical EM surveys are loop antennas wrapped around dielectric cores and lowered down well boreholes, often using several different wells and/or heights within these wells as locations for transmitter and receiver loops. Broadband pulses on these antennas maximize resolution and minimize overprinting of a modest response of an ore target by a large incident field. Infinitesimal magnetic and electric dipoles can be used to model transmitter and receiver loops that are commonly used for * Formerly with Department of Geology and Geophysics, 717 WBB, University of Utah, Salt Lake City, Utah 84112. surveys, but direct implementation in the FDTD grid is not sufficiently ac...

Comparison of FDTD Hard Source With FDTD Soft Source and Accuracy Assessment in Debye Media

“…From (3) and (4) it is apparent that the soft source field at a source location is the integral in time of current and terms. More precisely, because radiated from the soft source is proportional to the impressed current, can be expressed as (5) where is an unknown coefficient and is the permittivity in vacuum. From (1) and (5), the soft source is equivalent to a hard source whose impressed field is the integral of the impressed current , multiplied with coefficient in (5).…”

“…[2] presents the analytical solution of a pulse excitation without clear explanation of the inclusion of the effect of the medium in the analytical solution. [5] obtains the analytical solution by convolution of the quasi-static impulse response of the FDTD space and the frequency spectrum of source excitation. Instead, this paper explicitly includes the first order Debye media into the analytical solution.…”

“…However, the quantification of the deviation from the analytical solution is rarely performed in these works. 0018-926X/$25.00 © 2009 IEEE Some studies such as [2], [5] use the plot of the waveforms from both analytical solutions and results from numerical experiments without quantification. Although [4] quantified the error using the analytical solution, the quantification formula has utilised peak values of the electric field magnitudes from analytical solutions and numerical simulations.…”

Accuracy assessment of the soft source implementation for a dipole in FD-FDTD for UWB systems

“…However, the quantification of deviation from the analytical solution is rarely performed. Some studies such as [3] use the plot of the waveforms from both analytical solution and numerical simulations without quantification. Although [4] quantified the error using the analytical solution, the quantification formula has utilised peak values of the electric field magnitudes from analytical solution and numerical simulation.…”

“…These point sources are usually implemented as either a hard or soft source. When it comes to the validation of the code, a soft source dipole is applied because the analytical solution for the soft source in frequency independent media is known [1][2][3]. However, the quantification of deviation from the analytical solution is rarely performed.…”