Filter's geometrical details(M. Righi, C. Eswarappa, W. J. R. Hoefer)
Virtual Reality Model: go inside the filter, (best seen with Live3D plugin for Netscape)
Java Wire-Frame model of the filter
The electromagnetic simulation of passive components for PCS systems such as miniaturized ceramic bandpass filters is presented. We have used the Transmission Line Modelling (TLM), a general-purpose space-time discrete method, to perform the simulations. We have compared the computed scattering parameters of the filter with available measurement results. The features of the filter responses agree well.
Introduction Wireless RF components are getting more and more complex in nature because of miniaturization which has lead to densely packed circuits. Traditional circuit simulators alone cannot predict the behavior of such complex circuits; Maxwell's equation's must be solved in the structure. Of the several numerical methods available, Finite Difference Time Domain (FDTD) [1] and Transmission Line Modelling (TLM) [2] are becoming increasingly popular because they can solve structures of arbitrary shapes and can handle any transient waveform. In this paper, we use the TLM method which employs a spatial transmission line network filling the computational domain. Voltage impulses are injected into the mesh and their propagation in the mesh is tracked. These voltage waves are then mapped onto the desired electromagnetic fields in the structure. The transient analysis is performed by exciting the mesh with a band-limited time domain waveform which covers the frequency range of interest. After the convergence is reached, the time-domain data are Fourier transformed into the frequency domain.
The structure considered in the paper is a ceramic bandpass filter for PCS hand phones at 1.9 GHz. It was designed, fabricated and measured by Sumitomo Metal Industries, Japan [3-4]. (See Fig.) . It's size is as small as 3mm*2mm*1.5mm. The filter consists of broadside-coupled striplines sandwiched between ceramic blocks of relative permittivity 75, inserted between microstrip lines on ceramic substrates of relative permittivity 15. The filter was measured by placing it on the feeding lines etched on the glass-epoxy substrate. A simplified lumped element equivalent circuit representation of this filter is shown in (See Fig.). We have used the 3D symmetrical condensed node (SCN) TLM to discretize the filter. The main problem we encountered was fitting the exact dimensions of the filter into the TLM mesh while maintaining a computational domain of a manageable size. For this reason both uniform and variable meshes were used. The input feeding line was excited with a Gaussian pulse beneath the strip. The time-domain responses of the filter obtained at the input and output feeding lines are plotted in(See Fig.) Note that this transient analysis reveals a portion of the injected signal which travels in the substrate and couples to the output port without being affected by the above filter. Only later, the signal which entered the filter couples to the output port to yield a very long time domain response, typical of a filter. This long response is due to the multiple reflections in the resonating structure. The time step used in these simulations is 0.166 ps. The scattering parameters obtained after Fourier trans forming these time-domain waveforms are plotted in (See Fig.). We can see that the centre frequency of the passband is slightly higher than 1.9 GHz versus the measured value of 1.9 GHz. The difference between the measured and the simulated center frequencies is attributed to dispersion and coarseness error of TLM modelling as well as to the finite metallization thickness of the striplines, which was not included in the numerical model. From our experience it appears that the center frequency of the filter is extremely sensitive to the length of the coupled stripline. A change in the stripline length of as little as 200 mm, translates in a shift in the passband of the filter of more than 100 MHz. This findings are in agreement with the experience gained at Sumitomo Metal Industries during the realization of this filter. The advantage of an electromagnetic simulator in this case is the possibility of gaining this experience without actually realizing the filter. In addition, the simulated response contains all the features of the measured response, features which are not present in a simple lumped element model, as Kobayashi and Saito pointed out in [3].
The electromagnetic analysis of a miniaturized ceramic filter for PCS applications has been presented. This time domain analysis leads to results which agree very well with the measured values. Critical features of the filter such as the two poles above and below the passband frequency, are highlighted to give valuable insight into the filter operation.
The authors wish to acknowledge M. Fuji, K. Saito and Dr. S. Kobayashi from the Advanced Technology Research Laboratories, Sumitomo Metal Industries, Japan, for providing the details and the measurement results of the filter.
[1] K. S. Yee, "Numerical Solution of Initial Boundary Value Problems Involving Maxwell's equations", IEEE Transactions Antennas and Propagation, vol. 14, no. 3, pp. 302-307, May 1966.
[2] W.J.R. Hoefer, "The Transmission Line Matrix (TLM) Method", in T. Itoh: Numerical Techniques for Microwave and Millimeter Wave Passive Structures, John Wiley & Sons, New York (1989).
[3] S. Kobayashi, K. Saito, "A Miniaturized Ceramic Bandpass Filter for Cordless Phone Systems", IEEE MTT-S International Microwave Symposium Digest, Orlando, FL, June 1995, pp. 391-394.
[4] M. Fuji, S. Kobayashi, K. Saito, Private Communication.
G. Tardioli, L. Cascio, M. Righi, March 5, 1997
