Filter designers have a wide choice of commercial software design tools at their disposal. But the latest version (Version 3.0) of the MMICAD filter synthesis software suite from Optotek Ltd. (Kanata, Ontario, Canada) now includes user-defined physical surroundings as an integral part of the synthesis operation. The software synthesis tool is actually a set of 35 dedicated programs for the design of microstrip, lumped element, stripline, and wave-guide filters. The design interface and common menu format seamlessly integrates with the MMICAD linear simulator and physical layout in a user-friendly design environment. A design example of a parallel coupled bandpass filter will be illustrated in this article.

The parallel coupled-line bandpass filter is one of the more popular microwave-filter designs.¹ It is suitable for implementation in microstrip and surface-mount construction. However, there are practical limitations to its design. Design bandwidths are limited to about 20 or 30 percent around the center frequency due to decreasing input coupling gap that is a function of increasing bandwidth and decreasing passband ripple.

There are also practical considerations in construction that includes the width of the housing, which should be kept less than one-half wavelength of the highest frequency in order to suppress the possibility of the TE₀₁ waveguide mode propagation that can propagate energy around the filter and limit the attainable stopband attenuation. The presence of a cover over the circuit can cause the passband frequency to be shifted, and this should be compensated for in the design synthesis. At higher frequencies, microstrip dispersion² must be accounted for if the passband is to be centered on the design frequency.

Other circuit discontinuities, such as microstrip metalizaztion,³ and the capacitance at the ends of the coupled resonator lines,² must also be included in the filter synthesis. All of these factors should be included in the synthesis if multiple fabrication trials are to be minimized and possibly eliminated.

The interface to the synthesis software, the menu system, is organized and arranged to match the sequence of calculations a designer would normally follow. This gives the designer access to the intermediate steps in the calculations, and allows the designer the opportunity to control, interact, and optimize a design during the calculation process. For example, an electrical design may have been arrived at, and the microstrip dimensions can then be separately calculated and optimized by the designer varying the substrate dielectric constant and thickness, the metal thickness, the cover height, and the type of input coupling.

The requirement is for a filter that operates from 10.0 to 13.0 GHz with greater than 30-dB rejection at 9.0 and 14.0 GHz will be used as a design example. A filter with eight resonators and a passband ripple of 0.1 dB (return loss of 16.3 dB) will meet this requirement. The design bandwidth is set to 3.5 GHz centered at 11.5 GHz, which is an F1 of 9.75 GHz and F2 of 13.25 GHz, representing a fractional bandwidth of 30 percent.

Figure 1 shows the dialog window for entering electrical information for a desired filter, including the number of resonators, passband ripple, center frequency, bandwidth, rejection frequencies, and impedance. Figure 2 shows the resulting calculated even- and odd-mode impedances for this parallel-coupled-line filter design. The synthesis software can also calculate the filter frequency response, including dissipative losses. The response calculation can be used to verify that the basic electrical design will meet a set of requirements, including the increased passband insertion loss at the band edges due to dissipative losses. The calculated response for this design will be generated by Version 3.0 of the MMICAD linear circuit analysis program.⁵

Figure 3 shows the dialog box for entering the physical parameters of substrate dielectric constant and thickness, metal thickness, and the type of input coupling. For this particular filter design, the substrate is 0.015-in.-thick alumina with 0.125-mil-thick thin-film metalization. The unloaded quality factor (Q) for this substrate obtained though experiment is about 225 at X-band.

Because of the tight input coupling required for this large bandwidth, interdigitated coupling lines were chosen for this filter. The filter's calculated dimensions are shown in Fig. 4. For noninterdigitated lines, the line width would be 11.1 mils although the line spacing would be only 0.4 mils, somewhat less practical to fabricate compared to the interdigitated design.

In Fig. 5, a dialog box is used for specifying the circuit layout and the circuit mask. There is an intermediate window (not shown) that allows choosing the arrangement of the circuit input and output lines. Here, input and output lines are selected on a common centerline. This produces the narrowest layout and substrate width, and maximizes the TE₀₁ mode cutoff frequency for a metal enclosure. The calculated cutoff frequency is part of the dialog box and is 33.723 GHz for a substrate width of 175 mils. This width is chosen for mechanical packaging purposes. The overall length of the substrate is 1500 mils. Once all required values have been entered into the dialog box, the software yields a circuit layout that can be written to a DXF file for direct generation of the photo mask. A photograph of the filter fabricated from this synthesized file is shown in Fig. 6.

The filter-synthesis program generates a series of netlist files with the calculated nominal dimensions of the circuit for design verification, as well as files for analysis of changes in filter response with temperature, files for worst-case and yield analysis, and netlist files of circuit discontinuities to improve performance predictions.