New Microstrip Bandpass Filter Topologies
Marinus (Ron) Korber, Jr.
Watkins-Johnson Co.
Microwave miniature receivers require numerous microstrip filters for such functions as picket selection, preselection in the case of a receiver front end, and suppression of mixer spurious products and doubler fundamental frequency feedthrough. The sheer quantity of filters required in such systems makes filter size an important consideration. For most requirements the filter type is limited to two choices: either edge coupled or interdigital.
Microstrip interdigital filters are compact because they are an arrangement of shorted parallel quarter-wavelength resonators. 
They also possess the important advantage of having the nearest spurious response at 3f0. However, microstrip interdigital filters suffer from severe asymmetry of the filter response due to the effect of coupling between nonadjacent resonators (this skewing of the pass band is simulated poorly with most commercial simulators). The shallow low side rolloff, which is illustrated in the frequency response plot of a typical seven-section interdigital filter, shown in Figure 1 , can degrade adjacent picket rejection (in multipliers, for example) and cause insufficient suppression of unwanted signals entering the receiver preselector. Edge-coupled filters have a more symmetrical response and require no grounding vias. However, these filters are large, have a spurious response at 2f0 and are more likely than interdigital filters to require a following lowpass to suppress the 2f0 response. 
This article introduces two new compact microstrip experimental filter topologies. One filter has a more symmetrical frequency response and much steeper low side rolloff than an interdigital filter; the other filter has steep initial rolloff due to the addition of transmission zeroes introduced by cross coupling. A third new topology, a physically narrow filter demonstrated at 1 GHz with a steep frequency response also due to internal cross coupling, is presented. In addition, a compact folded version is discussed.
Parallel-/Capacitive-/Via-coupled Filters
A 5.5 GHz six-section interdigital filter can be transformed into two new filter types. 
Figures 2 and 3 show a six-section microstrip interdigital filter and its derivative, respectively. The second filter is formed by sliding resonators three and four and five and six up and to the left to form a parallel-coupled/capacitive (gap)-coupled filter. The inter-resonator couplings are designated Kxx. The overall length is only slightly longer at 5.5 GHz than the interdigital filter. The filter’s frequency response is shown in Figure 4 . 
Figure 5 shows a parallel-/capacitive-/via (inductive)-coupled filter that is a derivative of the modified interdigital filter. This new filter type is formed by moving resonators four, five and six over resonator three. With this move, one more degree of freedom is added to the filter design due to the inductive via-to-via coupling that has been introduced. 
This modification forms transmission zeroes on both sides of the pass band.
The zero location is a strong function of the distance between vias (solid circle). Normally, extra zero-forming resonators would be used to accomplish this objective. But here, trans-mission zeroes have been created through cross coupling some of the existing resonators. Figure 6 
shows the frequency response for the parallel-/capacitive-/via-coupled filter with a 15 mil via spacing and a 30 mil via spacing. The substrate material for these filters is 15 mil alumina. The side walls of the test fixture extend 10 mils beyond the substrate edge. The cover height is 215 mils. All resonators are a quarter-wave long and 25 mils wide. All three filters have 2 dB insertion loss.
Parallel-/Via-/Capacitive- Coupled Filters
The 1 GHz interdigital filter, shown in Figure 7 , has a frequency 
response as shown in Figure 8 . This filter is transformed into the parallel-/via (inductive)-coupled filter with capacitive cross coupling. The length and width of the shunt line connecting the via (plated through hole) to the resonators and the via inductance determine the coupling coefficients between resonators two and three, and also between resonators four and five. The zero location is determined by the capacitive gaps (solid circles). 
Once again, transmission zeroes are accomplished through cross coupling and not by the addition of extra zero-forming resonators. Figure 9 shows the parallel-/via-/gap-coupled filter.
The two plots shown in Figure 10 are for 50 and 10 mil gaps, respectively. The effect of changing gap width is seen clearly. The frequency responses compare favorably with that of the interdigital filter where nonadjacent coupling skews the low side of the frequency response. 
The resulting filter structure is narrow and long. The actual input and output (not the tap location, obviously) can be located virtually anywhere along the filter’s length or can come up from underneath the board, making the filter even narrower. A folded version, shown in Figure 11 , is also possible and brings the filter’s size more in line with the interdigital filter. 
The filter’s frequency response is shown in Figure 12 . The dielectric medium for the three filters is 31-mil-thick FR4. All the FR4 filters have resonators 50 mils in width and a quarter-wave long at 1 GHz. Insertion loss for both filters is 2 dB.
Conclusion
Several new experimental filter types have been proposed and discussed. 
First, two six-section, 5.5 GHz filters, both created by rearranging the resonators of a six-section interdigital filter, were built and their frequency responses were compared to the response of a seven-section interdigital filter of the same frequency. One of the new filters has transmission zeroes created by internal cross coupling. Next, a six-section, 1 GHz filter with transmission zeroes created by internal cross coupling was built. 
The frequency response of this filter, a derivative of the 1 GHz interdigital filter, was compared to that of the six-section interdigital filter. These 1 GHz filters were constructed on FR4 dielectric.
All of the filters were experimental; some were aligned with tweaks, so much more work needs to be completed. In the future, electromagnetic field solvers such as Sonnet will have to be used to model all of the parasitic coupling that takes place in the cross-coupled versions accurately and to develop coupling coefficient vs. via spacing information. The proposed filters provide engineers with some options offering steeper stop-band attenuation and no compromise in insertion loss. The 5.5 GHz alumina versions are also compact and, in fact, would be more compact than interdigital filters at higher frequencies.
Acknowledgment
The author wishes to thank Richard Ranson and Dan Swanson, as well as Todd Patterson for auto routing the FR4 boards.
Marinus (Ron) Korber is a staff scientist at Watkins-Johnson Co. and head of YIG devices engineering. He has 30 years of experience in various microwave product areas. In the last four years, Korber has been a designer in the microwave miniature receiver group, designing various microstrip and coaxial resonator components, particularly filters. Previously, most of his time was spent designing YIG filters and oscillators.