The SSSSIR Filters

In this section, two design examples of broadband bandpass filters using SSSSIRs are presented. The filters are fabricated and measured to demonstrate the validity of design procedures and the outstanding stopband performance. The specifications and structural parameters of the filters are listed in Table 4.2, where i represent the sequential number for the composing elements shown in the circuit model. The extracted fringing capacitance values of each capacitor are listed at the bottom of Table 4.2. In these experimental examples a substrate material, Rogers RO4003 with єr=3.38, is applied to each layer.

Table 4.2 Specifications and dimensions of the experimental filters of Filter structure B

Coupled line sections High impedance Lines Parameters in circuit model Dimension (mm) Ohm mm mm Filters Order FBW%

Fringing Capacitors in Filter E: C01=0.18pF, C12=0.025pF, C23=C34=0.05pF

Fringing Capacitors in Filter F: C01=0.18pF, C12=0.025pF, C23=0.045pF, C34= C45=0.045pF

Fig. 4.7 and Fig. 4.8 depict the simulated and measured results of filter E and F. The filter E is a third-order Chebyshev filter with 0.15dB of passband ripple, 3GHz of center frequency f0, and 70% of fractional bandwidth. The filter F is a fifth-order Chebyshev filter with same electrical parameters as filter A except its passband ripple to be 0.1dB.

It can be observed in Fig. 4.7 and Fig. 4.8 that the spurious passband are pushed up to more than 5f0. Moreover, better than –30dB for filter A and –40dB for filter B stopband rejection level are achieved. Again, each figure contains three different data that are acquired by circuit model, EM simulation, and experimental measurement respectively. Both filters show good agreement between these three data. The photographs of fabricated filter E and F are depicted in Fig. 4.9 (a) and (b) respectively.

Fig. 4.7 Simulated and measured responses of filter A.

Fig. 4.8 Simulated and measured responses of filter B.

(a) (b) Fig. 4.9 Photographs of fabricated (a) filter E and (b) filter F.

Chapter 5

Conclusions

In this dissertation, we have proposed two types of three-dimensional extremely tight coupling structures, namely VIP structures and SSS structure. Two modified VIP coupling structures have been proposed to realize multi-section cascade quadrature hybrid with multi-octave bandwidth. The purposes of modification are to compensate the unequal modal phase velocities, to further increase the coupling to as tight as -0.8 dB, and to provide layout freedom of minimizing the discontinuity effect. The proposed structure type I not only can compensate the unequal modal phase velocity but also provide the flexibility of physical layout to minimize the junction effect that occurred between different coupled-line sections.

However, the proposed type I coupling structure can not provide an extremely tight coupling as tight as -0.8dB and that is necessary when realize a quadrature hybrid with multi-octave bandwidth. Therefore, the type II coupling structure has been successfully proposed to achieve an extremely tight coupling by adopting the ground aperture technique. Both coupling structures use dielectric blocks beside the vertically substrate to compensate the modal phase velocities over a wide frequency range. Moreover, the vertically substrate and the main substrate in proposed coupling structure can be the low dielectric constant PCB substrates where the conventional PCB etching and assembly process can be used for mass production.

Following the systematical design procedures accompany with the design charts, a design example of a five-section quadrature hybrid with passband ripple of +/- 0.5dB and frequency range of 1-9GHz is realized. The measured amplitude imbalance is less than +/-0.65dB and phase difference is keeping at 90⁰ +/-5⁰ over the designed frequency of 1-9GHz. The experimental data confirm very well with the simulation result to demonstrate the validation

of proposed structure and design procedures.

For a broadband filter, the extremely tight coupling structure is also an essential circuit element. Fortunately, the unequal modal phase velocities could not be a problem for filter application because filter is not sensitive to phase velocity difference. Therefore, the conventional VIP coupler can be used. Moreover, we have proposed a modified stripline structure with groung plane opening. The modified structure has been named as SSS structure.

Simialr to the conventional VIP coupled-line structure, the SSS coupled-line structure is also suitable for broadband filter application.

Besides the tightly coupled line portion, an extremely high impedance uncoupled line is required to implement a SIR filter. Fortunately, both conventional VIP and SSS structures can realize an extremely high impedance uncoupled lines.

Therefore, two filter structures using VIPSIRs and SSSSIRs are proposed to realiz bandpass filters with a bandwidth of broader than 1:2, and a wide and good upper stopband suppression. Same as multi-section coupler, the two proposed filter structures are both suitable for standard PCB process and low cost low dielectric constant material. Both proposed filter structures can realize extremely low impedance values of coupled line section and really high impedance values of un-coupled transmission line section. Therefore, a SIR with very high impedance ratio R (R=Z0H/Z0L) can be realized. As a result, the harmonic passband could be pushed far away from the passband and a wide and good upper stopband clearance could be achieved. Several experimental examples using proposed filter structure have been designed and fabricated. The fractional bandwidth of the designed examples can be as wide as 70% and the nearest harmonic passband of most cases are located more than 5f0. Excellent upper stopband suppression (better than -30dB and -40dB suppression for three and five order filter respectively) has been accomplished. Systematical, fast, and accurate filter design procedures using commercial circuit simulator accompany with the design charts have been introduced. Again, the experimental data confirm very well with the simulation results to demonstrate the validation of proposed structure and design procedure.

Proposed three-dimensional coupling structure not only can apply to quadrature coupler and filter design. In the future work, the proposed coupling structures also can also be used in different RF/microwave circuits such as broadband balanced mixer, broadband balun, and balanced amplifiers. Moreover, the vertically installed planar configuration can be a good solution for the circuit crossover problem between two arms of microstrip lines. For example, two modified rate-race rings can be cascaded together to form an interesting performance [42].

In the modified rat-race ring a VIP cross-over is used. We believe that more study could be done on this kind of circuit where a cross-over is required for the implementation of the circuit.

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