Circuit fabrication and Measurement…

Fig. 3.12 shows the photos of the multilayer wideband bandpass filter. The circuit size is 1313 mil×366 mil (33.4mm×9.3 mm).

(a) Front-side view

(b) Back-side view

Fig. 3.12 (a) Front-side view and (b) back-side views of the circuit photos

From the circuit photos, we see that the two substrates are fixed by six screws. This can cause fabrication errors because the six screws can not fix every part of the two thin substrates

very tightly. The measured and simulated results are both shown in Fig. 3.13 for comparison.

The solid and dotted lines are measured and simulated results, respectively. The measured and simulated results are matched well, except the measured passband return loss is not as well as the simulated one. The return loss degradation might come from the fabrication errors. The fabrication errors include air gaps between two screw-fastened substrates and misalignment of the circuit. A very wide upper stopband clearance of -23dB up to 20GHz is achieved.

Fortunately, the spurious response at 13GHz in the measured results is not as high as the results in the simulation.

0.01 2 4 6 8 10 12 14 16 18 20

Frequency (GHz)

Graph 1

-60 -50 -40 -30 -20 -10 0

DB(|S(1,1)|) measurement DB(|S(2,1)|) measurement DB(|S(2,1)|) simulation DB(|S(1,1)|) simulation

Fig. 3.13 Comparison between the measured and simulated results

Chapter 4 Conclusion

This thesis has demonstrated two kinds of microwave circuits namely, multi-section ultra-broadband quadrature hybrid coupler and multilayer wideband bandpass filter.

In chapter 2, miniaturized ultra-broadband quadrature hybrid coupler has been realized by using 5-section cascaded CPW coupler structure. The synthesis techniques based on the concept of odd- and even-mode analysis has been developed. The VIP structure to achieve a extremely tight of coupling has been proposed. The unequal modal phase velocities, however, are a serious problem in the VIP structure. Putting two dielectric blocks with the same dielectric constant as the VIP substrate at both sides of the VIP substrate to compensate the modal phase velocities of a VIP coupled-line has been successfully developed. Although two dielectric blocks improve the directivity, they lower the even-mode characteristic impedance.

Tradeoff between directivity and coupling should be made. Finally, the measured results have shown a good agreement with the simulated results. The bandwidth reduction mainly results from the insufficient even-mode characteristic impedance in the tight-coupling section, the discontinuities between each section, and the air gaps between the dielectric blocks and VIP substrate. There still has space to improve the performance.

In chapter 3, the wideband bandpass filter has been realized by using the parallel coupled stepped impedance resonators (SIR). Utilizing the SIR to push the spurious frequency far away from the passband center frequency has been successfully developed. The multilayer structure to achieve a high to low even-mode characteristic impedances has been proposed.

According to the design parameters from the simulated results, a wideband bandpass filter can be easily designed. Finally, the measured and simulated results have been matched well. A very wide upper stopband clearance of -23dB up to 20GHz has been achieved.

Reference

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