Design of Compact Size Microwave Filters and Duplexers Using Vertically Interdigitated Resonators
Date Issued
2015
Date
2015
Author(s)
Tung, Chen-Yeh
Abstract
This thesis proposes design concepts for compact microwave filters and duplexers by vertically interdigitated resonators (VIRs), implemented on low temperature co-fired ceramic (LTCC) substrates. Filter designs with transmission zeroes near the passbands are firstly proposed using VIRs to achieve small size and high selectivity. By connecting two VIR filters in different frequency with T-junction, a duplexer design is then realized. Finally, the T-junction can be replaced with a dual band vertically interdigitated common resonator to achieve maximum size reduction. The proposed VIR is constructed with multi-layer shunt interdigitated capacitors and a shunt metal line inductor. Multi-layer structure makes filters smaller and introduces two transmission zeros near passbands to enhance selectivity. Therefore, these filters can be good solutions for design of duplexer having frequency ratio from 1.1 to 1.2 with acceptable isolation. First of all, a 2nd order Chebyshev response filter with center frequency 2.45 GHz and 7.8 % fractional bandwidth (FBW) is designed. This filter is implemented on ACX LTCC substrates with dielectric constant and loss tangent 7.5 and 0.006, respectively. Measured results of the filter show an in-band insertion loss less than 2.1 dB and a return loss greater than 14 dB. Compare with [17], the proposed filter halves the area in wavelength and lowers the insertion loss for about 0.4 dB. For realizing a duplexer which is used in mobile communication system, this thesis also designs two 2nd order Chebyshev filters with FBW 4.5 % and center frequencies 1.95 GHz and 2.14 GHz, respectively. Measured results of the 1.95-GHz filter show an in-band insertion loss 4.5 dB and a return loss greater than 14 dB, while the measured results of the 2.14-GHz filter show an in-band insertion loss less than 4 dB and a return loss greater than 14 dB. Connecting the two aforementioned filters with T-junction creates a duplexer with two 4.5%-FBW passbands atcenter frequencies 1.95 and 2.14 GHz. Measured results show insertion losses of two channels about 4.8 dB and 5.2 dB, respectively. Both the return losses of the two channels are over 14 dB and channel isolation is over 20 dB. It should be noted that although VIR reaches size reduction in filter design successfully, the area increase introduced by the T-junction makes the duplexer unable to achieve compact size. Since the T-junction occupies almost 92 % area of aforementioned duplexer, size reduction can be achieved by replacing the T-junction with a vertically interdigitated common resonator. Extra insertion loss caused by T-junction can also be avoided in the meanwhile. The proposed duplexer using vertically interdigitated common resonator can reach commercial size (3.2 × 2.5 mm^2). Simulation results show the insertion losses of the two channels about 3 dB and 5 dB, respectively. The return losses of the two channels are over 16 dB and 10 dB, respectively. Channel isolation is over 25 dB.
Subjects
vertically interdigitated resonator
low temperature co-fired ceramics
smartphone applications
compact microwave filter
compact microwave duplexer
Type
thesis
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