使用垂直指叉共振器之縮小化微波濾波器與雙工器設計

Abstract

本篇論文提出以垂直指叉共振器實現應用於微波頻段之小尺寸濾波器以及雙工器，並將電路製作於低溫共燒陶瓷(low-temperature co-fired ceramic, LTCC)基板上。首先以垂直指叉共振器搭實作濾波器，該濾波器具有鄰近通帶的傳輸零點，提升選擇性；其次則利用T接面(T-junction)組接兩個不同頻段的濾波器，完成雙工器設計；最後則以共用共振器架構取代T接面。 垂直指叉共振器由多層指叉電容並聯金屬線電感組成。利用多層特性大幅縮小共振器尺寸，且此濾波器在通帶附近有兩個零點，加強通帶選擇性。如此在製作頻率比介於1.1到1.2之雙工器上仍能保有不錯的隔離度。 本論文首先於WiFi頻段設計一中心頻率2.45 GHz，比例頻寬7.8 %之二階柴比雪夫響應(Chebyshev response)濾波器。使用基板為璟德電子之低溫共燒陶瓷，介電常數為7.5，正切損耗為0.006。量測的插入損耗約為2.1 dB，工作頻段內|S11|低於14 dB。與相關文獻比較[17]，此濾波器包含2個傳輸零點，波長面積縮小一半，且插入損耗減少0.4 dB。此外，本論文為了實作行動通訊頻段之雙工器，亦設計了中心頻率分別為1.95 GHz 與2.14 GHz，比例頻寬皆為4.5 %的兩個二階柴比雪夫響應濾波器。1.95 GHz濾波器之量測插入損耗約為4.5 dB，工作頻段內|S11|低於14 dB；2.14 GHz濾波器之量測插入損耗約為4.0 dB，工作頻段內|S11|低於14 dB。 將前述兩個不同頻段的垂直指叉共振器濾波器以T接面連接，即可組成兩通帶中心頻率分別位於1.95 GHz與2.14 GHz，比例頻寬皆為4.5 %的雙工器。此結構兩通帶量測插入損耗分別約4.8 dB及5.2 dB，工作頻段內的|S11|皆低於14 dB，隔離度20 dB以上。雖然以垂直指叉共振器成功將濾波器微小化，但於此頻段使用T接面實作雙工器卻會大幅增加電路面積，無法達到電路縮小化之效果。 觀察前述雙工器可發現T接面約佔整體電路92 %的面積，若能將T接面以垂直指叉共用共振器取代，即可大幅縮小電路面積，同時減少因T接面造成的插入損耗。使用此架構實作之雙工器，可把電路縮小化至商業尺寸(3.2 × 2.5 mm^2)。模擬結果顯示1.74與2.14 GHz插入損耗分別約為3 dB與5 dB；1.74 GHz工作頻段內的|S11|低於16 dB，2.14 GHz工作頻段內的|S11|最高可達10 dB，隔離度25 dB以上

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.