ABSTRAKSI: Coupler 3dB selalu digunakan pada rangkaian-rangkaian praktis seperti, balanced mixer, balanced amplifier, modulator quadrature, multiplexer/duplexer, dll. Pada modulator quadrature dibutuhkan dua sinyal yang berbeda fasa 90o sedangkan osilator yang ada hanya satu, untuk itu dibutuhkan penggeser fasa 90o
Branchline Coupler dapat membagi daya sama besar dan menghasilkan keluaran yang beda fasanya 90o tanpa harus ada pergeseran fasa lagi. Hanya saja pada Branchline coupler konvensional secara inherent bandwidthnya sempit (BW<20%) dan untuk frekuensi dibawah 2 GHz ukurannya menjadi besar yang mengakibatkan redaman menjadi lebih besar. Untuk memperlebar bandwidth beberapa branchline di cascade sehingga ukurannya menjadi tambah besar dan redamannya makin besar. Untuk mengatasinya dilakukan modifikasi terhadap branchline konvensional dengan cara memberikan beban kapasitif sehingga walaupun di cascade ukurannya menjadi relatif kecil dengan redaman yang kecil pula.
Pada tugas akhir ini telah dirancang dan direalisasikan sebuah Branch-Line Hybrid Coupler menggunakan saluran mikrostrip yang ukurannya tereduksi dan bandwidth lebar dengan cara pemberian beban kapasitif berupa stub open circuit. Diharapkan coupler hybrid ini bekerja pada frekuensi 1.7-2.5 GHz (DCS-WiMax) dengan coupling -3dB, fasa 90o dan return loss ≥ 15 dB. Dari realisasi dihasilkan return loss ≥ 15 dB dan pada frekuensi tengah konvensional dengan coupling -3.69 dB dan.fasa 90.148o dan pada frekuensi tengah open stub dengan coupling -3.22 dB dan.fasa 87.855o.
Kata Kunci : Branch-Line Hybrid Coupler Konvensional dan Open Stub, Coupler 3dB, beda fasa 90oABSTRACT: 3 dB coupler is always used in many practical electrical circuits such as balanced mixer, balanced amplifier, quadrature modulator, multiplexer/duplexer, etc. in quadrature modulator two signal with 90° phase difference is needed, while oscillator only need one, thus a 90° phase shift is needed.
Branchline coupler can distribute power equally and produce output with 90°phase difference without any phase shifter. But conventional branchline coupler inherently has narrow bandwidth (BW<20%) and for frequency under 2 GHz, its dimension becomes larger which resulting in higher attenuation. To extend the bandwidth, a few branchlines is cascaded, making its dimension become even bigger and the attenuation also higher. To overcome this, conventional branchline is modified by giving capacitive load, so even though it’s cascaded, its dimension becomes relatively smaller and the attenuation becomes lower.
In this final project, a branch-line hybrid coupler had been designed and built using reduced micro strip line and wide bandwidth by giving capacitive load in form of open circuit. The result was response that is almost flat from the frequency of 1700-2500 MHz, with return loss≥15 dB. Isolation 13 dB from the frequency 1700-1760 MHz and ≥ 15 dB at frequency of 1780-2500 Mhz. on frequency of 1700-2500 MHz, conventional circuit has vary value output [S21] averagely -3.79 dB, coupling [S31] averagely -3.63 and phase difference between [S21] and[S31] averagely 90.148o and for open stub circuit for output [S21] averagely is -3.36 dB, coupling [S31] averagely -3.42 and phase difference between [S21] and [S31] averagely 87.885oKeyword: Branch-Line Hybrid Coupler Conventional dan Open Stub, Coupler 3dB, fasa 90oe