A Compact Coupled-Line Butterworth Bandpass Filter for Sub-6 GHz Applications

Öz

This paper presents the design, fabrication, and experimental validation of a compact microstrip bandpass filter operating at 3.5 GHz for sub-6 GHz 5G wireless communication systems. The proposed filter is realized using a fourth-order Butterworth response implemented through a parallel coupled-line topology, aiming to achieve a maximally flat passband response, low insertion loss, and sufficient selectivity within the n77 frequency band (3.3-3.8 GHz). The design procedure starts with the synthesis of a normalized low-pass Butterworth prototype, followed by bandpass transformation and its implementation as an equivalent distributed microstrip structure. Electromagnetic analysis and parametric optimization are employed to accurately model coupling effects, transmission line behavior, and fabrication constraints. The filter is fabricated on a low-cost FR-4 substrate with a dielectric constant of 4.3 and a thickness of 0.8 mm using standard printed circuit board manufacturing techniques. Experimental characterization is performed using a vector network analyzer to evaluate the frequency response. Measurement results demonstrate a center frequency of 3.5 GHz with an impedance bandwidth of approximately 300 MHz, showing good agreement with the predicted performance. The proposed bandpass filter offers a compact footprint, simple geometry, and reliable frequency response, making it a suitable candidate for integration into 5G radio-frequency front-end modules and other modern wireless communication systems.

Anahtar Kelimeler

• Butterworth response
• Coupled-line topology
• Microstrip bandpass filter
• Sub-6 GHz 5G systems

Sub-6 GHz Uygulamaları için Kompakt Kuplajlı Hat Butterworth Bant Geçiren Filtre

Öz

Bu çalışmada, sub-6 GHz 5G kablosuz haberleşme sistemleri için 3.5 GHz merkez frekansta çalışan kompakt bir mikroşerit bant geçiren filtrenin tasarımı, üretimi ve deneysel doğrulaması sunulmaktadır. Önerilen filtre, maksimum düz geçiş bandı cevabı, düşük ekleme kaybı ve n77 frekans bandı (3.3–3.8 GHz) içerisinde yeterli seçicilik sağlamak amacıyla paralel kuplajlı hat topolojisi kullanılarak dördüncü dereceden Butterworth tepkisiyle gerçekleştirilmiştir. Tasarım süreci, normalize edilmiş bir alçak geçiren Butterworth prototipinin sentezi ile başlamakta, bunu bant geçiren dönüşüm ve eşdeğer dağıtılmış mikroşerit yapı olarak gerçekleştirilmesi izlemektedir. Kuplaj etkileri, iletim hattı davranışları ve üretim kısıtlarını doğru şekilde modellemek amacıyla elektromanyetik analiz ve parametrik optimizasyon çalışmaları yapılmıştır. Filtre, dielektrik sabiti 4.3 ve kalınlığı 0.8 mm olan düşük maliyetli FR-4 alttaşı üzerinde standart baskılı devre kartı (PCB) üretim teknikleri kullanılarak üretilmiştir. Deneysel karakterizasyon, frekans cevabını değerlendirmek üzere bir vektör ağ analizörü kullanılarak gerçekleştirilmiştir. Ölçüm sonuçları, yaklaşık 300 MHz empedans bant genişliğine sahip 3.5 GHz merkez frekans elde edildiğini ve ölçülen sonuçların öngörülen performansla iyi bir uyum gösterdiğini ortaya koymaktadır. Önerilen bant geçiren filtre; kompakt boyutları, basit geometrisi ve güvenilir frekans cevabı sayesinde 5G radyo frekans ön uç modülleri ve diğer modern kablosuz haberleşme sistemlerine entegrasyon için uygun bir aday sunmaktadır.

Anahtar Kelimeler

• Sub-6 GHz;
• bant geçiren filtre
• kuplajlı hat
• Butterworth filtre
• mikrodalga filtre
• RF devreleri

Kaynakça

1. [1] Hsieh L.-H., Chang K., “Compact, low insertion-loss, sharp-rejection, and wide-band microstrip bandpass filters,” IEEE Transactions on Microwave Theory and Techniques, Vol. 51, Iss: 4, (2003), pp.(1241-1246).
2. [2] Shin K.R., Arendell J., Eilert K., “Compact 5G n77 bandpass filter with through silicon via (TSV) IPD technology,” in Proceedings of the 2019 IEEE 20th Wireless and Microwave Technology Conference (WAMICON), (2019), pp.(1-3).
3. [3] Lee K.C., Su H.T., Haldar M.K., “A review of centrally loaded multimode microstrip resonators for bandpass filter design,” AEU-International Journal of Electronics and Communications, Vol. 69, Iss: 10, (2015), pp.(1533-1540).
4. [4] Shome P.P., Khan T., Koul S.K., Antar Y.M.M., “Two decades of UWB filter technology: Advances and emerging challenges in the design of UWB bandpass filters,” IEEE Microwave Magazine, Vol. 22, Iss: 8, (2021), pp.(32-51).
5. [5] Li C., Ma Z.-H., Chen J.-X., Wang M.-N., Huang J.-M., “Design of a compact ultra-wideband microstrip bandpass filter,” Electronics, Vol. 12, Iss: 7, (2023), pp.(1728).
6. [6] Dokmetas B., Arican G.O., Akcam N., Yazgan E., “A compact bandstop filter design using DMS-DGS technique for radar applications,” ACES Journal, Vol. 36, Iss: 11, (2021), pp.(1460-1467).
7. [7] Dokmetas B., Arican G.O., “Design of dual-band SIW antenna for millimeter-wave communication,” in 2023 31st Telecommunications Forum (TELFOR), Belgrade, Serbia, (2023), pp.(1-4).
8. [8] Dokmetas B., Belen M.A., “Microstrip antenna design for 2.4 GHz RF energy harvesting circuits with artificial neural networks,” Journal of Artificial Intelligence and Data Science, Vol. 4, Iss: 1, (2024), pp.(33-38).

9. [9] Keskin S.E.B., Güller C., Uslu B., “Optimization of Monopole Consecutive Square-Shaped Patch Antenna with ANN for Wireless Communication Systems,” El-Cezeri Journal of Science and Engineering, Vol. 10, Iss: 3, (2023), pp.(492-505).
10. [10] Kis G.E., Okan T., “Compact Power-Symbol Shaped Microstrip Antenna for Healthcare Monitoring Systems,” El-Cezeri Journal of Science and Engineering, Vol. 9, Iss: 1, (2022), pp.(232-240).
11. [11] Guernine R., Sahed M., Kenane E., Djahli F., “A High-Efficient Single-Band Rectenna Design Using Dielectric Resonant Antenna for ISM Applications,” El-Cezeri Journal of Science and Engineering, Vol. 13, Iss: 2, (2026), pp.(135-145).
12. [12] Cheng J., Alphones A., “Compact interdigital microstrip bandpass filter,” Microwave and Optical Technology Letters, Vol. 52, Iss: 9, (2010), pp.(2128- 2132).
13. [13] Chen C.-J., “A coupled-line coupling structure for the design of quasi-elliptic bandpass filters,” IEEE Transactions on Microwave Theory and Techniques, Vol. 66, Iss: 4, (2018), pp.(1921-1925).
14. [14] Bisu A.A., “Design and simulation of a fourth-order high-frequency bandpass filter for radar communication systems,” The Journal of the Nigerian Association of Mathematical Physics, Vol. 66, (2024), pp.(169-176).
15. [15] Yumnam K., Singh S., “A comprehensive analysis of bandpass filters for mmWave and sub-6 GHz 5G wireless communications,” in Proceedings of the International Conference on Computational Electronics for Wireless Communications (ICCWC), (2023), pp.(331-342).
16. [16] Sheta A.F., Coupez J.P., Tanne G., Toutain S., Blot J.P., “Miniature microstrip stepped impedance resonator bandpass filters and diplexers for mobile communications,” in 1996 IEEE MTT-S International Microwave Symposium Digest, Vol. 2, (1996), pp.(607-610).
17. [17] Ali M., Liu F., Watanabe A., Raj P.M., Sundaram V., Tentzeris M.M., Tummala R.R., “First demonstration of compact, ultra-thin low-pass and bandpass filters for 5G small-cell applications,” IEEE Microwave and Wireless Components Letters, Vol. 28, Iss: 12, (2018), pp.(1110-1112).
18. [18] Michalkiewicz M.L., Mariano A.A., Dartora C.A., Castaldo F.C., “Bandpass filter using SIW technology in inductive window topology with low-cost FR-4 substrate,” Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol. 21, Iss: 1, (2022), pp.(83-101).
19. [19] Pozar D.M., Microwave engineering: theory and techniques, John Wiley & Sons, (2021).
20. [20] Zhu L., Sun S., Li R., Microwave bandpass filters for wideband communications, John Wiley & Sons, (2011).
21. [21] Martin P., Ness J., “Coupling bandwidth and reflected group delay characterization of microwave bandpass filters,” Applied Microwave and Wireless, Vol. 11, (1999), pp.(86-96).
22. [22] Gruszczynski S., Staszek K., Wincza K., “Parallel-coupled bandpass filters with directly coupled input and output resonators,” AEU-International Journal of Electronics and Communications, Vol. 172, (2023), pp.(154974).
23. [23] Darwis F., Rahayu E.A.B., Sutrisno S., Madiawati H., Taufiqqurrachman T., Setiawan A., Kurniawan E.D., Wijayanto Y.N., “Cross-coupled line bandpass filter based on modified parallel-coupled line structure,” Jurnal Elektronika dan Telekomunikasi, Vol. 22, Iss: 1, (2022), pp.(8-13).
24. [24] Jamsai M., Angkawisittpan N., Nuan-On A., “Design of a compact ultra-wideband bandpass filter using inductively compensated parallel-coupled lines,” Electronics, Vol. 10, Iss: 21, (2021), pp.(2575).
25. [25] Sani A.H., Jianwen C., Khan M.T., Yussif S.B., Nahar S., Ewald K.E., Ukwuoma C.C., “Design and simulation of a 3.9 GHz microstrip parallelcoupled line bandpass filter,” in 2019 16th International Computer Conference on Wavelet Active Media Technology and Information Processing, (2019), pp.(429-433).
26. [26] La D.-S., Guan X., Wang M.-Y., Mi R.-Q., “Compact wideband bandpass filter based on coupled-line stub with high selectivity,” AEU-International Journal of Electronics and Communications, Vol. 138, (2021), pp.(153872).