Primitive and Improved LPF
To increase the transition band sharpness, four radial stubs are added to the combined resonator (see Figure 8). The primitive LPF has a sharp transition band; however, the stopband is narrow. The physical dimensions of this structure (in mm) are: W13 = 6.73, W14 = 0.88, W15 = 2.33, W16 = 0.64, H10 = 0.25, R1 = 4.06 and θ1 = 20 degrees.
According to Figure 8b, the stopband is limited by transmission poles. To achieve a wider stopband, four open-circuited stubs are added as suppressing elements, producing several transmission zeros (see Figure 9). The circuit layout and EM simulation are shown in Figures 10a and 10b, respectively. This improved LPF has a high return loss, and the stopband is extended to more than 35 GHz. The physical dimensions (in mm) are: W17 = 1.5, W18 = 0.1, W19 = 1.52, H11 = 2.79 and H12 = 0.76.
The final filter design employs four semi-circular stubs (see Figure 11). It has an ultra-wide stopband (2.38 to 65 GHz) with high attenuation (24 dB) up to the 34th harmonic. The physical dimensions (in mm) are: W19 = 0.69, Wm = 1.17, Hm = 1.5, R2 = 0.25 and θ2 = 180 degrees.
SIMULATION AND MEASUREMENT
The LPF was simulated using ADS software, fabricated on RT-Duroid 5880 (εr = 2.2, h = 0.381 mm, loss tangent = 0.0009) and tested with a Keysight 8757A network analyzer (see Figure 11c). It exhibits an ultra-wide stopband from 2.38 to 65 GHz, with 24 dB suppression up to the 34th harmonic. The final filter has a 3 dB cutoff frequency of 1.89 GHz and high return loss (14.8 dB) in the passband. Overall dimensions are only 17.7 mm x 7.5 mm (0.153λg x
0.065λg). With these features, it is useful for wireless applications, according to the specifications listed by Hayati et al.17 This filter and other reported works are compared in Table 4. This filter has the widest stopband, the smallest size and the highest FOM.
In Table 4, the transition band sharpness (ξ) is defined as
where αmin and αmax are suppression points of −3 and −40 dB, respectively. fs is the frequency corresponding to αmax, and fc is the frequency corresponding to αmin. The relative stopband bandwidth (RSB) is
The suppressing factor (SF) is
The normalized circuit size (NCS) is
For 2D and 3D circuits, the architecture factor (AF) is defined as 1 and 2, respectively, and the FOM is
CONCLUSION
An ultra-wide rejection band LPF using symmetrical modified T-shaped and flag-shaped resonators has been manufactured and tested. The stopband width is 62.62 GHz (from 2.38 up to 65 GHz) with more than 24 dB suppression. This configuration has the best performance for harmonic suppression (from the second to 34th) and the smallest size compared to other recent work.
References
- D. M. Pozar, Microwave Engineering, Third Edition, Wiley, New York, 2005.
- J. S. Hong and M. J. Lancaster, Microstrip Filters for RF/Microwave Applications, Wiley, New York, 2004.
- P. M. Raphika, P. Abdulla and P. M. Jasmine, “Planar Elliptic Function Lowpass Filter with Sharp Roll-Off and Wide Stopband,” Microwave and Optical Technology Letters, Vol. 58, No. 1, January 2016, pp. 133–136.
- Gh. Karimi, A. Lalbakhsh, Kh. Dehghani and H. Siahkamari, “Analysis of Novel Approach to Design of Ultra-Wide Stopband Microstrip Lowpass Filter Using Modified U-Shaped Resonator,” ETRI Journal, Vol. 37, No. 5, October 2015, pp. 945–950.
- P. Zhang and M. Li, “A Novel Sharp Roll-Off Microstrip Lowpass Filter with Improved Stopband and Compact Size Using Dual-Plane Structure,” Microwave and Optical Technology Letters, Vol. 58, No. 5, May 2016, pp. 1085–1088.
- A. K. Verma, N. Chaudhari and A. Kumar, “Improved Performance Step Impedance Lowpass Filter,” International Journal of Electronics and Communications (AEÜ), Vol. 67, March 2013, pp. 761–770.
- M. Kufa and Z. Raida, “Lowpass Filter with Reduced Fractal Defected Ground Structure,” IET Electronics Letters, Vol. 49, No. 3, January 2013, pp. 199–201.
- H. R. Khakzad, S. H. Sedighy and M. K. Amirhosseini, “Design of Compact SITLs Low Pass Filter by Using Invasive Weed Optimization (IWO) Technique,” Applied Computational Electromagnetics Society Journal, Vol. 28, No. 3, March 2013, pp. 228–233.
- K. Li, M. Zhao, Y. Fan, Z. Zhu and W. Cui, “Compact Lowpass Filter with Wide Stopband Using Novel Double-Folded SCMRC Structure with Parallel Open-Ended Stub,” Progress In Electromagnetics Research Letters, Vol. 36, November 2012, pp. 77–86.
- S. S. Karthikeyan and R. S. Kshetrimayum, “Compact and Wide Stopband Lowpass Filter Using Open Complementary Split Ring Resonator and Defected Ground Structure,” Radioengineering, Vol. 24, No. 3, September 2015, pp. 708–711.
- A. Boutejdar, A. Omar and E. Burte, “High-Performance Wide Stop Band Lowpass Filter Using a Vertically Coupled DGS-DMS-Resonators and Interdigital Capacitor,” Microwave and Optical Technology Letters, Vol. 56, No. 1, January 2014, pp. 87–91.
- M. Xiao, G. Sun and X. Li, “A Lowpass Filter with Compact Size and Sharp Roll-Off,” IEEE Microwave and Wireless Components Letters, Vol. 25, No. 12, December 2015, pp. 790–792.
- Y. Zhang, L. Jin and L. Li, “Design of LPF Using Hi-Lo Interdigital DGS Slot,” Institute of Electronics, Information and Communication Engineers Electronics Express, Vol. 13, No. 9, March 2016, pp. 1–6.
- F. C. Chen, H. T. Hu, J. M. Qiu and Q. X. Chu, “High-Selectivity Lowpass Filters with Ultrawide Stopband Based on Defected Ground Structures,” IEEE Transactions on Components, Packaging and Manufacturing Technology, Vol. 5, No. 9, September 2015, pp. 1313–1319.
- A. Boutejdar, “Design of Broad-Stop Band Low Pass Filter Using a Novel QUASI-YAGI-DGS-Resonators and Metal Box-Technique,” Microwave and Optical Technology Letters, Vol. 56, No. 3, March 2014, pp. 523–528.
- S. Majidifar, “Design of High Performance Miniaturized Lowpass Filter Using New Approach of Modeling,” Applied Computational Electromagnetics Society Journal, Vol. 31, No. 1, January 2016, pp. 52–57.
- M. Hayati, M. Gholami, H. S. Vaziri and T. Zaree, “Design of Microstrip Lowpass Filter with Wide Stopband and Sharp Roll-Off Using Hexangular Shaped Resonator,” IET Electronics Letters, Vol. 51, No. 1, January 2015, pp. 69–71.
- M. Mirzaee and B. S. Virdee, “Realisation of Highly Compact Planar Lowpass Filter for UWB RFID Applications,” IET Electronics Letters, Vol. 49, No. 22, October 2013, pp. 1396–1398.
- S. Liu, J. Xu and Z. Xu, “Compact Lowpass Filter with Wide Stopband Using Stepped Impedance Hairpin Units,” IET Electronics Letters, Vol. 51, No. 1, January 2015, pp. 67–69.
- S. Roshani, “A Compact Microstrip Low-Pass Filter with Ultra Wide Stopband Using Compact Microstrip Resonant Cells,” International Journal of Microwave and Wireless Technologies, September 2016, pp. 1–5.
Amirhossein Ghaderi received his B.S. degree in electronics engineering in 2015 from Islamic Azad University (with honors), and is currently working toward his M.S. degree. His current research interests include microwave passive circuits and RF integrated circuit design.
Saeed Roshani received his B.S. degree in electrical engineering from Razi University in 2008, his M.S. degree in electrical engineering from Shahed University in 2011 and his Ph.D. in electrical engineering from Razi University in 2015. He is currently an assistant professor in the Department of Electrical Engineering at Islamic Azad University. His research interests include microwave and mmWave devices and circuits, as well as low power and small size integrated circuit design.