The design of conventional stepped-impedance microstrip line low pass filter (LPF) is based on high (ZH) to low impedance (ZL) ratio. The width of ZH line, for ZH > 100 Ω, becomes critical and challenging, especially on high dielectric constant substrates. A concept of air-filled recessed ground plane below a microstrip line is introduced in this paper. The effect of dimensions of recessed ground on characteristic impedance, attenuation and propagation constant of a microstrip line are first studied. This simple approach is utilized to design the ZH line of stepped-impedance microstrip line LPFs. Prototypes of recessed ground stepped-impedance microstrip line LPFs with ZH/ZL (keeping ZL constant as 20 Ω)ratio in the range 6 to 10 are designed and developed on Rogers 4350B of height 0.508 mm with εr = 3.66 at 3 GHz. For LPF with ZH/ZL = 10, the measured 3-dB cutoff frequency (fc) is achieved at 3.12 GHz with return loss (RL) > 12 dB and insertion loss (IL) < 0.28 dB in its passband whereas the stopband attenuation (SBA) is better than 38 dB. In comparison to recessed ground LPF, the simulated results of conventional LPF with ZH/ZL = 10 (critical width of ZH line =) are as follows RL > 10 dB and IL < 1.07 dB in passband at fc = 3 GHz. The size of recessed ground LPF is reduced by 25%, when ZH/ZL is increased to 10 from 6. The approach of recessed ground microstrip line avoids the fabrication issues, reduces size, and improves the performance of LPF, which in turns confirms the advantages of recessed ground over conventional microstrip line.
2. Zhang, J., B. Cui, S. Lin, and X.-W. Sun, "Sharp-rejection low-pass filter with controllable transmission zero using Complementary Split Ring Resonators (CSRRs)," Progress In Electromagnetics Research, Vol. 69, 219-226, 2007.
3. Chen, Z.-Y., L. Li, and S.-S. Chen, "A novel dumbbell-shaped defected ground structure with embedded capacitor and its application in low-pass filter design ," Progress In Electromagnetics Research Letters, Vol. 53, 121-126, 2015.
4. Chen, J., Z.-B. Weng, Y.-C. Jiao, and F.-S. Zhang, "Low pass filter design of Hilbert curve ring defected ground structure," Progress In Electromagnetics Research, Vol. 70, 269-280, 2007.
5. Kumar, A., et al., "Design of nine pole microstrip low pass filter with metal loaded defected ground structure ," 2016 IEEE MTT-S Latin America Microwave Conference (LAMC), 1-3, Puerto Vallarta, 2016.
6. Lu, K., G.-M. Wang, Y.-W. Wang, and X. Yin, "An improved design of Hi-LO microstrip lowpass filter using uniplanar double spiral resonant cells," Progress In Electromagnetics Research Letters, Vol. 23, 89-98, 2011.
7. Wang, C. J. and T. H. Lin, "A multi-band meandered slotted-groundplane resonator and its application of low-pass filter," Progress In Electromagnetics Research, Vol. 120, 249-262, 2011.
8. Jiang, M. and W. Hong, "An approach for improving the transition-band characteristic of a stepped-impedance low-pass filter," 2012 International Conference on Microwave and Millimeter Wave Technology (ICMMT), 1-4, Shenzhen, 2012.
9. Drayton, R. F., S. Pacheco, J. G. Yook, and L. P. B. Katechi, "Micromachined filters on synthesized substrates," 1998 IEEE MTT-S International Microwave Symposium Digest (Cat. No. 98CH36192), Vol. 3, 1185-1188, Baltimore, MD, USA, 1998.
10. Sharma, P., S. K. Koul, and S. Chandra, "Design and development of microstrip low pass filters at K-band using MEMS technology," 2007 Asia-Pacific Microwave Conference, 1-4, Bangkok, 2007.
11. Eudes, T., B. Ravelo, and A. Louis, "Transient response characterization of the high-speed interconnection RLCG-model for the signal integrity analysis," Progress In Electromagnetics Research, Vol. 112, 183-197, 2011.
12. Ravelo, B., "Modelling of asymmetrical interconnect T-tree laminated on flexible substrate," Eur. Phys. J. Appl. Phys. (EPJAP), Vol. 72, No. 2, 1-9, id 20103, Nov. 2015.
13. Eudes, T. and B. Ravelo, "Analysis of multi-gigabits signal integrity through clock H-tree," International Journal of Circuit Theory and Applications (Int. J. Circ. Theor. Appl.), Vol. 41, No. 5, 535-549, May 2013.
14., , http://ntuemc.tw/upload/file/20110321102525847f2.pdf.