A novel microstrip lowpass filter (LPF) composed of high impedance transmission lines loaded by radial split-ring resonators and radial patch resonators is compact in size and exhibits a wide stopband, sharp roll-off and low insertion loss. The demonstrated filter has a ‐3 dB cutoff frequency of fc = 1.77 GHz with a roll-off rate of 121.4 dB/GHz. Its stopband is from 1.91 to 16 GHz with attenuation greater than 20 dB.

Figure 1

Figure 1 Resonator 1 layout (a) and simulated frequency response (b).

Microstrip LPFs have found wide application in microwave communication systems for suppression of spurious signals and unwanted high frequency harmonics. Various structures have been designed in recent years to achieve good characteristics, such as compact size, sharp roll-off, wide stopband and low insertion loss. Hayati and Moghadam1 used modified circular resonant patches and folded lines for obtaining a wide stopband, but their structure is large and the cutoff is gradual. Velidi and Sanyal2 report on a microstrip LPF with a wide stopband and high roll-off rate, but it is also large. Another wide stopband LPF uses triangular patch resonators, radial patch resonators and meander transmission lines,3 but its roll-off is not sharp and has low stopband attenuation. Karimi et al.4 describe a microstrip LPF with a wide stopband and sharp cutoff frequency, however, it is large and has high loss in the passband. A microstrip LPF with a radial resonator, triangular patch and open stub, introduced by Hayati et al.,5 has a high roll-off rate, but the stopband is narrow. A compact microstrip lowpass filter with a quasi elliptic response, by Wang et al.,6 exhibits a wide stopband, but its roll-off is not sharp and has low suppression. A LPF based on a resonator with slow-wave effects7 has a wide stopband and high roll-off rate but is large. A microstrip LPF using a tapered microstrip resonator cell8 has low insertion loss, but its roll-off is not sharp and it is also large. A microstrip LPF based on stepped-impedance resonators using semicircular structures to reduce its size9 is still relatively large, with a narrow stopband. The use of a defected ground structure (DGS)10,11 improves stopband characteristics but increases the total filter size and creates radiation losses.

Figure 2

Figure 2 Impact of resonator 1 L2 (a) and t2 (b) on |S21|.

In this work, a novel microstrip LPF with compact size using high impedance transmission lines loaded by radial split-ring resonators and radial patch resonators provides a wide stopband, sharp roll-off and low in-band insertion loss.

Figure 3

Figure 3 Resonator 2 layout (a) and simulated frequency response (b).

LPF DESIGN

The filter comprises three types of resonators. Figure 1a shows the layout of resonator 1, which consists of high impedance transmission lines loaded by radial split-ring resonators. Its simulated frequency response is shown in Figure 1b. Resonator 1 generates one transmission zero at 2.25 GHz with 43 dB of attenuation, which provides for a relatively narrow stopband with low attenuation. Optimal dimensions are L1 = 10.5 mm, L2 = 0.08 mm, L3 = 0.74 mm, W1 = 5.96 mm, W2 = 1.6  mm, r1 = 6 mm, r2= 6.7 mm, r3 = 7.4 mm, t1 = 88 degrees and t2 = 120 degrees. The transmission zero can be controlled with L2 and t2. By increasing the value of L2 from 0.08 to 0.32 mm, the transmission zero is shifted to the right, as shown in Figure 2a. By increasing t2 from 60 degrees to 120 degrees, the transmission zero is shifted to the left (see Figure 2b).

Figure 4

Figure 4 Layout of combined resonators 1 and 2 (a) and simulated frequency response (b).

Figure 5

Figure 5 Resonator 3 layout.

Resonator 2 along with its simulated frequency response is shown in Figure 3. Its dimensions are L4 = 0.68 mm, r4 = 5.6 mm, r5 = 5.8 mm and t3 = 45 degrees. As shown in Figure 3b, resonator 2 generates two transmission zeros located at 3 and 4.8 GHz, with attenuation levels of 60 and 59 dB, respectively. The cutoff is not sharp and the stopband is narrow. Figure 4 shows the combination of resonators 1 and 2 and the resulting frequency response. This structure has good attenuation in the stopband and a sharp transition; however, the stopband bandwidth is still relatively narrow. To suppress unwanted harmonics at high frequencies, a suppression cell is needed.

Resonator 3 (see Figure 5) acts as the suppression cell to enhance the stopband bandwidth at high frequencies. Its dimensions are W3 = 0.792 mm, W4 = 0.08 mm, W5 = 0.3 mm, r6 = 5 mm and t4 = 45 degrees. By adding resonator 3, the final filter structure is formed (see Figure 6a). Figure 6b shows the simulated frequency response. The ‐3 dB cutoff frequency is around 1.77 GHz. The stopband region extends from 1.91 to 16 GHz with attenuation greater than 20 dB across the band. Maximum insertion loss and minimum return loss in the passband are 0.16 and 14.35 dB, respectively.

Figure 6

Figure 6 Final filter layout (a) and simulated frequency response (b).

SIMULATED VS. MEASURED

The filter was fabricated on an RT/Duroid 5880 substrate with dielectric constant of 2.2, thickness of 20 mils and loss tangent of 0.0009. The circuit was simulated using Keysight’s Advanced Design System (ADS) and measured with an HP8757A network analyzer. Figure 7a shows the final filter and Figure 7b compares measurements with simulation, showing close agreement.

Figure 7

Figure 7 Fabricated LPF (a) and simulated vs. measured performance (b).

Table 1 compares the performance of the proposed filter with other reported LPFs, using the following definitions:

Table 1

The roll-off rate is

Math 1

where αmax is the 20 dB attenuation point, αmin the 3 dB attenuation point, fs the ‐20 dB stopband frequency and fc the ‐3 dB cutoff frequency.

The relative stopband bandwidth (RSB) is

Math 2

The suppression factor (SF) is based on the stopband suppression. A higher degree of suppression equates to a greater SF. If, for example, the stopband bandwidth requirement calls for 20 dB attenuation, then the SF is 2.

The normalized circuit size (NCS) is

Equation 3

where λg is the guided wavelength at the ‐3 dB cutoff frequency.

The architecture factor is known as the circuit complexity factor, which is 1 or 2, for a 2D or 3D structure, respectively.

Finally, the figure-of-merit (FOM) is the overall index

Equation 4

Table 1 shows the proposed filter exhibits a much higher FOM than the other reported designs.

CONCLUSION

A novel microstrip LPF consists of high impedance transmission lines loaded by radial split-ring resonators and radial patch resonators. The filter has a 1.77 GHz cutoff frequency with desirable features such as compact size, wide stopband, sharp roll-off and low insertion loss, resulting in a high FOM compared to other LPFs.

References

  1. M. Hayati and P. Karami Moghadam, “Compact Microstrip Lowpass Filter with Ultra-Wide Stopband Using a Modified Circular Resonator,” Microwave Journal, Vol. 58, No. 4, April 2015, pp. 152–158.
  2. V. K. Velidi and S. Sanyal, “Sharp Roll-Off Lowpass Filter with Wide Stopband Using Stub-Loaded Coupled-Line Hairpin Unit,” IEEE Microwave and Wireless Components Letters, Vol. 21, No. 6, June 2011, pp. 301–303.
  3. J. Wang, H. Cui and G. Zhang, “Design of Compact Microstrip Lowpass Filter with Ultra-Wide Stopband,” Electronics Letters, Vol. 48, No. 14, July 2012, pp. 854–856.
  4. G. Karimi, A. Lalbakhsh and H. Siahkamari, “Design of Sharp Roll-Off Lowpass Filter with Ultra Wide Stopband,” IEEE Microwave and Wireless Components Letters, Vol. 23, No. 6, June 2013, pp. 303–305.
  5. M. Hayati, S. Naderi and F. Jafari, “Compact Microstrip Lowpass Filter with Sharp Roll-Off Using Radial Resonator,” Electronics Letters, Vol. 50, No. 10, May 2014, pp. 761–762.
  6. J. Wang, L. J. Xu, S. Zhao, Y. X. Guo and W. Wu, “Compact Quasi-Elliptic Microstrip Lowpass Filter with Wide Stopband,” Electronics Letters, Vol. 46, No. 20, September 2010, pp. 1384–1385.
  7. J. L. Li, S. W. Qu and Q. Xue, “Compact Microstrip Lowpass Filter with Sharp Roll-Off and Wide Stop-Band,” Electronics Letters, Vol. 45, No. 2, January 2009, pp. 110–111.
  8. Y. Yousefzadeh and M. Hayati, “Compact Lowpass Filter with Wide Stopband Using a Tapered Microstrip Resonator Cell,” Microwave Journal, Vol. 55, No. 3, March 2012, pp. 122–128.
  9. L. Wang, H. C. Yang and Y. Li, “Design of Compact Microstrip Low-Pass Filter with Ultra-Wide Stopband Using SIRs,” Progress In Electromagnetics Research Letters, Vol. 18, January 2010, pp. 179–186.
  10. Y. Yang, Y. He and L. Sun, “Design of a Novel Compact Lowpass Filter with Defected Ground Structure and Open Stubs,” IEEE International Symposium on Radio-Frequency Integration Technology, August 2014, pp. 1–3.
  11. S. H. Fu, C. M. Tong, X. M. Li and K. Shen, “A Compact Ultra‐Wide Stopband, Low Insertion Loss, and Sharp Cutoff Low-Pass Filter,” Microwave and Optical Technology Letters, Vol. 52, No. 3, March 2010, pp. 568–570.

Shiva Khani received her B.Sc. degree in electrical engineering from Razi University, Kermanshah, Iran, in 2011 and her M.Sc. degree in electrical engineering from Kermanshah Science and Research Branch, Islamic Azad University, Kermanshah, Iran, in 2014. Her research interests include the analysis and design of microstrip filters and antennas.

Mohsen Hayati received his B.E. in electronics and communication engineering from Nagarjuna University, India in 1985 and his M.E. and Ph.D. degrees in electronics engineering from Delhi University, Delhi, India, in 1987 and 1992, respectively. He is currently a professor in the Electrical Engineering Department, Kermanshah Branch, Islamic Azad University, Kermanshah, Iran. His research interests include microwave and mmWave devices and circuits.