A compact microstrip lowpass filter with harmonic suppression exhibits ultra-wideband rejection. It is based on triangular patch resonators and a butterfly patch resonator with two 120 degree radial “wing” patches. A filter of this design with a 3 dB cutoff frequency at 1.78 GHz achieves a harmonic suppression bandwidth of 158 percent, enabling it to suppress the twelfth harmonic response. Its small size of 14.5 mm × 18 mm corresponds to 0.133 λg × 0.165 λg, where λg is the guided wavelength at 1.78 GHz.

Planar lowpass filters with compact size and high performance are frequently required in microwave communication systems to suppress harmonics and spurious signals. Conventional design methods utilize high and low impedance lines with shunt stubs and semi-lumped elements. These methods, however, yield low stopband rejection and relatively flat roll-off characteristics while large in size.1-2

Figure 2

Figure 2 Simulated lowpass filter response with only resonator 1 (a), only resonator 2 (b) and both resonators 1 and 2 (c).

Techniques to reduce size and enhance performance have been widely studied in recent years.3-7 Li et al.3 cascaded multiple stepped-impedance hairpin resonators to realize a sharp roll-off and wide stopband suppression, at the cost of large size and high passband loss. Hayati and Lotfi4 cascaded multiple, semi-circular and semi-ellipsoid patch resonators to achieve wide stopband suppression, but faced a tradeoff between size and performance. To further improve stopband performance, Ma et al.5 proposed a lowpass filter of cascaded LC resonant structures and transformed radial stubs, but this resulted in large circuit size and increased design complexity. Ma and Yeo6 replaced conventional low impedance stubs with radial stubs to realize wide stopband rejection, but roll-off performance was not ideal and stopband bandwidth could be improved. Defected ground structures and multilayer techniques have also been used,7 at the price of increased design complexity.

In this article, a microstrip lowpass filter with compact size and harmonic suppression is described. Both triangular patch and butterfly patch resonators are used in the design to achieve compact size and ultra-wideband rejection. Meander transmission lines are also employed to further reduce size. The resulting filter exhibits a harmonic suppression band from 2.83 to 21.6 GHz with better than 15 dB suppression, less than 0.3 dB passband insertion loss and a compact size of 0.133 λg × 0.165 λg, where λg is the guided wavelength at 1.78 GHz.

FILTER DESIGN

Figure 1 shows the filter layout. It consists of high and low impedance microstrip main transmission lines and two types of resonators. Resonator 1 is composed of a high impedance transmission line and a butterfly patch connected in series. Resonator 2 is a triangular patch. To illustrate the design theory, the frequency responses of the two resonators are discussed individually. Figure 2a shows the simulated frequency response of the lowpass filter with resonator 1, only. With the exception of a parasitic response in a narrow band around 7.5 GHz, the filter has wide harmonic suppression characteristics. To eliminate this parasitic passband a triangular patch resonator is introduced. As shown in Figure 2b, the simulated frequency response of the lowpass filter with resonator 2 has a wide stopband response. It has one transmission zero in the vicinity of 7.5 GHz, which can be adjusted by controlling the size of the structure. Both resonators are used to achieve wide stopband performance, shown in Figure 2c, which shows the lowpass filter response of resonators 1 and 2.

The microstrip filter (see Figure 3) was fabricated on an RT/Duroid 5880 substrate with a dielectric constant of 3.38, thickness of 0.813 mm and loss tangent of 0.0027. The dimensions (shown in Figure 1) are: l1 = 2.4 mm, w1 = 2.6 mm, w2 = 0.4 mm, w3 = 0.5 mm, w4 = 1.5 mm, w5 = 0.5 mm, l2 = 5.1 mm, l3 = 5.6 mm, l4 = 3.9 mm, r = 8.5 mm and θ = 120 degrees.

Figure 3

Figure 3 Fabricated microstrip lowpass filter.

Figure 4

Figure 4 Simulated vs. measured lowpass filter performance.

SIMULATION AND MEASUREMENT

Figure 5

Figure 5 Simulated vs. measured passband insertion loss.

The filter’s performance was measured with a Keysight N5244A vector network analyzer. The measured and simulated responses, shown in Figure 4, are in good agreement. The measured 3 dB cutoff frequency is 1.78 GHz (see Figure 5). The filter suppresses up to the twelfth harmonic, as spurious frequencies are suppressed greater than 17 dB from 2.37 to 18.20 GHz. For comparison, Table 1 summarizes the performance of this and several other previously published lowpass filter designs.

SUMMARY

A microstrip lowpass filter with a cutoff frequency of 1.78 GHz and good harmonic suppression was designed, fabricated and measured. The demonstrated filter achieved very good performance: low insertion loss in the passband and compact size. The filter design has a very wide stopband, able to suppress the twelfth harmonic. With this performance, the proposed structure has potential applications in modern communication systems.

References

Table 1

  1. D. M. Pozar, “Microwave Engineering, 3rd Edition,” Wiley, N.Y., 2005, pp. 412–415.
  2. X. B. Wei, P. Wang, M. Q. Liu and Y. Shi, “Compact Wide-Stopband Lowpass Filter Using Stepped Impedance Hairpin Resonator with Radial Stubs,” Electronics Letters, Vol. 47, No. 15, July 2011, pp. 862–863.
  3. J. Li, Z. F. Li and Q. F. Wei, “Compact and Selective Lowpass Filter with Very Wide Stopband Using Tapered Compact Microstrip Resonant Cells,” Electronics Letters, Vol. 45, No. 5, February 2009, pp. 267–268.
  4. M. Hayati and A. Lotfi, “Elliptic-Function Lowpass Filter with Sharp Cutoff Frequency Using Slit-Loaded Tapered Compact Microstrip Resonator Cell,” Electronics Letters, Vol. 46, No. 2, January 2010, pp. 143–144.
  5. M. Hayati, A. Sheikhi and A. Lotfi, “Compact Lowpass Filter with Wide Stopband Using Modified Semi-Elliptic and Semi-Circular Microstrip Patch Resonator,” Electronics Letters, Vol. 46, No. 22, October 2010, pp. 1507–1509.
  6. K. X. Ma and K. S. Yeo, “New Ultra-Wide Stopband Low-Pass Filter Using Transformed Radial Stubs,” IEEE Transactions on Microwave Theory and Techniques, Vol. 59, No. 3, April 2011, pp. 604–611.
  7. A. Bouteidar, A. Batmanov, A. Omar and E. Burte, “Design of Compact Low-Pass Filter Using Cascaded Arrowhead-DGS and Multilayer Technique,” Asia Pacific Microwave Conference, December 2008.
  8. L. Ge, J. P. Wang and Y. X. Guo, “Compact Microstrip Lowpass Filter with Ultra-Wide Stopband,” Electronics Letters, Vol. 46 , No. 10, June 2010, pp. 689–691.