LOWPASS FILTERS
Lowpass filters are essential ingredients of microwave and wireless devices and systems. However, they suffer from a limited harmonic rejection in the stopband. A remedy which is traditionally used by microwave engineers is to add attenuation poles to extend the stopband bandwidth. Unfortunately, such workaround usually increases the overall size of the filter which is, definitely, undesirable and hinders the ever-increasing demand for miniaturization. In recent years, a new technique to limit the harmonic rejection in the stopband, has become popular among microwave engineers. This technique, which consists of etching-out slots or defects on the ground plane of a printed microstrip board, is called a Defected Ground Structure (DGS). You can actually think of a DGS as a simplified form of Electromagnetic Band Gap (EBG) structure.
THE DGS SOLUTION
Numerous authors have reported a significant improvement in the frequency response of various lowpass filters by using DGS. We used the popular EMWorks’ high frequency simulator, HFWorks, to test a microstrip lowpass filter and study the efficiency of DGS in improving the frequency response of a microstrip lowpass filter.
The filter structure is shown in Figure 1. It consists of three stepped impedance resonators and two uniformly shaped low impedance stubs.
Figure 1: Top and bottom faces of the lowpass filter
The frequency response of the basic lowpass filter is shown in Figure 2. The cut-off frequency of the filter is about 2.03GHz, and the frequency at 20 dB suppression level is 2.54GHz.
This filter has a stopband bandwidth from 2.54GHz to 9.57GHz at 20 dB suppression level.
In this range of frequencies, S11 is close to 0dB which means that the signal is fully reflected.
Figure 2: Frequency response of the microstrip low pass filter
The filter’s major drawback is the existence of out of band spurious frequencies. To improve the performance of the basic lowpass filter without compromising the size, defected ground structures are used. DGS helps realize a wider and deeper stopband bandwidth. It consists of two symmetrical H-shaped slots etched on the ground plane of the filter.
Figure 3 shows the top and bottom views of the modified lowpass filter.
Figure 3: Top and bottom faces of filter with H-Shaped DGS
A good selectivity and a wide stopband bandwidth are obtained after adding the DGS. Figure 4 shows the frequency responses of the filter after the addition of the DGS.
Figure 4: Frequency response of the microstrip low pass filter with the DGS
The passband and stopband characteristics of the basic microstrip lowpass filter have improved by using the DGSs. A wide stopband bandwidth up to 16.5GHz at 15dB suppression level is obtained. The simulated 3 dB cutoff frequency of the filter with H-shaped DGSs is reduced to 1.98 GHz.
SUMMARY AND TAKEAWAYS
The integration of Defected Ground Structures (DGS) into the design of lowpass filters presents a significant advancement in addressing the challenge of limited harmonic rejection within a compact form factor. By employing DGS, engineers can achieve wider and deeper stopband bandwidths, effectively improving filter performance without succumbing to the drawbacks of increased size. This technique, validated through HFWorks simulations, underscores the importance of innovative design approaches in meeting the demands for miniaturization in microwave and wireless devices.
