How to Enhance the Performance of a Basic Low-Pass Filter

low pass filter

Low-pass filters are an important component of microwave and wireless devices and systems. However, they suffer limited harmonic suppression in the stopband. One remedy traditionally used by microwave engineers is to add attenuation poles to extend the stopband bandwidth. Unfortunately, this workaround usually increases the overall size of the filter, which is undoubtedly undesirable and hinders the growing need for miniaturization. In recent years, a new technique for limiting stopband harmonic suppression has become popular among microwave engineers. This technology consists of etched grooves or defects in the ground plane of a printed microstrip board, called a defective ground structure (DGS). You can actually think of DGS as a simplified form of the electromagnetic band gap (EBG) structure.

DGS solution

Many authors have reported significant improvements in the frequency response of various low-pass filters through the use of DGS. We use the popular EMWorks high-frequency simulator HFWorks to test microstrip low-pass filters and study the efficiency of DGS in improving the frequency response of microstrip low-pass filters.

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 surfaces of a low-pass filter

The frequency response of a basic low-pass filter is shown in Figure 2. The cutoff frequency of the filter is approximately 2.03GHz, and the frequency at the 20dB suppression level is 2.54GHz.

The filter has a stopband bandwidth of 2.54GHz to 9.57GHz and a rejection level of 20 dB.

In this frequency range, S11 is close to 0dB, which means that the signal is completely reflected.

Figure 2: Frequency response of microstrip low-pass filter

The main disadvantage of this filter is the presence of out-of-band spurious frequencies. In order to improve the performance of the basic low-pass filter without affecting the size, a defective ground structure is used. DGS helps achieve wider and deeper stopband bandwidth. It consists of two symmetrical H-shaped slots etched into the filter's ground plane.

Figure 3 shows the top and bottom views of the modified low-pass filter.

Figure 3: Top and bottom surfaces of filter with H-shaped DGS

The addition of DGS resulted in good selectivity and wide stopband bandwidth. Figure 4 shows the frequency response of the filter after adding DGS.

Figure 4: Frequency response of microstrip low-pass filter with DGS

By using DGS, the passband and stopband characteristics of the basic microstrip low-pass filter are improved. A wide stopband bandwidth of up to 16.5GHz is obtained at 15dB rejection level. The simulated 3 dB cutoff frequency of the filter with H-shaped DGS is reduced to 1.98 GHz.

Summary and takeaways

The performance characteristics of a basic low-pass filter can be enhanced by using an H-shaped DGS resonator without affecting the filter size. DGS resonators behave as resonant elements at high frequencies, allowing for wider and deeper stopband bandwidths. Therefore, tools like HFWorks are essential for designing and improving the frequency response of low-pass filters.