Читать книгу Electrical and Electronic Devices, Circuits, and Materials - Группа авторов - Страница 57

As discussed in an earlier section, fractal DGS has become popular to reduce size and to improve the return loss. After designing hairpin bandpass filter, fractal DGS geometry is added in the ground plane to reduce the size of the filter. Two fractal hexagonal shapes connected with vertical rode as shown in Figure 4.4 are etched in the ground plane. Here, 3^rd iteration hexagonal fractal shape is used for better results.

Simulated result of the hairpin bandpass filter with fractal DGS is shown in Figure 4.5. Figure 4.5 (a) shows that center frequency is shifted towards lower side to 3.16 GHz. Response shifting in the lower frequency range indicates size reduction. Insertion loss in the case of fractal DGS filter is 1.93 dB, which is more compared to filter without fractal DGS (0.41 dB). The 3-dB bandwidth of the filter is 530 MHz. Similarly, as per Figure 4.5 (b) the resonance frequency is shifted towards the left side and return loss in fractal DGS filter is 39 dB, which is improved compared to hairpin bandpass filter without fractal DGS. Figures 4.6 (a) and (b) show a comparison of S₂₁ and S₁₁ of hairpin bandpass filter and hairpin bandpass filter with fractal DGS, respectively. Comparing the result, it is observed that shifting of the center/resonant frequency to lower side and return loss improvement for a filter with fractal DGS.

Figure 4.3 (a) S₂₁ and (b) S₁₁ of hairpin bandpass filter.

Fabricated hairpin bandpass filter with fractal DGS is tested with Anritsu MS2307C Vector Network Analyzer (VNA). Before testing, SOLT calibration was performed for the VNA. Figure 4.7 shows hairpin filter with Fractal DGS under test. The measurement results are shown for S₂₁ and S₁₁ of hairpin filter with the fractal DGS in Figure 4.8 (a) and Figure 4.8 (b).

Figure 4.4 Fractal DGS (Back/GND) portion of hairpin bandpass filter.

Figure 4.5 Simulated return loss characteristics (a) S₂₁ and (b) S₁₁ of haripin bandpass filter with fractal DGS.

Figure 4.6 Comparison of simulated response (a)S₂₁ (b) S₁₁ of bandpass filter with fractal DGS and without fractal DGS.

It can be seen from Figure 4.8 (a) S₂₁ and (b) S₁₁ that simulated and measured result matches with each other that verifies our design. Minor mismatch is observed in S₂₁ and S₁₁ of the measured results because of fabrication error, dielectric tolerance of the substrate, soldering error, etc.

Figure 4.7 Testing/measurement of fabricated hairpin bandpass filter with fractal DGS (a) S₂₁ measurement setup with VNA (b) Enlarge view of setup.