hairpin bandpass filter.
4.3.2 Design of Hairpin Bandpass Filter with Fractal DGS
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, 3rd 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.
4.3.3 Design of Tunable Hairpin Bandpass Filter with Fractal DGS
To make a filter tunable, varactor diode or PIN diode has to be incorporated in the design. Here, two varactor diodes are inserted in outer hexagonal of fractal DGS as shown in Figure 4.9. For simulation purpose in CST MICROWAVE STUDIO® V. 2018, R-L-C components are chosen. To consider a perfect capacitor, R = 0 Ω, L = 0 H and desired values of C is selected using parametric sweep.
By using parametric sweep in simulation, various values of C (treated as varactor diode) were applied and simulation results are shown in Figures 4.10 and 4.11. As it is observed from the response, the center frequency of the band can be varied from 3.3 GHz to 3.58 GHz by changing values of C from 20 pf to 1.5 pf. As center frequency varies, variation in bandwidth is also observed from 360 MHz to 530 MHz. Tuning of center frequency is not much above 8 pf of capacitance value. For better visibility, magnified view of S21 is shown in Figure 4.10(b) and magnified view of S11 is shown in Figure 4.11(b). Also it is observed that insertion loss is minimized during the tuning range; it varies from 0.44 to 0.79 dB. Proposed filter is low insertion loss and a very compact filter. In S11 response, return loss stays around 20 dB to 25 dB, which is expected for any filter.
Figure 4.8 Comparison of simulated and measured result (a) S21 and (b) S11 of hairpin bandpass filter with fractal DGS.
Table 4.2 shows comparison of center frequency, bandwidth and insertion loss. As per simulation work, it is concluded that fractal DGS helps to make filter design compact.
Figure 4.9 Hairpin bandpass filter with fractal DGS with varactor diodes.
Figure 4.10 (a) S21 of tunable hairpin bandpass filter