(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 with fractal DGS (b) magnified version of Figure
Figure 4.11 (a) S11 of tunable hairpin bandpass filter with fractal DGS (b) magnified version of Figure
Table 4.2 Parametric comparison of simulated work.
| Filter | Center Frequency (GHz) | Bandwidth (MHz) | Insertion loss (dB) |
| Hairpin bandpass filter | 3.48 | 430 | 0.41 |
| Hairpin bandpass filter with fractal DGS | 3.16 | 530 | 1.93 |
| Tunable Hairpin bandpass filter with fractal DGS | 3.31-3.55 | 360 to 530 | 0.44 to 0.79 |
4.4 Conclusion
The hairpin bandpass filter offers compactness and good return loss. The proposed work of a hairpin filter with fractal DGS shifts the resonant frequency to lower frequencies, which reduces the size of the filter. Tunability along with hexagonal fractal DGS is achieved by using variable capacitance (varactor diode) inserted in fractal DGS. The simulation work shows that the proposed filter has a compact size and much less insertion loss.
Acknowledgement
The authors are thankful to ELARC Lab at Birla Vishwakarma MahaVidyalaya, V V Nagar, Gujarat, India, for providing the measurement facility.
References
1. Z. Awang (2014), Microwave systems design, vol. 9789814451246. Springer Singapore.
2. Jia-Sheng Hong (2011), Microstrip Filters for RF / Microwave Applications, 2nd Edition, Vol. 7.
3. I. A. Glover, S. R. Pennock, and P. R. Shepherd (2006), Microwave Devices, Circuits and Subsystems for Communications Engineering. Wiley.
4. T. K. Ramya, C. J. Bindu, A. Pradeep, B. Paul, and S. Mridula (2015), “Compact tunable filters for broadband applications,” Procedia Comput. Sci., Vol. 46, no. Icict 2014, pp. 957–964.
5. T. Pavlenko (2013), Tunable lumped-element bandpass filters for Cognitive Radio application, Master Thesis, Lappeenranta University of Technology.
6. A. F. Ali (2016), Differential Filters with Tunable Devices, Doctoral Theisis, University of Colorado.
7. J. Shivhare and B. V. R. Reddy (2015), “Compact and Small Sized Single, Double and Multi-Folded Hairpin Line Microstrip Bandpass Filters for RF / Wireless Communications,” International Invention Journal of Engineering Science and Technology, Vol. 2, no. 1, pp. 10–16.
8. Roberto Gómez-García
