design an electronically tunable filter, methods like microelectromechanical systems, semiconductor diodes (P-I-N diode and varactor diodes), ferroelectric films (Barium Strontium Titanate (BST) thin films) and RF Microelectromechanical Systems (MEMS) (MEMS tunable capacitor banks) are incorporated within a passive filtering structure. With integration of MEMS with planar filter, a size reduction can be possible. Hence, Microstrip tunable/reconfigurable filter is of larger interest [2, 4].
In tunable filter, bandwidth tuning is more difficult than frequency tuning. Also it is found that design of wideband tunable filter is more challenging than narrowband tunable filter for center frequency tuning range and bandwidth tuning. Nonlinearity observed in the performance of a tunable filter quite depends on the tuning element used. A piezoelectric transducer and RF MEMS switch used in tunable filter gives better linearity. Higher-order and narrowband tunable filters implementation can be restricted due to the lower value of the Quality (Q) factor of tuning element. Lower value of Quality (Q) factor of the tuning elements and other losses of the the filter can increase the insertion loss of the filter as the order of the filter increases and decreases bandwidth. Hence, all these factors contribute to increase insertion loss of a high-order and narrowband tunable filter. The tuning range is also one of the limitations for a higher-order filter. The design and implementation of tunable filters imply trade-offs like filter size, insertion loss and the complexity of the circuit [2].
Figure 4.1 Tunable filter replacing filter bank [5] (a) Receiver system with multiple filters (b) Receiver system with tunable filter replacing multiple filters.
Tunable microwave filters can have continuous tuning, discrete tuning or a combination of both. MEMS switches or PIN diodes are used to get discrete tuning in tunable filter. Varactor diodes, ferromagnetic materials and ferroelectric materials are utilized for continuous tuning of the filter. To get discrete and continuous tuning in the filter, designers combine the elements of discrete and continuous tuning as well. Semiconductor-based tuning elements are used for frequencies below 10 GHz [6].
The latest wireless receiver systems have constrained novel challenges for the design of tunable RF/microwave filters. Tunable filter imposes better optimization in filter parameters like insertion loss, return loss, selectivity, stopband attenuation, a percentage of bandwidth/ center frequency tuning, size and cost. Printed circuit technology makes it possible to reduce the size of the microstrip filter significantly and with this, it also reduces the cost of the fabrication. Microstrip circuits are made up of conducting material strip on dielectric substrate and a copper ground plane on the other side of the dielectric material. The proposed design used Hairpin microstrip design, which produces narrower bandwidth. Hairpin design gives better return loss, compact size and low cost [7]. Compactness is a demanding feature in the latest filter design. Defected Ground Structure (DGS) with fractal geometry offers a good size reduction. Proposed design also uses fractal DGS to get the advantage of size reduction. Varactor diodes are used along with fractal DGS to achieve centre frequency tuning.
The chapter is organized as follows: Section 4.1 is an introduction to tunable filter. Section 4.2 describes the literature review in this area. Section 4.3 discusses our designed filter with fractal DGS for size reduction and tunable filter with the use of varactor diodes. Section 4.4 is the conclusion.
4.2 Literature Review
Research in the microwave tunable filter can be classified into two major categories. First, to reduce effects on the performance parameter of the tunable filter like insertion loss, selectivity and linearity deterioration. Second, to attain better levels of tuning in terms of centre frequency tuning, bandwidth tuning and filtering type (e.g., switching between bandpass/ bandstop responses) [8].
As per reported literature, a tunable microwave filter can have mechanical tunning, magnetic tunning and electronic tunning. Mechanically tunable filters are designed using the coaxial cavity or waveguide resonators. Mechanical tuning is achieved by moving a tuning screw or plate to change the resonant frequency of a tunable filter. They can handle large power and they have low insertion loss. Mechanical tuned filters are large and bulky, and having slow tuning makes them unsuitable for current communication systems [9–11]. Magnetically tunable filters are popular in microwave communication systems due to the high quality factor, wide tunning range and low insertion loss. They use single crystal Yttrium-Iron-Garnet (YIG) spheres in the resonators and it can be tuned by altering the biasing current. Magnetically tunable filters offer low insertion loss but they are larger in size, consume more power and offer slow tuning speed [12, 13].
Electronically tunable filters incorporate variable capacitors, semiconductor diodes or RF MEMS switches, which are biased with DC voltage. The applied DC voltage changes capacitance or inductance loading in a resonator, which tunes the centre frequency of the filter. Electrically filters are more popular due to their better tuning range, miniaturization, speed of tuning and suitability to integrate with latest communication hardware [14–16]. The proposed work is based on planar structure, hence, the literature review related to it is presented.
4.2.1 Planar Reconfigurable Filters
Planar structure provides a simple structure and allows flexible filter design, and it is easy to integrate tuning with it. Cho, Y. H. et al. [17] proposed reconfigurable filter, which changes from bandpass to bandstop by changing the coupling coefficient of resonators controlled by RF MEMS switches and varactors. Bouyge D. A. et al. [18] proposed size reduction using split ring resonators (SRR) and frequency agility using vanadium dioxide switches. Different combination of switches adds a pole to the bandstop filter to change configurations of bandwidth and centre frequency. Bandwidth or center frequency switching is also possible with P-I-N diodes. Miller A. et al. [19] designed tunable bandpass filters with switchable bandwidths using short circuit coupled lines and short circuit stubs with P-I-N diodes. When the P-I-N diodes are in the ON mode, it achieves around 35% fractional bandwidth (FBW), while in OFF mode FBW is reduced to 16%, at a centre frequency of 1.9 GHz.
High-temperature superconducting (HTS) filters are popular due to their low insertion loss and they can be easily realized with higher orders. A HTS-based tunable bandpass filter with dual-mode resonator and Gallium arsenide (GaAs) varactors was proposed in [20]. The said work is designed for L-band tuning range from 0.784 GHz to 0.918 GHz (16%) while having small variation in fractional bandwidth. Hu Jiang et al. [21] proposed quasi-elliptic coplanar waveguide (CPW) tunable bandpass filter using BST varctors in Ka band. The filter exhibits area reduced by 35% with constant fraction bandwidth (FBW).
Filter size miniaturization is a demand in modern communication systems. Surface Integrated Waveguide (SIW) structure has low losses, higher Q value and is easy to integrate with other planar technologies. Bin You et al. [22] proposed tunable filter based on Half-mode SIW (HMSIW). HMSIW has the advantages of SIW along with size reduction to 50%. Bin You et al. [22] utilized reduced space to load JDV2S71E varactor (manufactured by TOSHIBA) as a tuning element. Designed HMSIW filter provides both center frequency and bandwidth tuning capabilities.
Filter size can be reduced in different ways in microwave circuit design. One of them is by using metamaterial. A metamaterial-based tunable is proposed in [23] for size reduction and ease in integration for
