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The filter is a very significant device in Radio Frequency Integrated Circuit (RFIC) design engineering, which passes certain frequencies and rejects certain frequencies from the applied input signal. It is used to separate channels in multiplexing communication systems, to remove certain harmonics in oscillator or amplifier circuits. It is also used to remove unnecessary disturbance termed as noise. “Filters are two-port devices designed in such a way so that a group of specified frequencies is allowed to pass with little attenuation, while unwanted frequencies are rejected” [1]. The latest wireless communications systems such as cognitinitive radio and mobile communication systems keep challenges on RF/ microwave filters with strict requirements like better performance, lower losses, smaller size, faster response, lighter weight, and lower cost [2]. The filters can be classified by their frequency responses. It is classified as stop band, high pass, low pass, band pass filters. In a low pass filter, signal frquencies below the cut-off frequency (fC) are allowed to pass without attenuation. Above the cut-off frequency (fC) all other frequencies are blocked or rejected. In the same way for high pass filter, signal frquencies above the cut-off frequency (fC) are allowed to pass without attenuation. For a band pass filter, a certain band of frequency ranging from a lower cut-off frequency (fL) to higher cut-off frequency (fH) is allowed to pass without attenuation. The range of frequencies from fL to fH is called the bandwidth of the filter. For a band stop filter, it rejects/blocks the band of frequencies fL to fH [1]. Filters characterstic response can be classified as Butterworth, Chebyshev, Bessel and Elliptic. Characteristic response is chosen according to application. Butterworth exhibit maximally flat behavious in the passband, but the trasition from passband to stopband is not steep. The Elliptic response gives abrupt transition from passband to stopband, but the passband and stopband contains ripples. Bessel response has poor amplitude response, but it gives linear phase behaviour. The Chebyshev response has an equi-ripple response in the passband [3].

Microwave planar filters can be made up of microstrip lines, waveguide or coaxial type. The planar microstrip filters offer superior performances like waveguide filters. However, microstrip filters are popular because of planar structure, which offers ease in volume production using circuit printing technologies [1].

Present communication systems demand an operation in multiple operating bands to meet the modern trends, which is not possible by a single filter prototype. Tunable/ reconfigurable filters can fulfil this requirement by using adjustable tuning elements used with filter topology to avoid the switching between different filters. Front end receivers use tunable filters where it has multiband operation like Cognitive Radio, radar applications and satellite communication systems [4]. Tunable/reconfigurable filters reduce complexity of the transceiver system in wireless communication systems. Tunable bandpass filter can be used to eliminate the filter bank and switching network as shown in Figure 4.1.

To 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.