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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.