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* Corresponding author: [email protected]
4 Tunable RF/Microwave Filter with Fractal DGS
Mehul Thakkar1,2*, Pravin R. Prajapati1,2 and Hitesh Shah2,3
1A.D.Patel Institite of Technology, New V V Nagar, Gujarat, India
2Gujarat Technological University, Gujarat, India
3G H Patel College of Engineeing and Technology, V V Nagar, Gujarat, India
Abstract
Due to the latest developments in multi-standard software defined radios and adaptive spectrum surveillance systems, over several years, the design of efficient high-frequency filters has become a popular and established research area in the field of application of electromagnetics and its component design. Without a doubt, the implementation of reconfigurable/tunable RF/microwave filtering components has been a trending area for recent research efforts in this field. Tunable/Reconfigurable filter design has been an active research topic due to challenges in tunable filters such as size reduction, selectivity, stop band reduction, insertion loss, center frequency tunability, bandwidth tuning, etc. Researchers are designing a filter to have the optimum parameter characteristics listed above.
The proposed work presents tunable microstrip filter with fractal DGS. Fractal DGS is used to reduce the size of the filter. The proposed filter is hairpin bandpass filter shape, which is designed for 3.5 GHz center frequency on the substrate of Rogers 3010 (εr = 10.2) with thickness of 1.27 mm. The hexagonal shape 3rd iteration fractal DGS is introduced in ground plane, which results in the shifting of resonant frequency on lower frequency and improvement in return loss at the resonant frequency. The simulated and fabricated results with fractal DGS are presented and discussed. Tunability can be achieved using varactor diodes in fractal DGS structure. In the proposed work, tunability is simulated by using variable capacitors. The simulation and optimization of the proposed work are done using CST MICROWAVE STUDIO® V. 2018.
Keywords: Microstrip filter, Tunable filter, Tunble filter with fractal DGS
4.1 Introduction
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
