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A compact conductorbacked CPWbased dual bandpass filter for satellite Sband and Cband
Journal of Electrical Systems and Information Technology volume 7, Article number: 5 (2020)
Abstract
An efficient and compact dual bandpass filter having single narrow rejection band between Sband and Cband using CPW arrangement is proposed. The half wavelength square resonator is used in ground surface to eliminate certain frequency bands. The transmission zero of rejection band is restricted by modifying the dimension of the resonator. The CPWbased DBBPF is simulated and tested with its equivalent prototype. This work emphasizes DBBPF with square resonator coupling constructed on CPW process. An innovative integration of circular resonator incorporated with interdigital line coupling in the top surface of the substrate and square resonator at the bottom surface of the substrate produces wide band and narrow stop band, respectively. The DBBPF has remarkable bandwidth from 2 to 3.8 GHz and 4.3 to 7.8 GHz as Sband and Cband, respectively. Hence, the overall bandwidth of the filter is 5.3 GHz (1.8 GHz and 3.5 GHz). The filter performances are evaluated and analyzed through the factors like Sparameters, Qfactor and group delay. The overall dimension of DBBPF is achieved to be (39 × 4.5 × 1.6) mm^{3}, which is very much compact in dimension. The filter characteristics show the return loss of − 35.2 dB and insertion loss of − 1.2 dB which are validated by measured results.
Introduction
RF filter circuits are generally in use in microwave system for the mitigation of spurious frequencies from other services. Bandpass filters are responsible for suppressing such spurious frequencies and higherorder harmonics in pass band [1]. The performances of filters are studied from the parameters like miniaturized geometry, intended pass band and attenuation band. Dualband filters are most inevitable element in designing satellite communication model. Sband and Cband range is predominantly allocated for mobile services in satellite communication. So, BSFs are designed by metallic pattern at the backside of coplanar waveguide (CPW) structure [2].
CPW planar method is the most appropriate for designing satellite band filters because of easy integration with active and lumped elements. Metallic patternbacked DBBPF with coupling structure has been developed and demonstrated in Xiao et al. [3]. Filter design deals with the optimization of stub length and slot width to generate discontinuity and thereby the filter achieves Cband frequency from (4 to 8) GHz [4]. Sharp selectivity with low loss is achieved by various lowpass and highpass stub employed between resonators where inductive and capacitive components exist due to strips on both sides. The size of the filter is reduced to a greater extent due to increased inductance and capacitance, named as slowwave effect [5].
CPW planar technology is defined as a series stub which is arranged by introducing discontinuity in the middle stub and slot generated from the middle stub on either sides of ground [6, 7]. An open circuit is created at the discontinuity which short circuits the input port whose length is λ_{g}/4 of the stub, where λ_{g} is the resonant wavelength [8]. For exhibiting bandstop response, this wavelength is more responsible. The distinctive features of the CPW model predominantly depend on the length and if the value of length is close to λ_{g}/4, then the circuit will act as a resonant circuit of resonating frequency and if the length is very small (< λ_{g}/10), then the circuit will perform as a capacitance circuit. Thus, the length of the stub should be necessarily to be λ_{g}/4 [9].
To achieve such highfrequency band, transition from microstrip to CPW technique will be the best choice to adopt [10, 11]. Transition from microstrip to CPW without connecting wires is an emerging new technique to create more interest among researchers. This hybrid MSCPW technology offered some advantage over traditional narrowband planar technology. To list few, they have very large bandwidth, low power requirements, less propagation delay, invulnerability in multipath propagation and compact circuit design. The famous known feeding mechanism is said to be CPW feed methodology which offers lower loss and low radiation leakage.
In this work, a compact CPWbased DBBPF with resonator integrated with coupling lines and backed with metallic pattern is designed, simulated, verified and investigated. DBBPF achieved response with sharp transition between pass band and attenuation band in reduced dimension. The proposed conductorbacked CPW DBBPF is unique in design and simple in geometry when compared with available filter geometry in the literature.
Filter design methods and analysis
Radius of circular resonator is,
Effective radius of circular resonator is,
So, resonant frequency for dominant TM^{z}_{110} is obtained from
Theoretical resonant frequency is expressed as
where λ_{g}—guide wavelength, a—mean side of hexagon, n—number of modes, f_{r}—resonant frequency, c—speed of light in free space, ε_{reff}—effective dielectric constant.
Filter topology has input/output feed lines, perfectly matched coupling lines and resonator for sustained oscillations [12]. Figure 1 shows concentric closed ring resonator model. Signal power is fed at one port, and output power is measured at other port since the circuit is symmetrical. Resonant frequencies of ring resonator will keep fluctuating unless the large gap is maintained between feed lines and resonator. This coupling is named as weak coupling [13,14,15,16] which has less value of capacitance between coupling lines. The coupling is strong when the cavity between feed lines and resonator structure is minimum which in turn increases capacitances [17]. This effect makes deviation in resonant frequencies from actual frequency of resonator. Resonance condition occurs only if the average value of all sides of ring resonator is identical to that of integral multiple of λ_{g} [18, 19] which allow the signal to propagate across the structure. In initial mode, field will be maximum at coupling discontinuity and no field at its normal plane.
Filter design
Geometric layout of compact metallic patternbacked CPWbased DBBPF using ring resonator for Sband and Cband is shown in Fig. 2. The DBBPF is simply comprised of circular resonator and square ring resonator structure at ground plane and constructed using FR4 of 1.6 mm height and relative permittivity 4.4. Typical geometric values of DBBPF are given in Table 1. The DBBPF circuit consists of four elements, namely circular resonator, interdigital coupling lines on both sides, feed lines at both ports for 50 ohms and backed square ring conductor. The interdigital link is coupled with circular MMR (multimode resonator) to produce widespread bandwidth and improved filter performances [20, 21].
Multimode resonator and interlinked coupling are combined together and produce continuous oscillation for the range of frequencies 2 GHz–7.8 GHz restricted to Sband and Cband. Tuning of desired bandwidth is obtained with support of resonator at ground plane which eliminates certain frequencies in pass band so that the filter responds as dualband BPF. Stop band width and extent of rejecting certain frequencies depend on number of rings in ground plane. Parametric optimization is carried out on filter circuit in order to obtain the desired pass band and stop band. Thickness and length of stubs are also a factor in determining the desired band. Simulation of filter geometry is carried out by Ansoft HFSS. Many complicated circuits with complex topology exist in the literature [22, 23], however, proposing that DBBPF topology performs well in terms of Sparameter, bandwidth and dimension on comparing with existing complex structures. DBBPF can also be referred as hybrid circular and square resonator circuits which are implemented in the form of transmission line and waveguide structures [24]. The CPW structure in the design is responsible for tuning the bandwidth. The topology is optimized for satellite band frequency.
Results and discussion
CPWbased DBBPF prototype is obtained for the above design values. The filter characteristics are investigated for S and Cband range by tuning various filter attributes on various elements that are analyzed. The filter geometry is optimized for different attributes like length, width and thickness of the strip for various values that are analyzed.
Scattering parameters (S_{21} and S_{11}), insertion loss (IL) and return loss (RL) of the proposed Sband and Cband filter based on CPW method are studied. The simulated frequency response of singlemode resonant cell with concentric rings in single cell is shown in Fig. 3. Sband and Cband filters are optimized for various parameters like length, width and strips values. The cell with five levels of ring is assumed to be the optimized design. Figure 4 depicts clearly that the Sband and Cband filters have excellent IL of − 1.2 dB and extremely low RL of − 35.2 dB is obtained.
DBBPF obtains remarkable pass band (2–3.8) GHz and (4.3–7.8) GHz as Sband and Cband, respectively. The phase variation response of Sband and Cband filter based on S_{21} values is evidently displayed in Fig. 5. It is obvious that the DBBPF has outstanding inphase characteristics in pass band. Figure 6 shows the comparison analysis of simulated results with mathematical modeling. The mathematical model of DBBPF is obtained from design expression given in Eqs. (1)–(6). Mathematical model is the theoretical analysis carried out for validating the filter response. The mathematical expressions satisfy the bandstop characteristics. However, these expressions will realize only approximate results when compared with simulated and measured results. As far as DBBPF is concerned, measured results obey the simulated results, and hence, those results are strongly considered for realtime implementations rather than mathematical results. However, for the sake of validating the filter design, the mathematic model results are verified with simulated results.
The Qfactor is a dimensionless quantity that shows the level of sustained oscillations in pass band for maximum duration. The Qfactor response of DBBPF over frequency using Eq. (7) is shown in Fig. 7. The analysis of uniform group delay is unavoidable and desired for satellite applications. Figure 8 depicts simulation group delay performance of DBBPF which implies excellent linear signal transfer. The typical value of group delay obtained is < 3 ns in pass band.
The prototype realization of top and bottom surface of DBBPF is displayed in Fig. 9. The insertion and return loss readings of filter prototype are studied using network analyzer, Model HP8757D, and compared with simulated outcomes, as shown in Figs. 10 and 11, respectively. A decent agreement prevails between simulated as well as measured outcomes. The proposed DBBPF has bandwidth (from 2 to 7.8) GHz at − 10 dB line with a stop band between 3.8 and 4.3 GHz (0.5 GHz) with fractional bandwidth of 120% calculated from bandwidth and resonant frequency.
Table 2 summarizes the comparative study on fundamental performance parameters like IL, RL, pass band and geometry of DBBPF with other existing filters. It is evident that the DBBPF performs well in terms of scattering parameter (S_{11} and S_{21}), and dual pass band with reduced size is achieved. The Sparameter values are also good when compared with the other LPFs listed in the reference.
Conclusion
A compact metallic conductorbacked CPWbased DBBPF using ring resonator is studied in this work. The filter has pass band (2–7.8) GHz with stop band (3.8–4.3) GHz in Sband and Cband. The circular resonator and interlinked coupling lines are optimized for the best performance in S and Cbands. Design, simulation and performance characteristics of DBBPF are deliberately analyzed and discussed. DBBPF demonstrates uniform bandwidth (2–7.8) GHz and fractional bandwidth of 120% with return loss and insertion loss of − 35.2 dB and − 1.2 dB, respectively. Thus, DBBPF shows better performance than the existing ones and is optimally suited for satellite band applications.
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Abbreviations
 CPW:

Coplanar waveguide
 MS:

Microstrip
 DBBPF:

Dualband bandpass filter
 IL:

Insertion loss
 RL:

Return loss
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Contributions
S. Oudaya Coumar (S.O.) and S. Tamilselvan (S.T.) conceived the presented idea on CPWbased BPF for satellite applications. S.O. developed the theory and performed the design computations dualband filter. S.T. verified the analytical methods and encouraged S.O. to investigate on CPWbased bandpass filter for S and Cband satellite applications and supervised the findings of this work. All authors discussed the results and contributed to the final manuscript. S.O. planned and carried out simulations and fabricated the CPWbased dualband filter for satellite band. S.O. and S.T. carried out the practical measurements of filter parameters. S.O. wrote the manuscript with support from S.T. S.T. supervised the project and contributed to the interpretation of the results. S.O. developed the theoretical formalism, performed the analytic calculations and performed the numerical simulations. Both S.O and S.T. contributed to the final version of the manuscript. All authors provided critical feedback and helped shape the research, analysis and manuscript. All authors read and approved the final manuscript.
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Oudaya Coumar, S., Tamilselvan, S. A compact conductorbacked CPWbased dual bandpass filter for satellite Sband and Cband. Journal of Electrical Systems and Inf Technol 7, 5 (2020). https://doi.org/10.1186/s43067020000138
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DOI: https://doi.org/10.1186/s43067020000138