Geometry parameters for the embedded slot structure
1. Introduction
In wireless communication systems, microwave planar bandpass filters are employed in most applications. The broadband and multiband applications are renewing the interest in the design of planar broadband filters with low loss, compact size, high suppression of spurious responses, and improved stopband performances. However, since the ultra-wideband (UWB) system covers very wide frequency range of 3.1 to 10.6 GHz and then may be interfered with the existing undesired narrow band from the 5.2 GHz or 5.8 GHz wireless local area network (WLAN) radio signals. Moreover, the WiMAX (3.5 GHz) and RFID (6.8 GHz) communications may interfere with the UWB system withinthe range defined by the FCC. Therefore, UWB bandpass filters with single- and multi-narrow notched bands are needed to avoid being interfered by the exiting RF signals. In order to obtain operations, several techniques have been reported in literatures based on UWB-bandpass filters with slotted resonators (Meeloon et al., 2007, 2008, 2009), UWB-bandpass filters with slotted resonators and embedded slotted feed (Meeloon et al., 2009, 2011), and UWB-bandpass filters with slotted resonators and embedded fold-slot feed (Meeloon et al., 2010, 2011).
In this chapter, many advanced UWB-bandpass filters are presented based on slotted linear tapered-line resonator (SLTR) and slotted step-impedance resonator (SSIR) structures for size reduction and improved stopband performances. A comprehensive treatment of slotted resonators and both ends of the resonator with interdigital coupled lines is described. The design concept is demonstrated using two filter examples including one with an SLTR and another one with an SSIR. These filters have not only compact size but also a wider upper stopband resulting from resonator bandstop characteristics. Single-SLTR and single-SSIR filters are designed and constructed and their performances are extensively investigated in simulation and measurement. The proposed filters demonstrate their capabilities in suppression of spurious responses. Also, two-SLTR and two-SSIR filters are designed and fabricated to prove that they improve the passband and upper stopband performances with sharpened rejection skirts outside the passband and widened upper stopband.
Then, UWB-bandpass filters based on SLTR and SSIR with embedded slot feed structure for notched band are presented. The embedded slot feed at the end of resonators will be comprehensively described. The proposed filters have narrow notches in the passband, resulting from the embedded slot feed. The center frequencies and bandwidths of the notched band can be easily adjusted by tuning the length and width of the embedded slot parameters. The wider upper stopbands caused by resonator characteristics have been also obatained.
After that, UWB-bandpass filters with single-notched and dual-notched bands and improved stopband performance are proposed using SLTR and SSIR as multi-mode resonator (MMR) and embedded slotted feed. To avoid the existing interferences in the UWB passband, two different embedded slotted feed are employed to obtain two narrow-notched bands. The center frequency and bandwidth of the notched bands can be controlled by adjusting the dimensions of the embedded slotted feed. To further suppress the upper stopband, the defected slot in λ/2 stepped impedance resonator fed by interdigital coupled line is introduced. Very good agreements between the measured and simulated filter characteristics have been obtained validating the proposed filter prototypes.
Finally, UWB-bandpass filters based on SLTR and SSIR with embedded fold-slot are presented. The proposed filters have narrow notches in the passband and size reduction, resulting from the embedded fold-slot. The length and width of the embedded fold-slot parameters resulting in their performances have been also studied.
2. UWB-bandpass filters with slot resonator
2.1. Interdigitalcoupled line characteristics
Fig. 1(a) shows a conventional interdigital coupled line which has been widely used as a capacitive coupling element in multi-stage bandpass filters. The optimized interdigital coupled lines must be performed to achieve design-specified coupling factor between two adjacent line resonators. The usual procedure is to reduce both strip and slot widths in order to achieve a tight coupling and lower insertion. However, it may introduce some difficulties into the design procedure and fabrication process as the coupling response is sensitive to the strip/slot widths configuration. In this work, we redesign and optimize the interdigital coupled line, as a result shown in Fig. 1(b). An RT/Duroid3003 substrate, which has a given dielectric constant of 3.0, a thickness of 1.524 mm, and a loss tangent of 0.0013 is used for designing the new interdigital coupled lines at a central frequency of 6.85 GHz and a fractional bandwidth of 100%. The conventional and new coupled lines are evaluated by using an electromagnetic simulation program, IE3D, which is based on the method of moments and proven to be quite accurate in its prediction. The response curves of both coupled lines are demonstrated in Fig. 2. It can be noticed that both interdigital coupled lines have almost the same resonant frequency of 6.85 GHz. Nevertheless, the new coupled line has superior performances with better
2.2. SLTR and SSIR characteristics
Fig. 3(a) shows a conventional λ
In order to obtain good stopband characteristics without passband perturbations of the desired UWB-bandpass filters, slot length
2.3. Filter designs and measured results
2.3.1. Single-SLTR filter
In the following, the two UWB-bandpass filters have been built using the MMR conventional λ/2 resonators and the proposed SLTR and SLTR with three slots fed by λ/4 interdigital coupled lines at both ends with a central frequency of 6.85 GHz and a fractional bandwidth of 100% as shown in Fig. 7(a) and (b). The RT/Duroid 3003 substrate with a dielectric constant of 3.0, a thickness of 1.524mm and a loss tangent of 0.0013 has been used. The optimized dimensions of the resonators and the interdigital coupled lines have been obtained in the previous section. Their electrical performances are then simulated by using IE3D program.
2.3.2. Single-SSIR filter
Fig. 7(c) shows schematics of the the proposed single-microstrip SSIR bandpass filters. The optimized dimensions of the resonators and the interdigital coupled lines are the same as resulting from the previous section.
2.3.3. Two-SLTR filter
Fig. 8 depicts schematics of the bandpass filters with two linear tapered-line resonators connected in cascade. Fig. 8 (a) and (b) was the proposed SLTR filters with single- and three-slotted structures, respectively. All dimensions of the resonators and interdigital coupled lines have been shown.
2.3.4. Two-SSIR filter
Fig. 8(c) depicts schematics of the proposed two-SSIR bandpass filter. The dimensions of the resonators and the interdigital coupled lines are the same.
2.4. Experimental verification
In this section, six UWB-bandpass filters are implemented on the RT/Duroid 3003 with a substrate thickness of 1.524 mm, and a dielectric constant of 3.0 at a central frequency of 6.85 GHz and a fractional bandwidth of 100%. Fig. 9 and Fig. 10 shows photographs of the fabricated SLTR and SSIR filters. Fig. 11 shows comparisons of measured and simulated responses of the SLTR and SSIR filters. In Fig. 11(a), (b) and (c), it can be found that the measured results agree very well with the simulation expectations, confirming that the proposed UWB-SLTR and SSIR bandpass filter is capable of reducing the insertion losses within the passband and also widening the upper stopband. The measured return and insertion losses are found to be higher than 14 dB and less than 2 dB over the UWB- passband, respectively. In Fig. 11(d), (e) and (f), two-SLTR and SSIR filter shows the improved upper stopband performance, with the return losses higher than 15 dB inside the passband. The lower and upper band skirts get sharpened to a great extent, while the upper stopband with the insertion losses above 15 dB occupies an enlarged range of 11.3–25 GHz. Also, the proposed two-SLTR filter with three slots has an improved upper stopband performance, as shown in Fig. 11 (e). A two- SSIR filter has improved the upper stopband performances, with the return losses higher than 15 dB outside the UWB-passband, and
insertion losses above 25 dB in a range of 19–25 GHz and above 47 dB at 24 GHz as shown in Fig. 11 (f). However, the measured passband insertion loss is higher than the simulationresult due to the dimension of the conductor and further the conductivity slightly deviated from the design. The group delay of both filters slightly varies between 0.2 and 0.3 ns in the passband.
3. UWB-bandpass filters with embedded slot
This section proposes new UWB-bandpass filters using slotted linear tapered-line resonators (SLTR) and slotted step-impedance resonator (SSIR) structures driven by interdigital coupled lines at both ends of the resonators for improving the stopband performances. Also, using embedded slot structure in the input and output feed line can create a notched band.
3.1. Embedded slot feed characteristics
The embedded slot at the feed line has been proposed in order to form a notch band. The RT/Duroid 3003 substrate has been used in this study. Fig.12 (a) shows the part of embedded slot feed and its frequency responses of
To verify the notched mechanism, the current distributions of embedded structure at 5.6 GHz notch frequency are shown in Fig.13. We can notice that in Fig. 13 (a) the current distribution passes through the conventional feed line but it cannot pass through the proposed structure as shown in Fig.13 (b).
W(mm) | L(mm) | BW(MHz)<-3 dB |
0.6 | 8.593 | 220 |
0.8 | 8.543 | 250 |
1.0 | 8.493 | 360 |
1.2 | 8.483 | 470 |
1.4 | 8.423 | 720 |
1.6 | 8.393 | 810 |
3.2. Filter designs and measured results
3.2.1. SLTR filter with three slots and embedded slot
The UWB-bandpass filters using slotted linear tapered-line resonators (SLTR) with three slots are proposed. The filters consists of the interdigital coupled lines at both ends of the resonators for improving the stopband performances. Also, using embedded slot structure in the input feed line can create a notched band. Fig.14 (a) shows the SLTR with three slots and one embedded slot at the input feed for notched band. The two-cascaded SLTR with three slots and one embedded slot at input feed is also shown in Fig. 14 (b). All dimensions of the UWB-bandpass filters have been shown.
3.2.2. SSIR filter with embedded slot
Fig.14 (c) shows the SSIR with one embedded slot at the input feed for notched band. The SSIR with embedded slot at input and output feed for dual-notched is also shown in Fig.14(d). This section proposes UWB-bandpass filters using slotted step-impedance resonator (SSIR) driven by interdigital coupled lines at both ends of the resonators for improving the stopband performances. Also, using embedded slot structure in the feed line can create a notched band.
3.3. Experimental verification
Fig.15 (a) shows photograph of the fabricated single-SLTR for notched band. Fig. 16 (a) shows a comparison of measured and simulated responses of the single-SLTR filter with a notched band. The measured results are agreed very well with the simulation predictions, confirming that the proposed SLTR filter with a notch is capable of narrow notched band, good insertion losses within the passband and also widening the upper stopband. The measured return and insertion losses are found to be lower than 10 dB and higher than 2 dB over desired UWB passband, respectively. Fig. 15 (b) shows photograph of the fabricatedtwo-SLTR filter. Fig. 16 (c) shows a comparison of measured and simulated responses of the two-SLTR filter, which a good agreement has been obtained for passband and sharp rejection skirt in upper stop band. The measured return and insertion losses of the two-SLTR filter are found to be higher than 2 dB and lower than 10 dB at the notched frequencies about 5.6 GHz which the bandwidth of notched band are about 276 MHz. Fig. 15 (c) and (d)show photograph of the fabricated SSIR filter with single and dual-notched band.Fig. 16 (c)and (d) show a comparison of measured and simulated responses of the SSIR filters with single and dual-notched band at the notched frequencies about 5.6 GHz and 8.3 GHz which the bandwidth of notched band are about 276 MHz and 300 MHz, respectively. The proposed filters show improved upper stopband performance with high insertion loss. The upper stopband with the insertion loss lower than 15 dB occupies an enlarged range of 12 to 18 GHz. As we can see that the proposed filter exhibits notched and dual- notched band, a wide upper stopband with values of
4. UWB-bandpass filters with embedded fold-slot
By modifying the UWB-bandpass filters with embedded slot, the embedded slot can be reduce size using embedded fold-slot. It usies slotted linear tapered-line resonator (SLTR) and slotted step-impedance resonator (SSIR)driven by interdigital coupled lines at both ends of the resonator to improve the stopband performances.
4.1. Embedded fold-slot feed characteristics
The embedded fold-slot at the input feed has been proposed in order to form a notch band. The RT/Duroid 3003 substrate has been used in this study. Therefore, by tuning the length and width of the embedded slot from previous section, center frequencies and bandwidth of notched band can be easily adjusted. Embedded fold-slot is thus suitable for use in the UWB- bandpass filter when a notched band is required. To creat the notched band at 5.6 GHz, the dimensions of the proposed embedded fold-slot feed include
4.2. Filter designs and measured results
4.2.1. SLTR filter with embedded fold-slot
The UWB-bandpass filters using slotted linear tapered-line resonators (SLTR) driven by interdigital coupled lines at both ends of the resonators for improving the stopband performances and using embedded fold-slot structure in the input feed line can create a notched band are proposed. Fig.18 (a) and 18 (b) show the SLTR and SLTR filters with three slots, using embedded fold-slot at the input feed for notched band. The two-cascaded SLTR with embedded fold-slot at input feed is also shown in Fig. 18 (c). All dimensions of the UWB-bandpass filters have been shown.
4.2.2. SSIR filter with embedded fold-slot
Fig. 18 (d) shows the SSIR with embedded fold-slot at the input feed for notched band.
This section proposes a new UWB-bandpass filter with simple structures using slotted step-impedance resonator (SSIR) driven by interdigital coupled lines at both ends of the resonators for improving the stopband performances. Also, using embedded fold-slot in the input feed line can create a notched band.
4.3. Experimental verification
Fig. 20 shows the photograph of the fabricated SLTR and SSIR filters for notched band. Fig. 21 shows a comparison of measured and simulated responses of the SLTR and SSIR filters with a notched band. The measured and simulated results have shown good agreement with a notch is capable of narrowing notched band, having good insertion losses within the passband and also widening the upper stopband. The measured return and insertion losses are found to be lower than 10 dB and higher than 2 dB, respectively over desired UWB-passband. The notched frequency of about 5.6 GHz has a bandwidth of about 276 MHz. The proposed filters show narrow notched band and improved upper stopband performance with high insertion loss. The upper stopband with the insertion loss lower than 10 dB occupies an enlarged range of 14 to 18 GHz. The group delay of both filters slightly varies between 0.2 to 0.3 ns in the passband. These superior stopband performances are caused by the stopband characteristics of the proposed slotted resonator structure, and narrow notched band is caused by embedded fold-slot structure.
5. Conclusion
In this chapter, the novel SLTR and SSIR UWB-bandpass filters with improved upper stopband performances have been presented and implemented. By properly forming SLTR and SSIR together with two interdigital coupled lines at both ends, the proposed filters are designed and constructed. The single-SLTR and SSIR filters show their performances in suppression of spurious responses. Also, two-SLTR and two-SSIR filters are designed and fabricated to show that they improve the passband and upper stopband performances with sharpened rejection skirts outside the passband and widened upper stopband. When comparing with SLTRs, we find that the SSIR structures are easier to design and fabricate and they also have better stopband characteristics.In addition, the SLTR and SSIR filters using embedded slot and embedded fold-slot with notched band, reduce size and improved upper stopband performances have been presented and implemented. The proposed filters demonstrate their capability in narrow notched band with the embedded slot, embedded fold-slot feed and suppression of spurious responses with slotted resonators. Also, the fabricated filters prove that they can create notched band and improve upper stopband performances with sharpened rejection and widen the upper stopbands.
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