Band-pass Filter with Harmonics Suppression Capability

Izni Husna Idris1, Mohamad Rijal Hamid2, Kamilia Kamardin3, Mohamad Kamal A Rahim4, Farid Zubir5, Huda A Majid6 i,2,4,5Advance RF and Microwave Research Group (ARFMRG), Faculty of Electrical Engineering, Universiti Teknologi Malaysia, Johor Bahru, Johor, Malaysia 3Computer Systems Engineering Group, Advanced Informatics School, Universiti Teknologi Malaysia, Kuala Lumpur, Malaysia 3Wireless Communication Centre (WCC), Universiti Teknologi Malaysia, Kuala Lumpur, Malaysia ^Research Center for Applied Electromagnetics, Faculty of Electrical and Electronic Engineering, Universiti Tun Hussein Onn Malaysia, Batu Pahat, Johor, Malaysia


INTRODUCTION
Many modern communication systems demand multifunctional RF front-end. Multifunctional antenna such as reconfigurable and filtering antennas have the advantage because it can reduce the overall size and complexity in the systems [1]. However, this antenna also exposed to unwanted harmonic frequencies which can cause interference in the communication system. Therefore, filter can be used to remove unwanted frequency by means o f being completely eliminated or partially suppressed them.
There are many types o f filter can be used as the filtering purposes in the RF front-end. Various filter structures are presented [2]- [13] such as parallel coupled line technique, stepped impedance resonator and multimode resonator.
Filter presented in [2], [3] have dual-band band-pass filter. Filter in [2] consists o f symmetrical Y-shaped central-loaded stepped-impedance, overlapped w ith the hexagon ring resonator. It has a center frequency at 1.9 GHz and 3.4 GHz, w ith the minimum insertion loss results o f 0.9 dB and 1.0 dB, respectively. The simple structure is based on parallel coupled line technique o f filter in [3], it has an insertion loss at 2.4 GHz for -0.6 dB while at 5 GHz for -2.1 dB. Filters w ith more than a quad-band are also been reported in [4]- [5]. By using multimode resonator (MMR), U4 resonator w ith mixed electric and magnetic coupling (MEMC), filter in [4] has a quint-band bandpass. Center frequencies are achieved at 2.1 GHz, 3.0 GHz, 4.0 GHz, 4.7 GHz and 7.2 GHz with the insertion loss o f 0.98 dB, 1.78 dB, 1.22 dB,

Single-, dual-, triple-and quad-line (s) rectangle-shape of filter
The forming o f modified U-shaped slot is based from rectangle slot. Figure 1 shows a single rectangle-shape slot and was given a name as configuration A. The substrate dimension is 150 x 100 mm2 (length, l x width, w). The single slot denoted as a stub1 has a dimension o f length=l1 and width=w1, placed in the middle o f CPW feedline w ith f=3 mm and g=0.5 mm. The slot length, l1 is approximately a quarter wavelength at the operating frequency. Thus creating a stop-band function.  when w1 o f stub 1 is fixed at 1 mm. W hen 11=9 mm, the -10 dB stopband bandwidth is 4.76 GHz to 6.12 GHz, with the center frequency, fc=5.4 GHz. W hen l1=10 mm, the bandwidth is covered from 4.21 GHz to 5.65 GHz, with fc=4.8 GHz. Bandwidth is ranging from 3.87 GHz to 5 GHz when l1=11 mm, with fc=4.3 GHz.
Meanwhile, Figure 2(b) shows S21 results when l1 is fixed at 10 mm and the value o f w1 is varied from 1 mm to 5 mm. Bandwidth percentage is increasing from 14.6% (w1=1 mm) to 20.8% (w1=3 mm) and 24.1% (w1=5 mm). From both o f the figures, we can notice that if the length of the stub is shorter, the center frequency (fc) is higher as shown in Equation (1). Meanwhile, when the width o f the slot is wider, the quality factor becomes low er and thus the bandwidth is increased.  Other rectangle slot shape such as dual-lines, triple-lines and quad-lines are shown in Figure 3, Figure 4 and Figure 5. Dual-lines, rectangle slot as shown in Figure 3(a) consists o f two slots known as stub1 and stub2. The structure is given a name as Configuration B. Stub1 has a dimension o f l1 x w1 while stub2 has a dimension o f l2 x w2. The distance betw een stub1 and stub2 is 1 mm. Figure 3(b) shows the insertion loss results o f Configuration B when l1=9 mm and l2=7 mm. Value o f w1 and w2 is fixed at 1 mm. The lowpass response is achieved from 2 GHz to 5 GHz. By adding an additional slot, the stop band frequency range is wider compared to having only one slot. For example, the band-stop is achieved from 4.8 GHz to 6.1 GHz using Configuration A (l1=9 mm). Meanwhile, by using Configuration B (l1=9 mm, l2=7 mm), the band-stop covers from 4.9 GHz to 8.0 GHz. The introduction o f stub2 has helped to create a stopband at higher frequency. (a) has three stubs where the size o f stub1 is l1 x w1, stub2 is l2 x w2 and stub3 is l3 x w3. The structure is denoted as Configuration C. The distance between each stub is 1 mm. Figure 4(b) shows the insertion loss result o f Configuration C when l1=12 mm, l2=9 mm, l3=7 mm. Value o f w1, w2 and w3 is fixed at 1 mm. W ith this configuration, the band-stop is now wider which covers from 3.7 GHz to 7.8 GHz.
Quad-lines, the rectangle shape shown in Figure 5(a) has one additional stub compared to the triple stub known as stub4. This structure is know n as Configuration D. The size o f stub 4 is l4 x w4. The rest o f the stubs are the same as previous Configuration C. The Band-stop from 3.7 GHz to 9.0 GHz is now achieved when l1=12 mm, l2=9 mm, l3=7 mm, l4=6 mm as shown in Figure 5(b). Value o f w1, w2, w3 and w4 is fixed at 1 mm. The distance betw een each stub is 1 mm. Therefore, it can be concluded that w hen an additional rectangular shape slot is added to the filter structure, multiple stopbands are achieved and the combination of these stopbands make the bandw idth o f the band-stop become wider.   Figure 7(a), both o f the rectangular slot length in Configuration B are added by 1 mm. As a result, Configuration E w ith l1=9 mm and l2=7 mm produced quite same bandwidth size w ith the filter o f Configuration B; l1=10 mm and l2=8 mm. W hen boxt and boxr is added to the antenna structure, the insertion loss o f the filter will produce a slight notch (blue circle) as shown in Figure 7. The notch occurs depending on the distance between each stub (e.g. wr and wy). The wider the w idth betw een the stubs (e.g. distance between stub1 and stub2 or between stub2 and stub3), the smaller the bandw idth o f the notch will be produced as shown in Figure 7(c).
The insertion loss result o f Configurations F and G are shown in Figure 8. Both o f the results are being compared w ith the same length o f stubs but different in configurations. Configuration F is compared w ith C while Configuration G is compared with D. As expected from Figure 7, results in Figure 8 also have a slight notch coming from the distance between each slot (e.g. wr and wy). This will prevent the filter to have wide bandwidth for the band-stop. Therefore, Configuration D is the best results so far in term o f bandwidth performance. However, the filter requires four pairs o f switch to turn on and off the filter later on.

Other configuration of filter
In this paper, another possibility o f configuration is investigated to achieve a better result in term o f the insertion loss, S21. For example, a combination o f one Configuration E and one Configuration A will produce the Configuration J as shown in Figure 9(a). Other configuration as shown in Figure 9 Based on Figure 10, we noticed that the Configuration L provide better performance in terms of bandwidth compared to Configuration K. Configuration J, w ith the size o f 11=12 mm, 12=9 mm and 13=7 mm have the stopband results from 3.39 GHz to 4.10 GHz and 5.12 GHz to 7.67 GHz as shown in Figure 10(a). The maximum ratio for Configuration J is 1.5:1.
Configuration K, w ith the size o f 11=12 mm, 12=9 mm, 13=7 mm and 14=6 mm has a stopband ranging from 3.44 GHz to 9 GHz w ith the ratio o f 2.62:1 as shown in Figure 10 Figure 11(b) and Figure 11(c). The dimensions o f the filter are listed in Table 1. In this design, the filter should have a band-pass result from 1.5 GHz to 3.0 GHz. This is because, the higher order modes will be eliminated when the filter is applied to an antenna structure. Figure 11 Figure 12 shows the simulated and measured results o f the final design of the proposed filter as shown in Figure 11(d). The results show good agreements betw een simulation and measurement. The insertion loss in Figure 12 shows that the band-pass is measured from 1.35 GHz to 3.31 GHz against that of the simulated result ranging from 1.38 GHz to 3.29 GHz. Two transmission zeroes are created near the passband. The 10 dB rejection band therefore is extended to 9 GHz, for both simulated and measured results.

CONCLUSION
A new design o f band-pass filter with harmonic suppression is presented. The proposed filter is achieved based on connecting two shape o f the rectangular-slot into one unit o f filter known as a U-shaped slot. By having two pairs o f U-shaped slots, the filter has a wider rejection band from 3.33 GHz to 9 GHz. The proposed filter shows that it can eliminate the harmonic frequencies effectively.