Compact and Wide Upper-Stopband Triple-Mode Broadband Microstrip BPF

A broadband microstrip bandpass filter (BPF) with compact size, sharp skirt and wide upperstopband performance is proposed using the triple-mode stub-loaded resonator. The resonator can generate one odd mode and two even modes in the desired passband. Due to the main path signal counteraction, two transmission zeros Tz1, Tz2 can be created near the passband edge to achieve sharp skirt. Two transmission zeros Tz4, Tz5 are created between two harmonic frequencies hm2, hm3 by the folded open stub and the interdigital coupling feeding lines, respectively. Owing to the source-load coupling, one zero Tz3 is introduced to deepen the stopband. Meanwhile, the zero Tz1 is shifted to the lower cut-off frequency and the other three ones Tz2, Tz4, Tz5 can be slightly turned to suppress three harmonic frequencies hm1, hm2, hm3. One filter prototype with the fractional bandwidth 34% is designed, fabricated and measured.


Introduction
High-performance wideband and broadband bandpass filters with the characteristics of compact size, wide stopband and high selectivity are essential for modern communication systems [1].Especially, excellent upper-stopband performance is highly demanded for a wideband and broadband BPF to suppress the unwanted interference or noise [2].In response to this need, numerous researchers have proposed various wideband and broadband BPFs with wide upper-stopband suppression [3][4][5].In [3], a wideband BPF with wide upper-stopband is given using stepped-impedance cascadable 180°hybrid rings.Nevertheless, the designed BPF suffers from large size with complicated implementation.In [4], a compact wideband microstrip BPF is realized using a transversal resonator.By means of introducing asymmetrical interdigital coupled lines, five transmission zeros are generated in the stopband, and the upper-stopband extends to 2.5f 0 .A more compact and simple wideband BPF with the total size of presented under multiple resonances of an asymmetric ring resonator, reaching a relatively wide 3dB fractional bandwidth of 64% [8].By stretching the paired stubs close to one-eighth of a wavelength, the first two even-order resonances move down to be quasi-equally located at two sides of the first odd-order resonance, thus forming a bandwidth adjustable BPF.However, all of the above-mentioned filters exhibit unexpected poor upper-stopband performance, since high order harmonic frequencies can hardly be rejected.In [11], a compact wideband BPF whose stopband exceeds to 3.6f 0 is designed.To achieve a wide stopband, two microstrip short-stubs are loaded to create one transmission zero to avoid the first spurious at the cost of poor in-band performance.Therefore, it is still a great challenge to design a compact broadband BPF with wide stopband and good in-band characteristics.
In this letter, a triple-mode stub-loaded resonator is adopted to design a compact broadband BPF with source-load coupling, as shown in Figure 1.The resonator can generate one odd mode determined by the high impedance line and two even modes flexibly controlled by the loaded stubs at the centre plane in the desired passband.Due to the main path signal counteraction, two transmission zeros T z1 , T z2 can be created near the passband edge.Owing to the source-load coupling, one transmission zero T z3 is created in the upper-stopband and the other four zeros T z1 , T z2 , T z4 , T z5 can be slightly adjusted to achieve sharper roll-off skirts and better upper-stopband performance.The proposed broadband filter exhibits good in-band performance, sharp skirt and wide upper-stopband characteristics.even-mode equivalent circuit.

Research Method
The tripe-mode stub-loaded resonator shown in Figure 1 is constructed by a high impedance line with length of 2l 1 and width of w loaded with one folded stepped-impedance open-circuited stub denoted by lengths (l 3 , 2l 4 ) and widths (2w, w) and one short-circuited stub with length l 2 and width 2w at the central plane.As can be seen, this resonator configuration admits an analysis in terms of even and odd excitations, for the T-T' plane behaves as an electric/magnetic wall for odd/even excitation [12], leading to the approximate transmission line circuit models represented in Figure 2, where 1 θ , 2 θ and 3 θ refer to the electrical lengths of the sections with lengths l 1 , l 2 , and l 3 +l 4 , respectively.And Y refers to characteristic admittance of the width w.
Thus the input admittances inodd Y and ineven Y of the odd-mode and even-mode resonators are as follows: As expected from formula (3), the resonant frequencies of odd excitation exclusively depend on the high impedance line while having nothing to do with the two loaded stubs，which are determined by formula (5).
where c is the speed of light and eff The characteristics of the triple-mode stub-loaded resonator are investigated in the case of weak coupling (d=1mm).Under the other parameters keeping unchanged, the resonant frequencies with varied l 2 are simulated and shown in Figure 3.The dimensions are chosen as: l 1 =12.9mm,l 3 =1.3mm,l 4 =11.1mm,g 1 =0.1mm, g 2 =0.22mm and w=0.3mm.The substrate herein is RT/Duroid 5880 with a thickness of 0.508mm, permittivity of 2.2 and loss tangent 0.0009.It can be figured out that there are three available resonant frequencies (one odd mode f m2 and two even modes f m1 , f m3 ) and four harmonic frequencies (h m1 , h m2 , h m3 , h m4 ) in the range of 0.1-15GHz.As l 2 increases, f m1 and h m2 move towards the lower frequency while the others remaining unchanged.Thus, the l 2 is mainly utilized to adjust the first resonant frequency f m1 into the desired passband while having no effect on the other two available resonant frequencies (f m2 , f m3 ).
Also the resonant frequencies varied l 3 +l 4 are interpreted in Figure 4 when choosing the parameter l 2 =1.9mm.According to Figure 4, f m3 , h m1 , h m2 and h m4 have the same trend of moving downwards while l 3 +l 4 is increasing, whereas the first two resonant frequencies f m1 , f m2 are remaining unchanged.Figure3.Resonant frequencies varied with l 2 .Figure4.Resonant frequencies varied with l 3 +l 4 .
In general, the resonant frequency f m2 can be adjusted in the centre passband frequency by reasonably choosing the length of the high impedance line, and the other two resonant frequencies f m1 , f m3 can be adjusted within the desired passband by simply varying the parameters l 2 , l 3 and l 4 .Herein, we may choose the parameters: l 2 =1.9mm, l 3 =1.3mm,l 4 =11.1mm.The first three resonant frequencies are 3.26GHz, 4.29GHz and 4.62GHz, respectively.If the resonator is properly fed with increased coupling degree [13], the first three resonant modes can be used to make up of a compact broadband BPF.
Under the tight coupling case of d=10mm, the simulated insertion losses (|S 21 | in dB) of the triple-mode broadband BPF with and without source-load coupling (shown in Figure 1 and Figure 5 respectively) are compared in Figure 6.The circuitry dimensions are depicted in Table 1.Two transmission zeros can be created near the passband edges due to the main path signal counteraction, which is explained in [14].As a result of the short-circuited stub is used to produce the transmission zero T z1 near the lower cut-off frequency, and the location of the transmission zero T z2 near the upper cut-off frequency is mainly controlled by the folded open stub.As demonstrated in Figure 6, two transmission zeros T z4 , T z5 are created between two harmonic frequencies h m2 , h m3 by the folded open stub and the interdigital coupling feeding lines, respectively.Owing to the source-load coupling, T z1 is shifted to the lower cut-off frequency and the other three transmission zeros T z2 , T z4 , T z5 can be slightly adjusted to suppress three harmonic frequencies h m1 , h m2 , h m3 .Furthermore, one zero T z3 is introduced in the upperstopband leading to deeper stopband.That is to say, a broadband BPF with sharp skirt, wide upper-stopband and good in-band performance can be achieved by introducing source-load coupling.

Results and Analysis
Based on the theories discussed above, the triple-mode broadband BPF with sourceload coupling is fabricated on the RT/Duroid 5880 substrate and its photograph is shown in

Conclusion
In this letter, a new prototype composed of a triple-mode stub-loaded resonator with source-load coupling has been presented for compact and wide upper-stopband broadband BPF.A fully integrated broadband BPF with a FBW of 34% is simulated, fabricated and measured.Significantly, the filter's upper-stopband in experiment is extended up to 3.7f 0 with a rejection level better than -12.5dB.Also the rectangular occupying area amounts to only about g g λ λ 237 .0 079 .0 × without considering the feeding lines.With all these advantages above, the broadband BPF is particularly suitable for future broadband applications.
[5]  to obtain 30dB rejection level in the stopband from 1.2f 0 to 3.8f 0 .Nevertheless the bandwidths in [4] and [5] are inconveniently adjusted.Recently, some multiple-mode resonators are employed to design compact broadband BPFs[6][7][8][9][10].A novel high selectivity triple-mode hexagonal BPF with capacitive loading stubs is designed in[6], with the radius of three radial-line stubs just affecting one even-mode resonant frequency while the other two odd-modes remaining unchanged.Another new broadband microstrip BPF isISSN: 1693-6930 Figure1.Schematic of the tripe-mode broadband BPF with source-load coupling.

Figure 5 .
Figure 5. Schematic of the triple-mode broadband BPF without source-load coupling.

Figure 6 .
Figure 6.Comparison of the BPF with and without source-load coupling.

Figure 7 .
Figure 7. Photograph of the fabricated filter.
The filtering performance is measured by Agilent network analyzer N5230A.The measured |S 11 | in dB and |S 21 | in dB are shown in Figure8(a) and illustrated good agreement with simulated results.The measured 3dB bandwidth is from 3.08 to 4.34 GHz, representing a FBW of 34% at the center frequency of 3.71 GHz.Further, its measured input return loss (|S 11 | in dB) is less than -20dB.The upper-stopband in experiment is extended up to 13.7GHz with an insertion loss better than -12.5dB.In addition, the measured in-band group delay in Figure8 (b) is varying from 0.79 to 0.93ns, which is quite small and flat in all the passband.If the feeding lines are ignored, the size of the fabricated filter is only Compact and Wide Upper-Stopband Triple-Mode Broadband Microstrip BPF (Wen Chen) 357 Figure 7.