Filtering Power Divider With Ultrawide Stopband and Wideband Low Radiation Loss Using Substrate Integrated Defected Ground Structure

In this letter, a novel substrate integrated defected ground structure (SIDGS) resonant cell is proposed. Such SIDGS resonant cells can not only introduce an ultrawide stopband for spurious suppression but also achieve low radiation loss in a wideband, which can be easily implemented in passive circuits with high performance. To verify this mechanism, a filtering power divider (FPD) using the folded stepped-impedance scheme is developed. Here, two coupled SIDGS resonant cells are cascaded at the arms of the power divider to achieve a filtering response with ultrawideband harmonic suppression. The fabricated FPD is operated at 2.87 GHz (i.e., the passband center frequency <inline-formula> <tex-math notation="LaTeX">$f_{0}$ </tex-math></inline-formula>) with a 3-dB fractional bandwidth (FBW) of 23%, which exhibits the 28-dB attenuation upper stopband with a low radiation loss even up to 25 GHz (i.e., 8.71<inline-formula> <tex-math notation="LaTeX">$f_{0}$ </tex-math></inline-formula>).


I. INTRODUCTION
W ITH the ever-developing demands of complex mi- crowave circuits and systems, passive components with wideband interference suppression are developed rapidly.As a key component in modern wireless communication systems, the power divider with filtering function and wide stopband has drawn great attentions recently.In [1], the substrate-integrated waveguide (SIW) is proposed for the power divider design with low radiation loss, which suffers from the narrow stopband.To extend the stopband bandwidth, the lowpass filter [2], stepped-impedance resonators (SIR) [3], and defected ground structure (DGS) [4] are proposed.However, the stopband performances of these methods are limited by the restriction of fabrication techniques.Recently, slow-wave DGS resonator [5] is presented to further enhance the stopband performance, such resonators can not only allocate the intrinsic fundamental resonance, but also introduce an ultra-wide stopband with high rejection level.Nevertheless, high radiation makes conventional DGS hard to be integrated in passive circuits.Therefore, the design of filtering power divider with ultra-wide stopband and low radiation loss for flexible integration still remains a great challenge.
In this report, a novel substrate-integrated defected ground structure (SIDGS) resonant cell is proposed.Such SIDGS resonant cell is consisted of a DGS resonator integrated between two substrate layers, where a bottom grounded plane and surrounding metal-vias are introduced as an integrated The authors are with the Center for Integrated Circuits, University of Electronic Science and Technology of China, Chengdu 611731, China.
package.Therefore, this SIDGS resonant cell can not only reserve the harmonic suppression characteristic of the DGS resonator, but also be easily integrated in passive circuits for the suppressed radiation.Based on the proposed SIDGS resonant cell, a filtering power divider (FPD) with ultra-wide stopband and low radiation loss is design and fabricated.Good agreement between simulation and measurement is achieved.

II. SCHEMATIC AND OPERATION
A. Substrate-Integrated Defected Ground Structure Fig. 1 depicts the configuration of the proposed FPD.Two pairs of coupling SIDGS resonance cells are cascaded on the output arms of a Wilkinson power divider while the microstrip T-stubs act as feed lines.A C-shape etched defect is located on the ground I, which is integrated between two substrate layers.Besides, a bottom grounded plane and surrounding metal-vias are introduced as a package.With such implementation, the proposed SIDGS resonance cell is similar to a λ/2 steppedimpedance resonator with two grounded ends [6].
The electric and magnetic fields of conventional DGS are distributed in an open space, which leads to a large electromagnetic radiation [7].Such radiation leads to an extra passband insertion loss and electromagnetic interference.For the SIDGS structure, the electric fields are mainly restricted in a quasicavity by ground II and metal-vias.With such characteristics, the SIDGS can not only effectively reduce the radiation loss compared to the conventional DGS, but also be convenient for integrated passive circuits.

B. Filtering Power Divider Design
To verify the aforementioned characteristics, a filtering power divider operating at 2.87 GHz with a 3-dB FBW of 23% is designed based on the SIDGS resonant cell.The design tools ADS, CST Microwave Studio, and dielectric substrate Rogers 4003C (i.e., ε r = 3.55, h 1 = 0.203 mm, and h 2 = 0.303 mm) are used.Such FPD is composed of two filtering parts and a stepped-impedance Wilkinson power divider.Here, a pair of SIDGS resonant cells and T-stubs constitute the filtering part.The coupling coefficient k for the required frequency response can be obtained by adjusting the distance d 1 and d 2 between the two SIDGS cells.Meanwhile, the demanded external quality factor Q e is determined by the width w 1 of the T-stub.Besides, a λ/4 open stub is added on the Tstub to generate a transmission zero at upper stopband, which could improve the passband selectivity.Then, the steppedimpedance scheme is utilized for Wilkinson power divider design to achieve good impedance matching with compact size.Moreover, the isolated resistor is optimized as 180 Ω to enhance the output-isolation.

III. FABRICATION AND MEASUREMENT
Based on the principles mentioned above, a FPD operating at 2.87 GHz with 3-dB FBW of 23% is fabricated.The prototype exhibits a compact core-circuit size of 29.8 mm by 14.9 mm (i.e., 0.46λ g by 0.23λ g , where the λ g is the microstrip guided wavelength at 2.87 GHz).The measured results depicted in Fig. 2 are performed using the network analyzer over the frequency range from 0.01 to 25 GHz.The measured minimal passband insertion loss is 1.1 dB, excluding 3 dB division loss.Meanwhile, the proposed FPD could achieve an ultra-wide upper stopband up to 25 GHz (i.e., 8.71f 0 ) with a rejection level greater than 28 dB.Besides, a transmission zero is created at 4.05 GHz to improve the passband selectivity.The in-band isolation greater than 20 dB is measured with the utilization of a 180 Ω resistor R. The performance summary and comparison with state-of-the-arts FPD are shown in Table I, which reveal that the proposed FPD shows competitive merits of the insertion loss, stopband performance, isolation, and radiation loss.
IV. CONCLUSION In this report, a SIDGS resonant cell is proposed for high performance integrated system design.Compared to the convectional DGS, such SIDGS can not only introduce a wide stopband performance, but also reduce the radiation loss in a wideband.Based on the proposed SIDGS resonant cell, an FPD with low insertion loss, ultra-wide band harmonic suppression, and low radiation loss is developed.With such good performances, the proposed SIDGS resonant cell is attractive for integrated circuits and systems with wideband spurious suppression.
Receiving the IEEE MTT-S Undergraduate/Pregraduate Scholarship has inspired me to develop the study on the mm-wave integrated circuit.It will also encourage me in the future study.I had received the undergraduate degree with the invaluable guidance of Prof. dr.Xun Luo at University of Electronic Science and Technology of China (UESTC).It is my honor to pursue the Ph.D's degree at the UESTC under supervision of Prof. dr.Xun Luo.

TABLE I COMPARISON
OF STATE-OF-THE-ART POWER DIVIDERS * : Technology.* * : Insertion loss.