Ultrawideband (UWB) communications in the band from 3.1 to 10.6 GHz holds great promise for effective data, voice, and video transmissions, but filtering and notching of unwanted signals in the band are essential to the effective use of such a wide swath of spectrum. To help make this band somewhat more usable, the authors developed an UWB power divider with the traits of a filter. It features strongly defined notch bands, based on simplified composite right/left-handed (SCRLH) resonators. The power divider is essentially a conventional Wilkinson design with two folded shunt quarter-wavelength short-circuited stubs separated by quarter-wavelength connecting lines at two symmetrical output ports. Both simulations and measurements show this design to achieve high notch-band rejection while maintaining good insertion loss and return loss in the passband.

Power dividers are used in many systems, including with antenna arrays for communications. The most popular power divider type is the Wilkinson divider, which provides good isolation with a typical fractional bandwidth of less than 20%.1 With the rapid growth of UWB applications, however, greater bandwidth coverage is needed and a variety of different UWB power divider geometries and design approaches have been recently proposed.2-5

For example, in ref. 2, multisection Wilkinson power dividers are cascaded to achieve an increase in bandwidth. Unfortunately, this approach requires an increase in the number of resistors for isolation with a corresponding increase in physical size and insertion loss.

Reference 3 features a three-way power divider with good UWB performance. However, it is based on a multilayer broadside-coupled structure that would require complex technology and is hardly compatible with existing microwave-integrated-circuit (MIC) fabrication techniques. In ref. 4, a UWB power divider was designed using a pair of stepped-impedance open-circuited stubs and parallel-coupled lines in two output ports. Recently, a novel UWB power divider with UWB bandpass filtering response based on multilayer slotline structure was proposed.5 It exhibits excellent power-splitting performance over the full UWB frequency range.

Although frequencies from 3.1 to 10.6 GHz have been cleared for UWB communications use by such regulatory agencies as the Federal Communications Commission (FCC) in the United States, these frequencies are not without signals from other systems. These other signal sources, such as 5-GHz wireless-local-area-network (WLAN) systems and some 8-GHz satellite-communications (satcom) systems, may interfere with UWB transmissions. Thus, a UWB power divider with multiple notched bands can play a key role in rejecting these potentially interfering signals. Work performed in ref. 6 reported on an UWB power divider with one notched band, but some applications may call for more than one notch to eliminate multiple interfering signals.

Building upon previous works,6,7 a novel UWB power divider with wide passband, two highly rejected notched bands, and relatively small size is proposed and implemented. The proposed UWB power divider is based on single-layer microstrip topology structure and one isolation resistor. Two notched bands are achieved by adding the SCRLH resonators. Measured results agree well with the simulated results.

The concept of a composite right/left-handed (CRLH) transmission line has been studied extensively for use in microwave components. An SCRLH resonator is comprised of high/low-impedance short-line and grounded stub with metalized via hole. Compared with a conventional CRLH transmission line, series capacitance CL is omitted in the SCRLH resonator, so designing the resonator is somewhat simplified.

According to the references,7-9 an SCRLH resonator has an inherent dual-mode property, and its two resonant frequencies can be found by applying Eq. 1 for resonances ?1 and ?2:

?1 = 1/(LLCR)0.5

?2 = (1 + 4LL/LR)0.5/(LLCR)0.5 (1)

where series inductance LR and shunt capacitance CR depend on high/low-impedance short-line elements. Shunt inductance LL is realized by a grounded stub with metalized via hole. The SCRLH resonator can obtain dual band-stop performance when placed next to the microstrip line, and it can be equivalent to two shunt-connected series LC resonance circuit, as shown in Fig. 1.

The frequency characteristics of the coupled SCRLH resonator with various dimensions were studied using Version 11 of the High-Frequency Structure Simulator (HFSS 11.0) electromagnetic (EM) simulation software from Ansys Corp., as shown in Fig. 2. It can be seen that the dual notched bands decrease simultaneously as the val ue of inductor LR1 decreases. Therefore, by appropriately adjusting the resonator dimensions, dual notched bands can be achieved at desired frequencies.

The basic UWB power divider design consists of two ?g0 short-circuited stubs that are placed symmetrically at the two outputs, where ?g0 is the guide wavelength at the center frequency of the UWB passband. The two short-circuited stubs are then joined through a pair of quarter-wavelength impedance transformers. To further reduce the filter size in the power divider, the short-circuited stubs are folded. Figure 3(a) shows the general circuit model for the UWB power divider, where ? = p/2, Z1 = v2Z0, and R = 2Z0. The two ?g0 short-circuited stubs can be modeled by a shunt-connected parallel LC resonant circuit. The second can be considered equivalent to a series-connected series LC resonant circuit because of the quarter-wavelength impedance transformer, as shown in the schematic diagram of Fig. 3(b).

To design a UWB power divider with two high-rejection notches, two SCRLH resonators were adding to the two output ports. Figure 4 shows the configuration of the UWB power divider. It features a simple and flexible structure for the purpose of blocking any unwanted signals that may appear in the UWB band and otherwise act as interference to an UWB communications system. The dimensions of the UWB power divider are as follows: W0 = 3.0 mm; W1 = 1.7 mm; L1 = 5.2 mm; L2 = 3.2 mm; L3 = 7.05 mm; L4 = 8.2 mm; L5 = 1.75 mm; Wshort = 0.45 mm; and Wgap = 0.15 mm.

The dimensions of the SCRLH resonator are: WR1 = 4.0 mm; WR2 = 0.6 mm; LR1 = 1.8 mm; LR2 = 3.7 mm; WL1 = 0.6 mm; and LL1 = 0.8 mm. The radius of all via holes is 0.2 mm. The substrate is RT/duroid 5880 high-frequency printed-circuit-board (PCB) material from Rogers Corp. with 1.0-mm thickness and relative dielectric constant of 2.2 at 10 GHz. The overall size of the fabricated UWB power divider is about 23 25 mm.

The fabricated UWB power divider was characterized with the help of a model N5230A microwave vector network analyzer (VNA) from Agilent Technologies. Models of the VNA are available for use from 300 kHz to as high as 50 GHz with as much as 108-dB measurement dynamic range. Measurements made with the model N5230A are compared with the results from the computer software simulations (Fig. 5), showing the two strong notch bands with center frequencies at 5.9 and 8.0 GHz and 3-dB bandwidths of 8.3% and 1.8%, respectively.

Better than 15 dB rejection was measured at the midband frequency of the notched bands. The measured return loss was better than 10 dB while the passband insertion loss was close to 3.5 dB. Isolation was better than 10 dB between ports 2 and 3 across the entire UWB frequency band. The simulated and measured group delays show good linearity.

In addition, a phase difference of 4 deg. was achieved between the two output ports across the entire UWB range, except in the notched bands. The deviations between the measured results and the simulations are expected mainly due to reflections from the connectors and the characteristics of the microwave substrate material. Figure 6 shows a top view of the fabricated UWB power divider.

In summary, the fabricated UWB power divider not only provides a power split, but also functions as a pair of notched filters to help alleviate or eliminate the effects of interfering signals from other microwave applications within the broad UWB frequency range. The dual notched bands can be readily tuned by means of the CRLH resonator characteristics. The divider's simple planar geometry also makes it a viable candidate for use with existing MIC designs and circuit fabrication methods.

LEI CHEN
School of Electronic and Information Engineering
Xi'an Technological University
Xi'an, People's Republic of China, 710032
e-mail: sumter1108@163.com

FENG WEI
National Key Laboratory of Antennas and Microwave Technology
Xidian University
Xi'an, People's Republic of China, 710071

Acknowledgments

This work was supported in part by the principal research fund project of Xi'an Technological University (XAGDXJJ1015) and the fundamental research funds for the Central Universities (K50511020013).

References

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