Adaptable Concurrent Dual-Band Symmetrical Stubbed T-Junction Power Splitter

Vivek Sharma, Nagendra Prasad Pathak Radio Frequency Integrated Circuits (RFIC) Group

Department of Electronics and Communication Engineering (ECE) Indian Institute of Technology (IIT) – Roorkee

Uttarakhand, India vivs@ieee.org, nagppfec@iitr.ernet.in

Abstract—A continuously tunable, concurrent dual-band stubbed T-junction power splitter is reported for mobile communication bands of GSM-900 and DCS-1800. Simultaneous dual frequency tuning is exhibited by the design through use of a variable capacitor that changes effective electrical length of open-ended stub in the structure. The fabricated hybrid microwave integrated circuit (HMIC) for the proposed design exhibits dual-frequency tunability of 16.8% and 6.8% in the first and the second bands, respectively. Due to symmetrical architecture, the prototype maintains equal power division, between its two output ports, throughout the tunable range, i.e., the input-to-output forward transmission coefficient is -3dB for each output port.

Keywords-concurrent; dual-band; frequency agile; microstrip; multi-band; power divider; reconfigurable; simultaneous; tunable

I. INTRODUCTION Advancements in wireless communication have led to

multi-band circuits and systems. Reported design modules allow operations at multiple frequency bands, either in switched mode [1] or simultaneously [2]. Primary focus in multi-band designs is to maximize hardware sharing and reduce power consumption, while operating over multiple bands.

Further incorporation of tunability into multi-band components, like, filters [3], couplers [4], power dividers [5], impedance matching networks [6], etc., not only makes them robust to fabrication errors, but also, enhances their versatility. In particular, reconfiguration permits desirable changes in operating bands. This, in turn, allows performance optimization at any desired channel of supported communication standards, even after fabrication glitches. Moreover, such system can be tuned to support other set of standards, whose frequency bands lie adjacent to the designed bands. Such frequency agile feature will enable future mobile equipments to support diverse communication standards, with different frequency spectrums, anywhere in the world.

Current work presents a modified stubbed T-junction power splitter that allows simultaneous frequency tuning of its two different operating bands. Previously reported power divider [5] neither presents theoretical analysis nor provides design relations, thereby, limiting design utility. Not only is the

structure of the tunable dual-band stubbed T-junction power divider proposed, its analysis along with design expressions are also detailed out in Section II. Its frequency agile characteristics are demonstrated in section III. Section IV provides measurement results of a fabricated prototype, designed for operation in two different GSM (Global System for Mobile Communications) bands, which validate the hypothesized concurrent dual-frequency tunable characteristics of the proposed structure.

II. CONCURRENT RECONFIGURABLE DUAL-BAND T-JUNCTION STRUCTURE AND DESIGN

Park and Lee [2] exhibited concurrent dual-frequency operability of a stubbed T-junction power divider. The structure is modified to introduce the proposed frequency tunability of both supported frequency bands. Specifically, a variable capacitor is appended at open end of the shunt stub, as shown in Fig. 1.

Figure 1. Tunable concurrent dual-band symmetrical stubbed

T-junction power splitter schematic.

978-1-4673-5952-8/13/$31.00 ©2013 IEEE

Analysis of the proposed concurrent reconfigurable dual-band circuit assumes the two design frequencies, f1 and f2, to be averages of their respective band limits that must be covered. Besides, all impedance values are normalized to the reference port impedance, Z0, and are denoted using small letters.

Normalized admittance seen at port 1, due to two transmission line (TL) sections, connected to ports 2 and 3, is:

(1)

Herein, both ports 2 and 3 are assumed to be individually terminated by Z0. Similarly, the normalized admittance exhibited by the shunt stub TL section, in series with the variable capacitor, is given as:

(2)

Total normalized input admittance at port 1 is sum of the two admittances, yIN1 and yINS. Since, yINS is purely imaginary; therefore, the real part yIN1 should be equal to unity to satisfy the impedance matching requirement at port 1. This leads to following relation for normalized characteristic impedance of each series TL section:

(3)

Additionally, in order to achieve concurrent impedance matching at the two desired frequencies, values of z1 must remain same for both the frequencies of interest. This leads to an expression for electrical length of each series TL section:

(4)

(5)

where, n is an arbitrary integer and θ1(fi) is electrical length of each series TL section at frequency, fi. Further, the shortest electrical length is achieved by considering positive sign in denominator in (5) while taking value of n in numerator as 1:

(6)

It turns out that the shortest electrical length of a series TL section is 90o at the center frequency of the two design frequencies, i.e., . Besides, synthesis equations (3) and (6) [or (5)] reveal that the characteristic impedance and the electrical length (at frequency, f1) of each series TL section are only dependent on frequency ratio, f2/f1. Fig. 2 depicts variations of the smallest electrical length (at frequency, f1) with the frequency ratio, f2/f1. Its value is always less than 90o for f2 > f1. Dependency of characteristic impedance, Z1 (with Z0 = 50-Ω), of each series TL section on f2/f1 is also shown in Fig. 2. As indicated by design relation (3), the minimum value of normalized characteristic impedance of series TL section, z1, is √2. This means that, for normalizing impedance, Z0, of 50-Ω, the characteristic impedance of series TL section, Z1, is always greater than 70.7-Ω., as also exhibited by the plot.

The shunt stub TL section in series with the grounded

variable capacitor is used to nullify the imaginary part of admittance from series TL sections, at the two frequencies of interest, to realize concurrent dual-band impedance matching:

(7)

Considering the shortest electrical length, as given in (6), z1 remains constant whereas tan(θ1) changes its sign as frequency of operation toggles between the two desired frequencies. This leads to a simplified expression that satisfies (7):

(8)

Simultaneously solving non-linear expressions of (7), adapted for frequency f1, and (8), provides values for the two design parameters of the shunt stub TL section, i.e., electrical length and characteristic impedance. While solving, the normalized capacitance value, cv (= CV * Z0), is taken as an arbitrary value near the geometric mean of its allowable maximum and minimum limits.

In sum, using (3), (6) and (7), a concurrent reconfigurable multi-standard stubbed T-junction can be designed for any two distinct frequency bands. However, feasible fabrication limits will restrict designable frequency ratios. For instance, in order to ensure that a TL section is feasible for fabrication in our laboratory, the maximum value of characteristic impedance is limited to 100-Ω. This limitation bounds ratio of the two simultaneously matched frequencies, using (3) and (6):

(9)

Same result can be observed from characteristic impedance plot of each series TL section in Fig. 2. Within feasible design limits, the proposed T-junction circuit can be employed in power division applications using ports 2 and 3 as output ports, while connecting input radio frequency (RF) signal at port 1.

III. TUNABLE DUAL-BAND CHARACTERISTICS Stubbed T-junction structure, realized using afore-

mentioned design relations, exhibits perfect input port matching at the two chosen frequencies, simultaneously, for a

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selected capacitance value. Furthermore, change in capacitance varies the input admittance, due to which, the two matching frequencies shift, simultaneously. Hence, the design exhibits concurrent reconfigurable dual-band characteristics.

As a design example, a concurrent adaptable dual-band T-junction power splitter is designed using the afore-mentioned procedure to support commercially popular GSM-900 (890.2MHz to 914.8-MHz) and DCS-1800 (1.7102-GHz to 1.7848-GHz) bands. Hence, the two design frequencies are selected as 902.5-MHz and 1.7475-GHz, which correspond to uplink center frequencies of the two selected GSM standards, respectively. Condition (9) is satisfied by current frequency specifications, as the frequency ratio is 1.94, approximately. Using design equations (6) and (3), the electrical length and the characteristic impedance of each series TL section are calculated as 61.3o (at 902.5-MHz) and 75.8-Ω, respectively.

Skyworks’ hyperabrupt RF varactor diode, SMV1236 [7], is chosen as tuning capacitor in the proposed circuit. With its total capacitance limits of 3.8-pF and 26.75-pF, an intermediate CV value of 7-pF is used to design the tunable dual-band stubbed T-junction. Design relations (7), adapted for 902.5-MHz, and (8) lead to electrical length of 136.5o (at 902.5-MHz) and characteristic impedance of 50.6-Ω for shunt TL stub.

Simulations of the proposed T-junction splitter with the calculated design parameters demonstrate concurrent dual-band input impedance matching. This is displayed in Fig. 3, wherein input reflection co-efficient (Γ11) characteristic of the designed T-junction splitter is presented with operating frequencies. Ideal input matching is achieved at the two design frequencies of 902.5-MHz and 1.7475-GHz, concurrently, for the capacitance value at which the circuit was designed, viz., 7-pF.

The plot in Fig. 3 also depicts simultaneous tuning of matching frequencies with variation in ideal capacitance values from 1-pF to 25-pF. This simultaneous variation of the two distinct frequencies, at which port 1 is matched, with changes in ideal capacitor’s value (within its allowed limits) are explicitly shown in Fig. 4. The two plots indicate decrease in dual-operating-frequencies with increase in capacitance value.

IV. EXPERIMENTAL REALIZATION

A. Prototype Implementation The above design is realized using hybrid microwave

integrated circuit (HMIC) technology. Microstrip TL sections are implemented on Neltec’s NH9320 substrate with relative permittivity (εr) of 3.2, 1.524-mm (or 60-mil) dielectric height and dissipation factor of 0.0024. Copper (with conductivity 5.813 x 107 S/m) forms microstrip and ground conductive layers, each with a thickness of 18-μm.

Apart from that, the RF tuning varactor diode is biased using a bias-Tee network. The network consists of a λ0/4 (at center frequency, f0) high-impedance microstrip TL, shunted by an RF bypass capacitor. The varactor is biased using a DC supply that can be varied from 0-V to 10-V. Using Agilent’s ADS EMDS, the design was optimized to mitigate effects of fringing fields and higher order modes, associated with junction discontinuity. Fig. 5 shows the fabricated prototype with optimized microstrip lines’ dimensions given in Table I.

TABLE I. MICROSTRIP LINES DIMENSIONS IN FABRICATED STRUCTURE

S. No. Microstrip Line Structure Width (mm) Length (mm)

1 Series TL section (each) 1.65 36.4

2 Shunt stub TL section 3.00 61.0

3 Bias-Tee network 1.00 36.9

4 50-Ω TL port extensions 3.64 20.0

B. Measurement Results and Discussion Concurrent dual-band performance of the fabricated

stubbed T-junction is displayed in Fig. 6 that plots magnitude of reflection co-efficient (|Γ11|) at port 1 while the other two ports are terminated with 50-Ω. Simultaneous existence of minimum values at two distinct frequencies indicates dual-band matching at port 1.

The plot also depicts effect on return loss as DC supply to the tuning diode is varied from 0-V to 10-V. It indicates that as the DC bias is increased, the RF varactor’s effective capacitance decreases. This in turn modifies the input admittance, resulting in simultaneous shifting of the two minimas in |Γ11| towards higher frequencies. Hence, desired concurrent tuning of dual-band impedance matching of port 1 is exhibited by the fabricated prototype. The tunable frequency range covers the required GSM uplink bands, i.e., GSM-900 and DCS-1800. In fact, a larger tuning frequency range from 0.852-GHz to 1.008-GHz and 1.7-GHz to 1.82-GHz is achieved for complete DC bias range. This leads to fractional frequency tunability of 16.8% and 6.8% in the lower and the upper bands, respectively.

Moreover, the two minimas in return loss are always greater than 20-dB for whole tuning range. With such minimum limit, the prototype gives extended tunable frequency ranges from 0.803-GHz to 1.054-GHz and 1.68-GHz to 1.85GHz, around the lower and the upper design frequencies. The lower tuning range covers the adjacent GSM-850 (824MHz to 849-MHz) band. Hence, the T-junction prototype exhibits effective performance for extended lower and upper fractional tuning ranges of 27% and 9.6%, respectively.

Furthermore, the proposed T-junction’s structural symmetry leads to equal power division between the two output ports. In other words, the input-to-output forward transmission coefficient is -3dB for each output port. The circuit maintains this -3dB input-to-output power transfer ratio at both frequencies, where |Γ11| are minima even when DC bias to RF varactor is changed, as depicted in Fig. 7.

The plot shows that power transfer ratio from the input to

an output port (while terminating the other output port with the port reference 50-Ω), for the two DC supply limits of 0-V and 10-V. Besides, as is typical of a T-junction topology, isolation between the two output ports (with input port matched) is observed to be 6-dB. Hence, the proposed structure, designed using derived synthesis relations, establishes the speculated concurrent dual-frequency tunable characteristics.

Although symmetrical structure led to equal power division, the design approach can be extended for unequal power division using unsymmetrical series TL sections. Furthermore, tuning can be realized through other variable reactive options, viz., ferroelectric, MEMS, transistors, variable inductors, etc.

V. CONCLUSION A modified T-junction power divider is proposed for

simultaneous tuning of multiple distinct operating frequencies. Inclusion of frequency tunability feature into a multi-band component allows performance optimization at multiple desired channels. Besides, tunability gives capability to nullify errors, introduced during fabrication. The proposed structure and its synthesis relations are validated through a fabricated prototype, designed to simultaneously cover two distinct GSM900 and DCS1800 bands. The symmetrical circuit displays input-to-output RF power coupling of -3dB, while electronically tuning dual-band input return loss through an RF varactor diode. For effective input match of atleast -20dB, the circuit extends its operating range, covering adjacent GSM-850 band. Such operation extension will allow reconfigurable concurrent multi-band systems to cater to the needs of upcoming wireless communication radios.

REFERENCES

[1] V. Sharma, R. Yadav, and N. P. Pathak, “Series switched resonator based dual-band oscillator,” XXXth URSI General Assembly and Scientific Symposium (GASS), pp. 1-4, 13-20 Aug. 2011.

[2] M.-J. Park, and B. Lee, “Dual-band design of single-stub impedance matching networks with application to dual-band stubbed T-junctions,” Microwave and Optical Technology Letters, vol. 52, no. 6, pp. 1359-1362, Jun. 2010.

[3] G. Chaudhary, Y. Jeong, and J. Lim, “Harmonic suppressed dual-band bandpass filters with tunable passbands,” IEEE Transactions on Microwave Theory and Techniques, vol. 60, no. 7, pp. 2115-2123, Jul. 2012.

[4] E. E. Djoumessi, E. Marsan, C. Caloz, M. Chaker, and K. Wu, “Varactor-tuned dual-band quadrature hybrid coupler,” IEEE Microwave and Wireless Components Letters, vol. 16, no. 11, pp. 603-605, Nov. 2006.

[5] D. Draskovic, and D. Budimir, “Varactor tuned dual-band Wilkinson power divider,” IEEE Antennas and Propagation Society International Symposium (APSURSI), pp. 1-4, 1-5 Jun. 2009.

[6] V. Sharma, and N. P. Pathak, “Continuously tunable concurrent dual-frequency impedance matching network,” 7th IEEE International Conference on Industrial and Information Systems (ICIIS), pp. 1-3, 6-9 Aug. 2012.

[7] Skyworks SMV1231-SMV1237 Hyperabrupt Tuning Varactor Data Manual, Skyworks Solutions Inc., Woburn, MA, 2005.