Dual‐band branch‐line coupler with two center‐tapped stubs

Abstract

Figure 5 shows the measured responses of the conventional and compensated couplers. The coupling of both couplers shows 3.3 dB at the center frequency. The measured return loss with the compensated broadside coupler exhibits an increase of 6.1 dB (from 17.2 to 23.3 dB) at 3.5 GHz. And the measured isolation shows an enhancement by 4.0 dB (from 18.3 to 22.3 dB) with the compensated design. Further improvement of return loss and isolation seems to be possible with more optimized design parameters. CONCLUSIONPerformance enhancement technique of the broadside coupler for the tight coupling has been realized with the LTCC technologies. Compensation approach by adding branch lines operates very well at C-band, resulting in 4.0 dB improvement of the isolation. The proposed compensation technique can be applicable at high frequency because the parasitic effects of branch lines are less than that of lumped elements. Recently, the advances in modern wireless system require the multi-band operation capability and the conventional single-frequency microwave circuit components are evolving with the dualor the multi-band functionality. In the case of branch-line couplers, several dual-band schemes are proposed in the literature [2][3][4][5]. These include the use of left-handed transmission lines [2], crosscoupled branch-line structure [3], and the use of stub lines at the input ports [4] or at the center of the branches [5]. DUAL-BAND BRANCH-LINE COUPLER WITH TWO CENTER-TAPPED STUBSThis letter proposes a new scheme for the dual-band operation of the branch-line coupler. The proposed structure is based on the use of center-tapped stub lines at one pair of branches of the conventional branch-line coupler. Although the existing stub-based dual-band schemes require four stubs in total for every ports [4] or branches [5] of the coupler, the proposed method requires only two stubs for two of the four branches of the coupler. The circuit complexity can be reduced accordingly and the structural change from the conventional branch-line coupler is minimized. As a matter of fact, the proposed structure reduces to the conventional branch line coupler for the specific band-ratio of 1-3.The proposed coupler structure is analyzed in detail using the scattering parameter formulation, and the exact design formulas are derived for the dual-band operation of the coupler. The validity of the proposed method is confirmed through the design, simulation and the measurement of a prototype dual-band coupler. Figure 1 shows the schematic representation of the proposed branch-line coupler for dual-band operation. For simplicity, only the open stub case is presented in the figure and will be analyzed in detail. The shorted stub case can be treated similarly, and the results will be presented at the end of this section. ANALYSIS AND DESIGNThe overall structure of the proposed coupler is quite similar to that of the conventional branch-line coupler except for two centertapped stub lines z 3 at two vertical branches of the coupler. The use of stub li...

Dual-Band, Unequal Length Branch-Line Coupler With Center-Tapped Stubs

“…The structure of the proposed coupler can be further simplified by eliminating the stub lines for some special cases, leading to the stub-less dual-band operation with great structural advantages. In performance, the same output phase relation can be obtained between the two bands, while some of the previous dual-band schemes [2], [5], [6] produce reversed output phase relation between two bands.…”

“…The proposed structure can be obtained by attaching the center-tapped stubs to the long branches of the unequal length branch-line coupler along with the careful choice of the lengths and impedances of each branch for the dual-band operation. This makes the proposed coupler the smallest one among similar dual-band structures based on center-tapped stubs [5], [6]. The structure of the proposed coupler can be further simplified by eliminating the stub lines for some special cases, leading to the stub-less dual-band operation with great structural advantages.…”

“…In the proposed coupler structure, the long, horizontal branches of the unequal length coupler in Fig. 1 are converted into the center-tapped stub topology for the dual-band operation [5], [6]. The vertical branches remain the same as those in Fig.…”

“…In the case of branch-line couplers, the dual-band operations have been reported using the left-handed transmission lines [1], the cross-coupled branch-line structure [2], the rectangular patches [3], and the use of stub lines at the input ports [4] or at the center of the branches [5], [6]. Most of the existing dual-band branch-line couplers are based on the conventional equal-length branch-line coupler with square shapes [2], [5], [6] and tends to have excessive stub impedances as the band-ratio approaches 1:3.…”

Dual band planar hybrid coupler with enhanced bandwidth using particle swarm optimization technique

“…Furthermore, the use of circuits able to be functional at multiple frequencies is of great interest for the integration of the circuitry in systems that can operate at multiple frequency bands. In the case of branch-line couplers, different techniques have been investigated to extend the bandwidth of the conventional hybrid such as using the dual stage hybrid coupler [1], left-handed transmission lines [2], the cross-coupled branch-line structure [3], the rectangular patches [4], and stub lines at the input ports [5] or at the center of the branches [6,7]. In addition, particle swarm optimization (PSO) is a population based stochastic optimization technique developed by Dr. Eberhart and Dr. Kennedy in 1995 [8], inspired by social behavior of bird flocking or fish schooling.…”