High-Selectivity Reflectionless Unbalanced-to-Balanced Filtering Power Combiner

A high-selectivity reflectionless unbalanced-tobalanced (UTB) filtering power combiner is proposed. The proposed power combiner composes of two absorbing branches, two filtering sections, two transmission lines, a grounded resistor, and a phase inverter. Two transmission zeros respectively located at the lower and upper sides of the passband are achieved by the filtering sections, resulting in high selectivity. The input-port reflectionless response in the bandstop region is obtained by the absorbing branch. A 1.0-GHz UTB filtering power combiner is designed and fabricated. Finally, the measurement performances are given in this paper. The input-reflection absorptive bandwidth and transmission bandwidth are measured as 545 MHz and 132 MHz, respectively. The absorptive bandwidth is 4.12 times the transmission bandwidth.


Introduction
Power combiner is capable of summing signal power, which has been widely used in balanced amplifier [1] and the linear amplification system using nonlinear components [2]. Furthermore, power amplifiers with high output power have become more and more important in 5G mobile communication and radar systems. As the operating frequency increases, the power handling capability of semiconductor solid-state devices is reduced. Combining power from multiple individual amplifier unit cells to obtain the desired power levels is in demand [3]. Such systems typically involve filters for frequency selection and attenuation of other bands [4]. Commonly, the combined signals in the passband are transmitted to the output port, and those in the stopband are reflected into the source. These undesired RFsignal-power reflections could cause additional unwanted intermodulation products through the remixing of such RF power reflections with the local oscillator or power amplifiers [5]. Hence, it is required to absorb the reflection power of the input signals in the stopband.
For the conventional filters, an additional branch is used to reduce the reflection in the stopband [6][7][8], but these structures cannot be directly applied to the filtering power combiner. Based on the Wilkinson power divider, absorptive branch [5] and parallel-LC resonators [9] are added to realize the reflectionless filtering power dividers. The heat-sinking capacity of these Wilkinson power divider-based structures can be further improved.
With the advantage of suppressing environmental noise and neutralizing electromagnetic interference, balanced components such as balanced filters [10][11][12], power divider [13][14][15], and coupler [16], have drawn more and more attention. However, a balun is needed to convert signals between unbalanced and balanced circuits in one system, which occupies an additional area. To solve this problem, circuits with both unbalanced and balanced ports are proposed, such as the unbalanced-to-balanced (UTB) filter [17], [18], diplexer [19], and balanced-to-unbalanced (BTU) power divider [20], these circuits also have the function of common-mode suppression (CMS).
Due to the good isolation between the two output ports, a Gysel power divider can work as a high-power combiner. Based on the Gysel power divider, branch lines with open-and short-circuited stubs are used to realize the filtering function of a BTU power divider [21], but the performance of the CMS and the isolation between the two unbalanced ports needs to be further improved. A BTU power divider is present in [22], whose filtering characteristics are realized by using coupled lines. Nevertheless, the 15-dB CMS bandwidth is narrower than the differential-mode operation bandwidth. Dual-mode ring resonators are used to replace the λ/4 transmission lines to obtain high selectivity filtering function in [23]. However, the good CMS and isolation bandwidth should be broadened.
In this paper, a high-selectivity reflectionless UTB filtering power combiner is proposed. It has two transmission zeros near the passband edges leading to high selectivity filtering characteristics.

Theory Analysis and Design
The schematic of the proposed high-selectivity reflectionless UTB filtering power combiner is depicted in Fig. 1. The input ports 1 and 4 are two unbalanced ports, the balanced output port A comprises of ports 2 and 3 terminated in Z 0 .
The mixed-mode scattering parameter S mm for the proposed UTB power combiner is expected to satisfy the matching condition at both balanced and unbalanced ports, good isolation between the unbalanced input ports, good CMS, and out-of-phase equal power combination. To meet the requirements, [S mm ] can be given as follows according to [24]   ) ( 2 φ 1 and φ 2 are the phase shifts of Block A and Block 2, respectively. The desired standard scattering matrix [S std ] of Block 1 can be derived as follows by using (1)

Theory Analysis of Block 1
As shown with dashed-dotted-dashed lines in Fig. 1, Block 1 is composed of two filtering sections (i.e., Block A), two transmission lines (Z 1 , λ/4), a grounded resistor (R 1 ), and a phase inverter (Z 2 , λ/2). The filtering section consists of two short-circuited stubs (Z 2 , θ 1 ), two transmission lines (Z 2 , θ 2 ), and an open-circuited stub (Z 2 , θ 3 ). The simplified circuits in terms of odd-and even-mode of Block 1 are given in Fig. 2. By decomposing the desired standard scattering matrix (9), the scattering matrixes of the odd-and evenmode equivalent circuits of Block 1 are needed as At the center frequency f 0 , the effects of the two λ/4 short-circuited stubs in Fig. 2(a) can be omitted, hence the odd-mode circuit is simplified to be a filtering section in Fig. 3(a), and it can be equivalent to one section of λ/4 transmission line as shown in Fig. 3(b) for satisfying the needed S-parameters as (10). Based on this equivalent relationship and ABCD matrix analysis, the electrical parameters of the filtering section can be derived.
The ABCD matrix of the filtering section can be derived as The ABCD matrix of the λ/4 transmission line with the characteristic impedance Z 0 is expressed as For simplicity, Z 2 is set to be equal to Z 0 .
By applying the ABCD parameters A a = A b , B a /C a = B b /C b , the conditions for the filtering section at f 0 equivalent to the λ/4 transmission line are found as      At f 0 , the filtering section in Fig. 2(b) can be replaced by the λ/4 transmission line, as shown in Fig. 4(a). For port 1, the effects of the λ/4 open-circuited stub (Z 2 ) and the λ/4 transmission line (Z 0 ) in Fig. 4(a) at the center frequency can be omitted. Thus, Fig. 4(a) is further simplified as Fig. 4(b).
To meet the matching condition in terms of evenmode (S e11 = 0), the relation between the characteristic impedance Z 1 and resistor R 1 in Fig. 4(b) can be obtained as To analyze the effect of the electrical lengths on the performance, five cases are given in Tab  (14)- (17). As shown in Fig. 5(a), when θ 3 decreases from 112° to 98°, the transmission zeros of |S dsA1 | become closer as a result of higher selectivity, but the transmission bandwidth is decreased. Therefore, there is a trade-off between selectivity and bandwidth. Furthermore, the center frequency of the UTB combiner can be flexibly controlled by the electrical lengths θ 1 , θ 2 , and θ 3 , as shown in Fig. 5(b).

Analysis of Block 2
To achieve the input-port reflectionless response, two absorbing branches (i.e., Block 2, as shown with dash line in Fig. 1) are separately connected at two unbalanced ports (i.e., ports 1 and 4). Each absorbing branch consists of a transmission-line section (Z 3 , λ/4), two transmission-line branches (Z 4 , λ/4), an absorbing resistor (R 2 ), and shortcircuited coupled lines (Z e , Z o , and λ/4). Owe to the absorbing branch, two additional reflection zeros are generated, which enhance the matching bandwidth of |S ddAA |, as shown in Fig. 6. Furthermore, the introduction of the absorbing branches will not affect the location of the two transmission zeros. Figure 7 gives the simulated responses of the proposed reflectionless UTB filtering power combiner with different values of Z 4 . As shown in Fig. 7(a), Z 4 has effects on the −10-dB input-reflection absorptive bandwidth (i.e., the bandwidth for |S ss11 | < −10 dB), and the input-reflection responses around 0.95f 0 and 1.04f 0 becomes poor when Z 4 decreases from 80 to 40 Ω. It can be seen from Fig. 7(b) that a smaller value of Z 4 causes poor responses of |S ddAA | and |S dsA1 | in the passband.  Figure 8 gives the simulated responses of the proposed reflectionless UTB filtering power combiner with different values of Z e . As shown in Fig. 8(a), weakly coupling (Z e = 80 Ω and Z o = 55 Ω) affects the input-reflection responses around 0.95f 0 and 1.04f 0 , and tightly coupling (Z e = 120 Ω and Z o = 55 Ω) degrades input-reflection response around 1.09f 0 . It can be found from Fig. 8(b) that Z e also has effects on the differential-mode return loss and transmission coefficient (|S ddAA | and |S dsA1 |) in the passband.
Based on the previous analysis, the following steps are suggested to determine the parameters in Block 2.
3) Select proper Z e and Z o to avoid the degraded input-reflection responses in the absorptive bandwidth, referring to Fig. 8.

Implementation and Performance
To demonstrate the theory in Sec. 2, a microstrip high-selectivity reflectionless UTB filtering power combiner is implemented on a microwave substrate (relative permittivity ε r = 2.65, thickness h = 1.5 mm) as shown in Fig. 9. The values of the isolation resistor and absorbing resistor are R 1 = R 2 = 50 Ω. The center frequency is f 0 = 1.0 GHz. The dimensions are finally obtained with the HFSS electromagnetic simulation, as given in Tab. 2.   Simulation and measurement curves of the proposed UTB filtering power combiner are given in Fig. 10. For the unbalanced-port responses, the measured 3-dB transmission band of |S dsA1 | and |S dsA4 | is 933-1065 MHz (thirteen-point-two percent). The −10-dB absorptive band of input-reflection is 700-1245 MHz (fifty-five percent), which is 4.12 times the transmission band. The isolation is 24.6 dB at f 0 , and better than 17.8 dB from 0.7 to 1.3 GHz. Two transmission zeros at 0.838 and 1.215 GHz are obtained. The attenuation rates (AR) are 137.1 and 140.5 dB, respectively, where the AR is defined as For the balanced-port responses, the CMS is better than 15 dB from 0.7 to 1.3 GHz, with excellent CMS of 40.1 dB at f 0 . The 10-dB return-loss band of the differential mode is 941-1061 MHz (twelve percent). Figure 11 gives the simulation and measurement curves of the phase difference between S dsA4 and S dsA1 . The measured phase difference between the unbalanced ports is 180°± 0.4° from 884 to 1100 MHz.
The comparisons among AR, CMS, isolation, 3-dB transmission bandwidth, 15-dB CMS bandwidth, circuit size, and whether with reflectionless response are summarized in Tab. 3. Compared with the previous works, the proposed UTB filtering power combiner has sharp edges at both lower and upper sides of the passband and wide CMS bandwidth. A reflectionless response is obtained to avoid the input-reflection.

Conclusion
In this paper, a high-selectivity reflectionless UTB filtering power combiner has been presented. The designed circuit can achieve the following advantages: (i) an inputreflection absorptive bandwidth is 4.12-times 3-dB transmission bandwidth, (ii) high-selectivity filtering characteristics, (iii) excellent common-mode suppression and isolation within the whole input-reflection absorptive band. In addition, the proposed power combiner also can work as a high-selectivity filtering power divider from the balanced port to the unbalanced ports. For high power applications, the grounded resistors of the proposed power combiner can be external to the packaging structure for efficient heat dissipation [25] or implemented with microwave highpower terminations [26].