This article has been accepted and published on J-STAGE in advance of copyediting. Content is final as presented. IEICE Electronics Express, Vol.* No.*,*-* Broadband analog phase shifter based on multi-stage all-pass networks Hsiao-Yun Li, Shiu-Cheng Chen, and Jia-Shiang Fu a) Department of Electrical Engineering, National Central University 300 Jhongda Rd., Jhongli City, Taoyuan 32001, Taiwan a) jsfu@ee.ncu.edu.tw Abstract: All-pass phase shifters using ferroelectric varactors are designed, fabricated, and measured. The design equations for the all-pass phase shifter are presented. The fabrication process of the ferroelectric varactors is described. Measurement results of single-stage all-pass phase shifters show that phase shift greater than 85 can be achieved under 10-V bias. At the frequency where maximum phase shift occurs, the insertion loss is less than 2 db and the return loss is greater than 12 db. Simulations based on the measured S parameters of the single-stage phase shifters show that, by cascading four stages of individually biased single-stage phase shifters, maximum phase shift of 180 can be achieved with a phase error as low as ±3 between 2.1 GHz and 3.25 GHz, corresponding to a 43% bandwidth. Keywords: phase shifter, all-pass network, ferroelectric capacitor Classification: Microwave and millimeter wave devices, circuits, and systems References IEICE 2013 DOI: 10.1587/elex.10.20130491 Received June 24, 2013 Accepted July 17, 2013 Publicized July 25, 2013 [1] B. Acikel, T. R. Taylor, P. J. Hansen, J. S. Speck, and R. A. York, A new high performance phase shifter using Ba x Sr l x TiO 3 thin films, IEEE Microw. Wireless Compon. Lett., vol. 12, no. 7, pp. 237 239, July 2002. [2] K. Miyaguchi, M. Hieda, K. Nakahara, H. Kurusu, M. Nii, M. Kasahara, T. Takagi, and S. Urasaki, An ultra-broad-band reflection-type phase-shifter MMIC with series and parallel LC circuits, IEEE Trans. Microw. Theory Tech., vol. 49, no. 12, pp. 2446 2452, Dec. 2001. [3] D. Adler and R. Popovich, Broadband switched-bit phase shifter using all-pass networks, IEEE MTT-S Int. Microw. Symp., July 1991, pp. 265 268. [4] X. Tang and K. Mouthaan, Design of large bandwidth phase shifters using common mode all-pass networks, IEEE Microw. Wireless Compon. Lett., vol. 22, no. 2, pp. 55 57, Feb. 2012. [5] D. Kim, Y. Choi, M. Ahn, M. G. Allen, J. S. Kenney, and P. Marry, 2.4 GHz continuously variable ferroelectric phase shifters using all-pass networks, IEEE Microw. Wireless Compon. Lett., vol. 13, no. 10, pp. 434 436, Oct. 2003. [6] L.-Y. V. Chen, R. Forse, A. H. Cardona, T. C. Watson, and R. York, Compact analog phase shifters using thin-film (Ba,Sr)TiO 3 varactors, IEEE MTT-S Int.
Microw. Symp., Jun. 2007, pp. 667 670. [7] S. Darlington, Realization of a constant phase difference, Bell Syst. Tech. J., vol. 29, pp. 94 104, Jan. 1950. [8] H.-Y. Li, S.-C. Chen, and J.-S. Fu, Ferroelectric thin-film integrated capacitor and its application in radio-frequency phase shifter design, submitted to 2013 IEEE Electr. Design Adv. Packag. Syst. Symp. 1 Introduction Phase shifters are important circuit blocks in a phased array, which is widely used in radars and radiometers for various applications. To implement a phase shifter, many circuit topologies exist, e.g. capacitively loaded transmission line [1], reflection-type [2], etc. Among them, all-pass network has been used to realize both digital [3, 4] and analog phase shifters [5, 6]. The advantages of all-pass phase shifter include excellent return loss and flat insertion loss responses over wide bandwidth and large phase shift under low capacitance tuning ratio [3]. Although large phase shift can be obtained, the phase shift is however quite narrowband. Cascading multiple stages of all-pass networks has been proposed to provide a constant (equal-ripple) phase shift over a wide frequency range [3, 4, 6, 7]. Nevertheless, the phase error reported in previous work is still large. For example, phase error of at least ±15 is observed for both the digital all-pass phase shifter in [3] and analog all-pass phase shifter in [6]. In this work, ferroelectric-based analog all-pass phase shifters are designed, fabricated, and measured. We have shown that, by individually biasing the single-stage all-pass phase shifters in a multi-stage all-pass phase shifter, maximum phase error as low as ±3 can be achieved over a 43% bandwidth. 2 All-pass network As shown in Fig. 1, there exist two configurations for second-order common-mode all-pass networks: series-l and series-c configurations [3]. Both configurations exhibit the same S parameters, which can be derived using even-odd mode analysis. Using the notation in [3], it is defined that and T ZT z, (1) Z 0 where 1 T and LC L Z T. (2) C ω and Z 0 denote angular frequency and system impedance, respectively. The S 11 and S 21 of the all-pass network can be then expressed as follows. and j( )( z z ) S 11 (3) 2 1 ( ) j( )( z z )
Fig. 1. All-pass networks: series-l, and series-c configurations. (after [8]). 2 S 1 ( ) 21 2 1 ( ) j( )( z z ). (4) As can be inferred from Eq. (3), if the L and C are chosen so that z = 1, or equivalently, L/C = Z 0 2, then the S 11 of the network would be equal to zero, regardless of frequency. Meanwhile, the magnitude of S 21 would be one if z = 1 holds. In other words, signal entirely passes through the network without reflection at all frequencies, resulting in an all-pass frequency response. 3 Single-stage all-pass phase shifter 3.1 Design As stated in Section 2, when z = 1 holds, the magnitude of S 21 is equal to one and is frequency-independent. On the other hand, the phase of S 21 can be as well derived from Eq. (4) and is found to be a function of frequency: S 21 2tan. (5) If L and C in the network are simultaneously varied while maintaining the condition that z = 1, then the network would remain all-pass but ϕ is changed. Therefore, the all-pass network can be used to implement phase shifters. As shown in the above analysis, the L and C must be simultaneously varied in order to maintain the all-pass frequency response. However, since variable inductors are not as commonly available as variable capacitors, only C is varied in practice. Although using a fixed inductance (L = L 0 ) results in nonzero reflection, S 11 at the frequency of operation, ω c, can be maintained below 10 db as long as C is varied between C 0 /2 and 2C 0, where C 0 1 Z and c 0 Z L 0 0. (6) c In this work, phase shifters are designed based on the all-pass networks. The schematic of the designed all-pass phase shifter is shown in Fig. 2. As shown in the Fig. 2, the inductance is fixed whereas the capacitance is variable. The bias voltage for the varactors is applied through one of the rf ports using a bias-tee. To properly bias the two series varactors, a large resistor R BB (> 2 k) is added, which provides dc path to ground while having little impact on the rf performance. Two all-pass phase shifters are designed at 2 GHz and 3.4 GHz and are designated
Fig. 2. Schematic of an all-pass phase shifter with fixed inductance and variable capacitance (modified from [8]), and photograph of the proposed ferroelectric-based all-pass phase shifter. as PHS-LB and PHS-HB, respectively. Using Eq. (6), the fixed inductance can be calculated to be 3.98 nh and 2.34 nh for the 2-GHz and 3.4-GHz phase shifters, respectively. Surface-mount chip inductors with 0201 case size are used to realize the desired inductance. On the other hand, the varactors are implemented using MIM (metal-insulator-metal) capacitors with ferroelectric BST (barium strontium titanate, Ba 1 x Sr x TiO 3 ) as the insulator. As a ferroelectric material, BST exhibits field-dependent permittivity. Therefore, the capacitance of a thin-film BST MIM capacitor can be controlled by changing the bias voltage across its two terminals. The fabrication process for the phase shifters with ferroelectric BST capacitors will be described in Section 3.2. Layout of the all-pass shifters is drawn and full-wave electromagnetic simulations are performed to account for the effects of pads and connecting lines. With full-wave electromagnetic simulation results, the frequencies at which maximum phase shift occurs shift slightly lower to 1.9 GHz and 3.1 GHz from the originally-designed 2 GHz and 3.4 GHz, respectively. 3.2 Fabrication The all-pass phase shifters with ferroelectric varactors are fabricated on a 430-μm-thick c-plane sapphire substrate. Bottom metal layer made of Cr/Pt (5/100 nm) is first formed using thermal/e-gun evaporation and standard lift-off process. Next, 200-nm-thick BST thin film is deposited using pulsed laser deposition (PLD) technique. The substrate temperature is 850 C. The composition of the target is Ba 0.5 Sr 0.5 TiO 3. The wavelength and pulse energy of the laser pulses are 248 nm and 180 mj, respectively. Constant flow of oxygen is supplied throughout the deposition. The pressure of the oxygen is maintained at about 250 mtorr. The BST thin film is then etched using a diluted HF solution to expose the bottom electrodes for further connections. Following that, top metal layer made of Ti/Pt/Ti/Au (5/100/5/1000 nm) is fabricated using e-gun and thermal evaporation techniques and standard lift-off process. Finally, surface-mount inductors and resistor are mounted on the substrate using silver epoxy. The photograph of one of the fabricated all-pass phase shifters is shown in Fig. 2. 3.3 Results According to measurement results of the fabricated ferroelectric capacitors, the capacitance density of the ferroelectric capacitors in our process is approximately
Fig. 3. The measurement results of the ferroelectric-based all-pass phase shifters: PHS-LB (after [8]), and PHS-HB. Top: S 11 and S 21 versus frequency for bias voltage swept from 0 V to 10 V; middle: phase shift versus frequency for various bias voltages; bottom: S 11, S 21, and phase shift versus bias voltage at frequency where maximum phase shift occurs. 19 ff/μm 2 at 0-V bias. The tunability of the capacitance reaches 50% as the bias voltage is increased from 0 V to 7 V. The measurement results of the ferroelectric-based all-pass phase shifters are plotted in Fig. 3. The results for PHS-LB and PHS-HB are shown in Fig. 3 and, respectively. By looking at the S 11 and S 21, it can be seen that both of the phase shifters exhibit all-pass frequency responses from dc to 6 GHz for bias voltage ranging from 0 V to 10 V. When biased up to 10 V, maximum phase shifts of 87.8 and 97.8 are achieved for PHS-LB and PHS-HB, respectively. The frequency at which maximum phase shift occurs is 1.75 GHz for PHS-LB and 3.15 GHz for PHS-HB. For bias voltage between 0 V and 10 V, the insertion loss is less than 2 db and return loss is greater than 12 db for both PHS-LB and PHS-HB at 1.75 GHz and 3.15 GHz, respectively.
Fig. 4. Simulation results of a four-stage all-pass phase shifter: S 11 and S 21, and phase shift. 4 Multi-stage all-pass phase shifter As shown in Fig. 3, the S 11 and S 21 of a single-stage all-pass phase shifter are fairly wideband. In contrast, the bandwidth for the phase shift is quite narrow. To improve the bandwidth for phase shift, one may cascade multiple stages of all-pass phase shifters that are designed at different frequencies [3, 4, 6, 7]. Based on this idea, a multi-stage all-pass phase shifter is simulated using the measured S parameters of the single-stage all-pass phase shifter presented in Section 3. Four single-stage phase shifters are connected in cascade as shown in the inset of Fig. 4. To produce a flat phase-shift response, PHS-LB and PHS-HB need to be individually biased. The combinations of the bias voltages for PHS-LB and PHS-HB are shown in Fig. 4. The bias voltage for PHS-LB is between 3 V and 8.5 V whereas that for PHS-HB is between 2 V and 9.5 V. Under these bias-voltage combinations, the multi-stage phase shifter exhibits an insertion loss less than 7 db and a VSWR less than 2 from dc to 3.7 GHz. Maximum phase shift exceeding 180 can be achieved. Furthermore, maximum phase error is as low as ±3 between 2.1 GHz and 3.25 GHz, which corresponds to a bandwidth of 43%. 5 Conclusion In this work, ferroelectric-based all-pass phase shifters are designed, fabricated, and measured. The fabrication process of the ferroelectric BST capacitors is described. The fabricated single-stage all-pass phase shifters exhibit wideband responses for insertion loss and return loss as well as large phase shift for a narrower frequency range. Cascading multiple single-stage all-pass phase shifters is proposed as a mean to achieve broadband phase-shift response. Simulation of a four-stage all-pass phase shifter is performed based on the measured S parameters of the single-stage phase shifters. Simulation results demonstrate that low phase error can be achieved over a wide bandwidth. Acknowledgments This work is supported by National Science Council of Taiwan under NSC 101-2220-E-008-003 and NSC 102-2220-E-008-010. The authors thank the National Center for High-Performance Computing for providing software.