PIERS ONLINE, VOL. 4, NO. 7, 28 726 On the Development of Tunable Microwave Devices for Frequency Agile Applications Jia-Sheng Hong and Young-Hoon Chun Department of Electrical, Electronic and Computer Engineering Heriot Watt University, Edinburgh, EH14 4AS, United Kingdom Abstract This paper presents a recent development of electronically tunable microwave devices for frequency agile applications such as emerging cognitive radios and ultra-wide band (UWB) wireless systems. 1. INTRODUCTION Since the demands for wireless communications increase and become complex, RF front-ends need multi-band or wideband circuits to satisfy several standards of wireless systems, which makes a tunable circuit essential [1, 2]. An adaptive controlled system which can enhance the performances of a wireless system also requires the tunable devices and circuits such as tunable phase shifters and tunable filters. In this paper, we review some our newly developed tunable microwave devices, which are based on new device structures and tuning elements [3 6]. For example, we have investigated new tunable device structures comprised of variable characteristic impedance (Z C ) transmission line which can be used as a distributed tuning circuit. Demonstrators for this type of tunable devices have been designed, fabricated and tested. Both simulated and experimental results are presented. In addition to semiconductor tuning elements used, ferroelectric materials are of great interest for the development of electrically tuned microwave components and circuits. Rapid tuning speed, moderate insertion loss at microwave frequencies, high power handling capacity and simple fabrication process make them more attractive. In this paper, as a practical application of ferroelectric materials, we will present some newly developed bandstop filters based on variable capacitors on BST thin film. A tunable bandstop or band reject filter, which is considered as an application of BST varactors in this work, has become more essential for wideband wireless communication systems as it can reduce incoming or emitted unwanted signals effectively. The designs using defected ground structures or electromagnetic bandgap structures will be described. 2. TUNABLE PHASE SHIFTER Figure 1 is a unit cell of conventional distributed phase shifter. Overall input impedance should be designed to vary around 5 Ω and its delay will be changed, while variable capacitance devices, such as varactor diodes, have different values of capacitance ( ). In this case, the return loss can be degraded due to varying while a transmission line, which is inductive, has a fixed value of Z C. It usually limits the tuning range of phase shifters. If we are able to implement a device in Fig. 1, wider tuning range of phase shifters can be achievable. Fig. 1(c) illustrates how a tunable Z C transmission line can be implemented with conventional devices such as CPW lines and lumped elements. Simulated results show that Z C can be electronically varied from 55 to 1 Ω at the center frequency of 8 MHz, which has higher value of Z C for impedance of a phase shifter. A fabricated tunable phase shifter of this type is photographed in Fig. 2. It consists of four tunable Z C lines and five tuning varactor diodes, D p, which are adjusted by dc bias, V C1. Varactor diode, D Z, with adjustable capacitance, C var2, is controlled by co have better return losses with consistent phase differences. All the varactors used are Infenion BB833. Its differential phase shift is demonstrated in Fig. 2 which shows the differential phase shift as a function of frequency for several bias values for the condition that the return losses are less than 12 db up to 85 MHz. It can be seen from this graph that, at 85 MHz, the differential phase shift is continuously variable from to 9 by adjusting the bias. When V C2 is fixed, tuning range with the same return loss property shrinks to 85. From the measured data the figure of merit (i.e., degree/db) for the phase shifter can be extracted. Some typical data are given as follows. For a fixed V C2 of 2 V, when V C1 varies the figures of merit are 86 degree/db, 14 degree/db and 114 degree/db for V C1 = 6.5,
PIERS ONLINE, VOL. 4, NO. 7, 28 727 Z C Z C_var Ground C var2 (D Z ) Z C_var C SE C S C SE (c) Ground Figure 1: Schematic diagram of a phase shifter using novel tunable transmission line; a unit cell of the conventional distributed phase shifter, a unit cell of a proposed phase shifter, and (c) a detailed block diagram for the realization of a tunable transmission line. 8 and 1 V respectively. As can be seen the figure of merit increases against V C1. This is because the capacitance of the varactors used is actually decreased when the bias voltage V C1 increases, resulting in a higher quality factor at a larger V C1. Using high Q varactors can improve the figure of merit. Also, a higher figure of merit can be obtained if referring to another V C2. Bias for phaseeshift (V C1 ) Varactors to adjust phase eshift Varactors to adjust just Z C Z C (D Z ) Bias fo rz C (V C2 ) Differential phase shift [Degree] 12 1 8 6 4 2 V C1 =6.5 V, V C2 = 2 V V C1 =8. V, V C2 = 2 V V C1 =1 V, V C2 = 2 V V C1 =2 V, V C2 = 1 V.2.4.6.8 1. Figure 2: Photograph of the fabricated tunable phase shifter. Differential phase shift versus frequency for selected values of varactors bias. The phase shift is with respect to the transmitted phase at V C1 = 2 V and V C2 = 1 V bias. 3. TUNABLE FILTERS 3.1. Tunable Bandstop Filters A bandstop filter (BSF) is designed at the center frequency of 1.5 GHz with the bandwidth of 2% which has a schematic diagram of Fig. 3. Its resonators can be easily replaced with stepped impedance line resonators of Fig. 3. Their frequency responses are identical at the center frequency. In order to make the circuit tunable, we modified it further as shown in Fig. 3(c) and (d). Whereas Fig. 3(c) can be straightforwardly realized by connecting varactor chips at the end of
PIERS ONLINE, VOL. 4, NO. 7, 28 728 shunt stubs, the realization of Fig. 3(d) would be sophisticated. We can change either the electrical length or the characteristic impedance, or both of them as indicated in Fig. 4. The first method is a capacitive loaded transmission line. It is quite frequently used to implement a phase shifter. The second method is a variable Z C transmission line which could adjust the characteristic impedance of the transmission line with slight change of electrical length described above. Z 12, 12 Z 1a, 1a Z 2a, 2a Z 1b, 1b Z 2b, 2b (c) (d) Figure 3: Schematic of a bandstop filter, its modified circuit with SIRs and tunable BSFs using a tunable SIR with lumped circuit (c) and distributed circuit (d). Figure 4: Realization methods of a tunable transmission line of : capacitor loaded transmission line and (c) variable Z C transmission line. Figure 5 shows a physical realization of the proposed tunable BSF, where the Ferroelectric Barium-Strontium-Titanate (BST) film varactor chips are used as the tuning elements. The measured results are plotted in Fig. 5, showing a tuning range that is around 1%. This type of BSF has not only good rejection, but also has an advantage to implement a DC bias circuit. In this case, the DC voltage is applied to an additional coupled line instead of a transmission line which is connected with input or output, which reduces spurious response from bias circuitry. Another newly developed BST-varactor tunable bandstop filter (BSF) is based on a defected or slotted ground structure as shown in Fig. 6. This structure can be explained as a kind of electro- (c)
PIERS ONLINE, VOL. 4, NO. 7, 28 729 Top View Variable Z C Transmission line Bottom View BST varactor chips DC Bias Supply S-parameters [db] -5-1 -15-2 -25-3 -35-4 -45-5 S 11- V S 21- V S 11-3V S 21-3V S 11-6V S 21-6V.6.8 1. 1.2 1. 4 1.6 1. 8 2. Figure 5: Fabricated tunable BSF. Measured results. magnetic bandgap (EBG) structure. The BST varactor chips are attached onto the fabricated BSF as illustrated. The fabricated tunable BSF with slotted ground structure has overall dimensions of 5. by 2.5 cm 2. Scattering parameter measurements are performed using Agilent 851B network analyzer over the frequency range from.5 to 2.5 GHz. Fig. 6 gives the simulated and measured responses of the bandstop filter in which we can observe that it operates at the center frequency from 1.2 to 1.4 GHz and has the bandwidth 1 MHz. Hence, the measured tuning range is 14%. -5 S21 [db] -1-15 BST varactor chips DC bias -2 Simulation (2pF) Simulation (3pF) Mea sured (V) Mea sured (35V) -25.5 1. 1.5 2. 2.5 Figure 6: Tunable microstrip BSF filter based on slotted ground structure. Simulated and measured results. Air Bridge So lid (Symbol) lines Measu red results BST chips S-Parameters [db] -1-2 -3 35V V C =V Dotted lines Simulated results S 11 S 21 Z S, θ S (Variable Z C Tx Line) DC bias -4 1. 1.5 2. 2.5 3. Figure 7: BST-varactor tunable bandpass filter. Simulated and measured results.
PIERS ONLINE, VOL. 4, NO. 7, 28 73 3.2. Tunable Bandpass Filters Figure 7 shows a recently developed BST-varactor tunable bandpass filter (BPF). This is a bandwidth-tunable dual-mode ring-resonator bandpass filter, which is designed by adopting a novel tunable circuit based on variable Z C or tunable impedance transmission line. The variable Z C transmission line is used for the perturbation of two degenerate modes which makes the bandwidth of filter adjusted as the value of characteristic impedance of the transmission line. A demonstrator of thin-film BST-based tunable bandpass filter with dual mode ring resonator is fabricated on CPW substrate and measured. Fig. 7 gives the simulated and measured responses of the bandpass filter in which we can observe that it operates at the center frequency of around 1.8 GHz and has the adjustable 3-dB bandwidth from 276 MHz to 318 MHz. Hence, the tuning ratio of maximum to minimum bandwidth is 1.15:1 for the given DC bias range. 4. CONCLUSIONS Several electronically tunable devices including tunable phase shifters, tunable bandstop and bandpass filters have been presented. It has been shown that the use of the variable Z C transmission line in association with the semiconductor or ferroelectric BST varactors can make the electronic tuning effectively. ACKNOWLEDGMENT The support from the U.K. Engineering and Physical Research Council (EPSRC) is acknowledged. The authors would also like to thank their project partners at the University of Birmingham and Cranfield University for fabricating the BST thin film devices. REFERENCES 1. Mitola III, J., Cognitive radio for flexible mobile multimedia communications, IEEE Int. Workshop on Mobile Multimedia Comm. Dig., 3 1, Nov. 1999. 2. Revision of part 15 of the Commission s rules regarding ultra-wide-band transmission system, ET-Docket 98-153, First note and order, Federal Communication Commission, Feb. 14, 22. 3. Chun, Y. H. and J.-S. Hong, A novel tunable transmission line and its application to a phase shifter, IEEE Microwave and Wireless Components Letters, Vol. 15, No. 11, 784 786, Nov. 25. 4. Chun, Y. H., J.-S. Hong, P. Bao, T. J. Jackson, and M. J. Lancaster, Tunable bandstop filters using BST varactor chips, 37th European Microwave Conference Proceedings, EuMC27, 8 12, Oct. 27. 5. Chun, Y. H., J.-S. Hong, P. Bao, T. J. Jackson, and M. J. Lancaster, BST-varactor tunable dual-mode filter using variable Z C transmission line, IEEE Microwave and Wireless Components Letters, Vol. 18, No. 3, 167 169, Mar. 28. 6. Chun, Y. H., J.-S. Hong, P. Bao, T. J. Jackson, and M. J. Lancaster, BST varactor tuned bandstop filter with slotted ground structure, 28 IEEE MTT-S, Jun. 28.