A Novel Thin Film Bulk Acoustic Resonator (FBAR) Duplexer for Wireless Applications

Transcription

1 Tamkang Journal of Science and Engineering, Vol. 7, No. 2, pp (24) 67 A Novel Thin Film Bulk Acoustic Resonator (FBAR) Duplexer for Wireless Applications C. H. Tai 1, T. K. Shing 1 *, Y. D. Lee 2 and C. C. Tien 2 1 Material Research Laboratory Industrial Technology Research Institute Jhudong, Taiwan 31, R.O.C. 2 Department of Electrical Engineering Chung Hua University Hsinchu, Taiwan 3, R.O.C. shing@delta.com.tw Abstract A duplexer comprising transmitter () filter and receiver () filter using thin film bulk acoustic resonators (FBARs) for use in 19 MHz PCS band is presented. The typical minimum rejection specifications for filter are > 5 db in the band of MHz and for filter are > 4 db in the band of MHz. The duplexer requires filters to pose very steep roll-off characteristic because of the very narrow guard band. In the meanwhile, the input impedance must have high impedance in the stopband of each filter to guarantee the receiving signal will go to the filter and the transmitting signal will go to the antenna only. This paper proposes a systematic methodology to design a duplexer to meet all the above requirements using FBARs with minimum auxiliary devices. Key Words: FBAR, Duplexer, Filter 1. Introduction A duplexer is a three-port device, in Figure 1, having a transmitting port, a receiving port and an antenna port. It provides the necessary connection while prevents the modulated transmitting signal generated by the transmitter from being reflected from the antenna back to the input of the receiver and overloading the receiver. A time switching system uses switches to select between the bands and the bands and provides good connections and attenuation properties. For a frequency division multiplexing structure, a duplexer instead a switch is used. Usually, the specification of the duplexer demands that the guard band between the transmitting signal and the receiving signal is about 1% of the carrier frequency and the bandwidth assigned to the transmitting and the receiving signal are about 3% * Corresponding author. (Currently with Delta Electronic) of the carrier frequency. This means that the filter and filter are required to have an extremely sharp roll-off. Although ceramic filter is able to meet these requirements, its large size and high cost make it incompetitive. As for surface acoustic wave (SAW) filter [1], its performance as a duplexer is not very good though it has the advantage of low cost and small size. FBAR duplexer can overcome these problems [2 5]. Nevertheless a quarter-wave transmission line is usually used between antenna and filter as the phase shifter and this may cause unnecessary parasitic effects and enlarge the device size. This paper proposes a novel design of a duplexer using FBARs with minimum auxiliary devices and requires no additional quart-wave transmission line. 2. Conventional FBAR Duplexers The duplexer proposed by Agilent shown in

2 68 C. H. Tai et al. ANT Port 1 Duplexer Port 2 A 2 A 4 A 1 A 5 A (a) filter Port Ant. Figure 1. The duplexer configuration. Chip Chip x A 1 A 2 A 4 A 5 (b) filter Figure 3. The S 21 and S 11 of filter and filter Quarter-wave Transmission Line Figure 2. Agilent FBAR duplexer configuration. Figure 2 consists of filter and filter; both are ladder type filters. One port of the filter is connected to the transmitter section and the other is connected to the antenna. The filter is connected to the antenna through a quarter-wave transmission line. Each filter has resonators with two frequencies wherein the shunt FBARs all have the same resonant frequency and series FBARs all have the same resonant frequency that is a few percent higher than shunt FBARs. The auxiliary inductors can be used to increase the bandwidth in the passband and enlarge the attenuation at stopband as shown in Figure 3. It should be noted that if these two bandpass filters are connected directly to the antenna, the performance of filter would degrade as in Figure 4. This is because the input impedance of filter is not high enough in its lower stopband (the passband of filter). The method proposed by Agilent [2,3] is to use the phase shifter that may be composed of lumped inductors and capacitors or a quarter-wave transmission line and this can transform the impe- A 1 A 2 A 4 A 5 A 1 A 2 A 4 A 5 A Figure 4. filter and filter are connected directly. dance into high impedance as in Figure 5 and improve the performance of duplexer as in Figure 6. However, this additional phase shifter may introduce parasitic effects and increase the size of the duplexer. 3. A Novel FBAR Duplexer To overcome the problems discussed above, a new strategy of designing duplexers is proposed. Similar to a conventional FBAR duplexer, a ladder-type filter structure is adopted. The auxiliary inductors are used to tune the passband bandwidth, sharper the roll-off in the guard band, and increase the attenuation in the stopband for each bandpass filter. The inductors can be connected to the shunt FBARs in series or to the

3 A Novel Thin Film Bulk Acoustic Resonator (FBAR) Duplexer for Wireless Applications 69 2 T/L = 18 mil 15 T/L = 1 mil f f r2 f a2 1 5 Figure 5. The effects of added transmission lines on the input impedance. A 1 A 2 A 4 A 5 A 1 A 2 T/L = 5 mil T/L = mil A 4 A 5 A db(s(2,2)) Figure 6. The simulated performance of Agilent FBAR duplexer db(s(3,1)) db(s(3,3)) dip f f r1 f r1 f a1 Figure 8. The effect of the auxiliary inductor Figure 9. Multiple dips caused by auxiliary inductors. 2. of the piezoelectric film, we can only tune f by changing the thickness ratio of piezoelectric layer to electrode layer or by using auxiliary components such as inductor or capacitor [6]. Here, the auxiliary inductor is used to enlarge the value of f. As shown in Figure 8, the original f is expanded to f, and correspondingly the bandwidth of the filter will become larger The Roll-off in the Guard Band and the Attenuation in the Stopband Figure 7. The possible configurations of the auxiliary inductors with FBARs. series FBARs in parallel as in Figure 7 to meet the above requirements. However, to deal with the impedance matching problem, both the number and positions of the auxiliary inductors must be taken into consideration. In the following, the systematic methodology will be described. 3.1 Limited Df of FBAR Since the bandwidth of the filter is related to f (= f a f r ) of FBAR which is the material property One way to increase the roll-off in the guard band and the attenuation in the stopband is to use multiple FBAR thicknesses. It will allow more choices on possible zeros. From fabrication consideration, it is not practical. Through the manipulations of the auxiliary inductors, however, the resonant frequency and dip phenomena of FBAR filter can be changed. Hence the roll-off in the guard band and the attenuation in the stopband can be modified by choosing the number and positions of dips as in Figure 9. As shown in Figure 9, the f r of shunt FBAR causes one dip and the other two dips are contributed by the two auxiliary inductors. By further tuning the position of these dips, the roll-off in the guard band and the attenuation in the stopband

4 7 C. H. Tai et al. Mag (Zin) Mag (Zin) Figure 1. The impedance of the FBAR filter without auxiliary inductors Figure 11. The impedance of the FBAR filter with auxiliary inductors. can meet the specification of PCS duplexer. 3.3 Impedance Matching Beside the two issues mentioned above, we must make sure the impedance in the stopband for each filter is high enough. Since the filter already has high impedance, it is not necessary to do anything. As for filter, it has very low impedance below the passband as in Figure 1. This will cause the transmitting signal to mix with the receiving signal at the receive port and may overload the low-noise amplifier. Therefore the performance of the duplexer may degrade as shown in Figure 4. Unlike the method adopted by Agilent, through appropriate arrangement of auxiliary inductors, the impedance of filter can be pulled up to high impedance region as shown in Figure 11. The effect is the same as the phase shifter used between the antenna and the filter with no additional transmission line required. This is good for parasit ic effects elimination and size reduction. However, from the study it is concluded db(s(2,2)) Figure 12. The proposed FBAR duplexer. that the position of the inductor must connect with the first series FBAR in parallel and there should be no shunt FBARs connected in front of this FBAR. 4. Simulation Results Based on the proposed method, a duplexer used for PCS system can be designed easily and effectively. An example is given as in Figure 12, wherein a series FBAR in the filter is connected with an inductor in parallel ( band: MHz) and series FBARs in the filter are connected with two inductors in parallel ( band: MHz). Ten FBARs are used in the duplexer totally. All specifications are met. The auxiliary inductors play multiple role s here such as enlarging f, adjusting roll-off in the guard band and attenuation in the stopband, and also acting as the phase shifter. 5. Conclusion In this paper, a new ladder-type FBAR duplexer without additional phase shifter has been proposed. The obtained frequency characteristic shows low insertion loss, high attenuation at stopband, and sharper roll-off. They meet all the required duplexer specifications for PCS system. In addition, the elimination of auxiliary phase shifter device is effective to remove unnecessary parasitic effects and reduce the size of the duplexer. References [1] Ikata, et al., A Design of Antenna Duplexer Using Ladder Type SAW Filters, IEEE International Ultrasonic Symposium, db(s(3,1)) db(s(3,3))

5 A Novel Thin Film Bulk Acoustic Resonator (FBAR) Duplexer for Wireless Applications 71 (1988). [2] Bradley, P. et al., A Film Bulk Acoustic Resonator (FBAR) Duplexer for USPCS Handset Applications, IEEE MTT-S International on Microwave Symposium, pp (21). [3] Lakin, K. M. et al., Duplexer Incorporating Thin-Film Bulk Acoustic Resonators (FBARs), 21 US patent [4] Ruby, R. et al., PCS 19 MHz Duplexer Using Thin Film Bulk Acoustic Resonators (FBARs), Electronics Letters, pp (1999). [5] Ruby, R. et al., High Rejection Filters for GSM Handsets with Wafer Level Packaging, IEEE Ultrasonic Symposium, pp (22). [6] Tai, C. H. et al., The Study on Electrical Performance of Thin Film Bulk acoustic Wave Resonator, Nano Tech. & MEMS Conference (22). [7] Rosenbaun, J., Bulk Acoustic Wave Theory and Devices, Artech House (1988). Manuscript Received: Dec. 29, 23 Accepted: Jan. 16, 24