SAW Filters with Reconfigurable Transition Bands Xiaoming Lu and Koen Mouthaan
Jeffery Galipeau, Emmanuelle Briot, Benjamin Abbott
ECE Department, National University of Singapore 9 Engineering Drive 1, 117576, Singapore
[email protected]
TriQuint Semiconductor Florida 1818 S Orange Blossom Trail Apopka, FL 32703-9419, USA
Abstract—A surface acoustic wave (SAW) filter with a reconfigurable upper transition band is presented. This is achieved through the reuse of SAW resonators by incorporating single pole double throw (SPDT) GaAs switches. The reconfigurable filter with two states is demonstrated around the 700 MHz frequency range currently allocated to LTE bands. The EM simulation results are obtained from a 3D EM model built in Ansys HFSS. Fabricated SAW dies and GaAs switch dies are assembled in a 7x9x1.4 mm3 SMP package. Measured results agree well with the EM simulations. In the first state the center frequency is 687.8 MHz and the BW is 2.4%. In the second state the center frequency is 691.1 MHz and the BW is 3.4%. The shift in the upper transition band is 0.95%.
I.
R1
SPDT1
R3
SPDT1
R2 C1
C1
R3
Fig. 1. Unit circuit of the reconfigurable filter.
INTRODUCTION
Multiband and multimode filters are in high demand for mobile applications [1]. The filters are required to cover different wireless standards in a single package [2]. Currently, most of the RF filters are based on high-Q acoustic resonators realized in either SAW or Bulk-Acoustic-Wave (BAW) technologies. Filter banks reported are typically bulky and costly [3], [4]. Research has been reported to change the coupling coefficient of the piezoelectric substrate electronically to obtain tunable acoustic filters. However this requires work at technology level and has not been effective enough [5]. SAW resonators in combination with lumped elements, such as inductors, capacitors or MOS transistors, can be used to obtain tunable or reconfigurable acoustic filters [6]-[9]. However, the performance of the reconfigurable filter deteriorates due to the filter topology. A reconfigurable resonance technology is proposed in [10]. It uses the same fixed SAW resonators working with different sets of lumped components to construct multi-mode filters. In conclusion, addition of lumped components and reuse of acoustic resonators are of great interest if the performance is not compromised too much. In the Long Term Evolution (LTE) technology for mobile communications, to switch between LTE band 3, band 4 and band 10 or LTE band 6 and 19, a SAW filter with a reconfigurable upper transition band is required [11]. Here SAW resonators are reused to reduce the total area of acoustic devices by introducing SPDT switches and capacitors to The research is supported by Temasek Defence Systems Institute in Singapore and Triquint Semiconductor Florida.
978-1-4577-1820-5/12/$26.00 ©2012 IEEE
Fig. 2. Complete circuit of the reconfigurable filter.
realize a reconfigurable upper transition band. In the demonstrators, the ladder type topology is adopted [12]. The center frequency of the filter is designed around the 700 MHz band newly allocated to LTE [13]. II.
FILTER DESIGN
A. Theory In a ladder type acoustic filter, connecting lumped components in series with a resonator branch shifts the resonant frequency of the resonator while keeping its antiresonant frequency the same. And connecting lumped components in parallel with a resonator branch will change the anti-resonator frequency of the resonator while its resonant frequency remains the same [9]. This principle is used to realize the filter with a reconfigurable transition band. Fig. 1 shows the unit circuit of the filter with reconfigurable upper
T ABLE I SAW R ESONATORS ’ D ESIGN P ARAMETERS
Parameters Resonant frequency Aperture width Duty factor IDT length
R1
R2
R3
R4
C1
Unit
712
703.6
688.6
688.6
-
(MHz)
18.3 50 65
26.1 50 50
17.2 50 50
21.3 50 84
45 50 22
Wavelengths % Wavelengths
Fig. 3. Assembly drawing of the reconfigurable filter with wire bonds.
Fig. 5. Photograph for the top view of the dies.
Fig. 4. The 3D EM model built in Ansys HFSS.
transition band. When SPDT1 and SPDT2 are switched to the R1 resonator branches, the circuit forms a filter at band 1 composed of resonators R1 and R3.When the SPDT switches are switched to the R2 resonator branches, the filter at band 2 is formed by R2, R3 and C1. In this case, the resonator R3 is connected in parallel with the capacitor C1. This results in a decreased anti-resonant frequency of the parallel branches which determines the center frequency of the filter. Also the series resonators are changed to R2. Compared to R1, the resonant frequency of R2 is shifted to lower frequency due to decreased anti-resonant frequency of the parallel branches. Thus by controlling the switches, a filter is realized with a reconfigurable upper transition band while for both bands resonator R3 is reused. It is worth mentioning that in case of band 2, R2 can also be connected in parallel with an additional capacitor to increase the shifting of the upper transition band if required or to provide a good matching in the passband. B. Implementation The complete circuit of the filter is shown in Fig. 2. It consists of two cascaded sections shown in Fig. 1 with the center SPDT switches combined. The parallel SAW resonators are merged and capacitors on the two sides are neglected. Fig. 3 shows the assembly drawing of the reconfigurable filter with wire bonds. The SAW resonators are designed in one single die as shown in the dotted block of Fig. 2 and connected with
the switch dies by wire bonds. Through bias control of the switches this filter can achieve two states with a reconfigurable upper transition band. The demonstrator is realized in a surface mount package (SMP) 35B which is 7x9x1.4 mm3. A 3D model is built in Ansys HFSS for EM simulation as shown in Fig. 4. The simulation includes the parasitics of interconnects, wire bonds, non-ideal switches and the package. III.
FILTER MEASUREMENTS
A. Measurements The SAW die is fabricated on a Lithium Tantalate substrate while the switch dies are high isolation absorptive GaAs SPDTs. The dies are assembled using 1 mil wire bond technique. The design parameters of the SAW resonators are listed in Table I. The top view photo of the dies in the package are shown in Fig. 5. The comparison of simulated and measured results is shown in Fig. 6 and Fig. 7 for band 1 and band 2 respectively. The measurements agree well with EM simulations, particularly in the stopbands, and there is no deterioration in the passbands. However, a slight shift of the passbands is observed in both states. This may be caused by the inaccurate modeling of the acoustic wave velocity in the substrate. Fig. 8 and Fig. 9 show the measured insertion loss and return loss for the two states. In band 1, the filter has a center frequency of 691.1 MHz and a bandwidth of 3.4%; while in band 2, the filter has a center frequency of 687.8 MHz and a bandwidth of 2.4%.
0
0 Measurement EM simulation
Band 1 Band 2 10 Insertion loss (dB)
Insertion loss (dB)
10
20
30
40
50 600
20
30
40
650
700
750
50 600
800
650
Frequency (MHz)
700
Fig. 6. The measured and simulated insertion loss of filter band 1.
800
Fig. 8. The measured insertion loss of filter band 1 and band 2. 0
0 Measurement EM simulation
Band 1 Band 2 Return loss (dB)
10 Insertion loss (dB)
750
Frequency (MHz)
20
30
10
20
40
50 600
650
700
750
800
30 600
650
700
750
800
Frequency (MHz)
Frequency (MHz)
Fig. 7. The measured and simulated insertion loss of filter band 2.
Fig. 9. The measured return loss of filter band 1 and band 2.
B. Discussion The basic principle to realize the reconfigurable upper transition band in this work is to decrease the effective coupling coefficient of the substrate using capacitors. Using lumped components to obtain a reconfigurable filter can also be seen in other works. The addition of capacitors and MOS transistors has been proposed to shift the lower transition band of a Bulk Acoustic Wave-Solidly Mounted Resonator (BAWSMR) filter [5], [6]. The simulated results in [5] and the measured results in [6] reveal that when the shift in the lower transition band increases, the rejection in lower stop band becomes worse. This is because in the ladder filter topologies, the reconfiguration is done by adding the lumped components and switches to the parallel branches only. The series branches are not changed when the switches are turned on and off for the two modes. Thus the performance of the reconfigurable filter deteriorates with worse out-of-band rejection and return loss. Here the SPDT switches are introduced in the filter topologies such that the series resonators for both bands are independent and can be optimized for the out-of-band rejection and return loss. Therefore, the out-of-band rejection
is approximately the same in the two measured states as shown in Fig. 8. IV.
CONCLUSION
This paper presents a SAW filter with a reconfigurable upper transition band. The fabricated filter has a center frequency of 691.1 MHz and a bandwidth of 3.4% in state 1 and a center frequency of 687.8 MHz and a bandwidth of 2.4% in state 2 respectively approximately keeping the same insertion loss of 1.5 dB and out-of-band rejection of around 20 dB. In the filter, the parallel resonators are reused which saves acoustic area compared to a filter bank using two individual filters. SPDT switches are also introduced in the filter topology. It allows the two bands of the reconfigurable filter having similar performance in out-of-band rejection and return loss. Thus this work further facilitates future exploration of the use of switches to obtain multi-band filters with a smaller area.
ACKNOWLEDGMENT
[6]
The authors thank TriQuint Semiconductor Florida Acoustic Design, Switch design and Manufacturing groups for support of fabrication/assembly and testing of the devices.
[7]
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