Reconfigurable Multiband SAW Filters for LTE ... - Semantic Scholar
Reconfigurable Multiband SAW Filters for LTE Applications Xiaoming Lu, Jeffery Galipeau+, Koen Mouthaan, Emmanuelle Henry Briot+, Benjamin Abbott+ ECE Department, National University of Singapore, 4 Engineering Drive 3, Singapore 117583 + TriQuint Semiconductor, 1818 South Highway 441, Apopka, Florida 32703 Abstract—Reconfigurable surface acoustic wave (SAW) filters are presented for the 700 MHz frequency band currently allocated to Long Term Evolution (LTE). Unlike traditional SAW filters with a fixed center frequency, the proposed filters have a reconfigurable center frequency. First, a novel modular ladder type topology is proposed incorporating SAW resonators and GaAs Single Pole Single Throw (SPST) switches within the basic reconfigurable filter. The measured center frequency in the low band state is 696 MHz and the BW is 3.4%. In the high band state the measured center frequency is 718 MHz and the BW is 2.6%. The center frequency shift is 3.2%. Spurious responses caused by switches are an important design consideration and the root causes of the spurious responses in high band and low band are identified. Then a novel filter topology using Single Pole Double Throw (SPDT) absorptive switches is proposed to remove the spurious responses in the high band state. Finally the root cause of the remaining spurious response is analyzed and a solution is proposed. Index Terms — SAW filters, switch, reconfigurable, LTE
Unit cell SW1
R2
R1
R1
R3 R4
SW2
SW1
R3 SW2
R2 R4
Fig. 1. Circuit of the basic filter.
I. INTRODUCTION In modern wireless communication systems, the need for multi-band and multi-mode devices is growing rapidly [1]. One of the most important emerging technologies in the 700 MHz band is the Third Generation Partnership Project’s Long Term Evolution (3GPP LTE) for mobile communications [2]. Most of the RF filters in the tranceivers for such applications are based on high-Q acoustic resonators realized either in SAW or Bulk-Acoustic-Wave (BAW) technologies. To realize reconfigurable acoustic filters, a switched filter bank can be used. However, the filter can be bulky and costly [3]. Non-resonant elements such as CMOS transistors, inductors and capacitors can also be used to obtain reconfigurable filters [4]-[6]. However, that solution suffers from the relatively low Q-factor of components which leads to high losses and deteriorated roll off in transition bands. Also, the change in bandwidth or frequency shift is limited when using such non-resonant elements. Thus reuse of SAW resonators is of great interest in future multiband/multimode applications. Such work has only been reported in [7]. However, the reconfigurable filters are demonstrated without considering the effect of the switches which is significant for the filters’ performance. In this paper, we present novel topologies to
Fig. 2. Photo of the basic filter in a 7x5 mm2 SMP package.
reuse SAW resonators in conjunction with GaAs switches. The spurious responses caused by switches are addressed and solutions are proposed. The demonstrated filters have reconfigurable bands with the same bandwidth. They are assembled using traditional chip and wire bond methods in surface mount packages (SMP). This modular approach has been used to realize two types of different reconfigurable SAW filters. One is a SAW filter with shifted center frequency and BW; the others are SAW filters with shifted band edges [8], [9]. Here we focus on SAW filters with reconfigurable center frequncy while keeping the BW the same. II. FILTER DESIGN A. Basic filter The circuit of the basic filter is shown in Fig. 1, where two unit cells are mirror-cascaded. In the high band state all the switches are on and the filter thus is formed by resonators R1, R2 and R3 in a π-topology. In the low band
0
Non ideal isolation (when switch off)
Measurement EM simulation
Insertion loss (dB)
10
SW1
Spurious responses
R3
R2
20
Lwire Lswitch (when switch on)
30
SW2
40
Lwire 50 600
R1
650
700
750
800
R4
Lgnd
Fig. 5. The parasitic inductance in unit cell of the basic filter.
Frequency (MHz)
Fig. 3. Insertion loss for the high band state of the basic filter. 0 Measurement EM simulation
Insertion loss (dB)
10 Spurious response 20
30
40
50 600
650
700
750
800
Frequency (MHz)
Fig. 4. Insertion loss for the low band state of the basic filter.
state all the switches are off and the filter is formed by resonators R2, R3 and R4 in a T-topology. It can be seen that R2 and R3 are reused as parallel branches in the high band state and as series branches in the low band state. The photo of the fabricated filter is shown in Fig. 2. Fig. 3 and Fig. 4 show the measured and simulated insertion loss for the high band and low band states. The measurements and simulations agree very well in the passbands and stopbands. The low band state has a center frequency of 696 MHz and a BW of 3.36%. The high band state realizes a center frequency of 718 MHz and a BW of 2.62%. The center frequency shift is 3.2%. However, GaAs switches as well as bond wires introduce parasitics as indicated in Fig. 5. Switches usually suffer from insertion loss and isolation problems, while the bond wires have parasitic inductance. Switch SW1 has non-ideal isolation when switched off which leads to a spurious resonance at 739 MHz in Fig. 4, due to the finite isolation of SW1 in series with the resonator R1. In the high band state, a spurious response is observed in the lower stop band at 666 MHz and another at 688 MHz. This is caused by the switch branch parasitic. The switch SW2 branch has switch inductance (Lswitch) and on
state bond wire inductances (Lwire). And the whole parallel branch has inductance (Lgnd) due to the wire bonding to ground. At the out-of-band frequencies resonator R4 behaves as a capacitor and forms a resonator tank with the inductance branch of Lwire and Lswitch. This tank is responsible for the spurious response at 666 MHz. Parallel branches R2 and R3 have a common ground path of the ground inductance Lgnd and the resonator tank of R4 with the switch path. This is responsible for the spurious response at 688 MHz. The spurious response in the low band state suggests the need for switches with higher isolation. The spurious responses in the high band state could be removed by using a different assembly to minimize the parasitic inductances, such as flip-chip. However, this may not be a cost-effective solution. A modified topology is proposed below to reduce these spurious responses. B. Modified filter Considering the properties of high isolation absorptive Single Pole Double Throw (SPDT) switches, a modified topology is proposed. The circuit is shown in Fig. 6. In the high band state, SW1 is on and SPDT is switched to output ①. In the low band state, SW1 is off and SPDT is switched to output ②. The photo of the fabricated filter is shown in Fig. 7. The equivalent circuits of both states are the same as in the basic filter. Fig. 8 presents the simulated and measured insertion loss for the high band state. It is seen that spurious responses are suppressed to better than 25 dB which is better than the rejection at 600 and 800 MHz. This modified topology removes spurious responses in the high band state without increasing the number of switch dies. The spurious response in the low band state of the above filters is caused by insufficient isolation of the switches. This can be verified easily by EM simulation. It is suggested that switches with higher isolation should be used in the future application. Other GaAs FET switches [10] and RF MEMS switches [11] represent good candidates.
0
Unit cell
2
R1 SPDT
R2 R4
R3 1
R1
SW1
SPDT
2
Measurement EM simulation 10
R2
R3 1
R4
Fig. 6. Circuit of the modified filter.
Insertion loss (dB)
SW1
20
Spurious responses
30
40
50 600
650
700
750
800
Frequency (MHz)
Fig. 8. High band insertion loss for the modified filter.
REFERENCES
Fig. 7. Photo of the modified filter in a 9x7 mm2 SMP package.
III. CONCLUSION SAW filters with reconfigurable center frequencies are presented. The basic filter is presented and root causes for spurious responses are identified. The spurious responses in the low band state are caused by parasitic inductances. The modified filter topology realizes improved lower stopband responses with minimized switch die area. The spurious response in the low band states is caused by the insufficient isolation of the switch off state. EM simulations show that the spurious responses can be suppressed by using higher isolation switches. The basic filter and modified filter realize two states with a BW of around 4% and a center frequency shift of around 3.4%. In the proposed reconfigurable filters, half of the total resonators are reused. This can serve as a guide for future exploration on the reuse of SAW resonators in multi-band filtering. ACKNOWLEDGEMENT The authors thank TriQuint Semiconductor Florida Acoustic Design, Switch design and Manufacturing groups for support of fabrication, assembly and testing of the devices.
[1] R. Aigner, “Filter Technologies for converged RF-fronted Architectures: SAW, BAW and Beyond”, Silicon Monolithic Integrated Circuits in RF Systems (SiRF), 2010. [2] A. LaMore, “The 700 MHz Band: Recent Developments and Future Plans”, http://www.cse.wustl.edu/~jain/cse57408/index.html. [3] J. Liu, S. He, S. Li, J. Liu, Y. Liang, “Switchable SAW Filter Bank with Both Narrow & Wide Channel Bandwidth and 10 Channels SAW Filter Bank”, 2007 IEEE Ulrasonics Symposium(IUS), pp. 2578-2581. [4] M. El Hassan, E. Kerherve, Y. Deval, J.B. David, D. Belot, “Tunability of Bulk Acoustic Wave Filters Using CMOS Transistors: Concept, Design and Implementation”, IEEE Radio Frequency Integrated Circuits Symposium (RFIC), pp. 241-244, 2010. [5] S. Aliouane, A. B. Kouki, R. Aigner , “ RF-MEMS Switchable Inductors for Tunable Bandwidth BAW Filters”, International Conference on Design & Technology of Integrated Systems in Nanoscale Era, pp. 1-6, 2010. [6] K. Baraka, E. Kerhervé, J.M. Pham, M. El Hassan, “Codesign for Tunability of a Bulk Acoustic Filter with 65nm CMOS Switch”, 2011 9th IEEE Int. New Circuits and Systems Conference (NEWCAS). [7] N. O. Fenzi et. al, “Multimode bandpass SAW filter using Reconfigurable Resonance Technology”, 2010 IEEE Ultrasonics Symposium (IUS) pp. 864-867. [8] X. Lu, J. Galipeau, K. Mouthaan, E. Briot, B. Abbott, “A Modular SAW Filter Design Approach for Multiband Filtering”, 2011 IEEE Ultrasonics Symposium (IUS). [9] X. Lu, K. Mouthaan, J. Galipeau, E. Briot, B. Abbott, “SAW Filters with Reconfigurable Transition Bands”, 2012 IEEE Frequency Control Symposium (FCS), pp. 1-4. [10] http://www.analog.com/en/rfif-components/rfswitches/products/index.html Last accessed on October 4, 2012. [11] http://www.radantmems.com/radantmems/products.html Last accessed on October 4, 2012.
Unit cell SW1
R2
R1
R1
R3 R4
SW2
SW1
R3 SW2
R2 R4
Fig. 1. Circuit of the basic filter.
I. INTRODUCTION In modern wireless communication systems, the need for multi-band and multi-mode devices is growing rapidly [1]. One of the most important emerging technologies in the 700 MHz band is the Third Generation Partnership Project’s Long Term Evolution (3GPP LTE) for mobile communications [2]. Most of the RF filters in the tranceivers for such applications are based on high-Q acoustic resonators realized either in SAW or Bulk-Acoustic-Wave (BAW) technologies. To realize reconfigurable acoustic filters, a switched filter bank can be used. However, the filter can be bulky and costly [3]. Non-resonant elements such as CMOS transistors, inductors and capacitors can also be used to obtain reconfigurable filters [4]-[6]. However, that solution suffers from the relatively low Q-factor of components which leads to high losses and deteriorated roll off in transition bands. Also, the change in bandwidth or frequency shift is limited when using such non-resonant elements. Thus reuse of SAW resonators is of great interest in future multiband/multimode applications. Such work has only been reported in [7]. However, the reconfigurable filters are demonstrated without considering the effect of the switches which is significant for the filters’ performance. In this paper, we present novel topologies to
Fig. 2. Photo of the basic filter in a 7x5 mm2 SMP package.
reuse SAW resonators in conjunction with GaAs switches. The spurious responses caused by switches are addressed and solutions are proposed. The demonstrated filters have reconfigurable bands with the same bandwidth. They are assembled using traditional chip and wire bond methods in surface mount packages (SMP). This modular approach has been used to realize two types of different reconfigurable SAW filters. One is a SAW filter with shifted center frequency and BW; the others are SAW filters with shifted band edges [8], [9]. Here we focus on SAW filters with reconfigurable center frequncy while keeping the BW the same. II. FILTER DESIGN A. Basic filter The circuit of the basic filter is shown in Fig. 1, where two unit cells are mirror-cascaded. In the high band state all the switches are on and the filter thus is formed by resonators R1, R2 and R3 in a π-topology. In the low band
0
Non ideal isolation (when switch off)
Measurement EM simulation
Insertion loss (dB)
10
SW1
Spurious responses
R3
R2
20
Lwire Lswitch (when switch on)
30
SW2
40
Lwire 50 600
R1
650
700
750
800
R4
Lgnd
Fig. 5. The parasitic inductance in unit cell of the basic filter.
Frequency (MHz)
Fig. 3. Insertion loss for the high band state of the basic filter. 0 Measurement EM simulation
Insertion loss (dB)
10 Spurious response 20
30
40
50 600
650
700
750
800
Frequency (MHz)
Fig. 4. Insertion loss for the low band state of the basic filter.
state all the switches are off and the filter is formed by resonators R2, R3 and R4 in a T-topology. It can be seen that R2 and R3 are reused as parallel branches in the high band state and as series branches in the low band state. The photo of the fabricated filter is shown in Fig. 2. Fig. 3 and Fig. 4 show the measured and simulated insertion loss for the high band and low band states. The measurements and simulations agree very well in the passbands and stopbands. The low band state has a center frequency of 696 MHz and a BW of 3.36%. The high band state realizes a center frequency of 718 MHz and a BW of 2.62%. The center frequency shift is 3.2%. However, GaAs switches as well as bond wires introduce parasitics as indicated in Fig. 5. Switches usually suffer from insertion loss and isolation problems, while the bond wires have parasitic inductance. Switch SW1 has non-ideal isolation when switched off which leads to a spurious resonance at 739 MHz in Fig. 4, due to the finite isolation of SW1 in series with the resonator R1. In the high band state, a spurious response is observed in the lower stop band at 666 MHz and another at 688 MHz. This is caused by the switch branch parasitic. The switch SW2 branch has switch inductance (Lswitch) and on
state bond wire inductances (Lwire). And the whole parallel branch has inductance (Lgnd) due to the wire bonding to ground. At the out-of-band frequencies resonator R4 behaves as a capacitor and forms a resonator tank with the inductance branch of Lwire and Lswitch. This tank is responsible for the spurious response at 666 MHz. Parallel branches R2 and R3 have a common ground path of the ground inductance Lgnd and the resonator tank of R4 with the switch path. This is responsible for the spurious response at 688 MHz. The spurious response in the low band state suggests the need for switches with higher isolation. The spurious responses in the high band state could be removed by using a different assembly to minimize the parasitic inductances, such as flip-chip. However, this may not be a cost-effective solution. A modified topology is proposed below to reduce these spurious responses. B. Modified filter Considering the properties of high isolation absorptive Single Pole Double Throw (SPDT) switches, a modified topology is proposed. The circuit is shown in Fig. 6. In the high band state, SW1 is on and SPDT is switched to output ①. In the low band state, SW1 is off and SPDT is switched to output ②. The photo of the fabricated filter is shown in Fig. 7. The equivalent circuits of both states are the same as in the basic filter. Fig. 8 presents the simulated and measured insertion loss for the high band state. It is seen that spurious responses are suppressed to better than 25 dB which is better than the rejection at 600 and 800 MHz. This modified topology removes spurious responses in the high band state without increasing the number of switch dies. The spurious response in the low band state of the above filters is caused by insufficient isolation of the switches. This can be verified easily by EM simulation. It is suggested that switches with higher isolation should be used in the future application. Other GaAs FET switches [10] and RF MEMS switches [11] represent good candidates.
0
Unit cell
2
R1 SPDT
R2 R4
R3 1
R1
SW1
SPDT
2
Measurement EM simulation 10
R2
R3 1
R4
Fig. 6. Circuit of the modified filter.
Insertion loss (dB)
SW1
20
Spurious responses
30
40
50 600
650
700
750
800
Frequency (MHz)
Fig. 8. High band insertion loss for the modified filter.
REFERENCES
Fig. 7. Photo of the modified filter in a 9x7 mm2 SMP package.
III. CONCLUSION SAW filters with reconfigurable center frequencies are presented. The basic filter is presented and root causes for spurious responses are identified. The spurious responses in the low band state are caused by parasitic inductances. The modified filter topology realizes improved lower stopband responses with minimized switch die area. The spurious response in the low band states is caused by the insufficient isolation of the switch off state. EM simulations show that the spurious responses can be suppressed by using higher isolation switches. The basic filter and modified filter realize two states with a BW of around 4% and a center frequency shift of around 3.4%. In the proposed reconfigurable filters, half of the total resonators are reused. This can serve as a guide for future exploration on the reuse of SAW resonators in multi-band filtering. ACKNOWLEDGEMENT The authors thank TriQuint Semiconductor Florida Acoustic Design, Switch design and Manufacturing groups for support of fabrication, assembly and testing of the devices.
[1] R. Aigner, “Filter Technologies for converged RF-fronted Architectures: SAW, BAW and Beyond”, Silicon Monolithic Integrated Circuits in RF Systems (SiRF), 2010. [2] A. LaMore, “The 700 MHz Band: Recent Developments and Future Plans”, http://www.cse.wustl.edu/~jain/cse57408/index.html. [3] J. Liu, S. He, S. Li, J. Liu, Y. Liang, “Switchable SAW Filter Bank with Both Narrow & Wide Channel Bandwidth and 10 Channels SAW Filter Bank”, 2007 IEEE Ulrasonics Symposium(IUS), pp. 2578-2581. [4] M. El Hassan, E. Kerherve, Y. Deval, J.B. David, D. Belot, “Tunability of Bulk Acoustic Wave Filters Using CMOS Transistors: Concept, Design and Implementation”, IEEE Radio Frequency Integrated Circuits Symposium (RFIC), pp. 241-244, 2010. [5] S. Aliouane, A. B. Kouki, R. Aigner , “ RF-MEMS Switchable Inductors for Tunable Bandwidth BAW Filters”, International Conference on Design & Technology of Integrated Systems in Nanoscale Era, pp. 1-6, 2010. [6] K. Baraka, E. Kerhervé, J.M. Pham, M. El Hassan, “Codesign for Tunability of a Bulk Acoustic Filter with 65nm CMOS Switch”, 2011 9th IEEE Int. New Circuits and Systems Conference (NEWCAS). [7] N. O. Fenzi et. al, “Multimode bandpass SAW filter using Reconfigurable Resonance Technology”, 2010 IEEE Ultrasonics Symposium (IUS) pp. 864-867. [8] X. Lu, J. Galipeau, K. Mouthaan, E. Briot, B. Abbott, “A Modular SAW Filter Design Approach for Multiband Filtering”, 2011 IEEE Ultrasonics Symposium (IUS). [9] X. Lu, K. Mouthaan, J. Galipeau, E. Briot, B. Abbott, “SAW Filters with Reconfigurable Transition Bands”, 2012 IEEE Frequency Control Symposium (FCS), pp. 1-4. [10] http://www.analog.com/en/rfif-components/rfswitches/products/index.html Last accessed on October 4, 2012. [11] http://www.radantmems.com/radantmems/products.html Last accessed on October 4, 2012.
