Multimode bandpass SAW filter using Reconfigurable Resonance Technology

Neal O. Fenzi, Patrick J. Turner, Balam A. Willemsen, James R. Costa, Edward R. Soares, Silverio Jimenez Superconductor Technologies Inc.

Santa Barbara, USA

Abstract— The trend to include more frequency bands in mobile phones has so far been addressed by adding more fixed frequency Surface Acoustic Wave (SAW) filters and switching the RF path to accommodate each band. As more bands are needed, a reconfigurable SAW filter is of great interest if performance is not compromised. Here we present the results of a new approach to designing multiband filters. The SAW resonators remain common to all the modes of the device, and non-resonant elements, (inductors and capacitors) are adjusted to reconfigure the passbands. A two-mode proof of concept is presented that uses the same fixed SAW resonators and demonstrates a six percent (6%) change in passband frequencies.

Keywords- filter, Reconfigurable Resonance (RcR), tuning

I. INTRODUCTION The number of frequency bands required in an individual

cell phone continues to grow. The currently-employed approach of switched single-band front end modules is becoming increasingly large and costly. In future handsets, multiband Power Amplifiers (PAs) offer a partial solution [1], [2]. Reconfigurable-passband SAW filters could further simplify the architecture of the RF front end. Approaches to tuning the passband by moving the resonant frequency of the resonators has received much attention and shows some promise [3], but tuning over ranges of interest for typical mobile phone bands (greater than five percent) remains elusive. Here we present results of a proof-of-concept SAW filter with a reconfigurable passband. Since the natural resonance frequencies of the filter network are reconfigured without tuning the individual SAW resonators, we call this technology Reconfigurable Resonance (RcR). While our proof of concept is for two modes, an arbitrary number of modes can be accommodated, both switching filter passbands between the transmission zeros and moving the passband slightly in frequency to adjust for temperature variations. The technology can be used with any low loss resonator technology and is not limited to SAW devices.

II. DESIGN In contrast to conventional coupled resonator filters [4] and

ladder type Impedance Element Filters (IEF) [5], where all resonators are resonant near the center of the passband, an RcR filter uses resonators that are often resonant far in frequency from the passband. Fig. 1 shows an RcR filter circuit topology. Non-resonant reactances are represented by xi,j and bk, and resonators are represented by R1-R5. The natural frequencies of

each mode of the filter (reflection zeros) are reconfigured by adjusting the xi,j and bk while keeping the resonators themselves fixed. In this way a multi-mode filter can be designed using fixed resonators. To demonstrate this approach, two filters having identical SAW resonators but different passbands were designed, modeled and constructed. Key results are presented here.

Each filter design (corresponding to a mode) consists of the same five SAW resonators connected by six inductors and five capacitors having values less than 50nH and less than 25pF, respectively. Both filters were designed for 20dB near-band rejection. The centers of the passband were designed for a more than six percent separation; and the bandwidths of the filters were designed to be three and four percent, similar to band V and band VIII filtering that is required for cellular phones intended to operate in both the US and Europe.

III. MODEL Simulations using a modified Butterworth van-Dyke model

[6] to represent SAW resonators (shown in Fig. 2) and inductors and capacitors to connect them a were produced using a circuit simulator, Agilent's Advanced Design System (ADS). The design inputs including the assumed Q values are shown in Table I. The results for each mode are shown in Figs. 3 and 4.

x01

b1

x12

b2

x23

b3

x34

b4

x45

b5b0

R1 R2 R3 R4

x56

b6

R5

Figure 1. Topology of an RcR filter

Lm, QR

CmRs

C0, QC0 Figure 2. The modified Butterworth van-Dyke Model used for

simulations

864 2010 IEEE International Ultrasonics Symposium Proceedings

10.1109/ULTSYM.2010.0220

978-1-4577-0381-2/10/$25.00 ©2010 IEEE

TABLE I. DESIGN INPUTS FOR DUAL MODE FIVE RESONATOR SAW FILTER

Parameter Mode 1 Mode 2

Passband Center Frequency (MHz) 837 898

Bandwidth (%) 3 4

SAW Resonantor Frequencies (MHz) 750, 815, 870, 932, 1000

Inductor Q 60

Capacitor Q 100

C0/Cm 13.5

QR 1500

QC0 140

Rs (Ohms) 0.5

Next a more detailed ADS filter model was made using separately measured or modeled components. SAW resonators were designed to correspond to the resonators used in the model shown in Figs. 3 and 4 and manufactured using 128 deg X-Y cut LiNbO3. Gold-wire air-core inductors and commercially-available capacitors were used for the passive elements. Measured SAW resonator frequencies deviated from the design frequencies by less than two percent. The individual components were connected in an ADS simulation together with parasitics associated with the connecting manifold. The results of this model are shown in Figs. 5 and 6.

0.75 0.80 0.85 0.90 0.95 1.00 1.050.70 1.10

-40

-20

-60

0

freq, GHz

dB(S

(2,1

))

m1 m1freq=dB(S(2,1))=-4.01Max

829.MHz

Figure 3. ADS model simulation of mode 1 filter uses an LCR model.

0.75 0.80 0.85 0.90 0.95 1.00 1.050.70 1.10

-40

-20

-60

0

freq, GHz

dB(S

(2,1

))

m1m1freq=dB(S(2,1))=-4.03Max

898.MHz

Figure 4. ADS model simulation of mode 2 filter uses an LCR model.

0.75 0.80 0.85 0.90 0.95 1.00 1.050.70 1.10

-40

-20

-60

0

freq, GHz

dB(S

(2,1

))

m1 m1freq=dB(S(2,1))=-4.48Max

838.MHz

Figure 5. ADS model simulation of mode 1 filter uses

measured/modeled components.

0.75 0.80 0.85 0.90 0.95 1.00 1.050.70 1.10

-40

-20

-60

0

freq, GHz

dB(S

(2,1

))

m1m1freq=dB(S(2,1))=-5.28Max

897.MHz

Figure 6. ADS model simulation of mode 2 filter uses measured/modeled components.

865 2010 IEEE International Ultrasonics Symposium Proceedings

IV. FILTER MEASUREMENTS The components (SAW resonators, inductors and

capacitors) were then connected using an alumina manifold and gold wire bonds, and mounted into a test package. Two devices were constructed, one for each mode of the filter. The same resonator design is used for each of the devices, and only the non-resonant elements (inductors and capacitors) change between the two devices. With all the components connected as shown in Figs. 7 and 8, measurements on the completed devices were taken, with VSWR better than 2:1 showing good alignment. Measured insertion loss for the devices is shown in Fig. 9 and Fig. 10. The measurement and models show good agreement.

Figure 7. Photograph showing the mode 1 filter.

Figure 8. Photograph showing the mode 2 filter.

V. CONCLUSIONS A proof of concept for designing reconfigurable filters

using Reconfigurable Resonance (RcR) technology has been shown. A more than six percent shift in passband frequency is shown using the same SAW resonators by only varying the inductors and capacitors connecting them. An electronically reconfigurable filter using fixed SAW resonators can, in principle, be made by adjusting the inductors and capacitors using switches such as GaAs or Si solid state switches, Micro Electromechanical Systems (MEMS) or adjustable capacitors using GaAs, or CMOS. The RcR technology is scalable to additional modes of operation (i.e. more bands). While still in its early stages, this technology may enable multiband SAW filters for cellular handset applications.

0.75 0.80 0.85 0.90 0.95 1.00 1.050.70 1.10

-40

-20

-60

0

freq, GHz

dB(S

2,1)

m1 m1freq=dB(S(4,3))=-4.08Max

838.MHz

Figure 9. Measured insertion loss for mode 1 filter

0.75 0.80 0.85 0.90 0.95 1.00 1.050.70 1.10

-40

-20

-60

0

freq, GHz

dB(S

2,1)

m1m1freq=dB(S(4,3))=-3.43Max

903.MHz

Figure 10. Measured insertion loss for mode 2 filter

ACKNOWLEDGMENTS The authors thank Victor Plessky, Genichi Tsuzuki, Robert

Hammond, Richard Silver, and Bill Pond for many productive discussions.

866 2010 IEEE International Ultrasonics Symposium Proceedings

REFERENCES [1] U. Kim, K. Kim, J. Kim and Y Kwon "A Multi-Band Reconfigurable

Power Amplifier for UMTS Handset Applications" 2010 IEEE Radio Frequency Integrated Circuits Symposium

[2] K. Y. Kim, J. H. Kim, and C. S. Park "A Single-Input Single-Chain Dual Band Power Amplifier for CDMA Mobil Application" Microwave and Optical Technology Letters Vol. 48, No. 5, May 2006

[3] R. Aigner "RF Filters for Converged Frontend Arcitectures in Mult-standard Phones" Proceedings of the Fourth International Symposium on

Acoustic Wave Devices for Future Mobile Communication Systems, March 2010

[4] G. Matthaei, L. Young, E.M. Jones, "Microwave filters, Impedance-matching Networks and Coupling Structures" McGraw-Hill, Inc 1964

[5] D. Morgan, "Surface Acoustic Wave Filters" Academic Press, 1985 [6] Larson, J.D., III.; Bradley, P.D.; Wartenberg, S.; Ruby, R.C.; ,

"Modified Butterworth-Van Dyke circuit for FBAR resonators and automated measurement system," Ultrasonics Symposium, 2000 IEEE , vol.1, no., pp.863-868 vol.1, Oct 200

867 2010 IEEE International Ultrasonics Symposium Proceedings