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Balanced Low-Loss Longitudinally-Coupled Double-Mode Resonator Saw Filters With Impedance Conversion

Doberstein S. A.

Omsk Research Institute of Communications & Electronics (ONIIP) Omsk, Russia e-mail: info@oniip.ru

Abstract— This paper presents the new wideband balanced lowloss SAW filter with impedance conversion from 50-100 ? to 200-300 ? in a wide fractional bandwidth of 2-6%. The two types of the longitudinally-coupled double-mode resonator filters on 42° YX LiTaO3, 64° YX and 41° YX LiNbO3 are used for this purpose. The first type of the filters is realized as a twotransducer scheme. The second type of the filters is built as the three-transducer scheme. The impedance conversion in the twotransducer filters is provided by a sectional output transducer. The impedance conversion in the three-transducer filters is attained by a series connection of the side transducers. The optimization of a SAW filter is provided with a computer simulation using an equivalent circuit model. The 240-361 MHz samples of the balanced SAW filters have shown an insertion loss of 2-3 dB, stopband attenuation of 20-50 dB. The filters were housed in the 5x5x1.35 mm SMD packages.

filters on 42° YX LiTaO3, 64° YX and 41° YX LiNbO3 [2, 3]. Our wideband balanced low-loss SAW filters with impedance conversion are offered for the front-end of the high frequency handheld transceivers for matching the low impedance antenna with modern high impedance double balanced mixer and for suppression of the local oscillator and image frequencies. II. BALANCED LOW-LOSS TWO-TRANSDUCER RESONATOR SAW FILTERS ON THE LONGITUDINAL 1ST AND 2ND MODES WITH IMPEDANCE CONVERSION The first type of the filters is realized as the two-transducer scheme on the longitudinal first and second resonance modes (Fig. 1) [2]. As will be seen from Fig. 1 the input/output interdigital transducers (IDTs) of the filter have not a common grounded busbar. So a symmetrical connection of the input/output IDTs to the loads is possible. Consequently, the two-transducer resonator SAW filter can be both the balanced/unbalanced structure. In order for a low input impedance to be converted to a high output impedance in the two-transducer filter we used a sectional output IDT with a series electrical connection of the 2 sections (Fig. 1). Then theoretically an output IDT impedance increased by a factor of 4. A theoretical analysis of the balanced two-transducer resonator SAW filter was provided using a computer simulation on a basis of an equivalent circuit model [4]. The equivalent circuit of the balanced two-transducer SAW filter is shown in Fig. 2. Here [P1] is a mixed matrix of the input IDT, [P2] is a mixed matrix of sectional output IDT, Z is characteristic impedance of the medium between IDTs and reflectors, V is a SAW velocity, R is an equivalent impedance for a reflector.

In

I.

INTRODUCTION

Using the double-mode resonator structures is an efficient method of realizing SAW filters meeting the latest demands of the market and providing the single-chip solutions for the low insertion loss, specified frequency response, balanced operation and impedance conversion [1]. This paper presents the new wideband balanced low-loss SAW filters with impedance conversion from 50-100 ? at input to 200-300 ? at output in a fractional bandwidth of 2-6% on a basis of the two types of the longitudinally-coupled double-mode resonator In

Out 1st mode 2nd mode

Figure 1. Balanced two-transducer resonator SAW filter with impedance conversion R Z,V [P1] Z,V [P2] Z,V R

Out

Figure 2. Equivalent circuit of the balanced two-transducer resonator SAW filter

978-1-4244-1795-7/08/$25.00 ?2008 IEEE

199

0 -10 -20 -30

Insertion Loss (dB)

-40 -50 -60 -70 -80 -90 -100 220

225.4

230.8

236.2

241.6

247

252.4

257.8

263.2

268.6

274

Frequency (MHz)

Figure 3. Simulated frequency response of the two-transducer resonator SAW filter on 42° YX LiTaO3

Figure 4. Measured frequency response of the two-transducer resonator SAW filter on 42° YX LiTaO3

Fig. 3 shows a simulated frequency response of the balanced two-transducer resonator SAW filter on 42° YX LiTaO3 in a 75-: – 300-: system. The filter has shown a 2-dB fractional bandwidth of 2% with minimal insertion loss and ripple, stopband attenuation around 20 dB at ±5% offset from a center frequency. The measured frequency response of the balanced two-transducer resonator SAW filter is shown in Fig. 4. The filter has been symmetrically connected to a 75-: input load and to a 300-: output load. Measurements were carried out by the network analyzer and balanced transformers. The losses of the balanced transformers were eliminated from the measured insertion loss of the filter. At the center frequency of 245.72 MHz the filter has shown an insertion loss of less than 2 dB, 2-dB fractional bandwidth of 5.1 MHz with a low ripple of 0.5 dB and stopband attenuation above 20 dB at r5% offset from the center frequency.

Fig. 3 and Fig. 4 show good agreement between the simulated and measured responses. A measured input impedance characteristic of the filter is shown in Fig. 5. As will be seen from Fig. 5 at the center frequency of 245.72 MHz the input impedance of the filter is close to real value of 70 :. The measured output impedance characteristic of the filter is shown in Fig. 6. As will be seen from Fig. 6 at the center frequency of 245.72 MHz the output impedance of the filter is close to real value of 300 :. Thus the impedance conversion from 70 : to 300 : was obtained experimentally in the lowloss two-transducer resonator SAW filter with the output sectional IDT (Fig. 1). It can be shown that the filters on 64° YX and 41° YX LiNbO3 provide the similar frequency responses with 4% and 6% fractional bandwidth respectively and impedance conversion from 100 : to 300 :.

Figure 5. Measured input impedance characteristic of the two-transducer resonator SAW filter on 42° YX LiTaO3

B B

Figure 6. Measured output impedance characteristic of the two-transducer resonator SAW filter on 42° YX LiTaO3

B B

200

Figure 7. Measured frequency response of the two-transducer resonator SAW filter on 41° YX LiNbO3

Figure 8. Measured frequency response of the cascaded two-transducer resonator SAW filter on 42° YX LiTaO3

The measured frequency response of the filter on 41° YX LiNbO3 is shown in Fig. 7. At the center frequency of 274 MHz the filter has shown an insertion loss of less than 2 dB, 3-dB bandwidth of 16 MHz, stopband attenuation over 20 dB at r12% offset from the center frequency in a symmetrical 100-: – 300-: system. To increase the stopband attenuation we can use cascading two filters. Fig. 8 shows the frequency response of the cascade of two balanced filters on 42° YX LiTaO3. The first filter is built without the impedance conversion. The second filter is built with the impedance conversion (with output sectional IDT). In a 75-: – 300-: symmetrical system the cascaded filter has shown an insertion loss of less than 3 dB, 2-dB bandwidth of 4.7 MHz, stopband attenuation over 40 dB at r10.7 MHz offset from the center frequency.

III. BALANCED LOW-LOSS THREE-TRANSDUCER RESONATOR SAW FILTERS ON THE LONGITUDINAL 1ST AND 3RD MODES WITH IMPEDANCE CONVERSION The second type of the filters is realized as the threetransducer scheme on the longitudinal first and third resonance modes (Fig. 9) [2]. As will be seen from Fig. 9 in the threetransducer filter the input and side output IDTs connected in series/parallel have not a common grounded busbar. So a symmetrical connection of the input and two side output IDTs to the loads is possible. Consequently, the three-transducer filter can be both the balanced/unbalanced structure similar to the two-transducer filter (Fig. 1). This construction allows to increase the output impedance in sufficiently large limits by a series connection of the side IDTs (Fig. 9). In this case as a simulation shows in the three-transducer filter the impedance conversion as 1:4 is achievable. The optimization of the balanced three-transducer resonator SAW filter with the impedance conversion was provided with the computer simulation using the equivalent circuit model [4]. The equivalent circuit of the balanced three-transducer resonator filter is shown in Fig. 10. Here [P1], [P2] are the mixed matrixes of the central IDT and side IDTs, Z is the characteristic impedance of the medium between IDTs and reflectors, R is the equivalent impedance for the reflector, V is the SAW velocity.

In

In

Out

R Z,V [P2] Z,V [P1] Z,V [P2] Z,V

R

1st mode 3rd mode

Figure 9. Balanced three-transducer resonator SAW filter with impedance conversion

Out

Figure 10. Equivalent circuit of the balanced three-transducer resonator SAW filter

201

0 -10 -20 -30

Insertion Loss (dB)

-40 -50 -60 -70 -80 -90 -100 220

225.4

230.8

236.2

241.6

247

252.4

257.8

263.2

268.6

274

Frequency (MHz)

Figure 11. Simulated frequency response of the three-transducer resonator SAW filter on 42° YX LiTaO3

Figure 12. Measured frequency response of the three-transducer resonator SAW filter on 42° YX LiTaO3

Fig. 11 shows a simulated frequency response of the filter on 42° YX LiTaO3 in a symmetrical 50-: – 200-: system. The filter has shown a 3-dB fractional bandwidth of 2.5% with minimal insertion loss and ripple, stopband attenuation of around 20 dB at r5% offset from the center frequency. It can be shown that filters on 64qYX, 41qYX LiNbO3 provide the similar frequency responses with 4% and 6% fractional bandwidth respectively and the impedance conversion from 50-75 : to 200–300 :. The measured frequency response of the filter on 42° YX LiTaO3 is shown in Fig. 12. The filter has been symmetrically connected to 50-: input load and to 200: output load. At the center frequency of 246.7 MHz the filter has shown an insertion loss of less than 2 dB, 3-dB fractional bandwidth of 2.5% with a very low ripple of 0.1 dB and stopband attenuation around 20 dB at r5% offset from the center frequency. Measurements of this filter were carried out similarly to the measurements of the two-transducer filter.

Fig. 11 and Fig. 12 show good agreement between the simulated and experimental responses. A measured input impedance characteristic of the filter is presented in Fig. 13. As will be seen from Fig. 13 at the center frequency of 246.7 MHz the input impedance of the filter is close to real value of 50 :. A measured output impedance characteristic of the filter is shown in Fig. 14. As will be seen from Fig. 14 at the center frequency of 246.7 MHz the output impedance of the filter is close to real value of 200 :. Thus the impedance conversion from 50 : to 200 : was obtained experimentally in the low-loss three-transducer resonator SAW filter with series connection of the output side IDTs. The measured frequency response of the filter on 41° YX LiNbO3 is shown in Fig. 15. At the center frequency of 276 MHz the filter has shown an insertion loss of less than 2 dB, 3-dB bandwidth of 15.8 MHz, stopband attenuation around 20 dB at r9% offset from the center frequency in a symmetrical 75-: – 300-: system.

Figure 13. Measured input impedance characteristic of the three-transducer resonator SAW filter on 42° YX LiTaO3

Figure 14. Measured output impedance characteristic of the three-transducer resonator SAW filter on 42° YX LiTaO3

202

Figure 15. Measured frequency response of the three-transducer resonator SAW filter on 41° YX LiNbO3

Figure 16. Measured frequency response of the cascaded three-transducer resonator SAW filter on 42° YX LiTaO3

To increase the stopband attenuation we can use cascading two filters. Fig. 16 shows the frequency response of the cascade of two balanced filters on 42° YX LiTaO3. The first filter is built without the impedance conversion (with the parallel connection of the side IDTs). The second filter is built with the impedance conversion (with series connection of the side IDTs). In a 50-: – 200-: symmetrical system the cascade filter has shown an insertion loss of less than 3 dB, 2dB bandwidth of 4.7 MHz, stopband attenuation over 40 dB at r4% offset from the center frequency. The similar approach can be used for realizing the cascaded balanced threetransducer resonator SAW filters with impedance conversion having 4% and 6% fractional bandwidth on 64° YX and 41° YX LiNbO3. The frequency response of the 361 MHz balanced cascaded filter on 64qYX LiNbO3 is presented in Fig. 17. The filter has shown an insertion loss of around 3 dB, 3-dB bandwidth of 14.4 MHz, stopband attenuation over 40 dB at r9% offset from the center frequency in a symmetrical 50-: – 200-: system.

IV.

CONCLUISON

Developed two-transducer and three-transducer resonator SAW filter with a low insertion loss of 2-3 dB have demonstrated the balanced operation and impedance conversion in a wide fractional bandwidth of 2-6%. These filters offer a single-chip solution for the front-end of the high frequency handheld transceivers for matching the low impedance antenna with modern high impedance double balanced mixer and for suppression of the local oscillator and image frequencies. The filters were housed in the 5x5x1.35 mm SMD packages. ACKNOWLEDGMENT Author thanks K. Nikolaenko for photomask manufacture, V. Dronova and N. Ryabova for manufacture of SAW filters, D. Gilfand for photomask design and editing manuscript.

REFERENCES

[1] O. Kawachi, S. Mitobe, M. Tajima, S. Inoue and K. Hashimoto, “LowLoss and Wide-Band Double-Mode Surface Acoustic Wave Filters Using Pitch-Modulated Interdigital Transducers and Reflectors,” IEEE Trans. on UFFC, vol. 54, ? 10, pp. 2159-2164, 2007. T. Morita, Y. Watanabe, M. Tanaka and Y. Nakazawa, “Wideband Low Loss Double Mode SAW Filters,” Proc. IEEE Ultrason. Symp., pp. 95-104, 1992. P. G. Ivanov, V. M. Makarov, V. S. Orlov, V. B. Shvetts, “Wideband Low Loss SAW Filters for Telecommunication and Mobile Radio Applications,” Proc. IEEE Ultrason. Symp., pp. 61-64, 1996. S. Doberstein, “High Frequency and High Selectivity Balanced Front-End SAW Modules for Handheld Transceivers,” Proc. IEEE Ultrason. Symp., pp. 1665-1668, 2007.

[2]

[3]

[4]

Figure 17. Measured frequency response of the cascaded three-transducer resonator SAW filter on 64° YX LiNbO3

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