Miniaturization Technology of RF Devices for Mobile Communication Systems

Transcription

1 Miniaturization Technology of RF Devices for Mobile Communication Systems Toru Yamada, Toshio Ishizaki and Makoto Sakakura Device Engineering Development Center, Matsushita Electric Industrial Co., Ltd. 6 Kadoma Osaka 57-85, JAPAN ABSTRACT Miniaturization of RF devices is a very important factor for success in the portable telephone business. The down-sizing technologies have been studied. The situation is still the same for the development of the next generation portable telephone terminals. There are two major approaches for miniaturizing the terminals. One is simplifying the RF circuit structure. For this purpose, the direct conversion technologies are reviewed. The other approach is the miniaturization of the RF devices. The miniaturization histories of RF filters are reviewed. Especially, the miniaturization technique of dielectric filter is described in detail by way of example. Their difficulties and new ideas are illustrated with their basic concept. system, which has been mainly used portable telephones, many RF components are difficult to be constructed in a semiconductor. This fact causes difficulty of miniaturization because of increment of external RF components. To overcome this problem, a direct conversion system has been attracted a great deal of attention. This system requires fewer external components than the super heterodyne system. However it still remain technical difficulties such as weak interference-resistance. Furthermore, very small RF devices, which cannot be constructed in the semiconductors, are still strongly expected to Hand Set INTRODUCTION Recently, the number of portable telephones is rapidly increasing. The main factor of this tremendous increment is greatly dependent on the miniaturization technique. Although the system has changed from the first generation analog system to the second generation digital system and further is changing to the third generation IMT-2 system, the demand for miniaturization is still important technology for portable telephones. The miniaturization of the base band circuit and some part of RF circuit could be achieved with semiconductor IC technologies. However, in a super heterodyne Volume (cc) VCO Duplexer TCXO YEAR Power Amplifier Module Figure Miniaturization trend of portable telephones and RF devices. 2

2 make super compact next generation portable telephones. In this paper, the authors review a newly proposed offset direct conversion system as compared with super heterodyne and direct conversion systems. The authors also illustrate the miniaturization technologies of RF devices. First, their histories are reviewed. The difficulty is explained for the case of dielectric filters. Then the new technologies and the new ideas are described. PROGRESS OF RF CIRCUIT CONFIGURATION Figure shows the miniaturization trend of portable telephones and the RF devices. In recent fifteen years, the handsets have become one-tenth in volume. And the RF devices have become onehundredth in volume. Therefore it is confirmed that the miniaturization of the RF devices greatly contributes to the miniaturization of portable telephones. Figure 2 shows the block diagram of the super heterodyne system. This system uses one or two IF frequencies with plural of voltage controlled oscillators (VCOs) and high selectivity IF filters, such as SAW filter and piezoelectric ceramic filters. It achieves excellent interference-resistant characteristics. However many external RF components are required. It is difficult to miniaturize the RF circuit block. Figure 3 shows the block diagram of the direct conversion system. This system has a clear advantage for miniaturization of RF circuits, due to the fact that it needs only one VCO and no needs of IF filters. However the RX signal is demodulated directly at the same radio frequency in a RF semiconductor IC. It is difficult to achieve important receiving TX-RF BPF PA Modulator TX st VCO 2 nd VCO Duplexer RX LNA Demodulator RX-RF BPF IF-BPF Figure 2 Block diagram of super heterodyne system. PA Modulator TX Duplexer VCO LPF RX LNA LPF Demodulator Figure 3 Block diagram of direct conversion system. PA Modulator TX /n VCO Duplexer LPF RX LNA LPF IC Demodulator Figure 4 Block diagram of offset direct conversion system. characteristics, such as low noise, high interferenceresistance and low power consumption. Furthermore, the VCO oscillates the same frequency as the radio frequency. High isolation characteristics in the RF circuit are required.

3 Table Comparison of RF conversion systems Super heterodyne Direct conversion Required number of devices VCO 2 TX-RF filter RX-IF filter 2 Characteristics TX spurious Fair TX interference-resistance Bad RX interference-resistance Bad Circuit size & cost Bad Offset direct conversion Good Good Figure 4 shows the block diagram of offset direct conversion system, which has been proposed recently[]. Here the relation of RF signal and local frequencies are shown like following equations; f f where frf Lo Lo2 n = frf n ± = fif = f n Lo = f n ± is RF signal frequency, f Lo is st local frequency and f Lo2 is 2nd local frequency. The 2nd local frequency is /n of st local frequency. Then, only one VCO is required. In this system, a receiving signal is converted to an intermediate frequency of /n frf at the st mixer. By use of IF frequency, an image frequency only has possibility to become the interference to the desired signal. In this case, the image interference frequency can be suppressed easily by simple RF filters. It doesn t require an image rejection mixer. As for the transmitting circuit, modulating characteristics does not degrade due to the difference frequency of VCO and transmission signal. And IF RF modulation frequency is /n frf, TX spurious is only occurred in integral multiples of the IF frequency. This means there is no spurious response adjacent to the transmission signal. High selectivity filters for spurious suppressing does not required. Table summarizes comparison of each system. As explained above, by sifting the local frequency from the RF signal frequency, the offset direct conversion system can achieve low spurious characteristics and simple RF circuit structure as the direct conversion system, and high RX interferenceresistance characteristics equally as the heterodyne system. MINIATURIZATION HISTORIES OF RF DEVICES So far, the portable telephone system has been changed from first generation analog system to second generation digital system. Further, in foreseeable future, third generation system will start. The required specifications for respective RF devices are greatly different. For examples, low distortions, wide signal bandwidth, high speed switching, 2GHz

4 Bandpass Filters Pin-type SMD-type Laminated planar filter Duplexers Connector-type SMD-type switch duplexer Laminated antenna Switch duplexer YEAR Figure 5 Miniaturization histories of dielectric filters and antenna duplexers. operations and so on are demanded for IMT-2 system. Thus, new RF devices have to be developed at the transition of the generations[2]. Figure 5 shows the miniaturization histories of dielectric bandpass filters and antenna duplexers. In 985, the configuration of bandpass filters was pin-type co-axial one. In 99, they became SMDtype. In 993, the laminated planar filter was introduced[3],[4]. It is very small comparing to the RF Filters conventional co-axial filters. The volume is only onetwentieth. As for antenna duplexers, in 985, the configuration of duplexer was connector-type coaxial one. In 99, they also became SMD-type. In 993, according to the advent of the digital system, the antenna switch duplexer was introduced[5]. In 998, it became laminated antenna switch duplexer. Figure 6 shows the miniaturization history of RF and IF SAW filters. In a beginning of 99 s the 5x5mm 3.8x3.8mm 3x3mm 3.2x2.5mm IF Filters 2.5x2.mm For TDMA 3x6.5mm 9x7mm 9x5mm 7x5mm For CDMA 9x6.5mm 3x6.5mm x5mm YEAR Figure 6 Miniaturization histories of RF and IF SAW filters.

5 Its difficulty is explained for the case of the dielectric filters in the following section. Unloaded Q 5 5 Diameter [mm] Figure 7 Unloaded Q of a quarter wavelength coaxial dielectric resonator (at GHz). RF SAW filter was introduced to the portable telephone. It was SMD-type with wire bonding connection. In 998, the volume was reduced to onefifth with flip-chip bonding technology. As for IF filters, transversely coupled resonator filters were mainly used for TDMA systems. The authors developed balanced type filters suitable for GSM system[6]. In 997, the size of this filter became 7x5mm. However the direct conversion system without IF filters is becoming a main stream especially for GSM system. On the other hand, in 996, transversal-type SAW filters for narrow band CDMA systems were introduced with 9x6.5mm package size. In 999, the volume was reduced to x5mm. For wide band IMT-2 system, new configurations of electrode pattern have developed to reduce the size to be 7x5mm or smaller[7]. As reviewing the figures, the, drastic miniaturizations are observed. But they were not easy ways[8]. Because, generally speaking, there is a trade-off between the sizes and the performances. DIFFICULTY OF MINIATURIZATION FOR DIELECTRIC FILTERS Figure 7 shows the relation between the diameter and the unloaded Q of a quarter wavelength co-axial dielectric resonator. Usually, the filter performance is evaluated by the insertion loss in the pass band and the attenuation in the stop band. A smaller insertion loss and a larger attenuation are preferable. These values of a filter are dependent on the unloaded Q of the resonator. The unloaded Q of a resonator is related to radiation loss, material tanδ and conductor ohmic loss. For the typical co-axial resonators, the conductor ohmic loss is a main factor determining the unloaded Q. The Q values can be read from Fig.7, as about 7 for a mm-diameter resonator and about 5 for a 2mm-diameter resonator. As mentioned, the resonator size becomes smaller, then the filter performance is degraded. The reason of the performance degradation is caused by the concentration of the current. Therefore the breakthrough of this trade-off is the main object to develop the very small dielectric filters. One approach to miniaturize the dielectric filters is using stripline resonators with ceramic lamination technique. Conventional co-axial resonators have a difficulty to make a small-diameter resonator due to the limitation of precision manufacturing process of ceramics. On the other hands, ceramic lamination technique has advantage to make precision stripline resonators with thin ceramic green sheets and screen printing process. However, for compact stripline filters, the gap between stripline

6 Input Output Stripline section Shield Electrode Electromagnetic coupling Transmission Line Fig.8 Equivalent circuit of SICF. Resonator resonators becomes very narrow. It means the electromagnetic coupling between resonators becomes too large. Thus it is very difficult to make a narrow bandwidth bandpass filter. To overcome this difficulty, laminated planar filter [3] and stepped impedance comb-line filter (SICF)[4] have been proposed. The laminated planar filter utilizes anti-resonance of the coupling circuit that consists of electro-magnetic coupling between striplines and capacitive coupling through a coupling capacitor. The equivalent circuit of the SICF is shown in Fig. 8. The resonators are stepped impedance type. The stripline section of the shortend is coupled magnetically. The stripline section of the open-end is coupled electrically. Thus by controlling the each coupling independently, the coupling between the resonators at the center pass band frequency becomes either electric coupling or magnetic coupling. Afterward, filter performance with attenuation pole under the pass band or filter performance with attenuation pole above the pass band can be obtained. By combining the techniques of the laminated planar filter and the SICF, the authors have developed small-sized and low-profile dielectric filter. The high selectivity has been achieved with very small size. NEW IDEAS FOR SMALL-SIZED DIELECTRIC FILTERS LAMINATED BAND ELIMINATION FILTER So far, all laminated filters were band pass type. The authors have developed laminated band elimination filter (BEF)[9]. Figure 9 shows the exploded view of 2 stage BEF. The feature of this filter is making use of electromagnetic coupling between resonators. The low insertion loss is easily obtained by BEF. Shield Electrode Figure 9 Exploded view of laminated band elimination filter.

7 FREQUENCY SHIFTABLE DUPLEXER TX For FDD systems that have narrow frequency separations between the transmitting band and the receiving band, the technique of the frequency terminal RX shiftable duplexer is very effective[]. The configuration and the concept are illustrated by Figs.(a), (b). The each band is divided into lower and upper bands. Then the band is selected by control signal applied to the control terminal. Thus the substantial frequency separation becomes broad. As the results, the number of resonator can be decreased and the insertion loss becomes very small. Figure shows the photograph. At this moment, the frequency shiftable duplexer consists of co-axial resonators. In the next step, the duplexer will be modified with the ceramic lamination technique to reduce the size. Control (a) <2MHz Tx Band (b) Rx Band Figure Concept of frequency shiftable duplexer. Figure Photograph of frequency shiftable duplexer. Transmission [db] Figure 2 Photograph of laminated duplexer for IMT-2 system Rx BPF -4 Tx BEF Frequency [GHz] Figure 3 Frequency response of laminated duplexer.

8 LAMINATED DUPLEXER For IMT-2 system, the authors have developed very small laminated duplexer[]. The volume is only one-tenth of conventional co-axial type duplexers. Figure 2 shows the photograph of the laminated duplexer for IMT-2 system. Figure 3 shows frequency responses of TX and RX filter. This duplexer has been developed by using many miniaturizing and loss-reducing technologies mentioned in this paper. This is one of the ultimate state of the art RF devices. CONCLUSION The miniaturization histories of RF devices are reviewed. The new concept of offset direct conversion system is described. The miniaturization technique and new ideas for developing dielectric filters of next generation portable telephones are explained. The RF devices will be changed by using ceramic lamination technique. REFERENCES [] H.Adachi, M.Sakakura, Y.Imagawa, H.Haruki, K.Nagase, T.Yasunaga and A.Daimou, A development of offset direct conversion transceiver, Proceedings of the 2 IEICE General Conference, B-5-22, p66, 2 (in Japanese). [2] T.Ishizaki and K.ogawa, Miniaturization Technique of RF Devices for Portable Telephone, MWE 99 Microwave Workshop Digest, WS3-3, pp , 999. [3] T.Ishizaki, M.Fujita, H.Kagata, T.Uwano and H.Miyake, A very small dielectric planar filter for portable telephones, IEEE Trans. Microwave Theory Tech., vol.mtt-42, No., pp , November 994. [4] T.Ishizaki, T.Uwano and H.Miyake, An extended configuration of a stepped impedance combline filter, IEICE Trans. Electron., vol.e79-c, No.5, pp , May 996. [5] USP5,442,82 [6] T.Yamada, K.Nishimura, T.Ishizaki and K.Ogawa, A broadband IF SAW filter, National Technical Report (Panasonic), vol.42, No.4, pp.4-8, 996 (in Japanese). [7] H.Nakamura, T.Yamada, T.Igaki, K.Nishimura, T.Ishizaki and K.ogawa, A practical SPUDT design for SAW filters with different-width split-finger interdigital transducers, IEEE Ultrasonics Symposium, PQ-2, 2. [8] K.Ogawa, T.Ishizaki and H.Kosugi, Downsizing technology of RF devices for portable telephones, The journal of the IEICE, Technical survey, vol.82, No.3, pp , March 999 (in Japanese). [9] H.Miyake, S.Kitazawa, T.Ishizaki, K.Ogawa and I.Awai, A study of a laminated band elimination filter comprising coupled-line resonators using low temperature co-fired ceramics, IEICE Trans. Electron., vol.e82-c, No.7, pp.4-9, July 999. [] USP6,85,7 []T.Ishizaki, H.Miyake, T.Yamada, H.Kagata, H.Kusutani and K.Ogawa, A first practical model of very small and low insertion loss laminated duplexer using LTCC suitable for W-CDMA portable telephones, 2 IEEE MTT-S Digest, TU3C-3, pp.87-9, 2.