SAW and MWC filters key components for mobile terminals and base-stations

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Dr. Quincy Chen and Mr. C. K. Lim

Epcos Pte Ltd., Singapore

Mr. Juergen Machui,Dr. Gerd Riha

Siemens Matsushita Components (S+M), Munich, Germany

Mr. Christian Block

Siemens Matsushita Components (S+M), Deutschlandsberg, Austria

Introduction

SAW (Surface Acoustic Wave) and MWC (Micro-Wave Ceramic) filters are key components for modern mobile communication terminals and base-stations. They are widely used in the RF and IF modules, and in both the Rx (Receiving) and Tx (Transmit-ting) paths of mobile transceivers. Their characteristics deter-mine to a great extent the overall performance of mobile termi-nals and base-stations. The extra compact size of the latest mo-bile phones is only possible with the miniaturization of modern SAW and MWC filters.

More and more RF SAW filters for GSM and PCN/PCS systems are housed in miniature 3.0x3.0 mm2_{SMD ceramic}

packages, and the even smaller Chip Sized SAW Package (CSSP) is also available now, with a dimension down to 2.0x2.0 mm2_{. IF SAW filters, usually much bigger than RF filters, can}

also be as small as 5.0x5.0 mm2_{now. SAW duplexers which}

can withstand the high output power of GSM terminals have been developed successfully, and the 2-in-1 SAW filters con-taining two independent channels find their way into many of the new mobile phones, especially the dual band / dual mode phones which are getting increasingly popular.

MWC filters are used in mobile equipment due to their ex-cellent power handling capabilities, extremely low insertion at-tenuation, small size and relatively low cost. They appear in mobile terminals and base-stations as duplexers, front-end filters etc. In PCN/PCS handset applications, MWC filters are widely used in the RF circuit due to their good duplex suppression. For power-hunger applications like the third generation mobile phones, low loss MWC filters can greatly extend the battery life. SAW filters are indispensable in base-stations and repeat-ers, particularly in the IF circuit. These filters are designed and produced with much stricter criteria in order that they have a better performance and can work in much more severe environ-mental conditions. SAW and MWC filters are also available for applications like wireless LAN, WLL and Blue-Tooth.

Basics of SAW and MWC devices

Like ICs and laser technology, the SAW filter is a new elec-tronic device invented in the 1960s. The centre of SAW filters is

SAW and MWC filters—

key components for

mobile terminals and

base-stations

the so-called interdigital transducers (IDTs). Surface acoustic waves can be generated electrically by these IDTs on the surface of piezoelectrically active substrates like quartz and lithium niobate single crystals, and the surface acoustic waves can also be converted back to electric signals by IDTs. Because of the much shorter wavelength of the acoustic waves compared with the electromagnetic waves at the same frequency, devices based on the acoustic waves can be much smaller. The basic structures of SAW devices are various kinds of IDTs on polished piezo-electric single crystal substrates, therefore the manufacturing pro-cesses of SAW devices are very similar to those of integrated circuits. As the electrode period of IDTs is inversely proportional to frequency, the high end in working frequencies of SAW de-vices is limited by manufacturing techniques while the low end is determined by component sizes and material costs. SAW de-vices are presently available in the frequency range of 30 - 3000 MHz, covering the majority of modern wireless communication systems’ RF and IF frequencies.

The main advantages of SAW filters are their excellent per-formance and compact size. The smallest SAW filter available now has a dimension of 2.0x2.0x0.85 mm3_{, thanks to the newly}

developed Chip-Sized-SAW-Package (CSSP). CSSP embodies the latest packaging technology for SAW devices, in which the SAW filter chip is mounted face down on a ceramic carrier and encapsulated with a patented technique. Conventional LC fil-ters and filfil-ters based on other techniques do not have the steep response and deep suppression or the small size and ease of use. SAW devices are also very suitable for mass production and this contributes greatly to their relatively low cost.

Microwave ceramic (MWC) filters are made of parallel-coupled resonators which are embedded in a high dielectric constant substrate. The substrate used has low loss at micro-wave frequencies. Due to the high dielectric constant of the substrate, the size of a typical mobile phone filter can be re-duced to 3x3 mm2_{. Quarter-wavelength resonators are usually}

used in the filter, but half-wavelength resonators can also be used. For quarter-wavelength resonators, one end of the reso-nator is grounded. For half-wavelength resoreso-nators, both ends are open. The input and output of the filters are capacitively

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Wireless Technologies China ‘99 • Conference Proceedings

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coupled to the resonators through the I/O pads. The I/O pads also form capacitors to the ground (both filter’s body ground and external PCB ground). The resonators in a MWC filter are often formed in steps. The narrow portion (step) can be viewed as an inductor and the broad portion (step) a capacitor. These two portions resonate to produce zeroes in the filter response. This is useful if a notch is required at a particular frequency (e.g. at the image frequency of a receiver front-end filter) or a steep roll-off skirt is required (e.g. in a duplexer).

Compared with SAW filters, MWC filters usually have lower insertion attenuation, higher power handling capability, better tem-perature stability and lower cost. But stop band suppression, steep-ness of skirts and size are generally not as good as SAW filters.

SAW and MWC duplexers and front-end filters

The first place filtering may be needed in a wireless transceiver is immediately after the antenna. In frequency division duplexing (FDD) systems, if the system shares one antenna, as is the case for most mobile terminals, a duplexer consisting of two band pass filters based on SAW or MWC is used. In time division duplexing (TDD) systems like PHS and DECT, transmitting and receiving occur in different time slots. Pin-diode pairs are usually used for this purpose due to their low switch transit time at high frequencies.

SAW duplexers are available for CT0 cordless phones (e.g. S+M B8200), and they have the advantages of smaller size, deeper stop band suppressions, greater ease of use and more stable characteristics compared with conventional LC modules. These duplexers are housed in a plastic package used also for most of TV IF filters, therefore their reliability is assured and production cost can be kept reasonably low.

SAW duplexers for other cordless phone systems have also been developed and successfully used, e.g. the extremely compact

SAW duplexer for CT ISM in the US, which is housed in a small 3,8x3,8 mm2_{ceramic package (S+M B4028 and B4029).}

Because of their superior performance, these relatively new devices have already become the preferred choice with cordless phone design engineers.

The high power handling capability of MWC filters make them suitable for RF front-end applications like cellular phone duplexers. Although SAW duplexers are also available in ap-plications like GSM and CT phones, it cannot handle high con-tinuous power. Therefore, in most multiple transmission slot applications, MWC duplexers are preferred. Due to its low temperature coefficient, MWC duplexers can have deep duplex suppression. Furthermore, MWC duplexers and filters can be designed to have very low loss. In third generation mobile phones (UMTS), using a low loss transmit filter or duplexer can greatly extend the battery life because the typical insertion loss of such a device is about 1 dB only and an stop band sup-pression of more than 50 dB can also be guaranteed.

SAW duplexers for cellular phones can have competitive performance, but one of the key issues for SAW duplexers is power handling. The maximum signal power to be generated in the antenna of a GSM mobile terminal is about 2 W. Given some passband attenuation in the Tx front-end filter itself this implies that the device must cope with a driving level of about 36 dBm. The Rx filter experiences a signal level of 33 dBm in its lower passband.

Conventional electrode beam evaporated Al electrodes can-not handle these power levels and extensive research has been

directed to the investigation of new electrode materials. Cu doped Al has become a popular approach. Today SAW duplexers for GSM mobile terminals have been developed successfully.

If a duplexer is not used or the duplexer does not have enough frequency selectivity, a receiver front-end filter (SAW or MWC filters) must be applied to prevent saturation of the sensitive low-noise amplifier (LNA) and assure its linear operation. The weak antenna signal has to be cleared of strong spurious signal, e.g. signals from other radio system and noises. It is equally impor-tant to suppress the strong Tx signal applied at the antenna.

Single low loss front-end SAW and MWC filters for both the Rx and Tx paths are available as options for RF design en-gineers to choose. In the transmitter path these filters can sup-press close-in noises and higher harmonics. This ensures the proper operation of the power amplifier and only the wanted signals are amplified.

The front-end SAW filters use a multiple resonator struc-ture (the Ladder type filter) to realize a very low insertion at-tenuation (as low as 1,5 dB) and steep passband skirts permit-ting excellent close-in rejection. Good suppression of the op-posite band (i.e. the Rx band for Tx filters and vice versa) can be achieved. Far-off rejections however are only moderate and do not improve with distance from the passband. MWC filters become more attractive at PCS/PCN bands than at the GSM band due to the size reduction at higher frequencies.

RF inter-stage filters

A second filter (SAW or MWC), the receiver inter-stage filter, is applied before the mixer to further suppress unwanted sig-nals, in particular those at the image frequency.

The RF inter-stage SAW filter design follows slightly differ-ent consideration than front-end filters. This becomes particu-larly evident in the case of the Rx inter-stage filter. Insertion attenuation is still very important yet requirements are relaxed compared to those for front-end filters. Suppression of spurious signals, especially in the image frequency becomes crucial.

Dual-Mode SAW (DMS) Filters usually consist of two reso-nant acoustic tracks electrically connected in series. They use two longitudinal resonance modes in each track to achieve a flat passband. The limited bandwidth of acoustic transmission in these filters leads to very attractive stop band rejections.

Two factors contribute to the successful application of dual-mode resonator SAW filters as inter-stage filters. First, their far-off rejection can be better than 60 dB and the filter can signifi-cantly suppress the image frequency band and other interferers. In inter-stage applications between a mixer with balanced termi-nation and the single ended front-end amplifier, some kind of balun is needed. This means additional costs, space requirements and signal deterioration by extra loss. Thus the second advan-tage of dual-mode resonator filters is that they allow for balanced or unbalanced operation and any combination of these two modes. Therefore they can naturally assume the functionality of a balun. MWC filters can also be used as inter-stage filters when balanced connection is not needed.

IF filters for channel selection

The receiver must process signals within a dynamic range of up to 120 dB with high linearity. For this purpose the re-ceived signal must be down-converted through one or more in-termediate frequency (IF) stages. Filtering requirement in the IF stages of the receiver path is mainly aimed to clear up the

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strong adjacent interferers from other users of the same com-munication system. The receiver IF filter must provide high selectivity and adequate bandwidth while preserving a high degree of linearity. Filters for these applications must provide moderate insertion attenuation to maintain a low noise level and to limit power consumption. To permit an undistorted pas-sage of the signal, low ripples in amplitude and group delay are required. For mobile terminals, size and cost of filter compo-nents are crucial and they must also offer temperature stability in the full temperature range.

The first IF filter is almost always a SAW filter based on quartz substrate and this filter is the key component deter-mining the channel selection obtainable by the system. Over the years IF filters for mobile terminals have experienced a dramatic miniaturization process. Early mobile terminals started with one or even two devices in leaded packages which occupy a surface area of 25 x 13 mm2_{on both sides of}

the printed circuit board. Today a high performance GSM IF SAW filter can be in a 5x5 mm2_{SMD package and}

fre-quencies for the first IF can be over 400 MHz. A higher IF is helpful for the resonator filters to achieve the desired ab-solute passband and reduce the filter size, but suppression of the first adjacent channels becomes more difficult and feed-through tends to deteriorate. To obtain temperature sta-bility the obvious choice of substrate material is quartz.

SAW IF filters are essential for GSM and PCN/PCS base-stations and repeaters too. To fulfill the much more stringent stop band suppression requirements, two or even three IF fil-ters are needed in the IF signal path, connected in series.

SAW IF filters are also used in some digital cordless phone systems like DECT and PHS. DECT uses fairly wide channels of 1,2 MHz and 1,7 MHz spacing. IF frequencies are usually 110,6 MHz or 112,3 MHz. To reduce cost, traditional transver-sal filter designs are used and these filters can be fabricated on cost optimized SAW production facilities for TV applications.

Two-in-one filters

Another new development is the so-called 2-in-1 filters. This type of filters has two separate filter channels housed in one compact package, resulting in a up to 50% reduction in size. The actual characteristics of the two channels vary accord-ing to applications. For examples, these can be the two re-ceiver path RF filters for a GSM phone (S+M B4209), or a receiver path RF inter-stage filter plus an LO clean-up filter for an AMPS phone (S+M B4202), or the two receiver path RF filters in PCN phones (S+M B4204). Dual band 2-in-1 filters have also been developed successfully. All these 2-in-1 filters come in a 3,8x3,8 mm2_{ceramic package. With the}

rapid adoption of the dual mode or dual band phones by the market, such 2-in-1 filters will become more widely used.

Application Considerations of MWC filters

As the MWC filter’s I/O tab geometry determines the way the signal couples to the internal resonators of the filter, care should be taken during the PCB layout and assembly in order to pre-serve the correct coupling. In general, the size of the PCB I/O pads should be slightly smaller that the MWC I/O tabs. This will prevent changing the coupling capacitance from the I/O pads to the resonators.

Most MWC filters have a large ground plane. For a relatively long MWC component, do not solder the whole ground plane to

the PCB (i.e. do not unmasked the whole ground pad on the PCB). Otherwise, the PCB ground pad will be peeled off when the PCB bends. Also, the masked area on the PCB can be used for placing plated through-holes for better grounding.

Some MWC filters have relatively small ground tabs and a significant amount of the substrate is directly exposed to the PCB. For this type of MWC filters, the layout of the ground plane (whether masked or unmasked) under the filter is critical. This is because the PCB ground plane directly under the high dielectric constant substrate will form stray capacitance to the I/O pads and the resonators. It is best to consult the supplier on the layout of the ground plane for such filter. Although not recommended, the user can “adjust” the filter responses, like the bandwidth, by varying the PCB track pattern under the filter.

The vertical distance between the PCB’s ground plane and the I/O pads should not be less than 0.5 mm. This is to prevent stray capacitance between the I/O and the ground. This may occur if multilayer PCB is used. If the PCB layer thickness is less than 0.5 mm, it is advisable to remove the ground below the filter’s I/O pads.

Although the top part of the MWC components are metal-lized and connected to ground, the boundary condition of per-fect ground does not always apply. Some MWC filters are de-signed and tuned to the required response even though the top of the filters are not perfect ground. If a grounded shield touches the top of such MWC filter, the filter response may be altered -especially at the notch frequency. If touching is unavoidable, then one should measure and ensure that the modified responses still meet the requirement. If not, one can either request for a customized filter or use a shielded MWC filter which can come either with a metal shield or an integrated shield. Integrated shield here refers to a MWC with a better-grounded top metal-lization. These filters’ responses are less sensitive to the exter-nal shielding. Some of these filters can be designed to have a very steep roll-off skirt, comparable to SAW filters, and a good harmonic suppression.

Other applications of SAW and MWC devices

SAW and MWC filters are widely used in many other applica-tions, e.g. wireless LAN, wireless local loop, wireless micro-phones and headmicro-phones, RF wireless remote controls, GPS sys-tems, Bluetooth syssys-tems, and wireless mice and wireless/cable modems for computers. Apart from filters, SAW and MWC reso-nators and other special SAW devices are also available in the market. SAW and MWC resonators are widely used in RF re-mote control systems, wireless microphones and headphones, CATV converters and wireless data transmission systems, etc. as frequency stabilization components for local oscillators. Ra-dar and electronic warfare systems and some electronic instru-ments use SAW devices for signal generation and processing.

Conclusions

SAW and MWC devices are key components in modern elec-tronics, especially in modern wireless communication systems. By keeping in pace with the rapid development of wireless tech-nologies, newer and better SAW and MWC devices matching novel market demand will be invented and fabricated. These can be in the forms of lower insertion attenuation, more stable and wider bandwidth filters, or higher frequency devices than pos-sible today, or smaller size or more-in-one filters. SAW and MWC component industry will continue to grow in the years to come.

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Presentation Materials

Author’s contact details

Dr. Quincy Chen

Principal Engineer Epcos Pte Ltd

166 Kallang Way, Singapore, 349249 Tel: 65 840 4851

Fax: 65 744 5978

E-mail: [email protected]

Christian Block

Manager, Product Development Siemens Matsushita Components

Siemens Street 43, Deutschlandsberg, Austria Tel: 43 346 2800 ext. 306

Fax: 43 346 2800 ext. 385

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