Akustik yüzey dalga esasına dayanan filtrlerin analizi, tasarımı ve GSM sistemindeki uygulamaları

Abstract

Haberleşme sistemlerinde, akustik dalgalara ilişkin özelliklere dayanan elemanların kullanılması, uzun yıllar önce düşünülmüş ve uygulamaya konulmuştur. Ancak akustik yüzey dalganın verimli bir biçimde uyanlıp kullanılmaya başlamasının otuz yıldan daha az bir geçmişi vardır. Piezoelektrik bir kristal yüzeyi üzerinde uyarılan akustik dalgalar kullanılarak 10 Mhz'den başlayan ve birkaç Ghz'e uzanan frekans bölgelerinde çalışabilen filtre, geciktirme hatları ve osilatör gibi elektronik elemanlar gerçekleştirilebilmektedir. Akustik yüzey dalganın ( Surface Acoustic Wave - SAW ) piezoelektrik bir kristalin bir yüzeyine yerleştirilen, karşılıklı konulmuş ve birbirlerinin dişleri arasına yerleştirilmiş iki tarağa benzeyen elektrot sistemi ile ( Interdijital Transdüktör, Interdigital Transducer - IDT ) uyanlabilmesi, diğer bir deyişle transdüktörün bir düzlem üzerinde gerçekleştirilmiş olması özelliği, akustik yüzey dalgaya dayanarak geliştirilmiş olan elemanların haberleşme sistemlerinde yaygın bir şekilde kullanılmasına imkan sağlayan önemli bir etken olmuştur. Birçok akustik elemanın gerçekleştirilmesinde temel olan düşük propagasyon kaybı ve küçük boyut düşüncelerine ek olarak, uyarılan dalgaya sürekli bir erişim olması açısından akustik yüzey dalga önemli bir avantaja sahiptir. Akustik yüzey dalga transdüktörünün uyardığı akustik dalga, transdüktöre uygulanan gerilimin frekansına bağlıdır. İki transdüktör belirli bir biçimde aynı kristal üzerine yerleştirilirse, bu transdüktörlerden birinin uyardığı dalgayı öteki alırsa, oluşturulan iki transdüktörlü bu yapının transfer fonksiyonu da frekansla değişecektir. Bu frekans karakteristiğini belirleyen transdüktöre ilişkin birçok parametre vardır. Frekans karakteristiğini etkileyen birçok parametrenin olması, filtre gerçekleştirilmesinde esneklik sağlamaktadır. Bu çalışmada, GSM ( Küresel Mobil İletişim Sistemi, Global System for Mobile Communications ) sisteminde kullanılmak üzere akustik yüzey dalga esasına dayanan filtre tasarımı konusu ele alınmıştır. Bu amaçla interdijital transdüktörlerin analizi ve tasarımı konusu gözden geçirilmiş ve ayrıca günümüzde yaygm bir şekilde kullanılmaya başlanan GSM sistemi incelenmiştir.

Surface Acoustic Waves ( SAW ), also known as Rayleigh waves have received considerable attention first in the early 1960's and many devices are now being incorporated in electronic systems. There has been a substantial growth of research into methods of generating and manipulating the waves, and in developing practical devices for use in a wide range of electronic applications. Acoustic waves propagating on the surface of a piezoelectric crystal provide a convenient means of implementing filters and delay lines at frequencies ranging from several megahertz to several gigahertz. In addition to the considerations of low propagation loss and microminiature dimensions which are basic to most acoustic devices, the surface wave mode has the important advantage of providing continuous access to the propagating acoustic wave. SAW devices make use of the surface acoustic wave which travels along the free surface of the material. The wave amplitude is a maximum at the surface and decays rapidly with depth into the material; almost all the energy is confined to a region extending about one wavelength into the bulk. As the velocity of the surface wave is around 3000 m/s the wavelength is less than the free space electromagnetic wavelength by a factor of about 105. For most of the SAW devices, the structure is simply a piezoelectric medium with a metal film on the surface, etched to give an appropriate geometry. Many of the advantages of SAW devices are derived from their physical structure. They are inherently very rugged and reliable. Because their operating frequencies and responses are set by photolithographic processes, they do not require complicated tuning operations nor do they become detuned in the field. The performance advantages realized with SAW technology are dependent on the application and frequently include small size, linear phase, low shape factor, excellent rejection and temperature stability. The semiconductor wafer processing techniques used in the manufacturing of SAW components permit large-volume production of economical and reproduciple devices. Owing to the simplicity of the structure, and the availability of convenient fabrication methods, nearly all surface wave devices use piezoelectric materials. In applications of the surface acoustic wave phenomenon to electronic devices, piezoelectric materials are required to convert the incoming IX electromagnetic signal to an acoustic one, and vice versa. Crystalline materials are usually chosen in order to obtain low attenuation of the waves. The materials most often used for SAW devices are quartz ( X direction on ST cut ) which has a low temperature coefficient of delay, and lithium niobate ( Y-Z ) which has a higher coupling coefficient. Lithium niobate is preferred for wide percentage band width structures. A major factor in the emergence of SAW was the invention of the Interdigital Transducer ( IDT ) which provides efficient transduction of electrical to acoustic energy and simultaneously enables the inherent versatility to be exploited. This transducer forms the basis for most of the wide variety of SAW devices in current use, including filters and delay lines. The most basic interdigital device is a delay line, consisting of two IDTs on a piezoelectric substrate. Bandpass filters, dispersive pulse-compression filters and tapped delay lines are all based on the same principle, but with more sophisticated transducers. The interdigital transducer consists of a series of interleaved electrodes made of a metal film deposited on a piezoelectric substrate, as shown in Figure 1. The width of the electrodes is equal to the width of the interelectrode gaps. An applied voltage will cause, through the piezoelectric effect, a strain pattern of periodicity L, the periodicity of the structure. If the frequency is such that L is close to the surface wave wavelength there is strong coupling into surface wave energy and, by symmetry, surface waves are launched in two opposite directions. The stressed pattern exited by the transducer corresponds to the sum of the stress of these two waves, i.e., a standing-wave stress pattern. A second transducer can be used to detect surface waves, thus forming a delay line. The unwanted surface waves are absorbed at the ends of the crystal by means of wax or adhesive tape. Input Transducer Wv\sxw\nx 0 Nb ^ E)N M ISI Rl ISI s ^ f^k^ksı i- Output Transducer SAW si s; S ^ Is R^C^Sk^ S Absorber T Piezoelectric Substrate Metal Film inductor Rl Figure 1 : The interdigital transducer structure on a piezoelectric substrate Practical transducers can be quite efficent, converting most of the available electrical power into surface wave power. However, half of the power is radiated in an unwanted direction giving a loss of 3 dB and in a device with two transducers this factor contributes 6 dB to the total insertion loss. Second-order effects, such as coupling efficiency, resistive losses and impedenca mismatch, raise the insertion loss of practical filters to 15-20 dB. Losses due to second-order effects can be reduced. The surface wave propagates with little attenuation, and diffraction spreading can be minimised by using a sufficently wide aperture ( W ), so that the output transducer is in the near field of the input transducer. For low loss one or more lumped components are usually added to match the transducer electrically to the source or load. The transducer impedance is largely capasitive and often it is sufficient to tune it using a series inductor, as shown in Figure 1. The aperture W influences the transducer impedance and the diffraction spreading, but can often be chosen such that minimal diffraction spreading and an impedance convenient for matching are both obtained. Typical apertures are 20 to 1 00 wavelengths, or a few mm, and are convenient for fabrication. With appropriate design, practical devices can give insertion losses 10 dB or less. However, the devices are usually designed to give larger losses in order to reduce reflections. It is consequence of the bidirectional nature of the transducer that, when it is well matched to an electrical source or load, it reflects incident surface waves quite strongly. This gives rise to an unwanted additional output signal known as the triple-transit signal, due to surface wave traversing the device three times. The triple transit signal is often supressed by delibrately avoiding a good electrical match to the source load, and in consequence the insertion loss usually exceeds 15 dB. However, some more complex types of transducer are unidirectional, generating surface waves in only one direction, and these enable low losses to be obtained while still suppressing the unwanted reflections. In SAW devices, the frequency response is determined by the finger spacing and overlap of the interdigital comb structures used as input and output transducers. Generally the SAW filter is configured with two IDT' s, one each for transmission and reception. One is the normal IDT shape, with fixed space and overlength between fingers, and the other is the weighted type IDT with a different pitch and overlength between fingers. The desired complicated frequency characteristics are achived by using the most suitable pitch and space between electrode fingers in the weighted type IDT. The transducers shown in Figure 1 are uniform, in the sense that the pitch and overlap of the electrodes do not vary in the transducers. Non-uniform IDT' s are the basis of a variety of SAW devices. If the electrode overlap varies, the transducer is said to be apodised. Figure 2 shows a device with one uniform and one apodised transducer. This is the basis of most bandpass filters. Both transducers have constant pitch. If an impulse is applied to the input transducer, a packet of SAW energy propagates along the device. Each inter-elecrode gap can be regarded as generating a delta function SAW XI waveform, with the form of a line whose length corresponds to the electrode overlap. At the receiving transducer, each line generates a voltage proportional to the length of the line, and hence proportional to the electrode overlap. The output waveform, which is the device impulse response, thus closely resembles the transducer overlap function. The uniform transducer must be wide-band, have few electrodes, for this approach to be valid. The frequency is obtained by Fourier Transformation. This is the basis of a first-order design procedure, the required frequency response is transformed to the time domain, and the result gives the required electrode overlap function directly. The apodisation has a ( sin x) / x form, giving a ( Sin Bt ) / Bt form for the envolepe of the impulse response; the frequency response then has the rectangular shape given by rect [( co0-co)/2B]. The direct relationship between the transducer geometry and the impulse response also applies to transducers whose electrode pitch varies. Impulse Response SinBt.Cosw0t Bt Figure 2: Bandpass filter using apodised transducer In this thesis a comprehensive circuit model characterization of dispersive interdigital transducers with nonuniform electrode spacing is presented. The model is an extension of a three-port circuit which has been useful for representing periodic transducers. The extended model includes the effects of strong piezoelectric coupling whereby the acoustic waves and electric circuits interact, and it also accounts for reflections of acoustic waves which result from perturbations of the crystal surface by the" metal electrodes. The circuit model is used to derive a transducer design procedure which determines the electrode positions and the apodization function required to reproduce a desired waveform. This prodecure is applicable to the design of weighted dispersive filters. By using presented circuit model for interdigital transducers, SAW filters are designed for GSM ( Global System for Mobile Communications ) IF and RF applications. In 1982 CEPT ( Conference of European Posts and Telegraphs ), the main governing body of the European PTT's, created the GSM Committee and tasked it with Xll specifying a cellular pan-European public mobile communication system to operate in the 900 Mhz band. A general objective of the GSM system is to provide a wide range of services and facilities, both voice and data, that are compatible with those offered by the existing fixed Public Services Telephone Networks ( PSTN ), Public Data Networks ( PDN ) and Integrated Services Digital Networks ( ISDN ). Another objective is to give compatibility of access to the GSM network for any mobile subscriber in any country which operates the system, and these countries must provide facilities for automatic roaming, locating and updating the mobile subscriber's status. Features of the GSM system: - Pan-European system - Efficient use of frequency spectrum for high subscriber capacity - SIM ( Subscriber Identity Module ) card for subscriber access - Emergency calling - Call forwarding - Electronic registration of radio equipment - VLSI ( Very Large Scale Integration ) technology for affordable equipment prices - Low power consumption extends operating times The SAW filter can be implemented in both Mobile Station ( MS ) and Base Transceiver Station ( BTS ) in GSM system. The GSM cellular radio network is shown in Figure 3. Figure 3: The GSM cellular radio network The Mobile Station is the equipment used by a subsciber to access the services offered by the system. MS is connected to BTS by Um interface. The Base Station Xlll System ( BSS ) is divided functionally into a BTS and Base Station Controller ( BSC ) and they are interconnected by the A-bis interface. The BSS is associated with the radio channel management including channel allocation, link quality supervision, transmission of associated signalling information and broadcast messages, as well as controlling transmitted power levels and frequency hoping. The BTS is the transmission equipment used to give radio coverage for a traffic cell. All control functions in the BSS are performed by the BSC. The radio equipment in a BSS may serve more than one cell, in which case the BSS will consist of several BTS' s under the control of one BSC. The Mobile Switching Centre ( MSC ) is linked to the BSC via the A interface and performs all the switching functions needed for the operation of the mobile stations in the group of cells it services. The functions of an MSC include call routing and call control; procedures needed for interworking with other networks ( e.g., PSTN, ISDN ); procedures related to mobile station's mobility management such as paging to receive a call, location updating while roaming and authentication to prevent unathorised access; as well as procedures required to implement handovers. The Home Location Register ( HLR ) is a data base unit for the management of mobile subscribers. Part of the mobile location information is stored in the HLR, which allows the incoming calls to be routed to the MSC in command of the area where the MS roames. The HLR contains the International Mobile Subscriber Indentity ( IMSI ) number which is used for the authentication of the subscriber by his Authentication Centre ( AUC ). The Equipment Identity Register ( EIR ) allows for stolen, fraudulent or faulty mobile stations to be identified by the network operators. The Visitor Location Register ( VLR ) is the functional unit that attends to a MS operating outside the area of its HLR. The visiting MS is automatically registered at the nearest MSC and the VLR is informed of the MSs arrival. Performance, size and cost are important for GSM system like all other communication systems. SAW filters have small size, light weight and low insertion loss. They are also inexpensive and especially suited for use in high-volume mobile and portable telephones. Because of this, SAW filters are used in GSM system. SAW filters which are used in GSM IF application have 70-71 MHz center frequency ( f0 ) and 200-400 KHz passband width ( BW, bandwith ) at 3 dB attenuation. SAW filters which are used in GSM RF application have 902.5 Mhz and 947.5 MHz center frequencies and 25 MHz passband width at 3 dB attenuation. In chapter 5 of this thesis, three bandpass SAW filters were designed at 71 Mhz, 902.5 MHz and 947.5 MHz center frequencies. In addition, the results of the computer simulation for designed bandpass SAW filters were given in chapter 5