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Understanding Surface Acoustic Wave (SAW) Devices for Mobile and W Understanding Surface Acoustic Wave (SAW) Devices for Mobile and W... http://www3.sympatico.ca/colin.kydd.campbell/ --> Understanding Surface Acoustic Wave (SAW) Devices for Mobile and Wireless Applications and Design Techniques by Colin K. Campbell, Ph.D., D.Sc. Session 19: "An Overview of SAW Devices For Mobile/Wireless Communications" ( 68 Questions and Answers for Year 2008 ) (Including Real-Time SAW Fourier Transformers ) (You may wish to print a copy of this web page for future reference ) WORLD-WIDE PRODUCTION LEVELS Question 1 . What is the current world-wide production level of surface acoustic wave (SAW) devices? Answer 1 : Major SAW manufacturers/suppliers include Japan, USA, Germany, mainland China, and Taiwan. While I have not been able to obtain up-to-date world-wide levels, my ownunofficial estimate is that these have to be several million SAW devices a year. For example, one company alone in one of these countries is reportedly producing 3 million devices per day ! SOME UNUSUAL PROPERTIES OF SAW DEVICES Question 2 : Before we go any further, tell me if surface acoustic wave (SAW) filters are analog or digital devices? Answer 2 : Tricky question! My own view is that some configurations (as in the basic bidirectional interdigital transducer (IDT) structure of Figure 1), can be considered to operate as passive HYBRID analog/digital devices! The basic SAW filter sketched in Figure 1 is indeed a passive analog device. It is just a thin metal film structure deposited on top of a piezoelectric crystal substrate, with no power supplies to worry about. However, this is not the complete answer! Now for the digital part. Look at the constituent input/output IDTs. The layout pattern of these input/output thin metal film patterns is designed to provide the desired bandpass filtering function H(f ) = Voutput/Vinput as the SAW propagates along the piezoelectric crystal surface. But these bidirectional IDTs may be considered to act asspatially-sampled versions of the corresponding time-evolving Inverse Discrete Fourier Transform (IDFT) h(t). (Remember that there is a unique correspondence between the frequency response H(f) of a filter, and its impulse response h(t). (Simple concepts for digital signal-processing engineers. Not so simple for old analog circuit designers like me !). Because of this, many digital signal processing techniques can be employed in the design of the IDT patterns. 1 of 43 28-04-2008 12:34 Understanding Surface Acoustic Wave (SAW) Devices for Mobile and W... http://www3.sympatico.ca/colin.kydd.campbell/ Additionally, SAW filters find applications in many digital communications systems. Question 3 : Give me three examples of the digital signal-handling equivalence of a SAW filter. Answer 3 : (a) Digital signal-processing window function techniques can be applied to shape the IDT patterns, and thereby shape the filter bandpass frequency response. Examples of these include Hamming, Cosine weighting, Kaiser, Kaiser-Bessel, Taylor-weighting, and Dolph-Chebyshev. (See Chapter 3 of my 1998 SAW book) . (b) The well-known (??) Remez Exchange algorithm - originally applied to the design of optimum Finite Impulse Response (FIR) linear-phase digital filters - can also be applied to the design of SAW bandpass filters. (See Chapter 8 of my 1989 SAW book Surface Acoustic Wave Devices and Their Signal Processing Applications ( Academic Press:Boston,1998 ), which also includes a FORTRAN Remez program for SAW applications. Also see: J. H. McClellan, T. W. Parks and L. R. Rabiner, "A computer program for designing optimum FIR linear phase digital filters," IEEE Transactions on Audio and Electroacoustics , vol. AU-21, pp. 506-526, December 1973.) (c) As a third hybrid-performance example, SAW Nyquist filters are employed in Quadrature-Amplitude-Modulation (QAM) digital radio modems. (See Chapter 19 of my 1998 SAW book) . Question 4 : Can SAW bandpass filters operate at harmonic frequencies? 2 of 43 28-04-2008 12:34 Understanding Surface Acoustic Wave (SAW) Devices for Mobile and W... http://www3.sympatico.ca/colin.kydd.campbell/ Answer 4 : Yes. They can operate at selected harmonic frequencies, depending on the metalization ratio h = a/b in Figure 2. Rayleigh-wave delay-line filters employing split-electrode IDTs on YZ-lithium niobate have been reported as operating efficiently up to the 11 th harmonic. (See: W. R. Smith, "Basics of the SAW interdigital transducer," in J. H. Collins and L. Masotti (eds.) Computer-Aided Design of Surface Acoustic Wave Devices . Elsevier: New York, 1976. Also see: W. R. Smith and W. F. Pedler, "Fundamental- and harmonic-frequency circuit model analysis of interdigital transducers with arbitrary metalization ratios and polarity sequences," IEEE Transactions on Microwave Theory and Technique s, vol. MTT-23, pp. 853-864, November 1975) . The IDTs in Figure 2(a) and Figure 2(b) can operate at selected odd-harmonic frequencies, while the IDT structure in Figure 2(c) can operate at selected even and odd harmonics, depending on the metalization ratio. Question 5 : But why would I want to operate a SAW filter in a harmonic mode? Answer 5 :: a) Say I am using SAW filters fabricated on single-crystal piezoelectric substrates. One good reason why I might want to use a SAW filter operating in a harmonic mode relates to possible interference from acoustic bulk waves, which may be generated to various levels by an excited interdigital transducer (IDT), in addition to the desired SAW. Bulk waves can propagate in any direction within the propagating single-crystal piezoelectric substrate on which the IDTs are fabricated. 3 of 43 28-04-2008 12:34 Understanding Surface Acoustic Wave (SAW) Devices for Mobile and W... http://www3.sympatico.ca/colin.kydd.campbell/ These can have three components: namely those for 1) longitudinal bulk waves, 2) fast transverse shear waves, and 3) the slow transverse shear waves. (See Chapter 2 of my 1998 SAW book ). Those components that arrive at the output IDT will generate interfering voltages there, in addition to the desirable SAW. These can cause undesirable passband as well as out-of-band degradation. If, however, I operate in a high-enough harmonic mode, it may be possible to "bypass" such bulk wave interference. (See References 37 and 38 in Chapter 6 of my 1998 SAW book) . b) Also, one good reason why I might need to operate at SAW filter (i.e., Rayleigh-wave or leaky-SAW (LSAW) type) in a harmonic-frequency mode relates to the operational frequency for my SAW filter. Remember that the SAW acoustic wavelength lois given by lo = v/fo , where v = SAW velocity and fo = fundamental operating frequency. This makes for very small SAW devices at frequencies above about 1.5 GHz. As an example, a packaged 1.880-GHz SAW Tx-filter for USA Personal Communications Services (PCS), (see Figure 1.4 in my SAW book), may only have an area in the order of 3 mm x 3 mm. (If you do not think this is a small filter, get out a millimeter scale and think about this!) Again consider that I want to use a high-frequency SAW filter design on a piezoelectric crystal substrate. If the operating frequency is to be above about 1.5 GHz, then I must be concerned as to whether or not I can have the desired photolithographic resolution in the fabrication of my IDT patterns. Recall that the acoustic wavelength lo at filter center frequency fo is given by lo= v/fo , where v = SAW/LSAW velocity. Remember from our previous web-page discussions that an electrode finger width in a SAW IDT is typically lo/4. So, in order to maximize my photolithography, I would want to use a SAW substrate with the largest acoustic velocity v . For frequencies above about 1 GHz this would suggest the use of a LSAW substrate cut, with acoustic velocity in the order of 4000 meter/sec. If the -4 filter fundamental frequency is to be f o = 2 GHz, this would give lo = 2.0 micron (1 micron = 10 cm). -4 For lo/4 IDT fingers this would result in required finger widths of only 0.5 micron (1 micron = 10 cm). If I want to make my own 2-GHz SAW filter with this fundamental frequency, I would require use of a high-resolution photolithographic camera. As well, I could encounter additional deterioration of the IDT finger edges in the follow-up microelectronic lithographic etching processes. If, as a result of these degradations, the fundamental frequency bandpass response was not achievable, or acceptable, I could try to use a suitable Mth harmonic-frequency design , while employing IDT finger dimensions as if for frequency f o/M . I have often fabricated SAW intermediate frequency (IF) filter designs for operation at the 5 th harmonic, because of lithographic resolution limitations. SAW DEVICE GENERAL CLASSIFICATIONS Question 6 : SAW devices my be classified into four (4) general groups, relating to their mobile/wireless signal processing applications. (a) List these four groups. (b) Give a few representative signal processing applications for each group. Answer 6 : (a) Group 1: Linear Resonator and Resonator-Filter Devices. Group 2: Linear Devices Using Unidirectional IDTs. Group 3: Linear Devices Using Bidirectional IDTs. Group 4: Nonlinear Devices. (b) Group 1 : Antenna duplexers (2 to 4 W) for mobile/wireless transceivers, RF filters for front-end interstage coupling, Resonator-filters for one-way and two-way pagers, Resonators and resonator-filters for medical alert transmitters, Resonators and resonator-filters for automobile keyless locks, Resonators for garage door openers, Fixed frequency and tunable oscillator circuits. Group 2 : Low-loss Intermediate Frequency (IF) filters for mobile and wireless circuits, Low-loss RF front-end filters for mobile/wireless circuitry, Multimode frequency-agile oscillators for spread-spectrum secure communications, Low-loss delay lines for low-power time-diversity wireless receivers.
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