Understanding Surface Acoustic Wave (SAW) Devices for Mobile and Wireless Applications and Design...

8/8/2019 Understanding Surface Acoustic Wave (SAW) Devices for Mobile and Wireless Applications and Design Techniques

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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 LEVELSQuestion 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 ownunofficialestimate 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 DEVICESQuestion 2: Before we go any further, tell me if surface acoustic wave (SAW) filters

are analogor digitaldevices? Answer 2: Tricky question! My own view is that some configurations (as in the

basic bidirectional interdigital transducer (IDT) structure of Figure 1), can beconsidered to operate as passiveHYBRIDanalog/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 functionH(f) = Voutput/Vinputas the SAW propagates along the


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piezoelectric crystal surface. But these bidirectional IDTs may be considered to act

asspatially-sampledversions of the corresponding time-evolving Inverse Discrete

Fourier Transform (IDFT) h(t). (Remember that there is a unique correspondence

between the frequency responseH(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 processingtechniques can be employed in the design of the IDT patterns. 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 thedesign 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 bookSurface 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).


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Question 4: Can SAW bandpass filters operate at harmonic frequencies?

Answer 4: Yes. They can operate at selected harmonic frequencies, depending on

the metalization ratio = a/b in Figure 2. Rayleigh-wave delay-line filtersemploying split-electrode IDTs on YZ-lithium niobate have been reported as

operating efficiently up to the 11thharmonic. (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 Techniques, 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.


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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.

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 2of 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 needto operate at SAW filter (i.e., Rayleigh-

wave or leaky-SAW (LSAW) type) in a harmonic-frequency mode relates to the


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operational frequency for my SAW filter. Remember that the SAW acoustic

wavelength is given by = 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, apackaged1.880-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 thisis 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 at filter center frequencyfo is given by = 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 /4. So, inorder 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 4000meter/sec. If the filter fundamental frequency is to be fo = 2 GHz, this would

give = 2 . 0 micron (1 micron = 10-4 cm). For /4 IDT fingers this wouldresult in required finger widths of only 0.5 micron (1 micron = 10-4 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 suitableMth harmonic-frequency design , while employing IDT

finger dimensions as if for frequency fo/M. I have often fabricated SAW

intermediate frequency (IF) filter designs for operation at the 5th

harmonic, becauseof lithographic resolution limitations.

SAW DEVICE GENERAL CLASSIFICATIONSQuestion 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


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frequency-agile oscillators for spread-spectrum secure communications, Low-loss

delay lines for low-power time-diversity wireless receivers.Group 3 : Nyquist filters for microwave digital radio, Voltage-controlled oscillators

(VCOs) for first or second-stage mixing in mobile transceivers, Fixed, or variable,

delay lines for path-length equalizers, Pseudo-Noise (PN)-coded delay lines for

combined Code-Division-Multiple-Access/ Time-Division-Multiple-Access (CDMA/TDMA), Clock-recovery filters for fiber-optics communication repeater stages,

Intermediate frequency (IF) filters for mobile/wireless receivers and pagers. (See

page 223 of my 1998 SAW book for variable SAW delay lines).Group 4: Synchronous and asynchronous convolvers for indoor/outdoor spread-

spectrum communications.

ANALOG CELLULAR TRANSCEIVERSQuestion 7: (a) By way of illustrating an analog-cellular type mobile

communications system, sketch the basic circuit for a dual-heterodyne 800-MHz

band Advanced Mobile Phone Service (AMPS) transceiver and illustrate whereSAW devices can be employed in it. (b) Briefly describe the functions and merits of

these components.

Answer 7: (a) Figure 3 shows the basics of such an AMPS transceiver, employing

six (6) possible SAW components. This operates as a narrow-band frequency-

modulation (FM) system, employing Frequency Division Multiple Access (FDMA).

(See Chapter 10 and Table 10.1 in my 1998 SAW book). Mobile Tx and Rx

bandwidths are 824-859 and 869-894 MHz, respectively, with 832 channels and a

channel spacing of 30 kHz.

(b) The antenna duplexer filters can typically be leaky-SAW (LSAW) low-loss

ladder-type filters. LSAW devices are normally preferred here over Rayleigh wave

structures, as they have greater sub-surface penetration than Rayleigh waves, whichallows for higher power handling capabilities (1-2 W) before the onset of device

degradation. As well, the receiver preselect filter Rx#1 requires 1) low insertion

loss (Less than about 3 dB), 2) a highly-selective bandwidth to prevent overloading

of the follow-up Low Noise Amplifier (LNA), and 3) a dynamic range capability of

about 120 dB. The follow-up RF filter RX#2, which can be a LSAW resonator-

filter type, is required to suppress (i) harmonics, (ii) image-frequency noise, and (iii)

noise generated by Class C (remember this ?) amplifier noise. The antenna-

duplexer transmit filter Tx#1 must handle power levels of up to 30 dBm. The

preceding RF filter Tx#2 , which can be a LSAW resonator-filter type, is required to

suppress close-in noise. The SAW component in the Voltage-Controlled Oscillator

(VCO) in the first mixer stage can typically incorporate a dual-mode SAWresonator-filter, or a wideband SAW delay line. Since the channel spacing is only 30

kHz here, the IF SAW filter must be very selective and also temperature stable.

Typically this could be a two-pole waveguide-coupled resonator- filter on a stable-

temperature cut (e.g. ST-X) of quartz piezoelectric-crystal substrate.


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DIGITAL CELLULAR TRANSCEIVERSQuestion 8: So far so good! Now sketch an illustrative transceiver for a digital-

cellular communications transceiver, such as for the Global System for Mobile

Communications (GSM). Again indicate the possible location of constituent SAW

components. Answer 8: Figure 4 outlines a basic European GSM digital cellular transceiver,

using In-phase/Quadrature-phase (I-Q) modulation/demodulation, and, showing up

to seven (7) possible SAW components. As given in Table 10.3 of my 1998 SAW

textbook, this system has a Tx band from 890-915 MHz, and an Rx band from 925-

960 MHz. In contrast to the analog transceiver of Figure 3, this digital system only

has 124 channels, with 8 users per channel, but with a carrier channel spacing of

1250 kHz. The access scheme here is TDMA/FDM with Gaussian Minimum Shift

Keying (GMSK) modulation. The SAW RF components are similar to those

discussed in Figure 3. The IF filter here is spectrally shaped, however, to cater for

the power spectral distribution of MSK signals. (See page 418 and Figure 15.2 of my

1998 SAW textbook).


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SAW NYQUIST FILTERS FOR MICROWAVE DIGITAL RADIOQuestion 9: (a) What microwave common carrier bands are used in North America

for long-haul and data communications traffic? (b) What is the purpose of a

Nyquist filter in a digital microwave radio system? (c) Sketch a block diagram

outline for the circuitry of a basic digital microwave transmitter employing

Quadrature Amplitude Modulation (QAM), showing the location of the SAW

signal-processing Nyquist IF filter. (d) Is the Nyquist filtering only carried out in

the transmitter section?

Answer 9: (a) North American microwave common carrier bands are 4, 6, 8, and 11

GHz.

(b) To attain freedom from Inter Symbol Interference (ISI).(See page 588 of my

1998 SAW textbook)(c) Figure 5 outlines the basic form of a typical microwave digital radio transmitter


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employing Quadrature Amplitude Modulation (QAM). Note that the SAW Nyquist

filter also incorporates an X/(sinX) filter to compensate for spectral distortion when

Non-Return-To-Zero (NRZ) binary signaling is employed. (See page 581 of my 1998

SAW textbook). (d) Not necessarily. Ifmatched filteringis required, the total

required Nyquist filter response is split evenly between IF stages in both the

transmitter and receiver. (See page 590 of my 1998 SAW textbook).

SIGNAL POWER LEVELSQuestion 10: In your response in Answer 7 you used the term "dBm". (a) What

does this mean? (b) Give some illustrative dBm numbers related to SAW front-end

components and oscillators for mobile/wireless systems.

Answer 10: (a) The term "dBm" is a base-10 logarithmic parameter and means

"decibels referred to 1 milliwatt (mW)". Thus 1 mW = 0 dBm.(b1) Consider an RF signal at the input to a wireless receiver with a voltage level of


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0.8 microvolt ( V) across a 50 ohm input impedance. The input power is (0.8 x 10-6)2/50 = 1.28 x 10-14 watts. The corresponding dBm value is dBm = 10 x log(1.28 x

10-14/10-3) = ~ -109 dBm. I have typically used this signal level output from a

frequency synthesizer when testing the required Signal-Noise-Distortion (SINAD)

performance specifications for a mobile radio receiver. (For SINAD information see

page 267 of my 1998 SAW textbook).(b2) "Off the shelf" Rayleigh-wave oscillators are typically limited to an upper

power level in the order of 15 dBm, while leaky-SAW oscillators perform up to

about 30 dBm.(See page 542 of my 1998 SAW textbook).(b3) Some wireless pagers are required to operate with input signal levels less than

-100 dBm. Figure 6 outlines one front-end circuit for achieving this. It employs a

low-loss leaky-SAW antenna duplexer, followed by a dual-mode leaky-SAW

resonator-filter. Down conversion to the IF stage is achieved using a differential

active mixer, a differential local oscillator, feeding a differential IF stage. The

merits of the conversion circuit in Figure 6 can include 1) low front-end insertion

loss, 2) good out-of-band rejection, 3) signal swings are doubled compared with

single-ended circuits, 4) improved common-mode rejection, 5) small package size, 6)no balance-to-unbalance transformer (Balun) required, 7) input/output impedance

matching capability, 8) reduced power consumption, and 9) frequency capability up

to 2 GHz.

( See Reference 112 in my web publication listing. Also see G. Endoh, M. Ueda, O.

Kawachi, and Y. Fujiwara, "High performance balanced type SAW filters in the

range of 900 MHz and 1.9 GHz,"Proceedings of 1997 IEEE Ultrasonics Symposium,

vol. 1, pp. 41-44.)


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SURFACE ACOUSTIC WAVE OSCILLATOR CONFIGURATIONSQuestion 11: Illustrate the types of SAW oscillators and configurations that can be

employed in mobile/wireless communications.

Answer 11: The artistic representation of Figure 7 hopefully serves to illustrate

the wide variety of available oscillator configurations. These include fixed-

frequency oscillators, tunable oscillators, frequency-hopping oscillators and

injection- locked oscillators. Moreover, oscillator types include those employing

Rayleigh-wave propagation, leaky-SAW propagation and Surface-Skimming BulkWave (SSBW) propagation. (For details of these see Chapter 18 of my 1998 SAW

book).


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Question 12: Why can STW or LSAW oscillators have a higher power outputcapability than Rayleigh-wave ones?

Answer 12: Because Rayleigh-wave sub-surface penetration is only about 1 acoustic

wavelength, excessive power densities can degrade the IDT metalization to the point

of destructive failure. Typically, STW oscillators can operate up to about 2 W ( + 33

dBm), before the onset of piezoelectric nonlinearities. They can also have excellent

far-out phase noise responses, and can be preferred for enhanced noise suppression

above 10-kHz Fourier frequency offset.

PHASE NOISE IN SURFACE ACOUSTIC WAVE OSCILLATORS

Question 13: How good is the phase noise capability and long-term stability of theRayleigh wave oscillators noted in Figure 7?

Answer 13: Typical noise floors of Rayleigh-wave resonator oscillators on ST-

quartz are down to about -176 dBc/Hz at a Fourier frequency offset of 20 kHz.

Long-term aging, attributed to random-walk processes, can be less than 1

ppm/year. Vibrational sensitivity capabilities are given as df/f= 1 x 10-9/g, (which

are at least as good as bulk-wave AT-cut devices). Multiple-pole oscillators can have

phase noises down to -80 dBc/Hz at 10-kHz Fourier frequency offset, with noise

floors of -183 dBc/Hz. Typically, four-pole VCOs can have flat group delay over

400 ppm, to compensate for 1) five-year aging, 2) temperature changes from -40 to

+70oC, and 3) frequency accuracy. The phase noise for hybrid Rayleigh-wave

VCOs can be about -100 dBc/Hz at 1 kHz offset. Power levels of Rayleigh waveoscillators are typically limited to less than about +15 dBm. (See Chapter 18 of my

1998 SAW book).

Question 14: In your Answer 13 you use the term "dBc/Hz". (a) What does this

mean ? (b) How do you measure this?

Answer 14: (a) This term means "Decibels below the carrier in a 1-Hz bandwidth."

It relates to phase-noise measurements, and is measured at a desired frequency

offset (called the Fourier Frequency Offset), usually anywhere from a 1-kHz offset

to a 1-MHz offset from the nominal carrier frequency. Figure 8 illustrates the

phase noise of a hybrid 422-MHz Rayleigh wave oscillator that I used for a wireless

application, before and after locking with a Phase-Locked Loop (PLL) forfrequency selection. The measured frequency stability of this particular locked

oscillator was +/- 10 Hz over a 1 second measuring period.(b) I used a commercial frequency stability analyzer which can be run to obtain the

stability in the phase domain or in the time domain. Time domain measurements

are quoted in terms of "Allan Deviation", or "Sigma y of tau" . (Note: If you want

to learn more about noise and noise measurements, a VERY good reference book is

the USA National Bureau of Standards Monograph 140, called "TIME AND


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FREQUENCY: Theory and Fundamentals," (B. E. Blair, Editor), U.S. Department

of Commerce, Issued May 1974, Library of Congress Catalog Number: 73-600299.

Also, for more information on definitions used in frequency and time

measurements, see: E. Ferre-Pikal, J. R. Vig, J. C. Camparo, L. S. Cutler, L.

Maleki, W. J. Riley, S. R. Stein, C. Thomas, F. L. Walls, J. D. White, ""Draft

revision of IEEE STD 1139-1988 standard definitions of physical quantities forfundamental frequency and time metrology - random instabilities, " Proc. 1997

IEEE International Frequency Control Symposium, pp. 338-357, 1997) .

Question 15: What kinds of phase noise do I have to consider in SAW oscillators?

Answer 15: i) White phase noise, ii) Flicker phase noise, iii) White frequency noise,

iv) Flicker frequency noise , v) Random walk. (See Table 18.1 on page 537 of my

1998 SAW text book).

Question 16: (a) What is special about the phase noise characteristic of an injection-

locked SAW oscillator listed in Figure 7? (b) What are some wireless applications of

injection-locked SAW oscillators?Answer 16: (a) As discussed in Chapter 18 of my 1998 SAW book, within themaximum injection-locking bandwidth, the oscillator tracks (and amplifies) the

input signal. Most importantly, the oscillator adopts the phase noise of the input

signal source. (See Ref #63 on my web publication page, as well as R. Adler, "A

study of locking phenomena in oscillators", reprinted in Proc. IEEE, vol. 61, pp.

1380-1385, Oct. 1973. (I THINK that these injection-locking relationships should

apply to ALL types of injection-locked electronic oscillators. )

(b) Applications of injection-locked SAW oscillators include 1) FM demodulation

at UHF frequencies which offers good signal-to-noise performance, as well as

fabrication simplicity over lumped Inductance-Capacitance (LC) tuning

networks, (See Ref. #49 in my web publication page), and 2) Carrier recovery atgigahertz frequencies in Binary Phase Shift Keying (BPSK) modulation systems,

with demonstrated Bit-Error Rates (BER) of about 10-7 at a Carrier-to-Noise (C/N)

ratio of 14 dB. (See Ref. #87 in my web publication page).


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LEAKY-SAW LADDER FILTERS FOR ANTENNA DUPLEXERSQuestion 17: Some of the circuits you sketched above related to front-end circuitry

employing leaky-SAW (LSAW) low-loss ladder filters with antenna duplexers. (a)

What are the merits of such LSAW ladder filters? (b) Sketch a LSAW "building

block" component of such a ladder filter. (c) Sketch an illustrative LSAW antenna

duplexer employing such building blocks, and illustrate a typical frequency

response for a 2.45-GHz ladder filter in a Wireless Local Area Network (WLAN)

circuit.Answer 17: (a) Merits include capabilities for 1) low loss operation (e.g., less than

about 3 dB for Tx and Rx stages), 2) high rejection at mutual frequency bands, 3)

power handling of at least 1 W, 4) good sidelobe suppression, 5) high rejection at

the image frequency and at second and third harmonic frequencies, and 6) very

small and light package sizes.

Question 18: What are the chief components of surface acoustic wave front-end

ladder filters and antenna duplexers?Answer 18: One-port resonators are configured in series-shunt combinations to act


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as Inductance-Capacitance-Resistance (LCR) Impedance Elements (IE). For energy

storage and resonator action the individual one-port resonators can either consist

of a long IDT with significant finger reflections, or a short IDT in conjunction with

end reflection gratings as shown in Figure 9. Because their relatively deeper sub-

surface wave penetration results in a higher power-handling capability, leaky-

SAW (LSAW) resonators (e.g. using 42

o

Y-X LiTaO3) are normally preferred overRayleigh-wave ones (e.g. using 128o LiNbO3). (See Chapter 13 of my 1998 SAW

textbook for more details).

Question 19: Sketch a basic LSAW ladder-filter antenna duplexer.

Answer 19: Figure 10 sketches an illustrative ladder-filter example. (There are

many possible variations, depending on the required filtering specifications). The

impedance/frequency characteristics for resonator elements IE-1, IE-2, IE-3, IE-4,

are selected to provide the desired Tx and Rx filtering responses.(See Chapter 13 of

my 1998 SAW textbook for more details).


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Question 20: Sketch an illustrative frequency response for a 2.45-GHz Wireless

Local Area Network (WLAN) as designed using LSAW ladder-filter technology.

Answer 20: Figure 11 illustrates such a response. Ladder filters can often be

recognized by the characteristic sidelobe "wings" shown here.


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WIDEBAND SAW IF FILTERS FOR SATELLITE

COMMUNICATIONSQuestion 21: a) Give an example of a communications system employing wideband

linear-phase SAW IF filters with fractional bandwidths of 50%. (b) What types of

piezoelectric substrates are used for the wideband SAW filter ? c) What finger slant

angles are typically employed ? d) What are some desirable response specifications?

Answer 21: (a) 70-MHz SAW IF filters with 50% fractional bandwidth have been

employed in digital data terminals in Mobile Earth Stations (MES) for

INMARSAT-C satellite communications. Such wideband filters have employedIDTs with slanted-finger geometry. Their characteristics include 1) extremely-flat

passband response, 2) excellent linear phase response across the passband, and 3)

large out-of-band suppression. Figure 12 illustrates one such response of a 70-

MHz slanted-finger SAW IDT structure with 50% fractional bandwidth.

(b) Typically, Y-Z LiNbO3 or 128 Y-X LiNbO3.(For more on wideband slanted-

finger IDTs see Chapter 8 of my 1998 SAW textbook).

(c) Finger slant angles of less than about 7 degrees are used to contain the SAW.


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(c) Passband specifications could typically require a passband amplitude ripple of

less than about 0.6 dB, a phase ripple of less than 5 degrees, and at least 50 dB out-

of-band suppression.(For detailed computer-aided design techniques for these slanted-finger structures

see: H. Yatsuda, "Automatic computer-aided design of SAW filters using slanted

finger interdigital transducers,"IEEE Transactions Ultrasonics, Ferroelectrics, andFrequency Control, vol. 47, pp. 140-147, January 2000.)

TIME-DIVERSITY (ASH) WIRELESS RECEIVERQuestion 22: (a) In your Answer 6 above, you mention "Time Diversity

Receivers." With reference to a basic block diagram circuit, outline the difference

between a time diversity (ASH) wireless receiver and a single-conversion

superheterodyne receiver. Briefly highlight the operation of the time-diversity

(ASH) wireless receiver. (b) What are some of the merits of this time-diversity

wireless receiver?


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Answer 22: (a) Figure 13 shows the basics of a time-diversity receiver, as compared

with those of single-conversion superheterodyne receiver. As shown, the time-

diversity receiver has no local oscillator for down conversion. Instead the incoming

RF data-modulated signal is time-gated into a low-loss SAW RF delay line. The

time-gating is controlled by a pulse generator which alternately switches on/off the

RF amplifiers at the input and output to the delay line. The low-loss (e.g. less than~ 3 dB) SAW RF delay line structure can typically employ Single Phase

Unidirectional Transducers (SPUDTs). It is designed to hold hundreds of samples

per incoming data bit. Typical delays are in the order of 0.5 microsecond. Since the

input/output RF amplifiers are not "on" at the same time, there is no undesirable

feedback to cause instability. The gating pulse signals can subsequently be removed

from the message data-bit signals in the output detector stage. Signal-processing

gains obtained with the time-diversity are comparable with that of the single-

conversion superheterodyne receiver.


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(b) Some of the target specifications applied to the time-diversity (Ash) wireless

receiver are as follows: 1) Center frequency 180 to 450 MHz, 2) -100 dBm

sensitivity at a 1.0-kb/s data rate, 3) 500 kHz minimum RF bandwidth, and 4)

very-low power consumption..

(For more on the time-diversity (Ash) receiver see 1) D. L. Ash, "New UHF receiver

architecture achieves high sensitivity and very low power consumption,"RF Design,pp. 32-44, December 1994, and 2) "1995 Product Data Book", RF Monolithics, Inc,

Dallas, Texas, USA).

(Also for more on low-loss SAW SPUDTs, see Chapter 12 of my 1998 SAW

textbook).

CLOCK-RECOVERY CIRCUITS FOR FIBER-OPTICS DATA-

COMMUNICATIONS NETWORKSQuestion 23: What are the merits of SAW-based clock-recovery circuits in digital

regenerative repeater circuits in fiber-optic data communication networks?

Answer 23: SAW-based timing-recovery modules can have excellent jitter-freeperformance in many instances. One example of their application is for digital

regenerative-repeater circuits for fiber-optic networks operating under an

Asynschronous Transfer Mode/Synchronous Optical Network/Synchronous Digital

Hierarchy (ATM/SONET/SDH) mode in data communications, as outlined in Figure

14. Bit-Error-Rate(BER) performance in each repeater is aimed at BER < 10-11, in

conjunction with reliability and long life. Depending on the fiber-optic Synchronous

Transfer Mode (STM) employed, these are clock-recovery SAW filters whose center

frequencies fb correspond to bit rates of 155.52 Mb/s (STM-1), 622.08 Mb/s (STM-

4) or 2488.32 Mb/s (STM-16). Effective Qs of these transversal SAW filters are

normally in the approximate range 700 < Q< 1500. Insertion losses for these linear-

phase clock-recovery SAW filters are typically in the range 15 to 20 dB, with verylow phase-slope ripple across the passband.


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Question 24: How are these SAW clock-recovery filters employed in regenerative

repeaters?

Answer 24: Figure 15 shows the basics of an illustrative regenerative repeater for a

fiber-optic data communications system employingNon-Return-To-Zero(NRZ)

modulation. (This outlines the circuitry for the timing-recovery "block" in Figure

14). One portion of the down-converted electrical signal is applied to a clock-

frequency extraction circuit. Since the power spectrum of an NRZ signal has nulls

at the signaling ratefb and a maximum at fb/2, an indirect method is used to extract

clock frequencyfb. As shown, the down-converted signal is first pre-filtered at the

power spectrum peakfb/2. This pre-filtered signal output is applied to a frequency-

doubling (squaring) circuit, for extraction of signaling frequency fb, which is thenapplied to the SAW filter with center frequency fo = fb. Timing comparisons and

"0" or "1" signaling decisions are obtained, following which the regenerated

electrical signal is up-converted to the optical output and passed "down the line" to

the next repeater stage. Note that in some applications the SAW filter and central

components in Figure 15 can be combined into anApplication Specific Integrated

Circuit(ASIC) for circuit packaging.


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Question 25: What limits the usable Q = fo/Df( f = 3-dB bandwidth) of these SAWfilters?

Answer 25: Static detuning over the entire repeater-circuit chain places a upper

limit on the usable Q value for the SAW filter. Additionally, the ringing time of the

SAW filters places a lower limit on usable Q.

Question 26: What types of SAW filters are used in these regenerative repeater

modules, and how difficult is their design?

Answer 26: Since the SAW clock-recovery filters are required to have extremely-

high phase linearity across the passband, transversal (i.e., delay-line) types of SAW

clock filters appear to be favored over SAW resonator filters. The design of SAW

filters operating at center frequency fo= 2488.32 MHz can be especially

demanding. To date, design techniques for SAW clock-recovery filters over 2 GHz

have included 1) delay-line structures operating at the fundamental center

frequency on piezoelectric crystal substrates, 2) filters operating at the third


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harmonic on piezoelectric crystal substrates, and 3) thin-film filters fabricated with

composite layers of Silicon Dioxide/Zinc Oxide/Diamond/Silicon. Figure 16 shows

the response of an illustrative SAW clock filter employing Silicon Dioxide/Zinc

Oxide/Diamond/Silicon, and operating at 2.488 GHz.

(See Chapter 19 of my 1998 SAW textbook).

REAL-TIME SAW CONVOLVERS FOR INDOOR/OUTDOORWIRELESS COMMUNICATIONSQuestion 27: What are real-time SAW convolvers used for?

Answer 27: They find application in indoor/outdoor spread-spectrum wireless for

packet-data and packet-voice communications. They also can be well suited to

combat multipath interference due to spurious reflections in indoor environments.


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Question 28: What are some of the merits of SAW convolvers for wireless

communications?

Answer 28: They can have the merits of broad bandwidth, large processing gain,

and small size. Also, as I just mentioned in Answer 27, they can provide improved

performance against multipath interference. Indeed, SAW convolvers with RF

bandwidths greater than the coherence bandwidth are well suited to indoor spread-spectrum communications in buildings with highly-reflecting structures. They can

also give good jamming protection if pseudo-noise spreading codes are employed.

( NOTE: SAW convolvers are used in IF stages, not RF ones!) .

Question 29: In their operation, are SAW convolvers designed to actually

implement the convolution of two signals ?

Answer 29: No! They are actually implemented to effect autocorrelationbetween an

incoming signal message bit and a locally-provided time-reversed reference replica

of the coding applied to the message signal.

Question 30: I really do not understand the difference between convolution andautocorrelation. Can you demonstrate this to me in a simple, non-mathematical

way?

Answer 30: Convolution and autocorrelation relate to the way the interaction

between two signals is processed as a function of time. Maybe Figure 17 will help to

demonstrate this. Here, an autocorrelation peakoccurs at a time when the animals

are identically overlapping one another.


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Question 31: Now give me a block-diagram sketch of a very basic real-time SAW

convolver.Answer 31: Figure 18 shows a basic real-time SAW convolver on a single-crystal

piezoelectric substrate. The input message-coded IF signal at frequency fis applied

at Port 1. A time-reversed replica of the message-coding sequence , also at

frequency f, is applied to Port 2. Their related SAW signals propagate under the

metal film, where autocorrelation takes place. The metal film must be long enough

to contain an entire code bit. The autocorrelated output at frequency 2fis obtainedat Port 3.

Question 32: Why is the Port 3 output signal at frequency 2f, while the input (Port

1) and reference (Port 2) signals are only at frequencyf?

Answer 32: The situation here is exactly the same as for an ordinary three-terminal

analog mixer component. Since the input and reference signals have to MIX, the

convolver has to operate non linearly! To do this, at least one of the input signals

(normally at Port 2 ) has to be large enough to drive the sub-surface SAW region

into nonlinearity. Note that a Rayleigh-wave crystal cut is therefore preferred,

instead of a Leaky-SAW (LSAW) one. The reason for this is that the sub-surfacepenetration of a Rayleigh wave is much less than a LSAW one. This means that

for the same input powers, the power DENSITY of the Rayleigh wave will be higher,

and make it easier to get in to nonlinear operation.


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Question 33: In Figure 18 you gave an outline of a very basic real-time SAWconvolver. Sketch an outline of a more sophisticated one, and mention some of its

relative advantages.

Answer 33: OK! Figure 19 shows the basics of a dual-track real-time IF SAW

convolver. Some such convolvers have been reported with correlation interaction

times of up to 22 microsecond. The design trick here is to arrange the polarities of

the interdigital transducers (IDTs) at Port 1 input so that they excited in-phase

SAWs in both tracks. However, the polarities of the IDTs at reference Port 2 are

arranged to excite 180oout-of-phase SAWs between the two tracks. The

autocorrelated signals in Track 1 and Track 2 can be summed by a differential

summer. This is not the end of the story, however! Any spurious undesirable SAW

reflections within Track 1 and Track 2 will be in-phase, and will therefore cancel outin the differential summer. Typical reported processing bandwidthsB for this

structure areB = 50 MHz at 350-MHz center frequency, with Tme-Bandwidth (TB)

products in the order ofTB = 150.


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Question 34: (a) Define the convolution efficiency c of a real-time SAWconvolver. (b) What are some typical values of convolution efficiency for real-time

SAW convolvers?

Answer 34: (a) This is normally defined as c = 10.log10[(Pout)/(Ps.Pr)]. It isusually expressed in (dBm)-1. In this evaluation the output power Pout at Port 3 is

normally measured with signal and reference powers PsandPr both set at 0 dBm

(i.e. 1 mW).(b) Representative values of convolution efficiency vary from about -70 dBm for a

dual-track basic convolver on a piezoelectric crystal substrate to about -46 dBm for

a layered structure involving ZnO/SiO2/Si. (See Chapter 17 in my 1998 SAW

textbook).

Question 35: A SAW convolver has a rated convolution efficiency c= -46 dBm. Ifthe signal input powerPs is 10 dBm (10 mW) and the reference powerPr is 20 dBm

(100 mW), what is the correlated output powerPout?

Answer 35: Expressed in dBm units we have Pout = c+ Ps + Pr . Before goingany further, however, we must remember that the IDTs at Port 1 and Port 2 are

bidirectional. This means that each IDT will lose 3 dB from the autocorrelation


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process. As a result the output power at Port 3 will be Pout = (-46) + (10 - 3) + (20 -

3) = -22 dBm = 6.3 microwatt.

Question 36: If the output noise floor level in the previous SAW convolver is -75

dBm, determine the output Signal- to-Noise (S/N) ratio.

Answer 36: This gives the output signal/thermal noise ratio (at output frequency 2f)asS/N= (-22) - (-75) = 53 dB, which also corresponds to the dynamic range in this

convolver example.

Question 37: Can a SAW convolver be used for synchronous or asynchronous

communications?

Answer 37: Yes, but a given design will only be for only one mode - not both. For

example, SAW convolvers have been designed for synchronous packet-data

communication using Binary Phase Shift Keying (BPSK)/Frequency Hopping (FH)

modulation and 255-chip orthogonal Kasami code sequences. Asynchronous types

have employed Direct Sequence (DS)/Frequency Shift Keying (FSK) or Direct

Sequence (DS)/Code Shift Keying (CSK) spectral spreading, using Pseudo-Noise(PN) 127-chip maximal sequence generators. (See Chapter 17 in my 1998 SAW

textbook).

Question 38: What are some of the frequency bands that modems with these IF

SAW convolvers have operated in ?

Answer 38: These include 1) the 900-MHz spread spectrum band using the DS/CSK

mode, 2) Full-duplex operation in the 2-GHz spread-spectrum band, and 3) the

licence-free spread-spectrum band in Japan below 322 MHz. (See Chapter 17 in my

1998 SAW textbook).

SAW WIRELESS BAGGAGE LABEL SECURITY

IDENTIFICATION "TAGS"Question 39: What are SAW wireless label identification "tags", and what are they

used for?Answer 39: SAW wireless label "tags" are used for identifying a wide range of

luggage or commercial shipping-container items. Instead of scanning an item with

an optical scanner, as at a supermarket checkout counter, the SAW inspection

transmitter circuit sends a high frequency radio signal pulse (e.g., at 1000 MHz)

from a transmitter to a SAW "tag" on the item to be inspected. The SAW baggage

tag itself is a passive component. Basically, it is a coded SAW interdigital transducer

(IDT) which has a small antenna attached to it . When excited by the interrogatingradio signal pulse from the nearby RF transmitter, it can radiate a coded RF signal

back to the source, for identification, as sketched in the basic circuit of Figure 20.

These tags can be very small indeed ! (For artistic illustration the size of the SAW

label sketched in Figure 20 is very greatly exaggerated here !)` A choice of different

code-length sequences can be employed in each IDT fabrication , depending on its

length (e.g., 128 bit-codes).


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Question 40: What are some of the reported merits of commercial SAW wireless

Radio Frequency Identification (RFID) "tags" ?

Answer 40: (a) SAW RFID tags are entirely passive. (b) They can be read with only

milliwatt levels of RF interrogation power. (c) They have a high level of radiation"hardness" under gamma-ray sterilization of medical and food products

requiring sterilization with gamma radiation. (d) "Read" ranges of 3 to 20 meters

depending on the system. (d) Good electromagnetic interference filtering. (d) Tag

temperature range capabilities from -100oC to over +200oC. (e) EPC compatible

with EPC-64 and EPC-96 RFID specifications. (f) SAW tag capabilities for 24-, 32-,

48-, 64- and 96-bit capacities. (g) Operational capabilities for operation in the 1.7

GHz and 2.5 GHz frequency bands.


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(For more on SAW wireless tags and their potential see, for example, C. S.

Hartmann, "Future high volume applications of SAW devices," Proceedings of

1985 IEEE Ultrasonics Symposium, vol. 1, pp. 64-73, 1985.

Question 41: When I check out my groceries at the supermarket, the optical scannerat the checkout counter can only scan one item at a time. In the case of SAW RFIDs

using electromagnetic wave interrogation, can the SAW inspection/detection circuit

only handle one RFID at a time? What happens if there are several RFID tags close

together, with the scope of the wireless detector circuit?

Answer 41: Another tricky question! When interrogated by a single wireless

transmitter/receiver, multiple reflection signals from RFID tags could of course

occur when several of these are close together ( such as placed on a number of

different jars of jam on a shelf), within the radiation pattern of the single

interrogating antenna. This would result in a multiplicity of received codes at the

interrogator in the same time interval! To overcome this, one reported technique

uses a phase modulation of selected finger pairs on each SAW RFID device, whichplaces a unique identifier on the signal returned to the wireless interrogator circuit.

(See, for example, P. J. Edmonson and C. K. Campbell, United States Patent No; US

6,827,281 B2, Dec. 7, 2004, "Encoded SAW RFID tags and Sensors for Multi-User

Detection Using IDT Finger Phase Modulation).

Question 42: In your answer to Question 40 you mention the terms "EPC-64" and

"EPC-96". What do you mean by these ?

Answer 42: (a) "EPC" stands for "Electronic Product Code" and represents a

numbering scheme for the unique identification of objects. EPC may be considered

as a Radio Frequency Identification (RFID) evolution of the Universal ProductCodes (UPC) currently used as optical-scanning barcodes in supermarkets and

elsewhere. There are several proposed standards of EPC, relating to the amount of

data stored in the interrogating transponder. Current EPC standards include EPC-

64 employing 64 bits of information data and EPC-96 employing 96 bits of

information data.

Question 43: Give an example of the coding distribution for an EPC-96 system.

Answer 43: Consider that we can divide the 96-bit code into four Segments from left

to right.Segment 1 is theHeader and is 8 bits in length (0 to 7 bits), This identifies the

EPC version in use.

Segment 2 is theEPC Manager, which employs 28 bits of data (8 to 35 bits). This is

used to identify the particularManufacturerof the product in question. The binary

number 228 gives us 228 = > 268 million identifiable Manufacturers !

Segment 3 is theProduct Object Class and is 24 bits in length (36 to 59 bits) . This

gives us 224= > 16 million products to identify.Segment 4 is theSerial Numberfor a given product, and is 36 bits in length (60 to 95


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bits). This gives us 236 = > 68 billion possible serial numbers!

Question 44: But before I figure out how the above EPC data be met by a SAW

RFID tag design, first of all sketch a simple binary-coded SAW wireless RFID label

tag, and explain its operation.Answer 44:I have sketched a simple illustrative SAW wireless tag in Figure 21,employing IDT reflector pairs configured, for example, as a 110011011 binary

code, as governed by the individual IDT relative "polarities". The antenna is

shown as a simple one-turn loop antenna. Note that input/output IDTs have a

common bus bar. The RF pulse transmitter in Figure 20 sends an interrogation

pulse to this SAW tag. After a short time delay the SAW tag re-radiates an RF

signal as a 110011011-coded RF waveform. This is subsequently detected by the

time-gated receiver and phase-detector circuit of Figure 20. Note that an

operational requirement for this particular circuit is that the free-propagation

distance between transmitter and SAW tag must be greater than the IDT code

length.


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Question 45: But am I restricted to the use of IDT sections as reflectors in Figure 21

above ?.

Answer 45:No. It is normally much easier if I use thin metal film reflectors strips -

each with modest SAW reflectivity capability - as shown in Figure 21a.

Question 46: How do these reflector strips work in the one-port device of Figure 21a

?

Answer 46:The IDT to the left is directly connected to the tag's antenna which

receives an interrogation RF signal. The RF signal is converted to a SAW which is

reflected sequentially from the various reflector strips and returned to the antenna.These reflector strips can be placed on the piezoelectric crystal substrate (typically

128o LiNbO3) to encode the RFID tag using amplitude weighting, phase weighting or

other variables.


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Question 47: (a) Give me an example of the level of bit encoding I can attain with

the RFID tag configuration of Figure 21a. Assume that I only have a maximum of

16 reflectors.(b) Highlight, (without giving mathematical details), how you could improve the

above simple 16-bit design to meet EPC tag specifications. Also give a reference to

such a designAnswer 47:(a) First of all, consider the simplest design where the 16 reflectors are

separated at fixed intervals. Further consider that the placement of each individual

reflector strip corresponds to a binary "1", while the absence of a reflector strip

corresponds top a binary "0". This will give us a capability of only 216= 65, 000

unique tags, which would not be of any use for the EPC-64 or EPC-96 tag numbers

mentioned above.(b) However, recent SAW design techniques involving a different type of data


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encoding - using a higher number of data bits for each signal pulse, together with

phase encoding of reflector strip placements and a higher data density -- have

shown that it is possible to attain 264 = 1.8 x 1019unique RFID tag numbers using the

same size SAW device as for the simple 16-bit one considered above.

For reference to the SAW design of part (b) see C. S. Hartmann, "A global SAW ID

tag with large data capacity, Proceedings of 2002 IEEE Ultrasonics Symposium,vol. 1, pp. 65-69, 2002.

"FBAR" (THIN) FILM BULK ACOUSTIC RESONATORS AND

FILTERS FOR THE 2 TO 5 GHz RANGEQuestion 48: a) What is an "FBAR" and b) where is it used in wireless/mobile

systems ?.Answer 48:a) "FBAR" stands for "(Thin) Film BulkAcousticResonator. By

themselves FBAR resonators can be employed as feedback elements in high

frequency VCOs. Bandpass FBAR ladder-filter modules constructed from FBARresonators can also be employed as front-end duplexer filters in the 2-GHz to 5-GHz

range. As well as a small package size (e.g., ~ 125 m3 in a PCS duplexer), FBAR

duplexers have good power-handling capability (e.g. > ~32 dBm in a PCS duplexer).

Question 49:What are the merits of FBAR filters compared SAW filters at these

frequencies - especially in the 5-GHz range?

Answer 49: SAW filter dimensions decrease with increasing frequency. As I noted

in Answer 5, apackaged1.880-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. And as we get up into the 5-GHz range, (and

unless we may choose to operate in a harmonic mode), SAW fabrication IDT linewidth dimensional limitations and tolerances become too severe for all but the most

sophisticated fabrication systems. But while SAW device fabrication resolution is

concerned with width parameters , the FBAR designs are dictated by depth

parameters thereby offering the potential for less stringent fabrication constraints.

Question 50: a) What piezoelectric thin-film materials are currently employed or

examined for FBAR filters? b) Give three important FBAR filter design

parameters? c) Why are these important?

Answer 50: a) These currently include Zinc Oxide (ZnO), Aluminum Nitride

(AlN), and PZT (PbZrxTi1-xO3). b) Three important parameters are i)

Electromechanical coupling factor k2

, ii) Temperature Coefficient of Delay (TCD),and iii) acoustic velocity v. c) i) Higherk2 values mean larger fractional bandwidth

capability ZnO has a larger k2 (~ 8.5%) than AlN (~ 6.4% in an epitaxial film),

while PZT has reportedly still-higher k2values. ii) However, the TCD of ZnO (~ 60

ppm/oC) is not as good as AlN (~ 25 ppm/oC). Low values of TCD are required for

maintaining frequency accuracy over a wide temperature range. iii) AlN has a

higher acoustic velocity (~ 10,400 m/s) than ZnO (~ 6330 m/s). A higher acoustic

velocity means that the device can operate at a higher frequency using the same


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physical dimensions.

(For more on FBAR resonators and filters, see, for example, a) K. M. Lakin, "Thin

film resonators and filters," Proceedings of 1999 IEEE Ultrasonics Symposium, vol.

2, pp. 895-906, 1999, b) H. P.. Lobl et al, "Piezoelectric materials for BAW

resonators and filters," Proceedings of 2001 IEEE Ultrasonics Symposium, vol. 1,

pp. 807-811, 2001).

Question 51: Why are we now talking about bulk acoustic wave (BAW) filters and

resonators, when we have been so far discussing SAW filters and resonators?

Answer 51: Their are many circuit equivalencies in the modelling of SAW and

BAW resonator and filter circuits. For example, one equivalent circuit for SAW

filter modeling employs the Mason Equivalent Circuitthat was first applied to

BAW filters and resonators.

(For more on the Mason Equivalent Circuit, see Chapter 4 of my 1998 SAW book

as well as, for example, J. F. Rosenbaum,Bulk Acoustic Wave Theory and Devices,

Artech House, Boston, 1988)


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Question 52: a) Sketch the basic configuration of one type of FBAR resonator, and

highlight its operating principles. b) Give some typical response and size

parameters for GHz frequency FBAR ladder filter front-end duplexers employing

series-shunt FBAR resonators.

Answer 52: Figure 22 shows the basics of one type of FBAR resonator. The

resonator itself is composed of a piezoelectric layer contained between input/output

connectors, which is excited to implement mechanical resonance. It is deposited on

top of a highly resistive wafer substrate, such as silicon (Si). For optimum

performance the design aim is to deposit an epitaxial (i.e., single crystal)

piezoelectric layer, with a particular crystal axis orientation for a given

piezoelectric. This can be tricky ! Analogous to a microwave resonator, thefundamental resonance frequency is that which results when the piezoelectric film

thickness is 1/2 acoustic wavelength. In order to minimize mechanical damping, the

resonator requires a large acoustic mismatch with outer boundaries. This is

achieved in the design of Fig. 22 by cutting away the bottom support base, using

micro machining or plasma etching.

b) A reported 5-GHz FBAR of this type on AlN had an unloaded series-

resonanceQs= 913 at 5.173 GHz, with a k2x Qs product of 58. Using such an FBAR in

a 5-GHz front-end ladder filter, (in the same way as for the SAW ladder filter of

Fig. 10 above), a fractional bandwidth of 5.0% was obtained, with a 2-dB

bandwidth of 210 MHz and a 3-dB bandwidth of 260 MHz, suitable for 5-GHz

WLAN applications. It was indicated that this particular FBAR responseoutperformed an equivalent SAW ladder filter in both the passband and out-of-

band responses. The filter package size in this design was 2.5 x 2.0 x 0.9 mm.(For further details of this particular 5-GHz FBAR resonator and filter see, T.

Nishihara, T. Yokoyama, T. Miyashita, Y. Satoh, "High performance and miniature

thin film bulk acoustic wave filters for 5 GHz," Proceedings of 2002 IEEE

Ultrasonics Symposium, (to be published).

Question 53:Is the FBAR filter structure of Fig. 22 the only design under study at

this time?

Answer 53: No. Instead of having a "free-space" piezoelectric membrane as in Fig.

22, another type of FBAR under development uses a Solidly Mounted Structure

(SMR), where the bottom resonator section is not "free", but is deposited on layered

films which are configured to act as a reflecting "mirror". This layered film

structure is known as a Bragg reflector. (For more on SMR filters, see, for example,

R. Lanz, M-A Dubois, P. Muralt, "Solidly mounted BAW filters for the 6 to 8 GHz

range based on AlN thin films," Proceedings of 2001 IEEE Ultrasonics Symposium,

vol. 1, pp. 843-846, 2001).


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Question 54:Sketch anLCR equivalent circuit, and illustrative frequency response

for an FBAR resonator.

Answer 54: As indicated in Fig. 23(a), the sameLCR equivalent circuit

representations can be used both for both FBAR and one-port SAW resonators.(For

more on one-port and two-port SAW resonators see Chapter 11 of my 1998 SAW

book). Resonator equivalent parametersCs,Ls, and Rs establish the series resonancewith minimumimpedance at notch frequency fs in Fig. 23(b). But the resonator is

also just a capacitor , with parallel capacitance Cp and tan(delta) dissipation loss

resistanceRp. At frequencies above fs, therefore,Cp andRpprovide aparallel

resonance withRs,Ls, resulting in an impedance maximum at frequency fp.

Rleadrepresents contact and lead resistance here. Depending on the design some

connection inductanceLleadmay also be present.


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SAW COMB FILTERS

Question 55: Is a SAW filter constrained to having just a single passband response,such as in the example of Figure 11?

Answer 55: No. This is where the analog/digital hybrid capability of the SAW filter

can be used, as mentioned in Answer 2 ! For example, we can apply digital-filter

concepts to the design of a SAW filter. One such sample design illustrated here

employed the Remez Exchange Algorithm used in linear phasedigital filter design.

This was derived in the early 1970s as a tool for designingfinite impulse response

(FIR) linear phase digital filters. (See, for example, J. H. McClellan, T. W. Parks

and L. R. Rabiner, "A computer program for designing optimum linear phase

digital filters,"IEEE Transactions Audio and Electroacoustics, vol. AU-21, No. 6, pp.

506-526, December 1973. Essentially, given a desired frequency response, it

supplies a finite set of impulse response coefficients for the digital filter synthesis,thus yielding an optimum approximation to the desired linear phase response. Its

application to SAW filters is covered in some detail in Chapter 8 of my earlier 1989

SAW book listed below. Figure 24 illustrates a prototypesingleSAW filter,

designed in this way to perform as a 10-band comb filter. Other Remez examples

are given in my 1989 SAW book.


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SAW WIRELESS BIOSENSORS FOR VAPOR DETECTION AND

IDENTIFICATION

Question 56: (a) Can SAW resonators be used as biosensors? (b) If so, give two

examples.

Answer 56: (a) Yes.(b) 1. Uncoated SAW resonators have been used in fast gas chromatography for

electronic nose simulation of olfactory responses. This is used to obtain a high

resolution visual image of specific vapour fragrances containing a variety of

chemicals. (See, for example, E. J. Staples, "Electronic nose simulation of olfactory

response containing 500 orthogonal sensors in 10 seconds" Proc. 1990 IEEE


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Ultrasonics Symposium.)

2. Bio-coated SAW resonators have been used for on-the-spot vapour phase

detection of plastic explosives containing nitro groups such as TNT, RDX and

others, using a SAW resonator immunosensor array. Detection sensitivity is

dependent on the biolayer deposited on the surface of the SAW resonator. ( See, for

example, S-H Lee, D. D. Stubbs, W. D. Hunt, and P. J. Edmonson, "Vapor phasedetection of plastic explosives using a SAW resonator immunosensor array"Proc.

IEEE Sensors Conference, Irvine, California, 2005).

Question 57: Sketch a basicuncoated two-port SAW resonator, and highlight its

important parameters for the sensor used considered here.

Answer 57: Figure 25 depicts the basics of a two-port SAW resonator. Reflection

gratings "bounce" back SAW that would otherwise "escape" from the IDTs.

Reflection gratings can be fabricated using open or shorted metal strips. Shorted

gratings, such as shown in Figure 25, can have better reflection qualities. SAW

resonators are generally designed to have low insertion losses in the range 1 to 3 dB,

and high-Q values (greater than 1000). Where Q = fo/ f at resonance frequencyfo, and f is usually measured at the 3-dB points in Figure 26. For temperature


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stability they are fabricated on temperature-stable substrates such as ST-cut

quartz. The resonance is critically dependent on the spacing between the IDTs and

the spacing between the reflections gratings and adjacent IDTs. The higher the Q,

the higher will be the resolution of the oscillator spectral response. Figure 26 shows

a typical frequency response, for a particular spacing between gratings and IDTs.

(See Chapter 11 of my 1998 SAW book).

Question 58: Sketch, and discuss, the basics of biocoated two-port SAW resonator

oscillator circuit, such as reported for plastic explosive detection as in your Answer

57, and highlight its important parameters.

Answer 58: Figure 27 depicts the basics of one biocoating configuration of a two-

port SAW resonator oscillator for vapor detection and identification. The biolayer


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comprises an antibody coating to detect the presence of target molecules from the

vapor of the small molecules from the gas phase . This causes an immobilization of

the antibody coating of the target molecules structure, and results in a baseline shift

of the oscillator frequency. The oscillator is transmitted to a test site for analysis. A

special analyses can subsequently be applied to identify the vapor in question. A

bank of such resonator oscillators with different identifying biolayer antibodycoatings (e.g., anti-TNT or anti-RDX antibodies) can be employed for identification

of more than one vapor. The normal linear relationship between frequency shift and

mass loading of the resonator surface has been extended to cater for the more

complex case of such antibody layer perturbations. (See, for example, W. D. Hunt,

D. D. Stubbs and S-H Lee, "Time-dependent signature of acoustic wave biosensors,"

Proc. IEEE, vol. 91, pp. 890-901, 2003)

SAW SENSORS AND IDENTIFIERS USING SELECTABLE

REFLECTOR ARRAYS


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Question 59: A "normal" SAW reflection grating, such as shown in Figure 25 can

be designed as an "shorted strip" one or as an "open-strip" one.(a) How many strips are typically used in these?(b) Are there any other ways we can effect the reflection of SAW waves, but in a

controllable "on" or "off" manner?

(c) Explain your above response in some more detail.Answer 59:

(a) An ordinary mirror only has one reflecting surface, as it reflects all of the

incident light. However a single SAW metal strip can only reflect about 1% of an

incident SAW. That is why we need many strips - usually 100 or more -

strategically separated - so that the combined SAW reflections reinforce each

other to totally reflect an incident SAW. (But in practice, things are not always

perfect!)(b) Yes, using split-electrode IDTs of the type shown in Figure 2(b)

(c) The split-electrode IDT shown in Figure 2(b), has some unusual properties

compared with the solid-electrode type of Figure 2(a). In Figure 2(a) each solid

electrode is one-quarter of a wavelength wide, while each of the split electrodes inFigure 2(b) is one-eighth of a wavelength wide. That means that the electrical

resonance frequency of the split-electrode IDT is one half of its mechanical

resonance frequency. So that the mechanical reflections cancel at exactly the

electrical resonance frequency. (See Figure 6.15 in my 1998 SAW book). However,

there will still be significant SAW reflections just off the electrical resonance

frequency. But if we put a short-circuit load across the split-electrode IDT, the SAW

will pass right under it with no reflections! (See, for example, A. J. DeVries,

"Surface wave bandpass filters", in text book,Surface Wave Filters, H. Matthews

(Ed.), Chapter 6, Wiley, New York, 1977). This gives us the means for controlling

SAW reflectivity by opening or shorting a load across the split-electrode IDT. With

intermediate magnitude and phase reflectivities by using other than a short-circuit

load.

Question 60:

(a) How can we in situ open or short load across a split-electrode IDT, in order to

control its reflectivity?(a) How many strips are typically used in these?

(b) Mention a wireless communication example of the above technique

Answer 60(a) A fluidic channel can be built into the surface structure of a split electrode IDT,

to inject a conductive fluid across a split-electrode pair, as sketched in Figure 28,

and thereby short out the IDT in question.(b) Individual split-electrode IDT in an array of these, as outlines in Figure 28, can

then be switched on or off, so that the output data from such an array resembles a

Pulse Position Modulation (PPM) type of data transfer. (See, P. J. Edmonson and C.

K. Campbell, US Patent No: US 6,967,428 B2, Nov. 22, 2005, "Selectable reflector

arrays for SAW sensors and identification devices')


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Question 61:

Sketch a basic SAW linear FM chirp filter and briefly describe its construction and

operating principles.Answer 61:

(a) Figure 29 illustrates the construction of a very basic SAW linear fM chirp filter.

Here the finger widths and spacings of the IDT electrodes are fabricated so that,

when impulsed, the detected signal at the (wideband) output IDT varies linearly

with frequency. This will be in the form of a frequency up-chirp or frequency down-chirp, depending on placement of the output IDT. (Note that the phase of the

output signal will have a linear term in time t, and a quadratic term in t2.) The

signal processing gain corresponds to the time-bandwidth (TB) product, where T =

chirp filter dispersion time (normally quoted in microseconds), and B = chirp

bandwidth (normally expressed in MHz), The linear FM chirp slope is givenas = B/T (in MHz/ ). Typical TB products for linear SAW linear FMchirp filters are TB = 10,000, while TB = 1 for a SAW filter with uniform finger


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spacing).

(See Chapter 8 of my 1998 SAW book for more on various types of SAW chirp

filters),

Question 62:

(a) What do we mean by the term Fourier Transform Pairas applied to signal

processing, and especially to SAW applications? Express in general terms, withoutequations.(b) What we mean by the term Convolution as applied to signal processing?

Answer 62:(a) The impulse response h(t) of any system is related uniquely to its frequency

response H(f) - and vice versa - by a Fourier Transform Pair. As one application to

basic SAW filter design, the IDT finger pattern is a sampled version of the impulse

response h(t) of the desired frequency response H(f), where h(t) represents the


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Inverse Discrete Fourier Transform

(b) The time-domain convolution of signal functions f1(t) and f2(t) corresponds tothe multiplication of their respective frequency response functions H1(f) and H2(f).

Convolution corresponds to a reversal of one of the time responses, together with a

relative time displacement of one of the responses, so that the two signals are

mathematically manipulated as moving towards one anther, and overlapping, as inFigure 17.

Question 63:

(a) Name three types of SAW real-time processors for mobile/wireless applications

utilizing SAW linear chirp filters and Fourier Transform techniques.(b) Where can I find more information on these Fourier Transform Processors?

Answer 63:

(1) Single-stage real-time Fourier-Transform Processor as a compressive receiver

for spectrum analysis of signals.

(2) Two-stage real-time Fourier-Transform Processor for Cepstrum Analysis.

(3) Two-stage Fourier-Transform Processor for real-time on-line filtering.(b) See for example, M. A. Jack, P. M. Grant, and J. H. Collins, "The theory, design

and applications of surface acoustic wave Fourier-transform processors,"Proc.

IEEE, vol. 68, pp. 229-247, 1980. Also see Chapter 16 of my 1989 book: Surface

Acoustic Wave Devices and Their Signal Processing Applications.

Question 64:(a) Sketch the basic circuitry for the single-stage real-time SAW Fourier Processor

mentioned in your Answer 63, and highlight its principles of operation. Exclude

circuit components such as compensation of inherent delays etc.(b) Give some typical operational parameters for such s single-stage Fourier

Transform Processors.

Answer 64:

(a) A very basic circuit for this Processor is shown in Figure 30, which employs two,

or three, linear FM chirp filters with the same chirp slope . This is based on themathematical trick that the Fourier Transform of the product of signal s(t) and the

impulse response time h(t) for the linear FM chirp filter can be expanded

mathematically into three separate terms involving a pre-multiplication,

convolution , and post-multiplication. The corresponding circuit is as shown in

Figure 30. Note that for convolution to be achieved the convolver chirp slope must

be the opposite of that for the pre-multiplier The optional output chirp filter servesto remove a residual quadratic phase term if both the magnitude and phase of the

output are required for network analysis.((b) These can have 100% duty cycle, with spectral resolutions, with analytic

bandwidths up to 1 GHz. Spectral resolutions can vary from the kHz to the MHz

range. IF frequencies can be in the GHz range with processing times in the 25 to 60

microsecond range. This can be much less than for digital Fourier Transform

Processors of the same price.


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Question 65:

(a) What is Cepstrum signal processing used for ?(b) State very briefly how Cepstrum signal processing can be achieved using a two-

stage real-time Fourier Transform Processor mentioned in Answer 62.

(c) Give a classic reference paper dealing with Cepstrum analysis

Answer 65:

(a) Cepstrum signal processing is a method for analyzing the power spectrum of a

signal which contains a periodic echo. It is based around the observation that the

logarithm of the power spectrum of a signal with a small echo component has an


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added periodic component due to that echo. Thus, the echo component should be

separable from the signal if a secondFourier transform is applied to the logarithmof

the power spectrum, (i.e., log(A.B) = log (A) + log(B) ).This gives the Cepstrum

response output in a pseudo-time domain, with the dimensions of seconds.

(b) The Cepstrum processor utilizes two cascaded processors of Figure 30, with a

logarithmic amplifier and detector located between the output of the first processorand the input of the second one. In this way high-speed real-time processing can

determine pulse durations and repetition rates from about 50 nanoseconds to 50

microseconds, as well as the bit rates of binary codes.(c) A classicCepstrum paper - with a most unusual title - is: B. P. Bogert, M. J. R.

Healey and J. W. Tukey, The quefrency analysis of time series for echoes:

Cepstrum, pseudo-autocovariance, cross-cepstrum and saphe cracking, " in M,

Rosenblatt (ed),Proc. Symposium on Time Series Analysis, Wiley: New York, pp.

209-243, 1963.

Question 66:

How do we achieve real-time on-line filtering, using a two-stage real time Fourierprocessor as mentioned in Answer 62?Question 66:

Instead of using a logarithmic amplifier and detector between the first a nd second

processors as in Answer 64, we use a third mixer, gated by a real-time filter function

H(2 t). This achieves amplitude-clipping or time-gating of the signal outputfrom the first Fourier processor, and so allows for on-line adaptive-filtering or

fixed-filtering of spread spectrum signals for suppression of narrow-band

interference. (See Chapter 16 of my 1989 SAW book)

SOME DEFINITIONS AND ABBREVIATIONS

Question 67: I do not understand some of the phrases used to describemobile/wireless handset units. Tell me what the following abbreviations mean,

namely (a) GPS-enabled, (b) Bluetooth-enabled, (c) Multi-band, (d) Multi-mode, (e)

3G?

Answer 67: (a) GPS stands for Global Positioning System. This enables accurate

position determination by means of the triangulation of signals from satellites, and

lets you locate where you are (e.g. while traveling in your car, or in a boat fishing in

one of Canada's many lakes, etc.). Typically, an integrated GPS unit can use 1 or 2

front-end SAW RF filters for enhanced detection of the satellite signals. AGPS-

enabledunit means that the GPS unit can be combined with other add-on services.(b) Bluetooth involves short-range wireless systems designed for operation in the

unlicensed 2.4-GHz Industrial, Scientific and Medical (ISM) band. Bluetooth-enabled systems are intended for portable linking to various units such as mobile

handsets or notebooks. A Bluetooth receiver can typically employ 1 front-end RF

SAW filter. (For more on license-free spread-spectrum bands see page 528 of my

1998 SAW book).

(c) Multi-band mobile/wireless transceivers can operate in more than one frequency

band. One example is for GSM three-band Worldphones that can operate in GSM,

DCS, or PCS modes. Recall that GSM stands for Global Systems for Mobile


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Communications, DCS stands for Digital Cellular System, whilePCSstands for

Personal Communications Services . The latter operate in the 1800-MHz and 1900-

MHz frequency bands. (See the Glossary definitions section on pages 613-618 of ny

1998 SAW book.)

***** Note that GSM is often referred to as the world'sfirstdigital wirelesstechnology. However, I personally consider it to be the world'sseconddigital

wireless technology - the first digital one being Morse Code wireless transmission,

that has been around for many, many years! ******

(d) Multi-mode mobile/wireless transceivers are those that can operate in more than

one mode of operation. These modes include AMPS, GSM, TDMA. (Again, see the

Glossary definitions section on pages 613-618 of ny 1998 SAW book.)(e) 3G refers to Third-Generation mobile/wireless systems operating in the 2100-

MHz band.

Question 68: What do we mean by Direct-Conversion Zero-IF?Answer 68: Direct Conversion Zero-IF (ZIF) receivers are those which directly

down-convert an incoming RF signal to baseband, as opposed to traditional

superheterodyne receivers that incorporate one or more intermediate-frequency

(IF) filter stages between RF and baseband. The use of ZIF stages will, of course,

depend on the mobile/wireless system involved, and on the sensitivity specifications

placed on incoming RF signals.

***********************************************************************************************************

My 1998 SAW text book is:

Colin K. Campbell,Surface Acoustic Wave Devices for Mobile and

Wireless Communications. Academic Press:

Boston, 633 pages, 1998. (ISBN Number 0-12-157340-0).

My 1989 SAW textbook, which includes chapters on the Remez

Exchange Algorithm, as well as on real-time SAW Fourier Transform

Processors is:Colin Campbell,Surface Acoustic Wave Devices and Their Signal

Processing Applications. Academic Press: Boston, 470 pages, 1989.

(ISBN Number 0-12-157345-1).

Check my 1998 book contents athttp://www.apcatalog.com/cgi-bin/AP?

ISBN=0121573400&LOCATION=US&FORM=FORM2
http://www.apcatalog.com/cgi-bin/AP?ISBN=0121573400&LOCATION=US&FORM=FORM2http://www.apcatalog.com/cgi-bin/AP?ISBN=0121573400&LOCATION=US&FORM=FORM2http://www.apcatalog.com/cgi-bin/AP?ISBN=0121573400&LOCATION=US&FORM=FORM2http://www.apcatalog.com/cgi-bin/AP?ISBN=0121573400&LOCATION=US&FORM=FORM2


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You can see my biographical sketch and photo at

http://www3.sympatico.ca/colin.kydd.campbell/ckcbiog.htm

Or find my author and co-author list of publications

athttp://www3.sympatico.ca/colin.kydd.campbell/ckcpub.htm

NOTE: Publication #76 in my list of publications is an Invited Review

Paper in the October 1989Proceedings of the IEEE, entitled

"Applications of Surface Acoustic and Shallow Bulk Acoustic Wave

Devices." This Review paper includes 322 References. (One of my SAW

illustrations in that paper was also used as the front cover design for

that Proceedings issue.) This paper may now be downloaded from the

IEEE web site for Ultrasonics, Ferroelectrics, and Frequency Control,

at Internet address:

http://www.ieee-uffc.org/index.asp?

page=freqcontrol/fc_reference.html&Part=5#top

:

My email address is [email protected]

--------------------------------------------------------------------------------------------

-----------------------------------------

Some - but not all - of the above SAW topics were discussed in previousSessions on this web-page site. These were:

Session 1: "How Many SAW Devices Can Be Used In a Typical AMPS

Mobile Transceiver ?"

Session 2: "Using a Leaky-SAW Differential Mode Resonator Filter in

Conjunction with a Differential

Active Mixer in the Front End of a Low-Power Wireless

Receiver."

Session 3: "On the Merits of Using SAW Convolvers For WirelessCommunications."

Session 4: "Example of a Fast Frequency-Hopping SAW Oscillator

Circuit."

Session 5: "Phase Noise in Surface Acoustic Wave Oscillators."

Session 6: "Leaky-SAW Front-End Ladder Filters and Antenna

Duplexers."
http://www3.sympatico.ca/colin.kydd.campbell/ckcbiog.htmhttp://www3.sympatico.ca/colin.kydd.campbell/ckcpub.htmhttp://www3.sympatico.ca/colin.kydd.campbell/ckcpub.htmhttp://www.ieee-uffc.org/index.asp?page=freqcontrol/fc_reference.html&Part=5#tophttp://www.ieee-uffc.org/index.asp?page=freqcontrol/fc_reference.html&Part=5#topmailto:[email protected]://www3.sympatico.ca/colin.kydd.campbell/ckcbiog.htmhttp://www3.sympatico.ca/colin.kydd.campbell/ckcpub.htmhttp://www.ieee-uffc.org/index.asp?page=freqcontrol/fc_reference.html&Part=5#tophttp://www.ieee-uffc.org/index.asp?page=freqcontrol/fc_reference.html&Part=5#topmailto:[email protected]


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Session 7: "SAW Nyquist Filters for Digital Microwave Radio."

Session 8: "SAW Clock-Recovery Filters for Fiber-Optic Data-

Communications Networks."

Session 9: "Wideband SAW IF Filters With Slanted Finger IDTs for

Satellite Communications."

Session 10: "So Far So Good, But How Do I Design a Basic SAW IF

Filter ?"

Session 11: "Why Would I Want (Or Need) To Use a SAW Filter

Operating In A Harmonic Mode?"

I have saved copies of each of the above web-page Sessions asMS 97

Word documents. Let me know if you would like me to e-mail you any

of the above session files.-----------------------------------------------------------------------------------------------------------

-----------------------------

Copyright Colin Campbell, 2008

In December 2004 this "hit" counter was reset to zero after a count of

38,000

Source: http://www3.sympatico.ca/colin.kydd.campbell/
http://www3.sympatico.ca/colin.kydd.campbell/http://www3.sympatico.ca/colin.kydd.campbell/

Understanding Surface Acoustic Wave (SAW) Devices for Mobile and Wireless Applications and Design...

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