PROCEEDINGS O F THE IEEE, VOL. 6 4 , NO. 5 , MAY 1976
and alternatingpolarityspaced one-half of aRayleigh wavelength apart. Note that there
are two sets of gratinglobesone corresponding to the longitudinal
waves and one to the
shear waves. The lateral wave amplitude at x = hl is now 33
dB below the Rayleigh wave in this example.
This last result is directlyapplicable tothe analysis of a
four pair interdigital transducer since the charges are localized
on the electrode edges.
SUPPRESSING
BULKWAVES
The bulk-wavegratinglobes
ofanID
transducerand of
lateral waves must be eliminated if low spurious SAW devices
are to be realized.Thiscan
be deducedfromtheplots
of
@ ( k , ) in Fig. 3 . Propagating bulk waves correspond to values
of I k , I < 1.657 kn for PZT. Therefore, it follows that the
charge distribution of an ID transducer must be designed such
that its Fourier transformis localized about the Reyleigh wavenumber. Since there is a one-to-onecorrespondencebetween
the fractional bandwidth and source spectrum in k space, this
localization implies a limit on fractional bandwidth. The wider
thefractionalbandwidth,themoreseriousthe
bulk-wave
problem.Additionally,
in crystallinematerialsoneshould
639
select a cut for which the surface-wave velocity is far from the
shear wave velocity in order to maximize the fractional bandwidth. Finally, if one has knowledge of the radiation pattern,
or grating lobes, it may be possible to design the SAW device
geometry so as to minimize the effects of bulk-wave reflections
from the bottom surface.
ACKNOWLEDGMENT
The author wdhes to acknowledge helpful
Dr. T. C. Lim and Dr. E. A. Kraut.
discussions with
REFERENCES
R. F. Milsum e t ai., “Comparison of exact theoretical predictions
andexperimentalresults
for interdigital transducers,” in R o c .
1974 Ultrasonics Symp., pp. 4 0 6 4 1 1.
R. S. Wagers, “Effect of finite aperture on spurious mode levels in
acoustic surface wave filters,” IEEE Trans. Sonics Ultrasonics, vol.
SU-22, pp. 375-379, Sept. 1975.
B. A. Auld, “Applications of microwave concepts to the theory of
acousticfields
andwavesinsolids,”
IEEE Trans.Microwave
Theory Tech., vol. MTT-17, pp. 800-811, Nov. 1969.
L. B. Felsen and R. Rosenbaum, “Ray optics for
radiationproblems in anisotropic regions with boundaries. I. Line source excitation,” Radio Sci., vol. 2, 1967; also L. B. Felsen, “Lateral waves,”
Res. Rep. PIBMRI-1303-65 (AD 630144).
Surface-Acoustic-Wave Devices for Signal
Processing Applications
J. D.MAINES AND EDWARD G. S. PAIGE
Abstruct-A survey of SAW devices is presented including delay lines,
frequency fisten, oscillators,matched filters,and Fourier transformers
Application of these devices to signal prowsing is discussed
basic property-the ability to store energylinformation on the
slowly propagating acousticwave.
The similarity is such that in some instances there
is competition between the bulk- and
surface-wave device(e.g., simple
I. INTRODUCTION
delay lines) but in
other instances they are complimentaryHE UTILIZATION OF acoustic waves in devices which one extending the range of the other as in frequency filtering.
perform signalprocessing functionsinelectronic
sys- However, in manyinstances the accessibility of the surface
tems has a history spanningseveral decades and has lead wave has lead to novel monolithic devices capable of performto some well establishedtechniques.Delay
lines, frequency ing more sophisticated signal processing functions than bulkfitters,dispersivedelaylines,andoscillatorsbased
on bulk wave devices (e.g., arrayprocessors). Of course, digital techacousticwaveshavebeenwidelyexploitedinthefields
of niques based on integrated circuits and, more recently, analog
communicationsandradar.They
are of considerablecomtechniques based on charge coupled devices offer current and
mercial as well as technological significance. It is not surpris- potentialcompetition.
However, withbandwidths of several
ing t o r i d the more recently developed surfuce-acoustic-wave hundredmegahertzanddynamicrangesextendingtowards
(SAW) devicesperformingsimilarfunctionsandbecoming
100 dB, passive SAW devices form strongly competitive signal
established in the same fields since both depend on the same processing elements in the IF frequency band.
In the decadesincetheirtechnologicalrelevancebecame
Manuscript received November 18, 1975.
appreciated,
SAW devices have moved from laboratory curiosThe authors are with the Royal Radar Establishment, Malvern, Worcs.
ities to established devices accepted in military and civilsysWR 14 3PS, England.
T
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640
PROCEEDINGS OF THE IEEE, MAY 1976
tems. During this time, confident predictions have been made
that surface-wave devices have a bright commerical
future. In
our view this optimism applies more particularlyto SAW-based
subsystems.
In this survey we shall briefly review SAW devices and their
application to signal processing, referring to other articles in
this issue. Section I1 introducessome basic informationon
surface waves and those surface-wave structures which repeatedly appear in surface-wave devices. Section 111 surveys SAW
devices relevent to signal processing. The applications of these
devicesarediscussedinSectionIV.Finally,SectionVpresentsconcludingcomments.
To alarge extentthis survey
builds on and updatesourmore
extensive review of SAW
devices [ 11 .
11.
BASIC INFORMATIONAND
COMMONSAW STRUCTURES
absorber
plezoekctrtc
,d
v57-3-d
(b)
(a)
Fig. 1. Asimplesurface-wavedelay-line.
Inset showsthe transducer
(b) Forpiezoelectric
cross-section.(a)Forpiezoelectricsubstrates.
layer on a nonpiezoelectric substrate.
SOME
Theessentialcomponents
of asurface-wavedevicein
an
electroniccircuitareshown
in Fig. 1. Theycompriseasubstratematerialwithanoptically
polishedsurfaceandtransducers for conversion between electrical and acoustical signals.
Exceptfor specialist devices, none of whichhavepassed
beyond the research stage, the substrate material is piezoelectric, usually quartz or lithium niobate. Neither of these materials fulfill all requirements and, in particular, there is an active
search for materials which combine
high K Z with a low temperature coefficient of delay [ 21 . ( K is for the electromechanical
coupling constant and K 2 is a measure of coupling efficiency.)
The simplest transducer is the periodic structure shown in
Fig. 1, normally formed photolithographically in an aluminum
film about 1000-8, thick. It has a conversion efficiency which
peaks at a frequency of u / A o , where u is the velocity of the
SAW, typically 3 X lo5 cm/s, and A 0 is the periodic length of
the transducer. Size of substrate, convenience of fabrication,
and
acoustic
attenuation
effectively
limit operating
frequencies to between 10 MHz and 1 . 5 GHz. Acoustically, the
periodic transducer has a fractional bandwidth
of N-' ,where N
is thenumber of finger pairs. Electrically,thetransducer
looks like a capacity shunted by a (radiation) resistance, which
2 by choice of finger length. If the transmay be made 50 f
ducer is inductively tuned and impedance matched, then a conversionefficiencyclose to 100 percent may
beachieved for
fractional bandwidths below 2K / 6 The 1 00-percent conversion efficiency is not fully exploited because
of the bidirectional of typical transducer structure.
In many devices the role of the transducer is not only to
convert from electric to acoustic energy but also to filter it.
The impulse response of a single transducer is directly related
to itsgeometry;the relativefingerpositions
determinethe
phase, and the source strength associated with the
fingers determines the amplitude of the response. (Techniques for weighting the source strength are discussed below.) Thus to the first
order of approximation, the frequency responseof a structure
such as shown in Fig. 1 is the productof the Fourier transform
of the impulse responseof each transducer.
Fig. 2 illustrates the relationship between impulse response
andtransducergeometryforthemuch
used technique of
fingeroverlapweighting
(apodization).The mainreasonfor
wishing to depart from this technique
arises because the impulse responses of the two transducers do not simply convolve
of the devicewhen both
toproducetheimpulseresponse
transducersareoverlapweighted.Largediffractioneffects
may provide another reason for interest in other approaches.
response
and
frequency
response. (a)Unweighted
Fig. 2 . Impulse
periodic transducer. (b) Sinc X weighted structure.
Alternativeweightingtechniquesincludefingerwidth
variation,fingerwithdrawal
weighting [ 31, capacitiveweighting
[41, and the series ("dog-leg") weighting [ 51 .
In the structure of Fig. 1, the surface wave is launched and
detected without any intermediate interference. Various
possibilities exist for redirecting and in some cases processing the
signalin transit. These include using reflecting arrays, multistrip components, and waveguides, topics which are briefly introduced below and are fully
covered in papers in thisissue
[61-[81.
The basicreflectingarraystructure(RAST)consists
of a
periodic set of reflectors, They may be set normal to the incident beam orinclinedatsomeother
angle. Thoughthe
intensity reflected from each reflector is usually low (-40 dB),
constructiveinterferencefromalargenumber(-100)leads
to goodreflectivity (-98 percent) overanarrow bandwidth
centered on u/2p, where p is the periodic length in the array.
Typical reflectors are metal strips, dielectric strips, or grooves.
A weighted response can be realized by variation
of the effective length of the strips or depth of the grooves. The primary
use of the RAST to date is in resonators (see Section 111-B)
and in dispersive delay lines (see Section 111-D).
Multistrip components (MSC) are based upon the multistrip
coupler [ 11. This coupler consists of a set of periodic metal
strips set normal to the propogating wave and arranged so that
the incident surface wave only fills half the aperture presented
by the coupler. As the wave propogates under the coupler, a
waveis generated in the unilluminated half of the structure
and by the process common to all forward directional couplers
the wave can be completely transferred to the adjacent track.
This action is basically broad band. The number
of coupling
strips to achieve 100 percent transfer is about ( r / K ) ' . Thus
applicationsrequiringstrongcouplingarelimited
to high K
3
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MAINES ANDSIGNAL
PAIGE: SAW
FOR DEVICES
PROCESSING
materials such as lithium niobate. A range of components [ 1 ]
based on the coupler enable a variety of functions to be performed; for example, beamwidth expansion or compression,
reflection,trackchangingwithreflectors,unidirectiontransduction,andtapping.
However it is featuresofthe
basic
couplersuchasbulk
wave suppression,removal of finger
length weighting effects, and beam splitting which are finding
widespread use in broad-band lithium niobate filters.
Waveguide structures stillremainintheresearchphase
despite obvious attractions as
a means of performing similar
functions to microwave waveguide components.Structures
considered include metal or dielectric overlays and topographical guides of various cross sections [ 81. The problems which
have retardedtheirdevelopmentincludedispersionofthe
wave, fabricationdifficulties,andlackofanefficient
design
for afeedingtransducer.Applicationswhich
have beenexplored with waveguides include delay lines, by forming a spiral
guide, and convolution in which
the nonlinear effects are enhanced by confinement of the acoustic power.
We concludethissectionbypointingoutthatthereare
numerousother
SAW structures ranging fromprismsand
lenses t o amplifiers and convolvers. Some of these will be introduced where appropriate.
111. SAW DEVICES
In this section, we review SAW devices which are relevent t o
signal processing. The emphasis is on their mode of operation,
if it is not self-evident, and current performances.
A . Delay Lines and Memories
Theability to store information is of fundamental importance in signal processing, and the use of acoustic waves with
their low velocity (relative t o EM waves) has been a feature of
delay line development for several decades. We may still say
that as a means of storing analog signals with passive componentstheyare
unrivalled. Competitionbetweenbulkand
SAW devices continues with advances on both sides. There is
a clear cut advantage to theSAW line when a tapped delay line
is needed.Here we review the progressof SAW fixeddelay
lines, tapped delay lines, variable delay and memory elements.
As evidenceof the strong interest in this field it should be
in this issue [ 6 ] ,
notedthat
five relatedarticlesappear
[91-[121.
1 ) Fixed Delay Lines-Tapped and Untapped: Delay times of
in proximityfuzeapplicainterestrangefromnanoseconds
tions to tens of milliseconds for, say, TV picture processing.
Both are difficult to achieve; reported delay times fall in the
100 ns to 1 ms range, with the range between 1 t o 5 0ps being
readily accessible using unfolded delay paths. Most attention
hasbeenfocused
onextendingdelay
to longertimeswith
acceptable bandwidths and this forms the subject
of Coldren
and Shaw’s paper in this issue. Long delay surface-wave structures investigated may be subdivided into i) thosein which the
wave is confined to asingle planar surface, e.g., spiral waveguide,foldedtrack
using multistripcouplers,and
ii) other
structureson
which the beam is wrappedaroundsuch
as
curved and/or planar surfaces in the form of a disk, cylinder or
plate with rounded ends.
Waveguide delay lines have so far failed to become competitive withalternativestructuresandremainasubject
of academic interest. A folded delay line with multistrip reflecting
track changers has been built with
usable performance [ 131.
It had a delay of 130 ps with 30-MHz bandwidth and spurious
64 1
signal level of -25 dB. Using the MSC approach, delays up to
500 ps are envisaged. One of the prime limitations of the MSC
approach is that it is necessary to use high coupling constant
materials; all work to date is being carried out on lithium niobate with its attendant problem
of high-temperature coefficient
[ 11. The problem of high-temperature coefficient
is not alleviated by the useofBilzGeOzo, amaterialwhichhasbeen
selected for wraparound delay lines because of its low surfacewave velocity. Impressive performance figures of 900-ps
delay, 65-MHz bandwidthcenteredon83
MHz, with 6 0 d B
dynamic range have been achieved [ 141.Inthe
diskdelay
line,a wave launchedfrom a transduceronto a disk with
smoothlyrounded edges followsa criss-cross pattern as it
loops round the surface; an article is devoted t o it in this issue
[ 111.On
a quartzdisk,somepatternsarestabilized
sufficiently by the anisotropy of the velocity and disk geometryso
that a low-loss long delay can be achieved. One
of these patterns has a temperature coefficient as good as that of ST-cut
quartz.Performancefiguresof220-psdelaywith
-57-dB
spurious signal level argues well for this novel approach.
To summarize, relatively short delay lines are readily made
andare used insystems(usuallyincorporatedinto
a more
sophisticated module). Longer delay lines pose problems but
current work holds out hope of competitive solutions. Looking furtherintothefuture,delay
lines withlonger TB and
large dynamic range become possible with surface-wave amplifiers.Indeed they provide the most convincing case for such
amplifiers, though their current development on temperature
sensitive lithiumniobatecouldbecomeanimportantlimitation of theiruse.
Most of the structures discussed above can be converted t o
tappeddelaylines,thoughtheremustbesomereservations
with regard to the disk delay line. The level of spurious signals
is generally greater in tapped lines because it is common
to
require equal tap spacing. Various techniques for suppression
of these signals in delaylineshasbeen
reviewed [ 11.Techniques t o reducetheir level arebased on either varying the
taps’ position relative to a wide aperture input transducer or
some formof“blooming”
in whichthereflected
signal is
removed by destructiveinterference.Theprimarylimit
on
tapped delay lines is the density
of taps; a limit imposed by
the density of electrical connections and the electrical interface. A 50-pm spacing is a typical limit. It may be overcome
by using parallel
delay
lines
and
distributing
the
taps
conveniently.
2 ) Variable Delay Lines and Memories,: Variable delay may
be achieved discretely or continuously. Discrete variable delay
has been obtained by switching between taps of
a typical delay
line [ 151.Continuous variabledelay canbe achieved using
dispersive delay lines [ 11 by variation of the frequency of a
local oscillator; performance is limited by available dispersive
lines (Section 111-D). Some results are shown in Fig. 3.
Surface-wave devices have recently been reported in which it
is possible to store a rephca of the signal for a variable time
before recall. A number of techniques use the electric fields
accompanyingthesurface
wave in apiezoelectricmaterial.
Thefieldsforceelectricalcharges
into trapping states of an
adjacent semiconductor.
By “switching-m” thetrappingprocessfor
a shorttime,
using a biassing voltage, a replica of the wave can be stored. In
the so-called memory correlator, an analog signal is stored in a
LiNb03-Si system in which the silicon is a continuous plate or
a Schottkydiode array suspended within a few thousand an%
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642
PROCEEDINGS OF THE IEEE, MAY 1976
Input pulse 200nS ; 50MHz
\
,B
LiNbO3
-
0
= 36MHz
2 9S
Tmax= 24pS
Fig. 3. Performance of SAW variable delay-line unit.
\
L
stroms of theniobatesurface
[ 121 . Thestoredreference
signal mixes with an input signal (at the same frequency) and
gives the correlation function as output at the plate terminal.
With a delta function input signal, a readout of the memory
is obtained.
10
o
l
o
1080
FREQUENCY
MHz
Fig. 4 . The SAW Q factor. Solid lines-intrinsic
Q of quartz and lithium niobate without air loading. Dashed lines-limit t o Q imposed by
1 single transit in a finite length of delay line.
B. Frequency Filters
Frequency filters have a variety of roles in signal processing.
They may be incorporated simply to prevent unwanted signals
entering the signal processor; theymay form a vital part of the
signalprocessor giving processing gain throughthe filter’s
ability to integrate by virtueof its storage capability; theymay
be present to identify frequency components as required for
spectral analysis.
The frequency filter has been one of the main foci of attention for those working on surface waves primarily due to the
potentialapplication
as a TV filter [ 161.Fromthisand
associated work, a rich diversity of techniques for matching,
(b)
weighting, and compensating second-ordereffects have evolved. Fig. 5 . Two basic forms of resonator structure. (a) One-port resonator.
(b) Two-port resonator.
The designer of any SAW filter is confronted with an array of
problems,some of thesecomplicate the design procedurediffraction, nonequivalence of sources (e.g., last fingers), the be made with high reflectivity “mirrors” and
do not involve
interdependence of sources,andcoupling
totheexternal
space consuming structures. The recent demonstration of high
circuit-otherssuch as multiple reflection, triple transit, and
reflectioncoefficient (-0.98) fromcompactperiodicstrucbulk wave generation may lead to complexity in the structure tures means that Q’s inexcess of lo4 arenowaccessible up
itself for effective suppression. However, it should be stressed
to frequencies close to a gigahertz on small samples. Achievthat excellentresultshavebeenachieved
[ 171without-ofable fractional bandwidths are limited to below K2. Two basic
band response and traps 60 dB down, insertion loss controlled forms of theresonatorformedfromperiodicstructuresare
to a fraction of a decibel, and phase error heldto a few degrees. illustrated in Fig. 5 . The one-port resonator has the important
The main limitation of SAW filters arises from the finite size attribute that, for the circuit engineer, isita direct replacement
of the SAW device or from attenuationof the wave; both have of the bulk-wave device [ 18J . On the other hand the two-port
abroad-bandingeffect
[ 1 J making it impossible to achieve resonator has a similar equivalent circuit to the one port and
very narrow-band response or shape factor very close to unity offers greater flexibility in design.
Frequencysorting
andselectivefiltersare
with a passive filter. Fig. 4 is an elaboration of an earlier graph
of particular
[ 11 showing the maximum Q to be obtained from quartz and
significanceinprocessingsignals
and will bediscussed later.
lithium niobate. The
highervalue for niobate arises from its
Severalapproachesareillustratedschematicallyin
Fig. 6.
Fig. 6(a) shows the most direct approach to frequency sorting
lower lossbut, unfortunately, its higher temperature coefficient
excludes its use in most ‘‘high$” requirements. Fig. 4 makes in which rectangular pass-band filters are arranged to give the
it clear that single transitoperation in the important region necessary frequency coverage.Obviously theyaresubject to
between100
to1000
MHz either significantlylimits
the thelimitations discussedaboveand
give aresolutiondeterachievable Q or forces the use of large substrates. Here lies the mined by Q (Fig. 4); a filter bank based on resonator strucmajor advantage of multiple transit schemes provided they can tures canbe anticipated.Acontiguousfilterbankinwhich
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64 3
AND MAINES
PROCESSING
SIGNAL SAW
FOR DEVICES
4
the frequency selection and channel separation is carried out
in theacousticmodeformsaninterestingdemonstrationof
the power of reflecting array structures for this type of application. A 16-channel device with channels 2
MHz wide in the
200- to 230-MHz band has been demonstrated with good performance [ 191. Fig. 6(b) introduces a frequency selector in
which the“state” of thefilter is changedelectronically. In
principle the number of selectable channels is the product of
et QI. [20] have
thenumber of states of eachfilter.Hays
demonstrateda
100 channelselectablefilter
of
0.5-MHz
channel bandwidth. Fig. 6(c) illustrates a tapped delay line as
a transversal filter. If the phase and amplitude of each tap are
controlled, then a degree of band pass shaping in addition to
selectivitycanbeachieved.
A means of obtaining the necessary programmability has been suggested and
is supported by
some preliminary experimental work [ 2 1 1. Fig. 6(d) indicates
one of the most recent approaches [ 221 to frequency selection
in which theinput signal is passed througha SAW Fourier
transformer, is gated (in the time domain), and then undergoes
an inverse Fourier transform. The position in time of the gate
determines
which
frequency
components
are
removed
or
passed. Resolution is determined by the dispersive delay time
of the delay lines in the transformer and by the width of the
gate.
C. Oscillators
FOURIER
NSFORM
INVERSE
FOURIER
OUTPUT
(dl
SAW devices.(a)
Fig. 6 . Somemethods of frequencysortingusing
Filterbanks.
(b) Electroniccontrol.(c)Tappeddelay-line
transversa1 filter. (d) A “time domain” filter.
An oscillator may be indirectly or directly involved in signal
processing. As a fixedlocaloscillator
it is involved tothe
extent that it is enabling other functions-wave form generation, filtering-to be performed at optimum asdistinctfrom
the transmitted frequency. As a swept frequency oscillator in,
for example, a spectrum analyzer, it may be at the heart of the
processor.
The conventional SAW oscillator consists of a quartz delay
line with output fed back to input through an amplifier with
sufficient gain to exceed the delay-line insertion loss [ 231. A
comb of frequencies satisfy the condition that the phase shift
around the loop is 2nN, with N an integer. In the single mode
oscillator, all but one of the possible modes are suppressed by
choice of transduceranddelaytime.Theadvantageous
features of the SAW oscillator have been widely stated; operation
up to low gigahertz frequencies without multipliers, freedom
fromsatellitemodes,
rugged,good short-termstability,and
easily frequency modulated are chief among them. The singlemodeoscillator is nowacceptedforapplication
in military
equipment and is actively being assessed for civil use.
Oscillators have been produced operating in the frequency
range 20 MHz to 1.5 GHz with transducers operated at their
fundamental[231.Kerbel[24]hasmadea
l G H z oscillator
with one transducer operated at its eighth harmonic and the
other at its eleventh harmonic, thereby relaxing the resolution
requiredinmanufacture.
By thischoice,responsesat
other
frequenciesweresuppressedwithoutexternaltuning.The
even harmonic was generated in a transducer with fingers
on
h/3 centers, two connected to one busbar
and one connected
to the other in a period length.
One of the most important aspects of theoscillator is its
stability. It is convenient to subdivide this into long term (-1
year), medium term (-1 h), and short term ( 7 1 s). Long term
stability or aging of close to 1 part per million per month has
been observed [ 2 5 1, which is comparable to a moderately good
bulk quartz crystal and an order
of magnitude worse than a
good one. Since the origins of aging have not yet been identi-
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644
PROCEEDINGS OF THE IEEE, MAY 1976
fied,possible
improvements are difficult to predictbut
it
wouldbe optimistic to think aging performances as good as
that obtained with bulk-wave devices will be achieved in the
near future. However, therearemanyapplicationsinradar,
forexample, where the aging is relatively unimportant. Mediumtermstability
is dictatedby“environmental”features
such as the temperature and voltage. The primary reason for
the choice of quartz is its low temperature
coefficient.
Voltage sensitivity can be reduced by incorporation of a SAW
discriminator controlled feedback loop and by maximizing the
acoustic path in the device. For some important applications
the short term stabilityis of primary importance since it determines the spectralpurity.
Fig. 7 showsaplot of the single
of off-set
sideband FM noisespectraldensityasafunction
of a490-MHzoscillatorwitha
from center frequency [33]
path length of 300 A. A Q of 4 X IO2 is realized (cf. Fig. 4).
Though optimized in certain respects for low noise
levels, an
improvement of more than 10 dB couldbe anticipated because
of the high (20-dB) insertion loss of the delay line. Nevertheless, the results shown in Fig. 7 compare favorably with those
obtained at a similar frequency with bulk-wave oscillators and
amultiplierchain.Clearly
thecompact SAW device has a
practical advantage over the bulk wave device with multiplier
chain in the VHF range-it is interesting to ask if it has a fundamentaladvantagewithrespect
to noise spectrum.Froma
model incorporating similar assumptions for bulk and surface
wave devices it has been concluded [ 261 that the high Q of the
low-frequencybulk-wavestructure
will offsetthe noiseenhancement of the multiplier chain giving the bulk-wave device
the advantage.Inclusion of the SAW resonatorstructurein
the argument enables the SAW device t o approach the bulkwave performance with a smaller size device. This is an area of
current research.
Trimming of the high-stability SAW oscillatorpresentsa
problemwhichmaybeovercomewithmorecomplicated
transducer arrangements such as one wide aperture transducer
of transducersover
slightly differentpath
feedingapair
lengths,trimming
(or FM)beingachieved
by the relative
power
split
between
the
pair. The
recently
investigated
traveling-wave transducer [27], byvariation of the electromagnetic wave transmission line velocity, may be employed to
advantage to trim the oscillator. The other virturesof this new
transducer-lowinsertion loss, smallmultiplereflection-also
favor itsuse.
Though the main emphasis has been
on fixedsingle mode
oscillators,programmableoscillatorshaverecentlyattracted
attention. In these oscillators it is possible to change from one
frequency to another in a controlled waywhilemaintaining
similar stability to that of the fixed single mode device. One
approach is to use wide-band transducers so that any one of
the comb of frequencies can be excited. A mode can be selectively excited to generate a particular frequency by injecting
theappropriatefrequencycomponentandswitchingonthe
amplifier [ 281. Another approach is essentially to replace the
delaylinebya
filterbankand to switchfromonefilterto
another allowing their passband to dictate the frequency response.Hartemann[29]has
describedaversion
ofsucha
A particularly ingeneous
device operating with ten channels.
programmable oscillator [ 301 uses the type of structure shown
in Fig. 8. The four output transducers are in echelon formation with a stagger of h/4, where h is the periodic length common to all transducers. In switching from olp A to B, say, the
mode number N remains constant but the frequency shifts due
-
CENTREFREOUENCY
‘ 4 9 0 MHz
SAW PATHLENGTH: 3001
INSERTION LOSS : Z O d B
A H ~ l L l f l E l NOISE T I S : 6 0
OUTrUT P O W E R : 5 n W
L
Y
c
=
-90
”
-P
IOhA
IllHz
IOOkHZ
OFFSET FREQUENCY
Fig. 7. Measured and calculated spectra o f a 490-MHz oscillator.
SCHEMATIC DIAGRAMOF A SIMPLE SAW
SYNTHESIZEREACH OUTRJT T R A N S W C E R
IS SEPERATED ay
Fig. 8. A scheme for electronicallyselectinga
multimode oscillator.
tothe
changein
pathlength.
particular modeofa
However,whenswitched
to
olp D note that it is possible to switch directly to olp A without change of frequency but with N changed to N - 1. The
processmaybe
repeated, giving arange of spotfrequencies
across the transducer bandwidth. Further subdivision or even
continuouscontrol
of the frequenciescanbeachievedby
replacing the simple switch with potential dividing networks
between transducers.
Oscillatorssuchasthesewhichcanbeprogrammedand
maintainhighstabilityareattractiveascheapsynthesizer
replacements.
D. Matched Filters
A major part of the surface-wave effort has been directed at
producing matched filters,i.e. filters which optimize theoutput
peak-signal/mean noise (power) ratio. In this respect a signal
s ( t ) will be optimally processed by a filter having an impulse
response which is the time reverse of s ( t ) . The flexibility in
design of SAW transducers permits precise control of their impulseresponse.Theyareconsequentlyconsidered
forboth
signal generation (ofs ( t ) ) and reception.
Matched filters have widespread application in signal processing buttypical surface-waveparametersmake
themreadily
acceptable in radar. Their impact is well illustrated by discussinggenerationandreception
of thefrequency-modulated
waveforms used in pulse compression radar. Linear frequency
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MAINES AND PAIGE: SAW DEVICES FOR SIGNAL
645
PROCESSING
dispersive
tmnsducer
a
j?
output
matching signal
(a)
etched grating
C
(c)
Fig. 9. Threebasic
forms of matchedfdter
for chirpsignals.
(a)A dispersivedelay line withdispersiondesignedintoone
dispersive delay line with dispersion in both transducers. (c) A reflective array compressor (RAC).
modulation is the commonest form of encoding in this application. It is characterized by pulse duration T and bandwidth B .
I ) Atlse Compression Devices: A“conventional”surfacewave structure for linear frequency modulation
is shownin
we shall refer to theseas
Fig. 9. (Bothhereandelsewhere,
dispersive delay lines.) In the receiver, a weighting filter is required in addition to the matched filter to suppress the time
sidelobes adjacent to the correlation peak. In the surface-wave
implementation this is usually built into the dispersive delay
line asadditional weighting.Agreatdeal
of detailedbasic
knowledge of generation, propagation, and receptionof surface
waves is now incorporated into the
design procedure ofdispersivedelaylines-see
forexamplethe
discussionby Paige
[31] and of Bristol [ 321.The success of thetechnique is
is close to
wellestablishedandexperimentalperformance
“ideal.”Maximumsidelobe
levels aslowas
- 40 dBwith
using routine and
respect to the mainlobe are now produced
automatic design procedure.
Theidealbehaviorcan
be achieved for time-bandwidth
m c t s of several hundred,
within
the
constraints
of
B < 100 MHz and T < 60 ps, using the conventional structure. The time-bandwidth product is an important parameter;
it is the pulse compression ratio, or the processing gain against
whitenoise.
The performance of SAW devicesmeets the requirements of the majority of radar systems as presently conceivedand it is worth stressing that the structure is simple,
reproducible, and reliable. We feel these and other attractive
features will ensure that the SAW device remains competitive
for several generations of radar systems.
There
are
some
specialized
radars
which
require
timebandwidth products which exceed afew hundred and in recent
years there has been some emphasis
on reflecting array com-
transducer. (b) A
pressors (RAC). These have been shown to be very effective in
meeting
this
need,
providing
compressors
with
timebandwidth up to 10 000. They are based on reflecting-array
structures introduced in Section I1 and are illustrated in Fig. 9.
Considerable
development
of these
simple
structures
has
occurred and is discussed in a specialist article of this issue [ 81 .
Low time-bandwidthproductcompressorshavepresented
specialproblems.However
the flexibility of design of the
SAW devicehasallowed
thebandpasscharacteristic
to be
shaped so that the effect of large ripples in the spectrum of the
transmitted pulse can be minimized. At IF, compressed pulses
have been produced with sidelobes more than 35
dB down on
the mainpulseand
time-bandwidth product aslow as eight
[331.
Waveforms other than linear FM can be designed with equal
ease. For example, Barker-coded devices are in production for
radarsystems.Particularlysignificantarethenonlinear-FM
waveforms that are being designed to overcome the mismatch
loss
which
occurs
when
linear-FM
filters
are
amplitude
weighted to reducesidelobes.Thespectrum
of thetransmitted waveform is “shaped”bydeviating
the phase of the
signal from the quadratic variation of linear FM. The receiving
transducer can be a matched filter for the modified waveform.
Thespectralshapingcanbeoptimized
to achievelow sidelobesandacceptableDopplersensitivitywithverylow
mi$matchloss, e.g., 0.05 dB [34]. Thistype of device will be
widely accepted in future
pulsecompressionsystems.Itcan
now be designed economically and accurately for the first time.
The performance of phase weighted generator and receiver is
shown in Fig. 10.
It wouldbehighlydesirable
to have transmittersand receivers capable of generating and receiving any waveform, so
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646
PROCEEDINGS OF THE IEEE, MAY 1976
--
-
--loa
- -
I
-2odB
- -
J
15
IO
1
6
5
0
-3OdB
5
10
15ps
Fig. 10. T h e sidelobes achieved using a SAW pulse compressor weighted by using a nonlinear-FM phase profile.
that transmission may be chosen to meet changing operational
conditionsor roles. Suchanadaptivesystem
is along way
from being realized but research into three types of surfacewave devicehasbeenaimed
atintroducingprogrammability
into systems.
2 ) ProgrQmmabIe Correlators: Three types of programmable
correlators will be discussed here. Two methods are based on
the ability of tapped delay lines to sample a waveform at discrete times, and the third uses nonlinear interactions between
surface waves to form the product of the signal and reference
waveform prior to integration.Importantdifferencesinthe
techniques distinguishbetween thetype of signal that each
processes optimally.
The programmable matched filter that has been most extensively investigated for processing biphase coded waveforms is
simply a tappeddelay line with taps that transfer portionsof a
signal t o a summing rail with a phase shift controlled by electronic circuitry. The impulse response of the line is thus set to
The
be thetime reverse of the waveform to bereceived.
ability to build such a device is not in question but geometrical
problems limit the bandwidth to less than 50 MHz. At relativelylow
bandwidthsthisprogrammable
device competes
directlywithconventional
digital processorsand the newer
charge-coupleddevices-botharealsoconvenientprogrammable processors of biphase coded signals. In the long term,
the SAW device is unlikely to be competitive where programmability is involved.
A different type of programmable device that will process
any waveform, within given B and T limits, is the SAW convolver. Nonlinear interaction between oppositely directed surface waves [= and &, produces an output signal which is the
two
signals
convolution of
and Eb. Correlation of the
occurs if [ b is time reversed with respect to
It is correlation which is usuallyrequired in systemsapplications.The
complexity of the time reversal circuitry, the necessity of generating the arbitrary waveform and its storage make thedevice,
as originally conceived, difficult
to sell to systems engineers.
Considerable effortis now being devoted to the memory correlator(Section 111-A-2). Suchadeviceovercomes
boththe
ea.
.-,
I
S
O
U
p
lt
l
.
(b)
Fig. 1 1 . (a)Thecommonformofactivecorrelator.
(b) .A tappfd
delay-line correlator which is less sensitive t o synchronlzatlon o f n g nal and reference.
problem of storage of the reference signal and of time reversal.
A third method, which has not yet received much attention
[35], is a simple extension of the standard method of active
correlation of two signals, i.e. multiplication followed by integration; the scheme is illustrated in Fig. 11. It has the virtue
of not requiringtime reversal and of correlatingverylong
sequences up to the integration time
ri (can be many milliseconds). It has the limitation of being completely synchronous, i.e., signal and reference must occur at the same time to
within one reciprocal bandwidth. The synchronous condition
can be removed by using a tapped delay line (see Fi g . 11) to
provide delayed versions of the reference signal. This tapped
delay-line correlator could have applications to a range of systems where integration times longer than 100 ps are required.
It is being
assessed
using
conventional
components, e.g.
balanced mixers and RC integrators, but should become a very
competitive device if a more compact implementation can be
found. One possible method has been reported [ 361.
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647
MAINES AND PAIGE: SAW DEVICES FOR SIGNAL PROCESSING
surface-wave implementation beingwidely adopted. We shall
return to SAW Fouriertransformers
in the discussion of
spectral analysis (Section IV-B).
IV. APPLICATIONS
TO SIGNAL
PROCESSING
We have indicated in preceding sections that SAW devices are
nowdeveloped
to anadvanced
stage. In thissection,
we
review
the
role
they
play
in
a
wide
variety
of
systems
including
GENERATOR (C1)
radar, communications, fuzing, sonar, and air-traffic control.
Abrief introductionto some of theimportanttechniques
Fig. 1 2 . Surface-waveimplementation of a Fourier transformer using
whichcontributetothe
signalprocessingintheseareas
is
the chirp transform.
presented here. Selected items will also be dealt with in some
depth in specialist papers.
The application of SAW devices to radar systems has been
rapid.
Thissituation
is partlydue tothe factthatretrofit
E. FourierTransformers
applicationshavebeenfoundbutmorebasicallybecause
Fouriertransformation
is animportant
signalprocessing
surface-wave device parameters are admirably suited to radar
use being inspectrum
function,itsmostextensivesystems
IF requirements-particularlypulselengthsandbandwidths.
analysis. Thedevelopment
of thefastFouriertransform
Typical requirementsforcommunicationandsonarsystems
(FFT) algorithm permits digital computers to perform Fourier
demand processing times that are too long and bandwidths too
transforms and this method
of signalanalysis is particularly
of SAW devices.Consequently
small forreadyexploitation
relevant to sonarandDopplerradarsystems.However,the
the trend has been to seek applications in wide-band commurequirement for real-time Fourier transformers has led to an
also insonarsystemsinwhichthe
nicationequipmentand
interest in the alternative methods of transforming offered by
bandwidth requirement of the signal processor is increased by
SAW devices (and CCD's). Because of their compactness, low
timecompression.Howeverin
thelatter,timecompression
cost,andlow-powerconsumption,theynotonlyfulfillthe
usually follows A/D conversion and, since the signal is then in
requirement of afastactingspectrumanalyzerbut
also the
digitalformandcan
be processeddigitally,thispresentsa
Fourier transformer can be considered as a building block in a
to haveslowed down acceptance of
sufficiently
large
barrier
more sophisticated, but still compact, processor. The use of
of sonar.
SAW
techniques
in
the
field
transformers in variable frequency filters [ 22 J (Section 111-B)
Application
to
military
systems
has been successful though,
andcorrelators [ 3 7 ] hasalreadybeenexplored.Closely
reat the present time, the only potentially large-scale application
latedstructures may also beused to expand, compress,and
of SAW devices to be publicised is the TV IF filter. The search
reverse pulses in time [ 3 1 J .
for other large volume SAW products continues to be a major
The Fourier transformation method now
being investigated
factor
channelingresearchintocomponentssuch
as oscilusing SAW techniques is the chirp transformation. As illusto the success of retrofit
trated in Fig. 12, the basic operation consists of multiplying latorsandresonators.Inaddition
of SAW devices is
the input signal s ( t ) by a linearly frequency-modulated wave- SAW deviceapplications,theversatility
now
influencing
systems
design.
We
shall
illustrate
this by disform (chirp C1) followed by convolution of the product with
cussing
particular
signal
processing
functions
such
as
correlaa filter whose impulse response is also a chirp (C?), having a
frequency-timerelation of theoppositeslope to C1. For a tion, spectral analysis, and clutter supression.
completecomplexFouriertransformthefurthermultiplicaA . CorrelationTechniques
tion by a chirp is necessary. This latter process is redundant
when only the power spectrum of the input signal is required.
Thereareincreasingpressures
to design signalprocessing
Theconfigurationshownin
Fig. 12 is easily implemented equipment with improved performance against noise and other
usingsurface-wavedispersivedelay
lines. Thechirp wave- interfering signals. In this respect, correlation techniques pay
forms are generated by impulsing SAW filters. Chirp C1 has an a vital role and here SAW devices are having a majorimpact.
impulse response of duration T, and for a single-shot chirp a
The basic principle of correlation reception is the encoding
continuous Fourier transform is performed over the time T,. of atransmitted signal witha waveformwhichcanbecorNormallythechirpgeneratorsarerepetitivelyimpulsedin
relatedwithareferencestoredinthereceiver.Thecrosswhich case continuous analysis' of the input signals occur and correlationbetweenthereceivedsignalandthereference
the output is the discrete Fourier transform (DFT) performed
providesa
means of recognizing the wantedsignalagainst
over thetimesamples
T,. An alternative SAW methodfor
others. Where transmitters are constrained to using a limited
performing the DFT has been demonstrated. Here, the chirp
peakpower,correlationonreception
will onlysignificantly
deviceswereessentially
tappeddelaylines
whoseimpulse
improve performance ( S / N after reception) if its use permits
response was a sequence of short bursts with phase shifts and anincreaseinenergy
is the
of thetransmittedpulse.This
amplitudes whichcorrespond
tothose of asampledchirp
basicargumentforpulsecompressioninradar,whereaninwaveform [ 381. This method requiresin-phase and quadrature creased pulse length at the same peak power encoded with frechannelprocessing
to give the DFT.It
wasessentiallya
quencymodulationprovidesenhanceddetectionprobability
surface-wavesimulation of themethodto
be used in CCD without loss in resolution.
chirp-transformprocessors.
We donot see this
particular
The impact of surface-wave devices in pulse compression is
considerable as we have indicated in Section
111-D. They are
now used to generate both the coded signal for transmission
'Strictly,forcontinuousanalysis
an identical, parallel chirp transformer is also required.
and to compress the return pulse from the target. The stored
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648
PROCEEDINGS OF THE IEEE, MAY 1916
referencewaveform is the impulseresponse ofthesurfacerelator is asynchronous.
Unfortunately,
the
technique
is
wave structure used in the receiver and the pulse-compressed restricted because of the limited processing time of the SAW
signal (for zero Doppler shift) is the autocorrelation function
device(-100
p s ) . Securecommunicationssystems
usually
of the waveform imposed on the transmitter. The waveforms requirelongcodesequences(severalhours)andhere
digital
usedinsuchsystemscanbeoptimallychosensince,
for the
codegeneratorsandactivecorrelationtechniquesareused
first time in practical systems, the vital signal processing com- withtheattendantcodesynchronizationproblems.Rapid
ponents can be flexiblydesigned,accuratelymade,andecosynchronization is a significant problem here and surface-wave
nomicallyproduced. High andlowtime-bandwidthproduct,
techniques are now being assessed as an aid to synchronization
as well as the nonlinear-FMpulsecompression
components acquisitioninspread-spectrumsystems.
For this application,
are good examples of SAW devices which are now influencing correlation times of several milliseconds are typically required.
systems design.
The limiting processing timesof SAW devices call for ingenious
[40], recirculationintegration
A furtherimportantconsiderationbroughtabout
by the solutionsincludingcascading
availability of these devices relates to the type of transmitter [ 4 1 ] , andtapped delay-line correlation [ 3 5 ] . Thenextyear
will establish whether these solutions are economic.
to be usedin thesystem.Forexample,alowpeak-power
Encoding of digital data followed by transmission and subtransmitter may not beable to meetrangeandresolution
sequent decoding (correlation) is also used for purposes other
specificationsunlessitsmeanpowercanbeincreasedand
thantheextraction
of signals from noise.Channelcapacity
.
correlation techniques used. A suitable solid-state transmitter,
redundancyandhence
such as a transistor oscillator has a longer
life than a tube so canbeincreasedbyreducingsignal
that its use in conjunction with SAW correlators will contrib- bandwidth of theinformationsource.Thedataundergoes
inverse
utedirectly t o increasing themean-timebetweenfailures
of a“transform”attheinformationsourceandthe
receiver.Karhuenen-Loeve,
Fourier,and
equipment.Someimportantapplications
of SAW devices transforminthe
Walsh-Hadamardareexamples
of transforms that havebeen
critically depend on this type of assessment.
process effectsanimprovementinnoise
Forradar,the
waveformsareusuallychosen
to optimize investigated.The
immunityandsusceptability
to interference(includingthat
detectionprobabilityandminimizefalse-alarmrateoverthe
fromother users).Channelcapacity
is thereforeincreased.
specified operationalconditions. Linear FM hastraditionally
been the choice for pulsecompression but withinthe usual This type of data transformation has been extensively studied
time-bandwidth limitations of SAW devices, any waveform is for image transmission using general purpose computers. SAW
the possibility of implementingsimilar
possible-forexample
O/n phase-coding. Theability to en- devicesnowoffer
schemes
operating
in
realtime.
Severalsurface-wave-based
of
codeandrecognizeaparticular
signal in thepresence
schemes have been discussed and a particular transform, chirp
differentsignals is oftenamoreimportantconsiderationin
is the subjectof
an articleinthis
communicationsthaninradar.It
appliesin
transponder/ z transformencoding,
issue [42].
ranging, in secondary surveillance radar, and in other systems
designed to replywhen
interrogated. A problemwiththis
B. Spectral Analysis
type of system is that the remote device may have a limited
Frequency analysis forms an important aspect of present-day
An signalprocessing butequipment is oftenbulkyandexpenlife since it dissipatespowerduring
its“quiet”period.
sive. The general purposelaboratoryspectrumanalyzerand
interestingsolutionforshort
rangeapplicationhasrecently
been reported which employs a passive transponder based on a filter banks as used in pulsed Doppler radar represent two such
examples.Consequently,there
is ageneralneed
for cheap,
surface-wave device [ 3 9 ] . The key element in the transponder
asdiverseassignalsorting
in
is acodedtransducer
whichhasamaximumresponse
to a miniatureunitsforpurposes
andinblood-flowdata
correspondingly coded interrogation signal. It ispassive since electroniccountermeasuresystems
analysis. SAW techniquesoffera richvariety of techniques
itneeds no clock to driveit-unlike othercorrelators.The
correlationpeakcanbeused
toinitiatetransponderaction.
for consideration.
It is certainlypossible to designdeviceswhich
give 20 dB
I ) FilterBanks:
Theability
to designminiature
SAW
wanted/unwanted signal discrimination. Such a device may findfilterswithpreciseandcontrollablephasecharacteristicshas
military application but the originators suggested use
is in the initiatedworkonbanks
of filterscontiguousinfrequency.
potentially largevolumemarket of monitoring traffic move- The
techniques
discussed
in
Section
111-B provide
simulment.
taneous resolution (typically
of afew MHz) with a 100-perA substantial amount of effort has been devoted to the use cent probability of detection. This approach
is not new but
of surface waves forspreadspectrumapplications(where
is nowargued that SAW filters w
li provideaCost-effective
correlation playssignificant
a
role).
In suchsystemsthe
solution for application in set-on jammers.
spreading of signalsacrosswide
bandwidths is usedasa
The filter-bank approach is expensive where high resolution
deliberate technique to provide anti-jamming capability, mulandlarge bandwidth coverage is requiredandtheselectible
tiple access, and selective calling.
filter approach of Hayes et al. [20] offers an alternative. The
In one form of spread spectrum, the data stream is modu- success of themethoddependsupon
reliablyandcheaply
latedbyafastcodewhichsubstantiallyincreasesthebandproducing low-loss filters which can be cascaded.
width of the transmitted signal. On reception the fast code is
2 ) DispersiveDelay-LineDiscriminator:
Dispersivedelayof several hundred MHz
removed provided the signal is demodulated by a similar fast linesoperatingoverbandwidths
discriminate
between
signals
having
differing
frequencies.
codegeneratoroperatinginsynchronismwiththeincoming
code. SAW devices can be used in the transmitter to generate The delay is designed t o be proportional to frequency and an
provides
reference
a
for
a fixed wide-bandwidth code and
in the receiver for correla- electronicchopperattheinput
tiondetection.Theirvirtue
is thatcorrelation is achieved comparison with the timing of the output pulses 1431. Since
the timing of the output signal is proportional to its center
independently of thetime of arrival of the signal-thecor-
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649
MAINES AND PAIGE: SAW DEVICES F O R SIGNAL PROCESSING
c\
t
I
I
‘
\
I
f\
--I----------I
frequency the technique can be used for frequency (spectral)
analysis, Notice,theoutput signal is not theFouriertransform of the input signal.
3 ) TheCompressiveReceiver:
The compressivereceiver is
essentially a fast-acting (few microseconds) spectrum analyzer
with the capability of analyzing single pulseseven in dense
signal environments. As canbeseenfromFig.13,
it is essentially theFouriertransformer
discussed inSection 111-E.
The diagram also shows the mechanism for the identifications
of two CW input signalshaving differentfrequencies. SAWbasedcompressive-receiverscanbedesignedwithin
the range
ofparametersapproximatelyboundedby:maximumbandwidth 500 MHz, resolution 3MHz and at the other extreme
bandwidth 2 MHz, resolution 20 kHz.
All the SAW techniquesforfrequency
analysisdiscussed
abovearelimitedinresolution.Equivalentcomponentsare
beingdesignedusingchargecoupleddevices
andarecapable
of resolving down to a few hertz. On the other hand, they
donot easilyprocess as large a bandwidth. Inthisrespect
CCD and SAW devices complement each other, and the first
combinedsubsystemhasalreadybeenreported
[ 4 4 ] . This
hybridsystem will providesimultaneous rangeandDopper
processingin real timeandcouldhaveamajorinfluence
on
pulse-Doppler radar development since it removes the need for
expensive filter banks on every range cell.
Application ofSAW bandpass filters to radar Doppler processing is noteasy,althoughattemptsare
being made as
reported in this issue. For typical radar systems the Doppler
shifts are too small and are more compatible with
CCD capabilities. However,in optical systemsDopplershiftsare
4-5
orders of magnitudelargerand
SAW devicesareexcellent
candidates for theiranalysis.
is a very active
Spectral analysis using surface-wave devices
area of research. The wide-range of possible applications and
successful miniaturization of some of the SAW devices is likely
to lead to substantial exploitation. This is a very fruitful area
for hybrid processors.
C.ClutterSuppression
In manysystems,extraction
of wanted signals from unwanted similar (and maybe larger) signals is a more important
problem than extracting the signals from noise. Multipath in
communications, reverberation in sonar,
and clutter in radar
areexamples.Inprinciple,therearecommonsolutions,
but
discussion of suppression of these signals in general
terms is
notappropriatehere.Cluttersuppressioninradar
is singled
out for discussion because it illustrates the techniques and, as
mentioned previously, radar parameters are more suitable for
IF processing by SAW devices.
Moving target indication (MTI) is the dominant method of
cluttersuppression sincemost of theunwanted signalsare
from stationary targets. A common requirement is to detect a
smallmovingtargetagainstastronglyreflecting
stationary
background. Cancellation of the background can be achieved
by techniques which display the difference between adjacent
returns. SAW delay lines provide a means of storing the return
signals prior to differencing.
Exploitation in delay-line cancellers is limited because of the
difficulty of achieving long delay, in fact development can be
questionedin view of the applicability of CCD anddigital
processors t o this problem. However, the large dynamic range
of MTI processors (-80 dB) under some circumstances make
these methods too complex.
In this case, it is argued that a
SAW solutionshouldbe
used to reducethedynamic
range
requirement of subsequent processors.
Delay-linecancellersare
in factband-stopfilterswhich
remove signals having zero Doppler shift. An alternative technique is to “time-order”returnsfromaparticularrangeaccording to frequency and then to remove those not required.
For example bydisplaying frequency as a function of time and
using gating techniques or, after A/D conversion, performing a
fast Fourier transform (FFT) and retaining only the nonzero
Doppler components for subsequent processing. The process
must be carried out with
largedynamic-rangesignals and on
each range cell. The digital computer is effective in this
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PROCEEDINGS OF THE IEEE, MAY 1976
650
respectbutformanyapplications,itsspeed,powerconsumption, and size areprohibitive.Combined
SAW CCD
subsystemsofferasolution.
In the schemesuggestedby
Roberts [44], CCD techniques areused tostoreandthen
speed-up the videoradar signal. The increased bandwidth of
theDoppler signals is then analysedusing the SAW Fourier
transformer.The CCD/SAW hybridunitthusdisplays
Dopof time.UnwantedDoppler
plerfrequenciesasafunction
signalsare then removedbytimegating.This
technique is
valid for moving as well as stationary platforms and, since it
processes all ranges in real time, it provides a viable alternative
to the FFT.
Surface-wavedevicesshouldbeconsidered
in several other
areasrelated to cluttersuppression.Theinverse
(or clutter)
filter is designed to give optimum detection of a target in extended clutter where MTI is not valid. Such a filter has been
implemented using surface-wave techniques [ 4 5 ] . In addition,
frequency changing the transmitter frequency is a technique
which helps decorrelate clutter returns. If advances in surfacewave oscillators lead to cheaper, morereliable, miniature oscillators with frequency agility, then they too could contribute
to clutter suppression.
D. Related Activities
There are a number of activities related to signal processing
for which SAW device development is important. Some of the
more important onesare discussed here.
1 ) Electronic Warfare: A variety of surface-wave devices are
beingdeveloped
to improvetheperformance
of systems
against electroniccountermeasures (ECM). Improvementin
performance is achievedby using suchtechniques as pulse
compression,frequencyagility,arraysignalprocessing,and
spreadspectrumtechniques.Surface
wave devices are applicable to each of these areas.
to beparticularlyuseful for
SAW delay-lineshaveproved
storing signals for times up to a few hundred microseconds.
Theyshouldhaveapplicationinlarge-bandwidthconfusion
jammers to generate repetitive wave forms with large energy
components within the bandwidth
of the intercepted pulse.
Againstunsophisticatedradars,gating
of the output can be
used to achieverange deception. However,where
faithful
reproduction of the signal is necessary other techniques are
considered.
The use of complex signal waveforms makes it necessary to
adopt techniques which do not distort the retransmitted
signal.Inthisrespect,
wide-band tappeddelaylinesare
being
assessed for range-gatepulloffapplications.Programmable
devices have been developed in which apparent range is vaned
[431. A more
by connecting each tap in turn to the output
complex programmable system has been reported which uses
both fixed and switchable tapped delays to achieve discretely
variable timing of the output in steps of 40 ns up to 40 p~
[46]. Instantaneousbandwidth
is 200 MHz andswitching
speed 15 ns. Techniques for producing continuously variable
delay using dispersive delay lines (Section 111-A-2) may also be
applied. Combined with switchable fixed delay lines they offer a powerful technique of target simulation. Target simulation can of course be used either for the disruptive measure
discussedabove
or as an aidinsystem
developmentand
automatic testing.
Surface-wavedevicesare
also beingconsideredfor
use in
set on noise jammers. They are unlikely
to beused directly
atradarfrequenciesbutaresponsivejammingtechnique
has been proposed which utilizes a
SAW filter bank. In this,
afixedfrequency
microwaveoscillatordownconvertsthe
incomingsignalwhich
is thendetectedinthefilterbank,
followed by repetitive impulsing
of the individual filter used
to identify the signals [ 471. In an extension of this technique,
the same filter could be converted
into an oscillator usinga
suitable feedback circuit.
ECM developmentsareatanearlystage
but it is already
clear that the cost/size advantages of some of these new signal
processing components will cause some basic rethinking. Considerable payoff could occur in relation
to expandable jammers.
2) Miniature Radars: There are several developments which
aresignificantforminiatureradars.FullycoherentIFpulse
compressionunitscannowbeproducedwithoutexcessive
cost/size power consumption penalties. .They offer new signal
processing opportunitiesforlowandmedium
rangeradars.
For MTI applications, the low-noise characteristics of a compact SAW oscillator/multiplierRFsourceshouldprovide
powerful competition against more conventional approaches.
Spread-spectrum techniques are also a possibility and the use
of tappeddelaylinecorrelators(Section
IV-A) shouldbe
particularly relevant for systems with relatively few rangecells.
Man-borneradarshavealreadybenefited
fromthe use of
SAW delay lines. An example is the improvement that accrues
using a simple surfacein a clutter reference radar system by
wave tapped delay line (481. Further advances in this area are
more likely when the potentialof SAW beamforming networks
is fully realized.
It is interesting to speculatewhat the impact of the new
technologies will be in the next few years; combinations of
SAW/CCD and digital techniques will surely create a wealth of
new miniaturebutsophisticatedsystems.Thesetechniques
should be particularly valid where power dissipation, size, and
cost are particularly important. Miniature radar is an example;
similar arguments should apply to lightweight communication
systems and radar fuzes.
3) Air TrafficControl:
The ATC market is potentially
very great. It is basically concernedwithcommunications,
surveillance,andnavigation.Eachareahasreceived
theattention of SAW engineers, but successful application is slow
due to the principle normally adopted that any change must
beminorandcompatiblewithexistingsystems.
This fact
means looking for retrofit applications such
as filters andoscillators which show economic advantages rather than requiring
major changes in philosophy. The recent acceptanceof biphase
coding for future ATC transponders, though, is almost certain
to require SAW device to demodulate the signal.
V. CONCLUSIONS
Surface-wavedeviceshavesuccessfullymoved
into asmall
though important niche in the
signalprocessing field. They
have become part of the accepted hardware for specialist requirements,particularlywherefixedfrequencyfiltersand
matched filters are needed with
large fractional bandwidths.
As we have stressed, these components are well suited to radar
requirements. In addition, there
is extensive speculative work
aimed at the broad spectrum of applications outlined in Section IV. In some of these applications we maybe confident
that the advantages of the SAW-based subsystems,'particularly
in relation to complex processing in a small volume, will ensure their success. In others, where they fail to
be competitive,
they may well have played an important role in establishing
the signal processing technique at a time when alternative de-
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MAINES AND PAIGE:
65 1
SAW DEVICES FOR SIGNAL PROCESSING
vices, which might ultimately be more suitable, were not sufficiently well developed.
The CCD is alsoananalogprocessorreceiving
muchattention.Atthepresenttime
devicesare not available for
operation abovea10-Mbitclock-rateandare
consequently
complementary to the SAW devices rather than competitive.
Theirextension to higherbitratesdoesnotautomatically
heraldthedemise
of the SAW device.On
thecontrary, regarding the CCD as a time compressor, the CCD opens up new
low bandwidth applications to the SAW device (e.g. Doppler
processing, sonar). We regard initial work in thisdirection
[44] as animportanttrend.Competitionfromdigital
circuitry is weakenedbypowerconsumption,cost,andthe
problems of achievinglarge dynamic range A/Dconverters
operating over
large
bandwidths.
As the
cost
diminishes
and the conversion problem
is overcome, it could be argued
that CCDwill be harderhitbythecompetitionthan
SAW
devices. We expect
these
and
more
searching
arguments
concerning the relativeroles
of
SAW
devices, CCD’s, and
digital devices to be taken up in the forthcoming conference
on “The Impact of New Technologies on SignalProcessing”
which is to be held at Aviemore, Scotland, in 1976.
Among the newer surface-wave devices, resonators form part
of the philosophyinitiatedbymultistripcomponentsand
RAC’s in which the response of the device is not dictated by
thetransducersbutbysurfacestructureswhich
selectively
redirectthe waves. Thisapproach is attractiveinsituations
and
in which the transducer’s usual dualrole-totransduce
determineresponse-leads
to conflictingrequirementsand
consequent loss of performance. In the case of resonators, a
reduction in substrate size is achieved for a given Q. This is
anexample
of an importantaspect
in apotentially large
volume market where size becomes doubly important; besides
increasing the size of theequipmentit raises coststhrough
substrateareasanddifficulty
of packaging.Suchconsiderations may help to rekindle an interest in waveguide structures
with their potentiality to shrink at least one dimension.
Oneareawhich
is receivingincreasing attention is that of
transformers. We have seen that Fourier transformers play an
importantpart
in several of the morerecently
conceived
SAW devices (Section 111-E). This,likethat
of resonator
structures, is one of the “openended” areas of SAW research
whereinterestingfuturedevelopments
canbe
anticipated.
However,interestinthisareadoesincreasethepressure
for
relativelycheaplargetime-bandwidthdispersivedelaylines
to the extent that these stiU remain a center of active research
and development.
The interest in long delay lines has lead
to painstaking research and has produced elegant ideas. The current situation is
intriguingwith the Stanford’s tour de force
[ 141in competitionwith University College’s diskdelay line [ l l ] , both
being developed at a time when the rather different memory
correlator is also attractingattention. All this is proceeding
against a background of growing competition from alternative
solutions to storage of analog signals [ 91 .
Interest in the SAW oscillator is becoming more widespread
and the device more diverse. Though its applications without
improvement of its current ageing performance are numerous,
an order of magnitude improvement in agingwouldbevery
significant, for then a range of large volume communications
applications would be opened up.
Interestingandstimulatingthough
the newercomponents
and devices are, the impact of SAW devices only becomes of
majorsignificancewhentheirexistancebegins
to influence
system design. There are numerous cases where this is already
happening. Forexample, passive generationandnonlinear
chirpinpulsecompression,verycompactcoherentpulse
compression subsystems, clutter reference filters, and sophistication of fuzes. As systemsengineersbecome morefamiliar
with the properties and potentialities of SAW devices, we can
anticipate an extension of this list to include spread-spectrum
techniques
in
radar,
development
of sophisticated ECM
techniques, and transform encoding in wideband communication lines.
ACKNOWLEDGMENT
This paper is published by permission of the Controller of
Her
Majesty’s
Stationery Office
Copyright
0 Controller
HMSO, London 1975.
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Surface=Acoustic=Wave Devicesfor
Communications
RONALD M. HAYS, MEMBER,
IEEE, AND
Abstmet -Surface-wave components have demonstrated performance
capabilities in the V H F / U H F range which are available with no other
filtering technique. The major advantages of the filters are their smln
size, high relinbility, low cost, high Q,good reproduciiility, tempenture stability, wide dynamic range, and linear phase response, as well as
special cluracteristics relevant to spedfk appliations. Surfacercoustio
[ 1 1, matched filterwave (SAW) devices are used for bandpas tilting for rpdv pulsecompression [2], [3) or Sprepd-spectrum signal
pnxesing [4], delay lines [SI, and frequencycontrolelements [3],
[6]. Within the context of communication systems’ applications,this
paper renews thestate of the art for SAW componentsfrom highperformance fred-tuned devices to tunable or programmable filters
Major technological advancesdiscussedincludeunidirectional
SAW
filters [7] which eliminate bidirectionality loss and simultaneously
suppress in-band spurious responses to achieve an insertion loss b e l o w
1 dB with a 0.04dB peak-to-peakripple.A
n o v e l surface-wave subsystem [SI which offers revolutionvy signalprocessingfunctions
through transform processingis also descnlbed.
Manuscript received December 3, 1975;revised January 19, 1976.
The authors are with Texas Instruments,
Inc., Dallas, TX 75222.
CLINTON S . HARTMANN
I. INTRODUCTION
URFACE-ACOUSTIC-WAVE (SAW) devices
have
performed vital functionsinnumerousproductionsystems
since the beginning of the decade [ 9 ] , [ 101. The earliest
large-scale uses of SAW filters were communication and radar
systems. SAW devices in radar systems [ 2 1 I and signal processing [ 111 are the topics of other papers in this issue, while the
components and theiruses incommunication systemsis the subject of this article. Various systems companies have supported
moderateproductionfacilitiesforradar
pulsecompression/
expansionfiiters(see
Fig. 1)andbandpassfiltersformany
years. SAW device fabrication is simpleand compatible in
most respects with highly developed manufacturing techniques
within the semiconductor industry. Fig. 2 shows a patterned
slice of 300-MHzfilters on STquartz for acommunication
receiver front.end,which
was producedbyastandard
IC
production line in 1970. Such production of high-performance
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