ACOUSTIC SURFACE WAVE AND CHARGE TRANSFER DEVICES IN

SOME SIGNAL PROCESSING APPLICATIONS

Jeannine HENAFF *

ABSTRACT

Both acoustic surface wave and charge transfer devicestechnologies are now maturing. The present paper presentsa survey of few analog applications restricted to civil com-munication systems. Actually, the input signal is delayedand weighted in the same way in both devices. This yields twovery important applications, i.e. delay lines and filters.In spite of disjoined frequency operation fields,the designof these functions are very similar in the two technologiesand leads also to similar achievements in terms of BT product,dynamic range, etc... The present applications and the com-plementarity of both techniques are described.

I - INTRODUCTION

In 1965, the acoustic-surface wave (ASW) interdigitaltransducer was described by WHITE and VOLTMER [1i . Fouryears later, SANGSTER and TEER [2] suggested in a quitedifferent technique the first Bucket Brigade device (BBD)followed the next year by BOYLE and SMITH [3] who descri-bed the Charge Coupled device (CCD) concept.

Even, if the technological process seems strongly dif-ferent, both precision requirements in their realizationand actual or potential auplications are closely related. Wewill shortly recall the principles of the two techniques,but the purpose of this paper is mainly to discuss some as-pects of the applications restricted to linear analog signalprocessing in civil communications.

Usually, the devices are separated into memory, signalprocessing and imaging applications. The last applicationis of interest in communication systems e.g. for picturephone.Both Charge transfer devices (CTD) and ASW (via non lineareffects) may be used even if only CTD's exhibit presentlypractical realizations. The memory function is clearly defi-ned for digital systems. In analog devices a circulatingmemory is very similar to a signal processing delay line andwe will not distinguish a special function. At last, somesophisticated signal processing devices are based on chirpedanalysis using dispersive delay lines or non linear effectsthese applications will not be considered in this paper.

II - DESCRIPTION OF THE DEVICES

Acoustic surface waves propagate with a relatively lowvelocity (typically 3 mm/pS) and a very low attenuation

* CNET EST-DEF92131 ISSY-LES-MOULINEAUX - FRANCE

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2

(for example, 10 dB/ wavelength, i.e. 10-2 dB/pS on lithiumniobate at a frequency of 100 MHz). They can be easily gene-rated and detected on piezoelectric substrates by means ofinterdigital transducers (Fig.1 a) at frequencies up to 600 MHzwith conventional photolithographic techniques, but in specialcases, frequency operation up to 2 GHz has been reported. Com-manly such devices can realize tap-weighted delay lines witha few microseconds delay. However, active devices are capableof larger delay, up to 10 mS.

pczoXe/eotc suk/Jrok (b

(a/ b

Fig 1 :a. Interdigital ASW transducer

b. 3 phase CCD structure

Charge transfer devices are basically lines of integra-ted MOS capacitors on silicon substrates. An additional elec-trical charge can be moved from one capacitor to the next bymeans of an external applied voltage (Fig. 1 b). In BBD, thetransfer between charge- storage- capacitors is obtained bycoupled FET'S operating as switches and in CCD, the pulsedcontrol voltages are applied directly to the electrodes of thecapacitors. In both cases, the signal velocity depends onlyupon the clock rate and is typically of lmn/s1 but large varia-tions are possible according to the clock frequency. For simi-lar geometrical sizes, it is alwready clear that CTD will ope-rate at frequencies of roughly two orders of magnitude lessthan ASWD.III- DELAY-LINES AND RELATED DEVICES

In analog delay lines, three non independent parametersare of interest :the delay T, the bandwidth B and the time-bandwidth product BT of the device. If the BT product charac-terizes the complexity of the device, in pratical applicationsthe delay T is an essential fdature. With respect to this BTproduct, ASWD and CTD are quite comparable, but CTD admit lar-ger delays than ASWD.

218

3

III-1 ASW DELAY LINES

Up to BT products of a few hundreds the ASW delay linesare very easily designed and realized with a very good yield.In this range, many applications can be found : for example,Fig.2 shows a multidifferential demodulator used in phase-shift-keyed (PSK) transmission. The dynamic range is 60-80 dB.

in~ur

Fig 2 Multidifferential P.S.K. demodulator ASW delay-lines providing the 8 required delays

The delay available in such passive delay lines is limi-ted to 10-20 pS. However with more sophisticated structures,delays up to 100 pS can be obtained. For exemple, a foldedpath can be realized using reflectors [4] as in Fig.3The dynamic range of such a structure is 40 - 60 dB and thebandwidth is usually about 50-100 MHz. However, the yield israther poor.

~~~r reflectors 1CjfJ~~~~~~~~~~~~~~~~dic- ln

Fig 3 : Planar ASW delay-line Fig 4 : Helical ASW delay-line

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-The attempt to reach the millisecond delay range requi-res acoustoelectric amplification. Such a structure is repre-sented on Fig.4, using an helical path, amplification andlens guidance of the beam. However, in spite of some success-ful experiments [51 , the yield of the implementation israther negligible. It is theoretically possible to reach 10 mSdelay with 50 MHz-bandwidth and a dynamic range of roughly40 dB.

In addition, the stability of the delay with respect totemperature has to be considered ; this stability depends uponthe material. Presently three materials are extensively used :lithium niobate with a poor behaviour (91 ppm/°K), quartzwhich admits a fair orientation the ST cut [6] and morerecently, lithium tantalate which can be stabilized by deposi-ting a Si 02 layer [7] . On the other hand, the maximumrelative bandwidth available on quartz, LiTaO3 and LiNbO3 arerespectively 5%, 10% and 25%.

At last, such delay lines can be used to realize ASWoscillators [ 8,9 1 . This is a particular application of ASWwhich cannot be extended to CTD, since in that case, an inter-nal time standard no more exists. Such ASW oscillators can bebuilt with a good yield in the frequency range 100 MHz up to1 GHz, showing a short term stability of 10-9, i-10, a fairbehaviour with respect to temperature and above all an excel-lent ability for an electrical control of the frequency. Anexample is given on Fig.5

Central frequency 282 MHzVoltage control range = 1 MHz ie3,5 10<-342 voltsOutput pcwer = - 10 dBmn

Fig 5 Voltage-controlled ASWoscillator

111-2 ANALOG CTD DELAY-LINES

The CTD delay line behave like an analog shift register.For a N-stage device operating at a clock frequency Fc thedelay is N/F and the Nyquist bandwidth is Fc/2. Thecmaxi-mum BT produSt is of course N/2.

The limitation in terms of BT product is due to the trans-fer noise. One aspect of this noise is the so-called ineffi-ciency rate e , which means than only a fractional part (1-C)of the stored electrical charge is transmitted from one elec-trode to the following oni. Generally the magnitude of thisinefficiency is io0-3 10 . This effect leads to both amplitudeand phase dispersion. It has been shown [ 10] that an accep-table n'umber of transfers is roughly equal to 0.2/ s.

220

This gives a BT product of a few hundreds for a simple delayline at a moderate clock frequency. However, the BT productof the device can be substantifically increased by use ofmore sophisticated structures.mostly for digital memories_using regeneration of the information in serpentine or looporgani zations [11,121 . For analog delay lines Fig.6 showsa serial-parallel-serial organization of n1rows and n2

/n1out ob--

02 9q Wq~~~~~ ~~~~~~Ti3 Ct 10 /

I I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fig 6: Serial-parallel-serial organization of a CT memory

columns . The 'information is put 'into the memory ( and readout) by means of aserial register at clock rate f co but eachline of n2 cells is transferred at the same time 'in parallelthrough the matrix at a slower rate f /n x so with a smallercontrol power. The storage capacity ii then equal to n 1 n2while the nurnber of transfexs is only p (n1+n2) where p isthe number of phases.

In terms of time del1ay ,the main li'mitation of CTD isdue to the dark current. According to the technoloyical pro-cess the magnitude of this current is 10-100 nA/cm .Whateverthe geometrical size of the electrodes, this yields a 10 mSmaximum delay for a signal to noise ratio of 4 0 dB.

In terms of fre que ncy, the trans fer inefficiency increaseswith the clock rate. The above values are gilven for clock fre-quencies of a few MHz. It has been reported clock rate ashilgh as 100 MHz ; however the actual devilces seldom exceed10 MHz presently. At the highest frequencies, the dark currentis generally negligible : 'it is then interesting to use bulk-CCD, in which the channel is buried by ion implantation andwhich has a better transfer efficiency and so a lower trans-fer noise but a higher dark current.

III-3 COMPARISON CTD vs ASW DELAY LINES

~ ~ ~ ~ --A _

The simplest devices, easily built with a good yield havevery similar features in both technologies BT product of afew hundreds, dynamic range of 60 dB. The ASW delay lines aremostl passivel bandiass devices andoterate at a few hundredMHz of central frequency. The CTD are active, baseband devicesoperatingfron- DC to a few hundred kHz or a few MHz. The compa-

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rison is more difficult for the most sophisticated devices.However the chart of Fig.7 tr.y to give a synthetic view of thepresent state of the art.

Aondwcdth B

1611* [:j:~~~~~~:.:.:.:.. l4,noe cr0-

16Hz COSSi/cC ASW

InNZOcc/Ael AS vW

10111k-

ips5 Oy )Oc5 u 1116lOm n loOms Is lOs

Fig 7 Comparison between CTD and ASW delay lines

An other point of view is the behaviour with respect totemperature : the CTD, using external time reference have alow sensibility ; the ASW devices on the contrary have to becarefully designed in order to meet such requirements. On theother hand, their internal time standard allows the realiza-tion of very fair oscillators.IV- TRANSVERSAL FILTERS

In civil communications, filters are of primary interestfrom the economical point of view : about one half of thetransmission end equipments are consisting of filters. Anyimprovement using new technologies may have large economicalconsequences. Both ASW and CTD filters are generally designedas transversal non recursive filters. In such devices, thefrequency response is obtained as the Fourier transform of aset of samples A defined at sampled times Ike In terms ofmathematical synthesis, one have to find the set {Tk, A }of finite impulse response T. The inverse 1/T of the totalduration determines roughly the steepness of the frequencyresponse. However, the synthesis is usually achieved by use ofsophisticated CAD'S procedures.

IV-1 ASW FILTERS

The mathematical sy nthesis is directly suitable for thedesign of ASW filters if the specifications allow relativelylarge insertion loss (= 20 dB). The filter is then formed oftwo interdigital transducers (IDT); one of the ITD's is

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regular and the other apodized i.e., the lengbhof the k-elec-trode overlap is proportional to the relative weight Ak

Fig 8: ASW filter with an apodized central transducerand two unidirectional side transducers

(cf. center transducer on Fig.8). In that case, the groupdelay or rather, the phase linearity is generally good enough.On the other hand, for more stringent insertion loss specifi-cations (< 10 dB) the synthesis procedure has to account forelectrical reflections [13] including triple transit echoesand for a matching network. For example, amplitude responses of the above bandpass filter];_

(Fig.8) is given on Fig.9 taking\ into consideration these second

order effects. More sophisticatedIDT's, multiple combs, couplers

a \ and reflectors may be used. Forinstance, Fig.10 describes a chan-nel bank filter using unidirec-tional transducers and a multis-trip reflective array [141

o ~~~~~~~Inaddition, if the center fre-quency can be adjust in the range20-800 MHz, the relative bandwidthwith moderate insertion loss islimited by the electromechanical

i79-.0 6"40.00 3 000070P.0jl 7140.00 7210.0o coupling coefficient up to roughlyFREQUENCE 10 4% for quartz, 10% for lithium

tantalate and 25% for lithium nio-ofg 9 Frequency response bate.Of the filter of Fig.8. At last, the temperature coeffi-

cient of the structure is of maininterest for small bandwidth:the ST5quartz is broadly used inthose cases and leads less than 10 variation in the range0-50°C. Owing to unwanted modes, the dynamic range is general-ly limited to 60-70 dB.

IV-2 CTD FILTERS

The design of CT recursive filters is theoretically possible and has been experimented, by many authors. However, sucha synthesis requires a very precise amplitude stability of thetransfers and most of the practical filters are implementedas non-recursive transversal filters. The weighting functionscan be obtained by external resistors, but an improved techni-que uses split electrodes (cf. Fig. 11) . The electrodes.

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I

F MHz

track A~dA -10

T 7

trackB 1 -20..I -30 I9\A\Il'A0

-50 '-

Fig 10 ASW filter bank a : device schematicb: frequency response of 3 ports

connected to one phase are separated into two sets 4 andwith relative areas 1+Ak/2 and 1-Ak/2 (cf. Fig.129. The

differential current between + and f- gives the actual weightAk. [15] . The number N of cells is limited by the transfernoise to a few hundreds [16] . If the clock rate is fc

phase #,AYPL17T

S/14NAL

Fig 11 Split-electrode transversal CTD filter

the steepness of the frequency response is roughly fc/N. Inaddition, the dark current corresponds to the integration timeT = N/f : this yields limitations in the steepness describedby Fig.S3. For example, with T = 100 mS, one can obtain atransition width of 10 Hz if the required dynamic range isequal to 60 dB.

The amplitude response of the baseband telephonic filtercorresponding to the 64 weightsof Fi12 is given on Fig.14

AkA dS DynoniA* ronye

0,2 1 1 a_____

10~ ~ ~ ~~~~~64A7 0't-I S4 k~ ~ ~~~~~~04 X

I.21 Fig 13 - Maximum dynamic rangevs the steepness of the filter

Fig 12 : Tap-weights of a 64- (taking into account the darkcell baseband filter current only)

224

in the bandwidth (Fig 14 a) and out of band (Fig 14 b). Thedesign of the deviae has to account for quantification noiseand imprecision in the realization of the weighting functions[17] . By using large enough electrodes, the quantification

noise may be negligible (and in addition the transfer noisedecreases). However imprecision in the photolithographic pro-cess remains an ultima noise which limits the rejection ofthe filter (cf. Fig 15 a) and spoils the ripples in the band-TAwidth (Fig 15 b). This effect can be decreased by use of

A BSI A dB2 / ChesysAev sSo

2.~~~.... Au/forwort2h&Mr w,--1 ) U2

,2 =~ sAu/rDrA

Fig 14 Frequency response a:in bard 3 4 5b:out of bandExaxrple of 64 stages CCD filter. The synthesis starts from a Chebyshev(1) or Bulterworth (2) nolinal frequency response both convoluted withClph-C_eby~-no,

special synEtesis _procedures.At last, input - output circuits and clock switch leakage

induce additional noise and limit the dynamic range. Futureimprovements in both the circuit design and the synthesis

Refl relidon SdB, £i.p/es (s /

20-

4o -~~~~~~~~~~~~~

so * 2

0.' 70 60 70 6701 NC(a) (A

FiJ 15 : Monte-Carlo simulation of the irprecision of the tap weights. Thedrawn by lot values are assurred to be uniformly spread on an interval +e/2of the maxinnm weight. a) rejection of the filter (1) of Fig.14b) ripples in the band idth of the sam filter.proceaure should be expected in terms of this dynamic range.

225

Basically, the (CiL) are baseband filters [181 , the bandwidth being limited by the Nyquist frequency fc/2 and by thetransfer inefficiency s. The clock frequency is generally inthe range 10 kHz-10 MHz, but clock frequency as high as100 MHz has been reported. The sources of noise and correla-tive limitations of CTD filters are summarized on Table I.

sources of noise limitations

transfer noise nurber of cells ( < 300)dark current steepness 1/T or dynamic range (Fig.13)quantification negligible if correctly designedimprecision in process rejection of the filter (Fig.15 a)input-output circuits dynamic rangeclock switc:hes dynamic range

__-.gr,-

Table I: Limitations of CT fiitersIV-3 COMPARISON BETWEEN CTD AND ASW FILTERS

If the design and the performances of both filters arevery similar, it is clear that their frequency ranges arequite different: CTD filters are used in base band or lowfrequency operation and ASW filters are used for high fre-quency bandpass filters.

!Cm IASWcentral frequency Fo 0 - 10 mHz 20 - 800 MHzbandwidth < f /2 < 30% F8steepness 1 c 10 Hz 6F/F0< 18-dynamic range 60 dB 60 dBdissipation 10 - 100 pw/cell 0L- I . . .

I

Table II - Comparison between CIL and ASW filters

In civil communications and in terms of economical inte-rest, the CTD filters should have -a larger market as channelfilters (0-4 kHz) ; however ASW have typical applications inIF filters and in radio link multiplexers, pilot filters andgenerators. The temperature behaviour of both devices shouldbe good enough in CTD, one has to use an external stableclock ; in ASW, the ST-quartz and more recently the SiO2 sta-bilized - lithium tantalate should be satisfactory in mostapplications.V- CONCLUSION

The present paper has been limited to linear analog signalprocessing applications. The new technologies have to competewith more conventional techniques. It seems already clear thatspecial applications can be found in voice channel filteringin transmission and commutation for CTD and in channel radio-link multiplexing for ASW. In addition, VCX-ASW oscillators ha-ve also found applications in digital radio - link systems.The two technologies are complementary in terms of frequencyoperation. However, the realization of similar functions bydifferent ways gives rise to competition and allows stimula-ting comparison. At last, hybrid devices using both technolo-gies may be considered.

226

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