THE REFLECTIVE ASW MULTISTRIP ARRAY

A FREQUENCY SENSITIVE DEVICE FOR FILTER AND CHANNEL BANK

Michel FELDMANN and Jeannine HENAFF

ABSTRACT

A frequency sensitive ASW structure is presented,, operatingby reflection of acoustic surface waves. Such a structure is of interestfor frequency filters and filter banks. The response can be computedby scattering matrices leading to a good agreement with experimentalresults. Different configurations like fan shape electrodes can be used.In this paper, a 5-channels filter bank with a relatively goodfrequency separation between adjacent channels is presented. The relevantparameters like insertion loss, rejection, number of channels willbe discussed and the other possible applications are described. Thisdevice meets the requirements of a frequency division multiplex.

I - INTRODUCTION

The acoustic-surface-wave (a.s.w. ) multistrip coupler has beensuggested by Marshall and Paige ( 1) using the interaction between twoconductive arrays electrically connected. When the period of both arraysis the same, the device operates as broadband track changer bytransmission.

A new type of grating filter can be realized using a multistripcoupler with different spacings of the two tracks (cf. fig. 1). In fact,if these spacings are respectively equal to dAand dBthe reflected beamswill add if

d +d XA B

dA

4- S12

Fig.1 : Basic MRA structure

X Centre National d'Etudes des TeleDepartement EST/DEF92131 ISSY LES MOULINEAUX, France

where X is the acoustic surfacewavelength.

This principle can be used todesign a filter with roughlya (sin x/x) response, andrelative bandwidth of 1/N, Nbeing the number of strips ofthe coupler. Such a structure isvery similar to the gratingfilters, but, in the presentdevice, all the beams arecollinear. A thery has beendevelopped to compute the scatte-ring matrix of this kind ofmultistrip coupler. The theory,valid for arbitrary spacings ofthe strips, gives the insertion

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loss of the reflected beam. It is shown, that for regular spacing, thescattering parameter at the central frequency is roughly equal to

1s2 12 2

1 +y

where y = 0.3 Nk, N is the number of strips and k the electromechanicalcoupling coefficient. For Y-cut, Z-propagation lithium niobate, theconvenient number of strips is about N = 100. In a previous communica-tion (2), the basic theory has been presented together with the firstmeaningful experimental results. A second paper (3) described a fanshape configuration (fig. 2) wich allows a continuous matching of a setof wavelengths. This can be used to separate different frequencies. Thepresent paper describes an improvement of this basic device in whichthe frequency separation between adjacent channels is increased.Alternatively, the number of separated channels, or the shape factor ofeach channel can be improved.

II - FILTER BANK

Similarly t ref. 2,the actual device has beendesigned to meet a filter-bankspecification. The fan-shapedelectrodes on one track arecontinuously matched to a setof wavelengths running from

dA +B 1

Each frequencyreflected by athe array.

to dA + %n = X .

is selectivelylimited section of

Any section of trackB (fig. 2) is matched to afrequency

vF. =1 A .

1

track A | dA TI To

track B

dBn T

Fig.2 : Fan. shape MRAschematic

V

dA+dBwhere v is the a.s.w. velocity ; i.e. the matched frequencies run from

F = v/(d + dB,) to F1A %n)to v/(dA+%dB)

Let N be the number of electrodes on each track.

In terms of selectivity such a N-strips device is capable ofseparating elementary channels of bandwidth F /N (if F is the centralfrequency). On the other hand, the available input banawidth isA F = F - F1 X difference between the extreme matched frequencies ofthe fan track. The number of elementary channels is then

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N AF

F0

In the actual device, we have

AF = 5 MHz ; F = 150 MHz ; N = 450 and n = 15

These n elementary channels are spread over the entireaperture WB of track B, i.e. 8 mm.

On the other hand, each output transducer aperture is W.= OJ8mm,i.e. 1.5 elementary channel and the dummy distance between adjacentoutput transducers is 1 mm, i.e. 1.9 elementary channel.

The expected bandwidth of each output is

A.F = AF x = 0.5 MHz1 WB

The shape factor, corresponds to the summation of 1.5elementary channel and remains rather poor in the actual device.

An improvement of this shape factor is possible by allowinga larger number of elementary channels to each output transducer. Theinsertion loss of the MRA has t be computed for the elementary channelusing the parameter y :

2 wA w2 -2y = 0.3k N A'w+) b 1s2= 2OIS(WA+W) x by |l|2

where w is the elementary aperture W /n and where W is the A-track -B A.width ( 1 mm). We obtain y = 6,43. The expected insertion loss is thenless than 1 dB (fig. 3).

241

aodB

Fig.3 : Insertionloss of one MRAvs y = 0.3 N k2

5

10

15-

01 05 1 2 5 10

The actual insertion loss (10 dB) corresponds t 2 dB-lossesfor the MRA. This is attributed to,bulk wave conversion. TRe outputsignals of the 5 ports are plotted against frequencyan fig. 4.

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/12 /44 /46 /48 /So /52 FtIMZ

Fig.4 : Frequency response of the five ports of a5 - channel 450 strips MRA.

III - CONCLUSION

We have described a filter bank using a fan-shaped-MRA. Thisdevice is capable of selecting a large number of elementary channels.According to the size and position of the output transducers, thechannels can be arranged to obtaain a wide frequency separation betweenadjacent filters, or a large number of filters, or on the contrary a fewnumber of filters with a good shape factor. The in:sertion loss due tothe device itself is relatively low (I or 2 dB) and the 2-track-structure is favourable to eliminate the unwanted bulk responses.Similar devices are projected, in order to meet the requirements of afrequency division multiplex for a five channel 4 PSK - 2 M4B/s signal.

REE REN CES

1 - MARSCHALL F.G. and PAIGE E.G.S. "Novel acoustic-surface-wavedirectiona-l coupler with diverse applications", Electron. Lett., 1971,7, pp. 460 - 462.

2 - FELDMANN M., and HENAFF J. "A new multistrip acoustic surfacewave filter". Proceedings of the IEEE ultrasonics symposium. Milwau-keel, Wis , USA, 1974 , pp. 157y1-60 (catalog number 74 CHO 896-1 SU).

3 - FELDMANN M., HENAFF J., and CAREL M.: "Asw filter bank using amultistrip reflective array", Electron. Lett., 1976, vol. 12 no 5pp. 1 18- 119 E

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