GB2030411A - Improvements in or relating to surface acoustic wave filters - Google Patents

Abstract

In an In-Line Reflective Array Compression (ILRAC) filter having input and output acoustic surface wave transducers 1, 5 situated opposite respective dispersive reflective gratings 3, 4 with a multistrip coupler 2 between the transducers and gratings, amplitude weighting is achieved by shifting the reflective edges of one grating in relation to the reflective edges of the other. The coupler 2 allows the input wave (full lines in Fig. 1) to reach both the gratings and both reflected waves (dotted lines) to reach the transducer 5 where they are effectively summed. Fig. 3 shows finger groups 30, 31 of the respective gratings staggered by ???/4 for full cancellation of the output transducer 5. Groups 32, 33 are unstaggered for zero cancellation, and groups 34, 35 provide a desired characteristic by reduction of the lengths of successive reflecting fingers. <IMAGE>

Description

SPECIFICATION Improvements in or relating to surface acoustic wave filters The present invention relates to surface acoustic wave filters and more particularly to reflective array pulse compression surface acoustic wave filters.

A particular form of such filters are known as In Line Reflective Array Compressor (ILRAC) filters and comprise two surface acoustic wave filters constructed in a parallel arrangement on a single piece of piezoelectric material.

Known ILRAC filters have an amplitude weighting characteristic to achieve a desired amplitude response. Presently known methods of providing this characteristic are expensive and it is an object of the present invention to provide a design of ILRAC filter which is easier to manufacture.

The present invention provides an ILRAC filter comprising an input surface acoustic wave transducer situated opposite a first dispersive reflective grating, an output surface acoustic wave transducer situated opposite a second dispersive reflective grating, a 3dB multi-strip coupler interposed between said transducers and said reflective gratings, in which in one of said reflective gratings the reflective edges are shifted in relation to the reflective edges of the other grating.

In a preferred embodiment further fingers are interposed between the normal fingers and the lengths of the fingers are chosen to produce an amplitude and phase cancellation characteristic for each reflection grating.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a known in line RAC filter Figure 2 shows the provision of phase controlled amplitude weighting in a surface acoustic wave filter according to the present invention, Figure 3 shows diagrammatically a section of an ILRAC filter illustrating a first embodiment of combined phase and finger length weighting, Figure 4 shows in a second embodiment, combined phase and finger length weighting and Figure 5 shows an example of distribution weighting according to the present invention.

Referring now to Figure 1, in a known filter, a surface wave of a given frequency is launched on a piezoelectric substrate by an interdigital transducer ( 1 ) and passes through a 3dB multistrip coupler (2) where the acoustic beam is divided into two separate beams of equal amplitude. One beam continues straight through and continues along a dispersive reflective grating (3) which can have any dispersive function designed in, for example, by depth variation of the groove in the filter until the beam reaches a zone where the grating periodicity matches half the wavelength of the incident acoustic wave. Reflections from the several groove (finger) edges then add in phase and a strong reflection through 1 800 takes place.

The incident wave is shown as a solid line and the reflected wave as a dotted line.

Simultaneously the wave launched from transducer (1) is processed by the 3dB multi-strip coupler, the second acoustic beam being directed along the reflecting grating (4). This wave is also.

reflected through 1800 at the resonant zone of the grating which has the same dispersive characteristics as grating (3).

The reflected wave from grating (3) is track changed by the multi-strip coupler and is detected by surface acoustic wave transducer (5).

The second reflected wave from grating (4) passes through the multi-strip coupler and is also detected by transducer (5) where the two inputs are summed to provide the filter output.

When the two dispersive gratings are identical the two reflected surface acoustic waves have the same phase and will reinforce each other, see figure 2a. It has been realised in the present invention that should the positions of the groove (finger) edges in grating (4) be moved by 1/4(A= wavelength of the surface acoustic wave at any instantaneous frequency in the spectrum of the dispersive grating) relative to the groove edges in grating (3) then the two reflected signals will be 1800 out of phase and cancellation will take place and the resultant signal will have zero amplitude for equal strength reflections. Intermediate amounts of groove displacement give intermediate amplitudes.This cancellation technique can be used for amplitude weighting the device by varying the groove positions in a section of one grating while keeping the groove positions in the other grating constant. This is the basic invention and is augmented as shown in Figures 3, 4 and 5 by several other methods of restoring the phase error response to zero. This pulse cancellation is iliustrated in Figure 2b which shows phase cancellation of 100% on one track (4) only.

A first device according to the invention is illustrated in Figure 2, in which anti-symmetric phase weighting for IRLAC and other filters is shown.

The displacement of the reflecting element edges to achieve phase cancellation which results in a change in the amplitude response of the inline reflective array compressor is divided equally and anti-symmetrically in the two gratings (3) and (4) about their true positions. The resultant summed signal then has an amplitude change but no error is introduced into the phase response of the device.

If a groove in grating (3)Figure 2c-is displaced a distance Ax towards the multi-strip coupler and the corresponding groove in grating (4) is displaced Ax away from the multi-strip coupler then it may be shown that the amplitude of the vector sum of the two reflected waves from these grooves is given by 4nsx A= AoCos where A is the wavelength and Ao is the amplitude when Ax = n. Thus by varying dx between zero (no weighting) and lig (complete cancellation) any desired amplitude weighting can be realised.

Further embodiments of the invention are shown in Figures 3 and 4 in which a further method of amplitude weighting is employed.

In this method only 1800 phase cancellation is considered and the maskmaking resolution problems are reduced (the two gratings may be drawn as a single unit thus ensuring perfect phase matching between the corresponding groove edges).

When the grooves are full-length either zero amplitude or maximum amplitude can be obtained, the intermediate amplitude weights being achieved by adjusting the ratio of the lengths of adjacent (in phase and 1 800 out-of-phase) grooves. This may be done in two ways. Figure 3 shows the length weighting inserted symmetrically in both reflectors and Figure 4 shows the length weighting inserted in one reflector only, the second reflector being completely unweighted.

Referring now to Figuro 3 in which all reflecting elements, shown shaded, are anti-symmetrically displaced byA/4) at the left hand end 100% cancellation is present in finger groups 30 and 31, in the centre there is zero cancellation in finger groups 32 and 33. At the right hand end the reflecting elements 34, 35 are reduced in length and further reflecting elements are interposed in the intermediate positions as indicated by the cross hatching.

Referring to Figure4 in which again all reflector elements are anti-symmetrically displaced by /4, elements 40, 41 exhibit zero cancellation, elements 42 to 46 partial cancellation and element 47 100% cancellation.

The advantage of the arrangement of Figures 3 and 4 over the arrangement of Figure 2c is that in Figure 2c the displacement of the reflective edges must be controlled in micrometers and therefore, requires extremely accurate masking techniques whereas the arrangement of Figures 3 and 4 requires variations in the lengths of the fingers which may be controlled in millimetres rather than micrometres. Thus the masks are not required to be so accurate.

Figure 5 shows a single dispersive reflective grating 3 which is distribution weighted. In this form of weighting the reflective discontinuities are all either in phase or 1800 out of phase. The amplitude weighting is then determined by groups of fingers-the reflections from which either sum constructively or destructively. The fingers may be arranged in any sequence to give intermediate amplitude weights.

The weighting can be applied to one or both of the reflectors equally well. The discontinuities in the time domain response due to the non- uniform distribution of reflecting edges will be averaged out in the frequency domain.

Thus in Figure 5 finger distribution 51 gives zero cancellation, distribution 52 gives 1 800 out of phase with respect to the other track 4 (not shown) and 100% cancellation. Distribution 53 is in phase and distribution 54 shows reflectors alternately in and out of phase with respect to track (4) which will give over a number of reflectors a 50% cancellation.