Title of Invention	MINIMIZATION OF EFFECT OF DIFFERACTION IN SAW FILTERS
Abstract	Disclosed herein are surface acoustic wave (SAW) filters, more specifically to diffraction in SAW filters.

Full Text

The present invention relates to surface acoustic wave (SAW) filters, more specifically to diffraction in SAW filters. As known in the art, a surface acoustic wave (SAW) filter consists of two interdigitai transducers, (IDTs) formed by finger like metal patterns on a piezo electric substrate. One of the transducers converts electrical signal into acoustic waves which travel along the surface of the substrate and reach the other transducer which converts back the acoustic waves to electrical signals. The finger patterns of the two transducers will determine the net frequency response of the filter.The individual frequency response of each transducer is determined by the finger overlap pattern in the transducer, which is a spatial representation of the desired impulse response from each transducer. The desired impulse response is the Fourier inverse transform of the desired frequency response from the individual of the SAW filter is the multiplication of the two transducer transfer functions. As known in the art, the transducer with constant overlap is called unifom IDT and the one in which overlap varies is called apodized IDT. The uniform IDT generates "sine" response with a 4 dB bandwidth which is equal to the inverse of the impulse duration of the transducer. Amplitude weighting (or the apodization) of the IDT results in diffraction variations which destroy the Fourier transform relation between the envelope of the finger overlap function and the device frequency response. Consequently the stopbandrejection of the filter will not be as high as predicted. Diffraction of surface waves is a physical consequence of their propagation and can vary greatly depending upon which anisotropic substrate is chosen for a given application. Regions where the finger overlap is small yield significantly more diffraction loss than those where there is longer overlap. This results in distortion of time domain wave amplitude and phase at the receiving transducer. Consequently the actual frequency response of the SAW filter will have stopband rejection which is poorer compared to the theoretical frequency response where difraction effect is not considered. Thus in order to obtain the required stopband rejection it is necessary to consider the effect of diffraction at the design stage and devise a method to compensate this effect. As known in the art, the diffraction compensation involves modification of each finger tap in an interdigital transducer containing hundreds of finger taps. This techniques requires a vast amount of computer resources in terms of time and space (memory), resulting in expensive and long design cycles. As known in the art, diffraction in the isotropic materials, causes the acoustic beam profile to quickly lose its uniform amplitude and phase as it travels away from the input transducer to undergo significant fluctuations in shape and to eventually broaden to an infinite width. Very close to the source, small ripples will appear in the beam profile, then one or more pronounced cleavages appear as the beam progresses. Afterwards a peak will emerge and then a far field pattern is formed. A distinction between a near field or Fresnel region and the far field or Fraunhofer region can be made. A convenient figure for approximately defining the demarcation between these two regions is the far-field length (Dj:), given by D£ = A* T~ where A is the aperture of the transducer and A© IS the SAW wavelength. Actually the transition from near field to far field is not distinct and the transition region extends from D /4 to F 3D . F As known in the art, the isotropic diffraction theory cannot be applied in total for anisotropic substrates as the anisotropic surface wave velocity is a function of propagation direction (e). For small angles, the expression for anisotropic velocity can be given as 2 V (9) - Vo Cl+(T/2)(8-8) 1 where T is an anisotropy parameter, V is pure mode velocity and 0 is the angular orientation of the pure mode axis. Furthermore for these conditions the acoustic powerflow vector deviates from pure mode axis by the angle 0 = T (0-0 ) (3) with these changes for the anisotropic media, the diffraction integral reduces to Frensnal integral with the following scaling in the distance factor D" = D (1+1) which means that diffraction can be retarded or accelerated depending on the value and sign of T. Accordingly the far field length for anisotropic media is changed to \ot\u-rf) (5) As known in the art, design curves are generated in which diffraction correction for the finger tap is plotted against the scaled distance factor. When the finger tap is equal to the aperture of the transducer no correction is required and when the top is much smaller than the aperture the correction required is substantial in the region which is neither Fresnei region nor Fraunhofer region. Note that no correction is required whenever either near field or far field conditions prevail. However it is generally assumed that the SAW filters designed with far field conditions are not viable in practice due to long substrate required to satisfy far field conditions. Hence diffraction compension in SAW filters is either done by correcting individual taps which requires vast amount of computer resources or by near field design. However for narrow band SAW filters which have long impulse duration, which use quartz as the substrate with condition. For high frequency devices this results in fabrication of very thin and long electrodes which poses fabrications difficulties with low yields. These limitations in the prior-art, are overcome by an optimized far-field design approach in which substrate is not prohibitively long as well as the aperture width becomes small, thereby facilitating easier fabrication of high frequency narrow band SAW filters with good yield. In the present art described here the distance and the aperture of the transducers are related by the inequality expression given by : 2 The region satisfying above inequality will be in far field region requiring no diffraction correction, thus stopband rejection is not deteriated by diffraction. At the same time the substrate length will not be too long to become impracticable. The aperture is much smaller than the near filed design approach thereby facilitating short electrodes enabling high yield and ease of fabrication. There is no need for correction of individual finger taps, there by no need for vast computer resources in terms of computer time and memory. We Claim: 1. Surface acoustic wave interdigital transducers, formed by finger tabs on a piezo electric substrate, characterised in that (i) the effect of diffraction on the stopband rejection is minimized by optimal placement of the transducers in the far-field region to obtain the desired stopband rejection ; and (ii) optimal choice of the aperture which facilitates easier fabrication of high frequency narrow band SAW filter due to shorter electrodes to enable higher yield. 2. Surface acoustic wave (SAW) filter as claimed in Claim 1, wherein the individual finger taps are not corrected for diffraction. 3. Surface acoustic wave (SAW) filter as claimed in Claim 1 or 2, wherein the optimal placement of the transducer and the optimal choice of the aperture are related by the following inequality expression, where A is the aperture of the transducer, is the SAW wave length and T is an anisotropy parameter. 4. A method to minimize the effect of diffraction in Surface Acoustic Wave (SAW) filter comprising the steps of : optimal placement of the transducers in the far-field region to obtain the desired stopband rejection ; and (ii) optimal choice of the aperture which facilitates easier fabrication of high frequency narrow band SAW filter due to shorter electrodes to enable higher yield. 5. A method as claimed in Claim 4, wherein the optimal placement of the transducer and the optimal choice of the aperture are related by the following inequality expression, where A is the aperture of the transducer, is the SAW wave length and T is an anisotropy parameter. 6. Surface acoustic wave (SAW) filter substantially as hereinbefore described with reference to the accompanying drawings. 7. A method to minimize the effect of diffraction in Surface Acoustic Wave (SAW) filter substantially as hereinbefore described.

Documents:

0705-mas-1999 drawings granted.pdf

0705-mas-1999 abstract granted.pdf

0705-mas-1999 abstract.pdf

0705-mas-1999 claims granted.pdf

0705-mas-1999 claims.pdf

0705-mas-1999 correspondence-others.pdf

0705-mas-1999 correspondence-po.pdf

0705-mas-1999 description (complete).pdf

0705-mas-1999 description(complete) granted.pdf

0705-mas-1999 drawings.pdf

0705-mas-1999 form-1.pdf

0705-mas-1999 form-13.pdf

0705-mas-1999 form-19.pdf

0705-mas-1999 form-26.pdf