Weighted SAW reflector grating using dithered acoustic reflectors

A reflective grating (36) for a SAW filter (10) or resonator. The reflective grating (36) is formed by selectively dithering grating grid lines (38) with respect to a uniform spaced grid of M number of grid lines (38) per each N wavelength (λ) of the grating (36) (Nλ/M), where λ is the wavelength of the center of the frequency band of interest, M and N are integers and M>N. M and N are selected so that the grating (36) does not have a net reflection when all of the grid lines (38) are uniformly spaced. By controlling the dithering pattern of the grid line in each sampling period of Nλ, any desired net distributed reflectivity from the grating can be implemented in both magnitude and phase.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a weighted surface acoustic wave (SAW) reflector for use in a SAW reflector filter or SAW resonator and, more particularly, to a weighted SAW reflector for use in a SAW reflector filter or resonator, where the reflector includes reflector grid lines selectively dithered relative to uniformly spaced grid lines that have M grid lines per each Nλ, where λ is the wavelength of the center frequency of the frequency band of interest, M and N are integers and M>N.

2. Discussion of the Related Art

Intermediate frequency (IF) filters are employed for channel selection in mobile phone communications systems, such as CDMA and GSM. The IF filters must be small in size and provide narrow bandwidths with steep transition edges and good out of band rejection. One type of filter that provides these properties is known in the art as a surface acoustic wave (SAW) filter.

Conventional SAW filters include an input transducer and an output transducer formed on a piezoelectric substrate. The input transducer is electrically excited with the electrical input signal that is to be filtered. The input transducer converts the electrical input signal to surface acoustic waves, such as Rayleigh waves, lamb waves, etc., that propagate along the substrate to the output transducer. The output transducer converts the acoustic waves to a filtered electrical signal.

The input and output transducers typically include interdigital electrodes formed on the top surface of the substrate. The shape and spacing of the electrodes determines the center frequency and the band shape of the acoustic waves produced by the input transducer. Generally, the smaller the width of the electrodes, or the number of electrodes per wavelength, the higher the operating frequency. The amplitude of the surface acoustic waves at a particular frequency is determined by the constructive interference of the acoustic waves generated by the transducers.

The combined length of the transducers determines the length of the overall filter. To design a conventional SAW filter with ideal filter characteristics, the filter's impulse response needs to be very long. Because the length of the impulse response is directly proportional to the length of the transducer, the overall length of a conventional SAW filter having ideal characteristics would be too long to be useful in mobile phone communications systems.

Reflective SAW filters have been developed to satisfy this problem. Reflective SAW filters generally have at least one input transducer, one output transducer and one reflector formed on a piezoelectric substrate. The reflector is typically a reflective grating including spaced apart grid lines defining gaps therebetween. The acoustic waves received by the reflector from the input transducer are reflected by the grid lines within the grating so that the reflected waves constructively and destructively interfere with each other and the wave path is folded. The constructively interfered waves are reflected back to the output transducer having a particular phase. Because of the folding, the length of the transducer is no longer dependent on the duration of the impulse response. Reflective SAW filters are, therefore, smaller in size and have high frequency selectivity, and thus are desirable for mobile phone communication systems.

The frequency response of a reflective SAW filter is further improved by weighting the individual reflectors to achieve a desired net reflectivity. Existing weighting methods include position-weighting, omission-weighting, and strip-width weighting. Other methods of weighting reflectors include changing the lengths of open-circuited reflective strips within an open-short reflector structure. Weighting the reflector helps to reduce the physical size of the filter and to improve the filter's frequency response.

An ideal frequency response for a reflector SAW filter has steep transition edges. The reflective gratings in a reflector SAW filter are weighted by a suitable weighting function to provide the desired filter response. For example, a weighted sin(x)/x function can be implemented in each reflective grating to generate a filter response having very steep transition edges.

Existing weighting techniques include position-weighting, omission-weighting and strip-width weighting. Other methods of weighting reflective gratings include changing the length of open-circuited reflective strips within an open-short reflector structure. Weighting the reflective grating helps to reduce the physical size of the filter and improve the filters frequency response.

The weighted reflective grating acts as a key element in the reflector SAW filter by reducing the physical size of the filter and improving the electrical filter response. A size reduction of 70% and an insertion loss of around 8 dB has been reported in the art using a Z-path reflector filter compared to in-line filter structures. One known reflective filter is a Z-path IF SAW filter for CDMA mobile phones.

The known methods of weighting a reflective grating in a SAW filter are all dependent upon the critical dimension of the reflector structure. The critical dimension is the smaller of the reflective grating grid width or the gap width, and is inversely proportional to the operating frequency of the filter. As the operating frequency increases, the critical dimension decreases. Fabrication constraints limit the critical dimension, thus limiting the operating frequency of the filter. As the operating frequency of the filter increases, the known reflective gratings have a limited dynamic range when implementing a wide range of reflectivity, which is required for filters with high selectivity. A reflective grating that provides strong reflectivity at a given frequency and critical dimension would be advantageous.

BRIEF SUMMARY OF THE INVENTION

In accordance with the teachings of the present invention, a new type of reflective grating for a SAW filter or resonator is disclosed. The grating is formed by selectively dithering grating grid lines with respect to a predetermined uniform or periodic spaced grid lines defined by M grid lines per each N wavelength (Nλ/M), where λ is the wavelength of the center frequency of the frequency, fo, band of interest, M and N are integers and M>N. M and N are selected so that the grating does not have a net reflection when all of the grid lines are uniformly spaced, i.e., no dithering. λ is defined as V/fo, where V is the propagation velocity of the surface acoustic waves on the substrate. By providing a specific dithering pattern of the grid lines in each sampling period of Nλ, any desired net distributed reflectivity from the grating can be implemented in both magnitude and phase.

Additional advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION

The following discussion of the embodiments of the invention directed to a weighted surface acoustic wave reflector for a SAW filter, where the reflector includes dithered reflector grid lines, is merely exemplary in nature, and is in no way intended to limit the invention or its application or uses.

FIG. 1is a top view of a reflective SAW filter10fabricated on a piezoelectric substrate12, according to an embodiment of the present invention. The reflective SAW filter10includes a bidirectional input transducer14, an output bi-directional transducer16, and a reflective grating18, according to the invention. The input transducer14and the output transducer16include a plurality of uniformly spaced interdigital electrode fingers20attached at opposite ends by bus bars22. The reflective SAW filter10is excited by an input signal that is to be filtered applied to the input transducer14on input line24. The input transducer14converts the electrical signal into surface acoustic waves28. The surface acoustic waves28propagate outward from the input transducer12along the surface of the piezoelectric substrate12.

Approximately half of the acoustic waves28are received by the output transducer16where they are converted back into electrical energy on an output line30. The other half of the acoustic propagated waves28are received by the reflective grating18, and are then reflected back through the input transducer14to the output transducer16where they are converted back into electrical energy. In a working embodiment, the filter10would include two input transducers and two output transducers to correct for signal cancellation at the output transducer16, as is well understood to those skilled in the art. As will be discussed below, the reflective grating18employs dithered grating lines that optimize the reflecting ability of the grating18for a particular wavelength.

It is noted that in order for the filter10to operate properly, a second reflective track should be included that also includes an input transducer, an output transducer and a reflective grating. The two input transducers would have the same polarity and the two output transducers would have opposite polarities. Thus, the surface acoustic waves that are directly received by the output transducer from the associated input transducer, and are not reflected by the reflective grating, are in phase with each other at the output transducer of the filter, and thus cancel because the two output transducers have opposite polarities. The acoustic waves that are reflected by the reflective gratings reach the associated output transducers 180° out of phase with each other, and therefore add at the output because the two output transducers have opposite polarities. The 180° phase difference between the grating reflections can be provided by several techniques, including providing a delay in one of the tracks relative to the other track where the acoustic waves in the two tracks propagate a λ/2 difference in distance. This delay can be provided by an offset between the reflective gratings in the two tracks of λ/4.

FIG. 2is a graphical representation of the reflectivity function of the reflective grating18with λ/4 sampling. According to the invention, the sampling period is defined by M grid lines per each N wavelength (Nλ/M), where λ is the wavelength of the center frequency of the frequency band of interest, M and N are integers and M>N. Examples of sampling periods that satisfy this requirement include, but are not limited to, λ/4, λ/3, 2λ/5, 3λ/7, 3λ/8, 4λ/7, and 5λ/8. A characteristic of a sampling period meeting this requirement is that if the reflective grating18had grid lines that were uniformly spaced or periodic, i.e., no dithering, the grating18would have no net reflectivity. The critical dimension, CD, of the transducer is proportional to the grid line period, NUM. Since λ=V/fo, fo is proportional to N/(M*CD). The larger the ratio N/M, the higher the grating center frequency will be for a given CD.

As discussed above, a reflective grating having the Nλ/M orientation of grid lines would provide reflections of the surface acoustic waves within the grid lines that have the proper phase to destructively interfere and provide no net reflectivity. The desired reflectivity is achieved by dithering or changing the position of the grid lines relative to the uniform spacing according to a predetermined dithering function, discussed below. The dithering function is selected to control the magnitude and phase of the reflected acoustic waves so that they are coupled together to provide the desired reflection at the center of the frequency band of interest. The shape of the frequency response is an ideal reflectivity function that will produce a filter with ideal characteristics.

FIG. 3is a graphical representation of the frequency response for the reflective grating18that has the reflective characteristics shown in FIG.2.FIG. 3illustrates what is known in the art as a “brick wall” frequency response. The brick wall frequency response is an ideal response that has steep transition edges and a narrow bandwidth. A reflective grating with the reflectivity function ofFIG. 2will produce the “brick wall” frequency response shown in FIG.3.

FIG. 4is a top plan view of a reflective grating36that can be used in place of the reflective grating18in the filter10, according to the present invention.FIG. 5is a graphical representation of dithered reflector strength versus grating position for the grating36. A graph line48identifies the magnitude and phase of the reflected surface acoustic waves at that location in the grating36. As will be discussed below, the reflective grating36provides a reflectivity function so that an incident surface acoustic wave32is reflected back in the opposite direction as a reflected surface acoustic waves34having the desired amplitude and phase for the frequency band of interest.

The reflective grating36includes a series of spaced apart grid lines38defining gaps40therebetween. The width of the grid lines38and the gaps40are based on a uniform grid spacing of M grid lines38per each Nλ, as discussed above. For a uniform or periodic grid line orientation, each of the grid lines38and the gaps40all have the same width in the propagation direction of the surface acoustic waves32. The uniform grid line spacing thus provides no net reflectivity. In accordance with the teachings of the present invention, some of the grid lines38are dithered relative to the uniform spacing to provide the desired net reflectivity (phase and magnitude) for a particular center frequency. The dithered grid lines44in the grating36are identified from the non-dithered or uniform grid lines46by being shaded.

In this embodiment, the spacing of the grid lines38is identified by a sampling period42, where each sampling period42includes four grid lines38spaced across a distance equal to one wavelength (λ) of the center frequency. Further, in this embodiment, each grid line38and uniform spacing gap40has a width of λ/8 relative to the center frequency being filtered. The vertical graph lines inFIGS. 4 and 5identify the separation of the sampling periods42. In one embodiment, the grating36has 275 sampling periods or is 275λ long.

The dithering of the grid lines38follows a predetermined sequence. For example, between vertical graph lines50and52the dithering of the grid lines38in each sampling period42has one dithering orientation, and between the vertical graph lines52and54the dithering of the grid lines38in each sampling period42has another orientation, where the sequences alternate across the complete reflective grating36. The phase of the reflectivity function changes by 180° from one side of the line52to the other.

According to the invention, the dithering of the grid lines38in the sampling period42is defined by a number between −1 and 1. A zero means that the grid line38has not been dithered relative to the uniform spacing, a positive number means that the grid line38has been dithered to the right a certain amount, and a negative number means that the grid line38has been dithered to the left a certain amount. In this example, −1 and 1 dithering represent the maximum distance that the grid line38can be dithered. In this embodiment, a −1 and 1 dithering is halfway across the gap40. The amount of maximum dithering can be more than halfway across the gap as long as the critical dimension, which is the smallest gap in the grating, is within the feasible limit of lithography.

The dithering orientation in the sampling periods42between the vertical graph lines50and52is identified by the sequence (0, 0, 1, −1). The values in the dithering sequence show the relative displacement of each grid line within a sampling period. This means that the first and second grid lines38have not been dithered, the third grid line can be dithered to the right between zero and the maximum amount, and the fourth grid line38can be dithered to the left between zero and the maximum amount. However, both of the third and fourth grid lines within the same period will be dithered the same amount. The dithering orientation in the sampling periods42between the vertical graph lines52and54has the dithering sequence (−1, 1, 0, 0). This means that the first grid line38can be dithered between zero and the maximum distance to the left, the second grid line can be dithered between zero and the maximum distance to the right, and the third and fourth grid lines38have not been dithered relative to the uniform spacing. However, both the first and second grid lines within the same period will be dithered the same amount. The graph line48gives the magnitude and phase of the reflections for this dithered sequence. Since the SAW propagation velocity, V, is a function of the dithered magnitude, the λ of each sampling period must be adjusted according to the dithered sequence in order for the SAW to propagate properly throughout the grating.

FIG. 6is a top view of a reflective grating70that can be used as the reflective grating18in the filter10, according to another embodiment of the present invention.FIG. 7is a graphical representation of dithered reflective strength versus grating position of the grating70, as will be discussed below. The reflective grating70is similar to the reflective grating36discussed above, where like referenced numerals will be used to identify the same elements. The sampling period42of the reflective grating70provides the same reflectivity function as the reflective grating36, as shown by the graph line48inFIGS. 5 and 7.

The reflective grating70also includes a sampling period42of four grid lines38, where Nλ/M=λ/4. The dithered sequence of the grid lines38between the graph lines50and52is (1, −1, 1, −1), and the dithered sequence of the grid lines38between the graph lines52and54is (−1, 1, −1, 1). Therefore, all of the grid lines38in the grating70can be dithered between zero and the maximum amount, either to the right or to the left. However, all four grid lines within the same period will be dithered the same amount. Thus, the same reflectivity function can be provided by different dithered sequences.

The reflectivity phase changes by 180° by reversing the dithering direction of each reflection. In general, if the reflection center of one sampling period42is spatially offset by λ/4 with respect to that of another period, the reflectivities of the two periods will be 180° out of phase. It is clear fromFIGS. 4-7that as the values of the dithering sequence within each sampling period42decreases, the reflectivity magnitude of that period42decreases. Similar grating reflectivity can be achieved by using different sampling periods.

FIG. 8is a top view of a reflective grating80that can be used as the reflective grating18in the filter10, according to another embodiment of the present invention. Likewise,FIG. 9is a graphical representation of dithered reflective strength versus grating position of the grating80. As above, like referenced numerals represent like elements. The reflective grating80has a different sampling period82than the sampling period42shown inFIGS. 4 and 6that can be employed to provide the same reflectivity function as shown inFIGS. 5,7, and9. In this embodiment, each sampling period82includes five grid lines38and has a width of 3λ. Further, each grid line38and each gap40has a width of 3λ/10. The dithering sequence for the section of the reflective grating80between the graph lines50and52is (−1, −0.35, −0.35, 0.8, 1), and the dithering sequence for the reflective grating80between the grid lines52and54is (0.35, 1, 0, −0.8, 0.35). Again, the values in the dithering sequence show the relative displacement of each grid line within a sampling period.

The grid lines38can be formed on the piezoelectric substrate12by any suitable technique. For example, FIG.10(a) is a top plan view of a reflective grating90having dithered grid lines92defining gaps94therebetween. The grid lines92are coupled at their ends to opposing end bus bars96and98, as shown, to provide a grid short circuit. The reflective gratings36,70and80do not have end bus bars, and thus provide an open circuited design. In this embodiment, the grid lines92and the bus bars96and98are metal deposited on the substrate.

FIG.10(b) is a top plan view of a reflective grating100, according to the invention, including a series of spaced apart and dithered grid lines102defining gaps104therebetween. The grid lines102can be formed by any suitable grating material, such as metal, any suitable substance deposited on the substrate, an etched groove below the substrate, ion implantation into the substrate, or any kind of disturbance in the substrate that provides the particular desired dithered pattern.

FIG.10(c) is a top plan view of a grating110including tapered grid lines112. The tapered grid lines112provide a suitable reflectivity channel for adjacent frequency bands for multiple communication channels. The grid lines112are provided by connecting N channels of grating lines from top to bottom in an ascending or descending order for the desired wavelengths. Various embodiments of the reflective gratings depicted in FIG.10(a) and FIG.10(b) can also be applied to the tapered reflective grating112.