Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-10-11
2004-09-14
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S31300R
Reexamination Certificate
active
06791236
ABSTRACT:
FIELD OF INVENTION
The invention relates generally to surface acoustic wave (SAW) devices and more particularly to SAW transducers with overlapped electrode fingers.
BACKGROUND OF THE INVENTION
Surface acoustic wave (SAW) devices are often employed as filters or resonators in high frequency applications. A SAW device contains a substrate of piezoelectric material. For piezoelectric material, a propagating surface wave is accompanied by an electric field localized at the surface, and this enables the wave to be generated by applying a voltage to an array of metal electrodes on the surface. The electrode array is known as an interdigital transducer. Up until this point, prior art methods for tuning SAW devices have concentrated on the weighting of electrodes, as will be further described hereinbelow. However, the methods for determining how to weight the electrodes have heretofore been working on the assumption that SAW velocity dispersion effects can be ignored, and have assumed that SAW velocity value is an independent constant.
U.S. Pat. No. 5,831,492 to Solie discloses transducers which convert input electrical signals to surface acoustic waves propagating upon the surface of the substrate, and then reconvert the acoustic energy to an electric output signal. The input and output transducers are frequently configured as interdigital electrode fingers, which extend from pairs of transducer pads. Alternating electrical potential coupled to the input interdigital transducers induces mechanical stresses in the piezoelectric substrate. The resulting strains propagate away from the input transducer along the surface of the substrate in the form of surface acoustic waves. These propagating surface waves arrive at the output in an interdigital transducer where they are reconverted to electrical signals.
U.S. Pat. No. 5,818,310 to Solie discloses an electrical filter which transmits signals having frequencies within certain designated ranges or passbands, and suppresses signals having other frequencies outside the passband, or within attenuation bands. An ideal filter would transmit the signal within the passband without attenuation and completely suppress signals within the attenuation bands. In practice, known filters do attenuate the passband signal due to absorption, reflection, or radiation, which results in a loss of desired signal power. Further, such filters do not completely suppress signals within the attenuation bands. The use of surface acoustic wave devices as filters or resonators is well known for having the advantages of high Q, low series resistance, small size, and good frequency temperature stability when compared to other frequency control methods such as LC circuits, coaxial delay lines, or metal cavity resonators.
Trivial periodical uniform interdigital transducers have a sin(x)/x passband shape which is not a preferred filter shape because its transition bandwidth is equal to the filter bandwidth, and more importantly, the first sidelobe is typically only 13 dB below the main response. To synthesize arbitrary passbands, transducer weighting is employed. Filtering is thus accomplished in the process of generating the surface acoustic wave by the input transducer, and in the inverse process of detecting the wave by the output transducer. The most effective filtering is preferably accomplished if both input and output transducers are weighted, and thereby participate in the filtering process. Common transducer weighting techniques include apodization and withdrawal weighting. Apodization is typically used for wideband filters and either apodization or withdrawal weighting typically used for narrowband filters.
The term weighting as used herein refers either to the transducers topology or to the separate finger geometry and polarity. Prior art uses the topology weighting mechanisms which are directly associated with amplitude and phase of currents on electrode fingers and of voltages in the gaps between electrode. Such currents and voltages are considered as the sources of the launched SAW or the result of the detected SAW.
Weighting by apodization refers to the varying of the inter electrode overlapping length.
Capacitive weighting refers to a topology implementation, wherein the weights associated with fingers, are mainly defined by overlaps, both by some far fingers as well as by the nearest fingers.
Withdrawal weighting is the isolation or omission of selected fingers which roughly controls currents and voltages.
Series-block weighting is a kind of capacitive weighting.
Line-width weighting refers to an interdigital transducer, having fingers with different widths, and allows a weak weighting of currents and voltages.
Tapering of the transducer, i.e. weighting that leads to fanned topology, is a weighting in the direction along the electrode finger's length. Such a weighting controls the phase shift of the resonant frequency along the finger's length, because the period of the finger array changes along that direction.
Phase weighting, i.e. phase modulation that leads to chirp transducer, is achieved by varying the distance between fingers along the direction of SAW wave propagation.
There follows a more in-depth description of certain prior art techniques for weighting electrode fingers. Apodization varies the length of the electrodes to achieve an electrode weighting. With apodization, Fourier transform techniques can be readily applied for computing a filter impulse response when defining a special geometric pattern for the interdigital transducer fingers. It is well known that it is not practical to have an input apodized transducer launching a wave directly into an .output apodized transducer, because an apodized transducer launches a wave which has a non-uniform beam profile, and as a receiving transducer, it must see a uniform beam profile. If a surface wave incident upon an apodized transducer is not uniform over the entire width of the beam, the frequency response changes dramatically. For this reason, apodized input and output transducers can not be used to form a filter unless an added structure such as a multi-strip coupler is used. The multistrip coupler positioned between the apodized input and output transducers, transfers energy from a non-uniform beam into an adjacent track, in which a surface acoustic wave is launched as a uniform beam, and is thus compatible with an apodized transducer receiving the uniform beam. However, using an apodized input transducer for generating a surface acoustic wave, and transmitting the wave through a multistrip coupler to an apodized output transducer, widens the filter device, thus requiring increased space within electronic systems seeking to be ever more miniaturized. Further, apodized transducer-to-apodized transducer through a multistrip coupler is only useful on high coupling substrates such as lithium niobate, whereas, in fact, it is not practical on quartz.
Apodized interdigital transducer structure typically has a lot of small overlaps of the electrode fingers. This leads to diffraction spreading of the partial narrow SAW beams and consequently to additional insertion loss (i.e. diffraction loss) and distortion to the SAW device's response. The diffraction effect is not easy for mathematical simulation, and requires sophisticated techniques to be accounted in a synthesis problem. As described in the book by D. P. Morgan titled “Surface-Wave Devices for Signal Processing”, in order to simplify these difficulties, either the simplified diffraction model called “parabolic approximation” might be used for ST-quartz substrate, or the diffraction effect is ignored for the minimal-diffraction orientation Y,Z of the lithium niobate substrate. As described in U.S. Pat. No. 6,031,315 to Abbot, the important criterion in the choice of a piezocrystal cut, is the minimal-diffraction orientation, by way of example, for quartz it reduces degrees of freedom for choosing the cut of either the greatest temperature stability or the most appropriate coupling coefficient. An additional difficulty arises here
Dougherty Thomas M.
Langer, Pat. Atty. Edward
Shiboleth, Yisraeli, Robets, Zisman & Co.
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