Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1999-03-05
2001-04-03
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S31300R
Reexamination Certificate
active
06211600
ABSTRACT:
BACKGROUND OF THE INVENTION
a) Field of the Invention
The invention is directed to the field of electrical engineering and electronics. Objects in which the invention may possibly and advantageously be applied are components based on surface acoustic waves such as bandpass filters with a relative bandwidth of up to several percent and oscillators.
b) Description of the Related Art
Surface acoustic wave components are known in which two single-phase unidirectional transducers which are composed of base elements comprising a pair of fingers and a reflection finger are arranged on a piezoelectric substrate, wherein the two fingers of the pair of fingers have opposite polarities and are at a distance of one quarter wavelength from one another. One of the fingers of the pair of fingers and the reflection fingers are connected with ground potential, while the second finger of the pair of fingers is connected to a potential not equal to the ground potential (C. S. Hartmann and B. P. Abbott, 1989 IEEE Ultrasonics Symposium Proceedings, pages 79-89 [1]; E. M. Garber, D. S. Yip and D. K. Henderson, 1994 IEEE Ultrasonics Symposium Proceedings, pages 7-12 [2]). The single-phase unidirectional transducer is designated as such or, in abbreviated form, as SPUDT in English-language technical literature.
In a special construction (P. Ventura, M. Solal, P. Dufilié, J. M. Hodé and F. Roux, 1994 IEEE Ultrasonics Symposium Proceedings, pages 1-6 [3]), as also in [1] and [2], the distance from the center of the reflection finger to the center of the nearest finger of the respective pair of fingers is the same in all base elements of both single-phase unidirectional transducers. The distance of the effective location of wave excitation from that of reflection, referred to as the excitation center and reflection center, respectively, is ⅜ of the wavelength. The reflection centers are located in the center of the reflection fingers, while the excitation centers are positioned close to the center of those fingers of the pairs of fingers connected to a potential other than ground potential.
Due to the distance of one quarter wavelength between the two fingers of a pair of fingers, the pairs of fingers are non-reflecting, but excite surface acoustic waves, namely, effectively at the site of the excitation center. Reflections take place exclusively at the reflection fingers. Because of the distance of ⅜ of the wavelength between the excitation center and reflection center, there is a phase difference of 3&pgr;/2 between the directly emitted wave and the reflected wave. In this respect, the phase displacement of &pgr;/2 is summed due to the reflection at a finger with front edge and rear edge and therefore gives a total phase displacement of 2&pgr;, wherein the reflection factor at an edge is assumed to be real. Consequently, directly emitted waves and reflected waves overlap constructively in the direction pointing from the reflection finger to the pair of fingers of one and the same base element. Typically, a single-phase unidirectional transducer is formed of a periodic arrangement of base elements with a period length equal to a wavelength. Therefore, the distance of an excitation center (approximately the center of the finger of a pair of fingers not connected to ground potential) from the next reflection finger in the opposite direction is ⅝ of the wavelength, which gives a phase displacement of 3&pgr; between the wave emitted in the opposite direction and the wave reflected in the opposite direction. In other words, with respect to the directly emitted waves, the reflected waves are real. Consequently, in the direction pointing from the pair of fingers to the reflection finger of one and the same base element, the directly emitted waves and reflected waves overlap destructively. Accordingly, a greater wave amplitude is radiated in the direction pointing from the reflection fingers to the pair of fingers of one and the same base element than in the opposite direction and this is therefore designated as the forward direction.
Because of this characteristic, interdigital transducers which radiate in essentially only one direction can be constructed on the principle of the single-phase unidirectional transducer so as to prevent losses due to wave radiation in unused directions. Therefore, the principle is suitable for producing components with low insertion loss. However, it must be ensured that echoes due to reflections at the transducers as a whole do not interfere with the transmission behavior. In constructions [1] and [2] referred to above, the reflection of every transducer is suppressed. Apart from suitable selection of matching networks, this is accomplished primarily by the adjustment of spatially-dependent reflection factors of the reflection fingers from one base element to next base element. In construction [3] above, however, not only are the echoes resulting from reflections at the transducers not suppressed, they are even utilized for structuring the transmission behavior. Accordingly, filters with a small form factor (ratio of 30-dB bandwidth to 3-dB bandwidth), that is, with steep edges, can be realized without the very long layouts which are otherwise necessary.
However, construction [3] has the disadvantage that frequency dependencies of insertion loss and/or group delay time which are asymmetric relative to the center frequency, for example, sloping passbands, cannot be realized in a predetermined manner.
OBJECT AND SUMMARY OF THE INVENTION
It is the primary object of the invention to change surface acoustic wave components of the known type in such a way that the dependency of the insertion loss and/or group delay time on the frequency as asymmetric function in relation to the center frequency can be realized in a predetermined manner. Sloping passbands are particularly significant in this respect.
This object is met by the surface acoustic wave component in accordance with the invention in which two single-phase unidirectional transducers which are composed of base elements comprising a pair of fingers and a reflecting finger are arranged on a piezoelectric substrate, wherein the two fingers of the pair of fingers have opposite polarity and are at a distance of one quarter wavelength from one another and wherein at least one base element in at least one of the single-phase unidirectional transducers is distinguished from the rest of the base elements by its distance between the center of the reflection finger and the center of the nearest finger of its pair of fingers.
This feature means that the distance between the excitation center and the reflection center does not equal ⅜ of the wavelength in every base element and that, consequently, the phase difference between the reflected waves and the waves which are emitted directly without reflection is no longer an integral multiple of &pgr; for all base elements. Through a suitable selection of the spatial dependency of the deviation of the distance of the excitation center and reflection center from the value of ⅜ of the wavelength, which value is given in all of the base elements of solutions [1], [2] and [3], a determined dependency of said phase difference on the position of the base elements is adjusted. This contributes to the frequency dependency of insertion loss and group delay time which are asymmetric to the center frequency. As a whole, there results a transmission behavior which is asymmetric to the center frequency.
According to a first construction of the invention, every base element of each of the two single-phase unidirectional transducers is allotted the distance of the center of the reflection finger from the center of the nearest finger of its pair of fingers by means of a distance assignment. For this purpose, the distance assignment of one single-phase unidirectional transducer can differ from that of the other single-phase unidirectional transducer; however
Dougherty Thomas M.
Dover Europe GmbH
Reed Smith LLP
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