Reactance filter with surface wave resonators

Details Reactance filter with surface wave resonators Reactance filter with surface wave resonators

1999-03-19

2001-06-12

Pascal, Robert (Department: 2817)

Wave transmission lines and networks

Coupling networks

Electromechanical filter

C333S196000, C310S31300R, C310S31300R

Reexamination Certificate

active

06246302

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a reactance filter with surface wave resonators having interdigital transducers arranged between reflectors wherein at least one of the interdigital transducers is formed as an omission weighted normal finger transducer.
2. Description of the Prior Art
It is known in this field of art to employ reactance filters for broad band systems. It is known to employ such filters for broadband systems, particularly as filters in branching circuits (ladder type). For obtaining narrowband filters, attempts have been made to employ electrode materials other than aluminum wherein metallization heights were reduced. The variation of substrate materials was also incorporated into such attempts.
An object of the present invention is to create a reactance filter of the type initially described that is formed as a narrowband filter, particularly in a branching circuit (ladder type).
SUMMARY OF THE INVENTION
The respective resonator, in particular of the present invention, is formed as a one-port resonator, wherein the capacitance ratio (static to dynamic capacitance) is particularly dimensioned such that due to the arrangement and plurality of the active fingers of the interdigital transducer, a narrow filter bandwidth is achieved. Further, an improved stop band level can be achieved by the finger arrangement of the interdigital transducer of each and every resonator.
Low insertion attenuations can be achieved given the formation of the reactance filter as a filter in a branching circuit (ladder type) since no power is transmitted over acoustic paths. Al or an Al alloy, for example AlCu alloy, can be utilized as the electrode material. A suitable substrate material is LiTaO
3
(36° rotated YX) or LiTaO
3
(X112.2° Y). Other traditional piezoelectric substrate materials also may be used such as LiNbO
3
which exhibits, for example, the section (64° rotated YX) or (YZ). The relative metallization height (metallization height/wavelength at the center frequency) of the electrodes preferably amounts to approximately 0.05 through 0.1, particularly 0.07 or 0.09. A relative 3 dB bandwidth of 1.6% is achieved given a filter in a branching circuit. Resulting therefrom is an advantageous utilization of the present invention in duplexers; particularly, cordless telephone duplexers. The minimal insertion attenuation that is achieved lies at approximately 2.5 dB.
The ratio of static to dynamic capacitance of the one-port resonators employed for the filter can be varied by employing omission-weighted normal finger transducers such that more narrowbanded filters in branching circuits can be realized than heretofore known.
Additional features and advantages of the present invention are described in, and will be apparent from, the Detailed Description of the Preferred Embodiments and the Drawings.


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