Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
1999-05-10
2001-07-03
Lee, Benny (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S195000, C310S31300R, C310S31300R
Reexamination Certificate
active
06255915
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention is related to the field of surface acoustic wave (“SAW”) filters. In particular, the invention is related to SAW filters that are useful in UHF radio transceivers, either fixed or mobile, that require frequency selectivity and image rejection in the transceiver front-end prior to a differential mixer. However, the teachings of this invention are applicable to any type of electronic circuit that employs a SAW filter having balanced outputs or inputs as the case may be.
2. Description of the Related Art
Dual-mode leaky SAW structures were reported in the literature by Morita in 1992 (T. Morita, Y. Watanabe, M. Tanaka and Y. Nakazawa, “Wideband Low Loss Double Mode SAW filters,” Proc. 1992 IEEE Ultrasonics Symp., October 20-23) The layout of this type of SAW consists of a central inter-digital transducer (“IDT”) surrounded by two other IDTs connected in parallel with a external connection. On either end of the IDTs are reflectors. This resonator structure is usually then cascaded with another identical device to produce a filter with reasonably good attributes. Usual operation of this filter is to have a single input and a single output. In other words, these SAW devices do not provide a differential output signal.
Recently, the trend has been to design the front-end of a radio transceiver with a differential input to the first mixer. The advantage of this type of design is to take advantage of the common mode rejection between the two differential inputs. This method then requires a differential SAW filter before the first mixer. Such a differential SAW filter is shown in
FIG. 1
, described in more detail below. As with most such differential SAWs, the structure shown in
FIG. 1
suffers from an imbalance of the differential outputs.
FIGS. 2 and 3
depict the imbalance in the differential outputs in this type of SAW. This imbalance is due to the nonsymmetrical coupling within the IDTs of the output stage.
Therefore, there remains a general need in this field for a differential SAW filter having a pair of balanced outputs.
SUMMARY OF THE INVENTION
The present invention overcomes the problems noted above and satisfies the needs in this field for a differential SAW filter having a pair of balanced outputs. More specifically, the present invention provides a differential SAW filter having a cascaded structure including at least an input SAW stage and an output SAW stage. The SAW stages are configured so that the electrical distance of travel of the surface acoustic waves from a single unbalanced input of the input stage to the dual balanced outputs of the output stage is electrically equidistant, thus resulting in a balanced differential output.
For brevity of discussion, the description of the invention is directed primarily to a cascaded structure receiving a single input and outputting two balanced signals. It is to be understood, however, the invention is not limited to this particular use. The invention would work just as well in an environment calling for a structure receiving two balanced signals and outputting an unbalanced single output.
In the invention, the input stage includes an unbalanced input coupled to a pair of outer IDTs, both of which launch a surface acoustic wave to an input stage inner IDT. The input stage inner IDT connects to the inner IDT of the output SAW stage, which launches a surface acoustic wave to a pair of output IDTs, referred as the first and second outer output IDTs respectively, that form the differential, balanced output signals.
In a preferred embodiment of the invention, in the input stage, the distances between the pair of outer IDTs and the inner IDT are preferably the same, and the distances between the pair of outer IDTs and their respective reflectors are the same. In the output stage, the distances between the inner IDT and the pair of outer output IDTs are not necessarily the same. Further, the distances between the pair of outer output IDTs and the reflectors adjacent thereto are not necessarily the same. Preferably, each of the output IDTs in the output stage has the same number of interdigital elements. In the output stage, preferably, two particular distances are predetermined to properly balance the filter. In this preferred embodiment, a distance, defined as So, between the first outer IDT and the input stage inner IDT is predetermined so as to produce a 180 degree phase shift between first and second output signals and a distance, defined as Ro, between the first outer IDT and an adjacent reflector is predetermined with the cumulative effect of Ro and So producing an effective integer number of quarter wave lengths for the output stage. The values of Ro and So would vary depending on the particular structural scheme of the filter.
The present invention provides many advantages over presently known SAW filters. Foremost of these advantages is the ability to provide a balanced differential output. A secondary advantage of providing this type of balanced output is that the common mode rejection of the filter is increased, thus providing better noise immunity. This is particularly important when the filter is coupled to subsequent amplification or other processing stages, whose performance depends upon a truly balanced output stage. Another advantage of the present invention is that having the outer IDTs, in the input stage, as inputs in parallel allows for higher input powers to be used in the differential SAW.
These are just a few of the many advantages of the present invention, as described in more detail below. As will be appreciated, the invention is capable of other and different embodiments, and its several details are capable of modifications in various respects, all without departing from the spirit of the invention. Accordingly, the drawings and description of the preferred embodiments set forth below are to be regarded as illustrative in nature and not restrictive.
REFERENCES:
patent: 5568002 (1996-10-01), Kawakatsu et al.
patent: 5790000 (1998-08-01), Dai et al.
patent: 5835990 (1998-11-01), Saw et al.
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“High Performance Balanced Type SAW Filters in the Range of 900 MHZ and 1.9GHz”, Endoh, G et al., IEEE Ultrasonics Symposium, New York, NY, Oct. 5, 1997, pp. 41-44.
Radiation Conductance and Grating Reflectivity Weighting Parameters for Dual Mode Leaky-SAW Resonator Filter Design, Edmonson, P.J. and Campbell, C.K., Department of Electrical and Computer Engineering, McMaster University, Hamilton, Canada, IEEE 1994 Ultrasonics Symposium, pp. 75-79.
“Coupling-Of-Modes Studies Of Surface Acoustic Wave Oscillators And Devices”, Edmonson, P.J., A Thesis Submitte to the School of Graduate Studies in Partial Fulfillment of Requirements for Degree of Doctor of Philosophy, McMaster University, Feb. 1995, pp. 19, 83.
Jones Day Reavis & Pogue
Lee Benny
Meyer, Esq. Charles B.
Pathiyal, Esq. Krishna K.
Research In Motion Limited
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