Wave transmission lines and networks – Coupling networks – Electromechanical filter
Patent
1999-02-25
2000-06-27
Pascal, Robert
Wave transmission lines and networks
Coupling networks
Electromechanical filter
333195, 310313D, H03H 964
Patent
active
060811722
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a surface wave filter, hereafter referred to as OFW, which can be used in particular for RF applications.
In communication systems, signals are processed asymmetrically or symmetrically, whereby changes between the two operating modes often take place in the routing of the signal, and so a need arises for filters that, regarded at the input side or at the output side, can be operated asymmetrically/symmetrically or symmetrically/symmetrically. In addition, certain circuit designs require filters with the characteristics stated above, and in addition with different input and output impedances.
OFW filters that can be operated asymmetrically/asymmetrically, intended in particular for use in the RF range, are commercially available. The change from asymmetrical to symmetrical operating mode has up to now standardly been carried out by means of additional components, e.g., transformers. The impedances, i.e., the input and output impedances of these filters, are equal, and are matched by means of networks consisting of inductances and capacitances.
OFW filters that can be operated asymmetrically/symmetrically or symmetrically/symmetrically are also known. FIG. 1 shows a schematic representation of the design of such a filter, i.e., in the present case of a single-track filter with identical input and output impedances.
The structure of this OFW filter consists of an interdigital input transformer IDT3, centrically arranged, and of the reflectors REF1 and REF5, short-circuited in themselves, and interdigital output transformers IDT2 and IDT4 connected in parallel.
If the filter selectivity is to be increased, then, as FIG. 2 in turn schematically shows (identical elements are thereby designated with identical reference characters), the known single-track filter according to FIG. 1 can for example be connected in series with a filter of the same type. This filter also has identical input and output impedances and can be operated asymmetrically/symmetrically or symmetrically/symmetrically.
SUMMARY OF THE INVENTION
As already mentioned above, the underlying object of the present invention is to create OFW filters that can likewise be operated asymmetrically/symmetrically, symmetrically/symmetrically, or asymmetrically/asymmetrically, but have different input and output impedances.
For the solution of this object, in an OFW filter the invention provides that the interdigital output transformers are connected in series, and in addition are preferably mirrored to one another about the direction of propagation of the surface waves.
By means of the mirrored arrangement of the output transformers, their terminals can be applied to one side of the filter structure, and the corresponding contact paths can thereby be kept short. This contributes to a space-saving filter structure and, if necessary, reduces disturbing inductive and capacitive influences.
If, according to a further proposal of the invention, the above OFW filter is connected in series with an OFW filter consisting of a basic element with OFW single-port resonators connected in series and parallel hereto, there results a filter that can be operated asymmetrically/symmetrically or symmetrically/symmetrically, having different input and output impedances, which filter unites the high remote selectivity of the single-track filter structure with the high adjacent-channel selectivity and the low insertion loss of the connected single-port resonators.
Independent of the above OFW filter, according to a further inventive proposal a filter with identical input and output impedance, which be operated asymmetrically/symmetrically, symmetrically/symmetrically, or asymmetrically/asymmetrically, is created solely by means of the cascaded (if necessary) connection of the above basic element, which filter unites high adjacent-channel selectivity with an insertion damping that is low but that increases with an increasing number of cascaded basic elements.
BRIEF DESCRIPTION OF THE DRAWINGS
The features of the present
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Bauregger Josef
Strauss Georg
Pascal Robert
Siemens Matsushita Components GmbH & Co. KG
Summons Barbara
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