Wave transmission lines and networks – Coupling networks – Wave filters including long line elements
Patent
1983-12-29
1985-10-01
Nussbaum, Marvin L.
Wave transmission lines and networks
Coupling networks
Wave filters including long line elements
333208, 333230, 333232, H01P 1208, H01P 706
Patent
active
045449015
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to the realization of microwave bandpass filters with dual mode resonance cavities arranged so as to achieve asymmetrical transmission characteristics.
Microwave bandpass filters are widely used in terrestrial and space telecommunications systems in order to provide noise or interference rejection and in multiplexers where they are used for low loss combination or separation of different transmission channels. The majority of these characteristics are symmetric and have been realized in microwave structures that are synchronously tuned, i.e. structures in which all resonators are tuned to the same center frequency.
In some particular applications, however, it is desirable to provide asymmetric transmission characteristics. A first application, for instance is the outer channel filters in a contiguous-channel multiplexer where the absence of a neighbour channel on one side causes a severe asymmetric distortion of the in-band group delay and insertion loss characteristics. This asymmetric distortion can be very damaging to digital signals and, if uncorrected, will require higher transmitter powers to restore the bit error rate to that of the undistorted case. Another major application is within transmission systems which have asymmetric rejection specifications, for example in a receive channel with an adjacent transmit channel which has to be heavily rejected.
Considering the physical implementation an interesting construction uses in-line dual mode resonance cavities. FIG. 1 shows schematically an exploded view of a two-cavity implementation. The two cylindrical cavities 100 and 200 are separated by a plate 300 having a cruciform coupling iris 400 therein. Each cavilty supports two TE.sub.11 mode resonances, polarized orthogonally to each other and tuned individually by means of a tuning screw. These two resonances are coupled by means of a coupling screw located at 45.degree. to the tuning screws. Coupling between resonances in adjacent cavities is achieved with the cruciform coupling iris 400. This type of construction only realizes transmission characteristics which are symmetric about the center frequency because the starting point is always a folded prototype network which is essentially symmetric (FIG. 2). This is a folded ladder network allowing cross-coupling between non adjacent shunt capacitors. These cross-couplings are designated by the symbols K18, K27, K38. Such a network is in effect the electrical embodiment of the characteristics which are defined in purely mathematical form by transfer polynomials. The process for converting these transfer polynomials to the folded electrical network has been described by J. D. Rhodes in: "A Low-Pass Prototype Network For Microwave Linear Phase Filters (IEEE-MTT, Vol. MTT-18, June 1970, pp. 145-160).
In order to create asymmetric responses it is required to construct a structure corresponding to an electrical prototype network comprising diagonal couplings as shown at K17, K26, K35 in FIG. 3.
SUMMARY OF THE INVENTION
The object of the invention is a microwave filter using dual mode resonance cavities arranged so as to achieve asymmetrical transmission characteristics.
This object is achieved in accordance with the invention by a microwave structure comprising of cascade of dual mode resonance cylindrical cavities wherein each cavity is coupled to the adjacent cavity by a coupling iris set at a determined angle relative to the angular position of the tuning screws of the cavity, with the adjacent cavity set at a determined angle relative to the angular position of the coupling iris between said adjacent cavity and the former cavity.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an exploded view of a type of construction of microwave filter structure using dual mode resonance cavities;
FIG. 2 shows the electrical prototype network corresponding to a type of structure as illustrated in FIG. 1,
FIG. 3 shows an electrical prototype network which is able to achieve asymmetrical transmission characte
REFERENCES:
patent: 2541375 (1951-02-01), Mumford
patent: 2738469 (1956-03-01), Miller
patent: 2968771 (1961-01-01), De Loach, Jr.
patent: 3235822 (1966-02-01), De Loach, Jr.
patent: 3697898 (1972-10-01), Blachier et al.
patent: 4028651 (1977-06-01), Leetmaa
patent: 4030051 (1977-06-01), Shimizu et al.
Cameron Richard J.
Rhodes John D.
Agence Spatiale Europeenne
Keire Fred A.
Nussbaum Marvin L.
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