Using the branch-line directional coupler in the design of microwave bandpass filters

Grupo de Microondas y Radar, Univ. Politecnica de Madrid, Spain
IEEE Transactions on Microwave Theory and Techniques (Impact Factor: 2.24). 11/2005; 53(10):3221 - 3229. DOI: 10.1109/TMTT.2005.855140
Source: IEEE Xplore


This paper addresses the application of the branch-line directional coupler to the design of microwave bandpass filters. The basic idea consists of using the branch-line coupler as a transversal filtering section by loading the coupled ports of the coupler with suitable transmission-line segments ending in an open circuit and taking the isolated port as the output node. Thus, under the signal interference philosophy involved in classic transversal filter schemes, bandpass transfer functions with perceptible stopbands and sharp cutoff slopes are derived. Furthermore, the main characteristics of the synthesized filtering response, such as the bandwidth or the position of the out-of-band power transmission zeros, can be easily controlled by means of the design parameters of the transversal section. Hence, a large variety of bandpass filtering profiles different from those offered by classical filter schemes can be realized. Finally, the experimental usefulness of the transversal filtering section based on the branch-line coupler is proven with the design and construction in microstrip technology of two microwave bandpass filter prototypes at 5 GHz.

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Available from: J.I. Alonso, Aug 27, 2015
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    • "1-3. As the basic circuit mode in [13]-[16], two transmission paths are used to realize the signal transmission from Port 1 to Port 2; quarterwavelength open stubs , half-wavelength shorted stubs and open/shorted coupled lines are parallel connected to realize two different high-order wideband bandpass filters. Through a similar analysis as the filters in [13]-[16], the in-band/out-ofband transmission zeros and poles can be realized by the superposition of signals for the two transmission paths. "
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    ABSTRACT: Two high-order wideband bandpass filters with improved upper stopband based on transversal signal-interference techniques are analyzed and designed in this paper. By choosing different open stubs and open/shorted coupled lines, two wideband bandpass filters with several transmission zeros from 0 GHz to 2f0 (f0 is center frequency of the stopband) can be achieved respectively, due to the superposition of signals of the two transmission paths. The bandwidths of the two wideband bandpass filters can be easily adjusted by changing the characteristic impedance of the resonant structures of the two transmission paths. One wideband bandpass filter prototype with 3-dB fractional bandwidth of 80% (0.6-1.8 GHz) is designed and fabricated for demonstration. The theoretical and measured results are in good agreement and show good in-band filtering performance and high selectivity.
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    • "The transmission is between Ports 1 and 2. Fig. 4. Sub-networks connected to the coupled ports of 3-dB: (a) 90 and (b) 180 hybrid couplers. The transmission is between Ports 1 and 2. evident that the transversal BPFs utilizing branch-line couplers proposed in [7] are a special case of the Cul-De-Sac networks presented here. This is because branch-line couplers may also be utilized to design bandstop networks as proven above. "
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    ABSTRACT: A synthesis procedure is described for the design of Cul-De-Sac networks realizing either bandpass or bandstop filter characteristics. The prototypes comprise a pair of one-port ladder sub-networks each connected at one of the coupled ports of a 3-dB, 90deg or 180deg hybrid coupler. Numerical examples and simulated filter layouts are presented to illustrate the design methodology
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