Bandpass Filters on Orthogonal Resonators
Keywords:bandpass filter, resonator, transmission line, open-circuited stub, short-circuited stub
Introduction. The rapid development of radioengineering, in particular telecommunication, systems leads to a constant tightening of requirements for bandpass filters, and this, in turn, the search for new, more efficient design solutions. In the article, a structure based on two orthogonal resonators (OR) is proposed as the basic structure of a bandpass filter. The transmission response (TR) of the OR-structure and filters on it has been studied.
1 Structure based on OR. The structure consists of two resonators.The longitudinal (relative to the direction of wave propagation) resonator is formed by a half wavelength transmission line section, and the transverse one — by two open-circuited or open- and short-circuited stubs. The TR of the OR-structure has been studied. With a decrease in the characteristic impedance of the longitudinal resonator, the bandwidth of the OR-structure expands. Since the TR zeros near the passband are fixed by stubs, in this case the steepness of the characteristic increases.
2 Filter based on OR with sections. The design of the OR-structure with two additional quarter wavelength sections of the main line adjacent to the OR-structure is considered. This solution leads to an improvement in the side lobes level and a lowering of the requirements for the characteristic impedance values. The parameters of the considered OR-structure with sections are sufficient for its use as a simple filter.
3 Filters based on coupled ORs. The TR of filters based on two and three coupled OR-structures have been studied. The coupling is made by a quarterwavelength section of the main line. The structure of the filter is symmetric, the transverse resonators are identical. Filter parameters: relative bandwidth 20%, ripple in the bandwidth 0.3 dB, sidelobe level –40 and –69 dB, respectively for filter on two and three OR-structures. Parameters of TRs correspond to the elliptic characteristic of the sixth and eighth orders, respectively.
4 Filter with extended suppression bands. The disadvantage of the filter based on open-circuited stubs is the presence of responses at zero and double frequencies. Such responses are absent if in OR-structure one of the stubs is shorted. A filter based on two OR-structures with short-circuited stubs is considered. The filter TR has extended suppression bands.
5 Results discussion. Coupling of longitudinal and transverse resonators in the OR-structure is such that its bandwidth expands as the characteristic impedance of the longitudinal resonator decreases. The high steepness of the TR with zeros in the immediate vicinity of the passband is formed by a transverse resonator based on open-circuited stubs. For a high-rectangular quasi-elliptic TR of a bandpass filter, several OR-structures are sufficient. Proposed structure can be used in filters of various design implementations (microstrip, coaxial, etc.).
Conclusion. In the considered filters the number of OR-structures is equal to 1...3. The parameters of a single OR-structure with sections are sufficient for its use as a simple filter.
The parameters of the synthesized filters correspond to elliptic characteristics of order 4, 6 and 8. The length of the filters is within (1...2)λ0 (λ0 is the center frequency wavelength) and width — (0.201...0.53)λ0.
The filter based on the OR-structure with a short-circuited stub has extended suppression bands.
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