D.V. Kholodnyak, V.M. Turgaliev, I.V. Munina, P.A. Turalchuk, I.B. Vendik
Quadrature directional couplers based on coupled transmission lines provide a wide operational bandwidth but have a large size, of the order of the guided wavelength. The size can be significantly reduced by replacing the distributed quarter-wavelength coupled lines section with lumped elements. Using the multilayer low-temperature co-fired ceramics (LTCC) technology for device implementation enables an additional miniaturization due to the three-dimensional allocation of the components. The design of the miniature quadrature directional coupler with the 3-dB coupling has been laid out for the frequency range 2–4 GHz.
The phase difference between the output signals of a rat-race ring can be equal to 0° or 180°, depending on which port of the device is employed as the input. The conventional rat-race ring involves an electrically long 270° transmission line section, giving rise to a strong frequency dependence of the characteristics and hence limiting the operational bandwidth. Moreover, using distributed transmission lines leads to a large size of the device. The designed miniature broadband rat-race ring consists of three sections of artificial right-handed transmission line and one artificial left-handed transmission line section. All transmission line sections were realized as a cascade of two single Π-cells on quasi-lumped elements The size reduction was sixfold compared to a conventional rat-race ring design on the same substrate whereas the operational bandwidth was enhanced in 1.5 times.
A capacitive loading allows a significant decreasing the size of LTCC cavity resonator that can be as low as one eighth of the guided wavelength and even smaller while its Q-factor still remains much higher as compared to a quasi-lumped-element LTCC resonator. This gives possibilities to design low-loss and small-size narrowband LTCC filters for low-frequency applications. A novel very compact dual-mode resonator consisting of two capacitively loaded LTCC cavities nested one into another was proposed. The low-loss and small-size LTCC bandpass filters on capacitively loaded cavities for single- and dual-band applications were designed.
All the devices under consideration were implemented on DuPont Green Tape 951 LTCC.