Compact High-Isolation LTCC Diplexer Using Common Stub-Loaded Resonator With Controllable Frequencies and Bandwidths

In this paper, a low-temperature co-fired ceramic (LTCC) diplexer is presented with compact size and high isolation. By using the common stub-loaded resonator shared by two channel filters, combining and matching networks are eliminated and only three resonators are used to design the two second-order channel filters, resulting in simple structure and miniaturized size. A novel transmission path is utilized to not only realize the higher channel filter but also prevent the signal at the lower frequency band. And a specific coupling region is selected to simultaneously realize the coupling strength required by lower channel filter and a transmission zero at higher-band frequency. In this way, each channel filter can generate a transmission zero at the passband frequency of the other channel filter, resulting in high isolation. Moreover, the bandwidths and center frequencies of the two channel filters can be individually controlled due to the design flexibility of the multilayer LTCC structure. For demonstration, a diplexer operating at 3.5 and 5.5 GHz is fabricated. The core circuit has the compact size of 1.9 mm × 1.7 mm × 2 mm or 0.054 λ
_g
×0.048λ
_g
×0.057 λ
_g
. Within the 3-dB passbands of the lower and higher channel filters, the measured isolation levels are better than 40 and 45 dB, respectively.