Single-to-balanced multi-coupled line LTCC bandpass filters with compact size and impedance matching capability

This paper presents the impedance matching capability of the single-to-balanced multi-coupled line LTCC bandpass filter. The proposed single-to-balanced filter is composed of a three-line coupled-line of 1/24 electric length with shunt capacitors loaded at suitable positions, which make them realizable in a LTCC chip size of 2012. Transformations from the 50 Ω single-ended port to balanced port impedances of 50 Ω, 100 Ω, and 150 Ω are demonstrated. Besides, broadside-coupled technique and meandered line are used for further size reduction. Specifically, a 2.4 GHz single-to-balanced filter with 50 Ω : 50 Ω impedance transformation ratio is achieved in a LTCC chip size of 1608, which is the smallest one in the market as far as we know.