High quality LTCC resonator for voltage-controlled oscillator

Design and technology of microwave conductor lines embedded in low-temperature cofired ceramic (LTCC) multilayer substrates are summarized with a focus on achieving the highest possible quality (Q) factor for a given line inductance. The work was initiated to test the integrability of base station voltage-controlled oscillators (VCOs) in ceramic multilayer substrates. This approach leads to a miniaturization of current versions by a factor of 2 to 4. However, base station specifications for phase noise and hence resonator Q are extremely demanding. Therefore, both the design and the processing technology were optimized. By choosing a twin-line design with two parallel lines vertically separated by a single LTCC layer, Q factors of 90 and 180 have been achieved for integrated 5.5 nH inductors at frequencies of 640 MHz and 1650 MHz, respectively. Application of this result to VCO modules in standard LTCC technology already yields low phase noise levels, e.g., -136 dBc/Hz at 100 kHz offset, which is suitable for base station applications. However, further noise reduction is expected from a dedicated high Q fabrication process that uses conventional via punching and filling steps to replace the ceramic material between the two lines by conductive silver paste. This raises the Q to 120 and 200, respectively, at the two frequencies and adds extra degrees of freedom to LTCC design for low-loss wireless solutions.