Voltage controlled oscillators (VCOs) usually require high Q and high tuning range varactors in their LC tanks for low phase noise and higher tunability. Most of the time, this is realized using off-chip passives. MEMS based tunable capacitors with high-Q and tuning range emerged in last years that are capable to replace off-chip varactors [1]. In this paper a MEMS based parallel plate dual gap varactor is presented to be integrated with –Gm LC voltage controlled oscillated that is implemented with AMS 0.35μm-SiGe BiCMOS process. Electrical and 3-D model /view of the dual gap parallel plate MEM varactor topology in this work is presented in Figure 1.
MEM varactor is fabricated using surface micromachining which includes six layers and five masks in Sabanci University's cleanroom. The SEM picture of the fabricated varactor is shown in Figure 1. This fabricated MEM-Varactor is integrated into a modified version of a formerly proposed VCO.
The integrated system is tested using Karl Suss RF&DC probe station and Agilent E4407B Spectrum Analyzer. The actuation voltage of the MEM Varactor is changed from 0 V to 10 V resulting 70 MHz frequency change from 7.79 GHz to 7.72 GHz as depicted in Fig. 6. The output power of -7.89 dBm and -5.868 dBm are measured for 7.80 GHz and 7.72 GHz respectively with the additional losses from the connecting cable and the RF probes which is around 6dBm. Tuning characteristic of the VCO is given in Figure 3
Figure 3: Changing frequency and capacitance values due to increasing and decreasing actuation voltages
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