Digitally Tuned RF-MEMS Varactors Implemented in an 800-MHz Low Phase Noise VCO

Abstract

This paper reports on the design, fabrication, and measurement results of a voltage controlled oscillator (VCO) in the 800-MHz band that uses LC (inductor and capacitor) resonance controlled by a newly developed radio frequency microelectromechanical systems (RF-MEMS) varactor. In the system, a pair of metal-insulator-metal (MIM) capacitors is integrated with an electrostatic actuator made of an electroplated gold layer over a TEMPAX^TM substrate. We used an insulator hinge made of Parylene-C for the first time to electrically isolate the switch contact from the electrostatic actuator body, thereby allowing an option to form a series of MIM capacitors. A packaged 2-bit MEMS varactor is operated at 35 V_dc to digitally control the capacitor values in four different states between 0.70 pF and 0.86 pF with a 0.05-pF-step at 1 GHz. A high-Q factor (electrical) of 60 is experimentally confirmed at 1 GHz with the MEMS varactor tuned to 0.86-pF. The VCO circuit was developed using the packaged 2-bit MEMS varactors and surface-mounted devices (SMDs) on a low-loss printed circuit board (PCB), and its free-running frequency controlled between 802 MHz and 822 MHz. It exhibited a low phase noise of -101 dBc/Hz at a 10 kHz offset from the carrier frequency.