Design of a broadband VCO for WSN applications

This paper designs a LC-VCO in the frequency synthesizer for wireless sensor network applications. The whole system is based on 1V low voltage with the process of TSMC 0.18μm RF-CMOS. The negative resistor pair is provided by cross-coupled PMOS transistors and the source of tail current is provided by NMOS transistor. Switched capacitor array is used to extend the frequency range. Technologies of second harmonic filter and large capacitor filter are introduced to further improve the phase noise performance. Switched currents source technology is also adopted to save the power. A output buffer with two stage of common-source is applied to drive the output signal and prevent external interference. Final on-chip test results show that the chip area is about 0.86mm × 0.73mm = 0.63mm², the core power of the VCO is from 2.7 to 4.1mW, the power of output buffer stage is from 5.7 to 6.1mW respectively, the output frequency range is from 4.66 to 6.78GHz with 37% large tuning range, and phase noise is as low as from -94 to -108dBc/Hz@ 1MHz and from -104 to -124dBc/Hz@3.5MHz respectively.