MEMS variable capacitor versus MOS variable capacitor for a 5GHz voltage controlled oscillator

MEMS variable capacitors (varicaps) are an alternative to MOS and PN junction varicaps as a tuning element in voltage controlled oscillators. There is however no comparative data available on oscillators with a MEMS and a MOS varicap. Therefore we did a design experiment with two 5GHz oscillators in 0.35µm CMOS. The worst case phase noise over the tuning range improves by 5dB when changing a MOS to a MEMS varicap while keeping power consumption and tuning range almost constant. There are however some requirements that need to be fulfilled before MEMS varicaps can be used in mainstream applications: the required tuning voltage should be obtainable in standard CMOS and accidental pull-in of the capacitor has to be avoided.