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Variable Capacitors

  
MEMS variable capacitors have the potential to replace conventional varactor diodes in many applications such as phase shifters, oscillators and tunable filters. While there are various configurations to realize MEMS variable capacitors, the parallel plate configuration exhibits a relatively high Q value and is very convenient to build due to simplicity of fabrication. Such type of MEMS capacitors, however have a maximum theoretical tuning range of 50% due to the collapse of the capacitor structure as the voltage is increased beyond the pull-in voltage.
 
  

An RF MEMs (micro electro mechanical system) variable capacitor has been demonstrated with a 22:1 tuning range, tuning from 1.5 to 33.2 pF of capacitance. This capacitor was constructed using bistable MEMs membrane capacitors with individual tuning ranges of 70:1 to 100:1, control voltages in the 30–55 V range, switching speeds less than 10 mS, and operating frequencies as high as 40 GHz. These devices may eventually provide a viable alternative to electronic varactors with improved tuning range and lower loss.

 
  
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. 
 

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