MPhil thesis of Lo Chi Wa - Department of Electronic & Computer ...

The finest tuning resolution also depends on the minimal size of the switchablecapacitor. The switchable-capacitor-array employs linear capacitors and donut NMOStransistors as the switches, as shown in Fig. 24a. The donut transistor is used because ofthe smaller drain parasitic capacitance 13 . The switchable-capacitor-array includes a 7-bitbinary-weighted array of unit switchable-capacitor and an extra half-size switchablecapacitoras the least significant bit. Totally, this 8-bit array can provide around300MHz tuning range (from 800MHz to 1100MHz) and 800kHz frequency resolutionwhich is equivalent to 4 frequency channel spacings. The available tuning range shouldbe good enough to compensate the center frequency shift due to the process variation orthe inaccurate modelings of the inductor and capacitors. Each of switchable-capacitorsin the 7-bit binary-weighted array includes a 26.7fF linear capacitor and a 11.5u/0.5udonut transistor while the half-size switchable-capacitor includes a 14.3fF and a8.5u/0.5u donut transistor. When the switches are off, there are about(2.8fF+2.8fF+3.95fF)=9.55fF and (1.5fF+1.9fF+2fF)=5.4fF remained in the normalsizedand half-sized unit switchable-capacitors due to the drain parasitic capacitance(Cd), the gate-to-drain overlap capacitance (Cov) and the Nwell-to-substratecapacitance of the linear capacitors (Cwell) respectively, as shown in Fig. 24c. Thechanges of capacitance are about 63% of the maximum value for both switchablecapacitorsof different sizes. Higher resolution of the switchable-capacitor array is alsopossible by using smaller unit linear capacitors and switches. However, the increase inthe parasitic capacitance will limit the overall tuning range of the array.36