TABLE OF CONTENTS Avaractor is a voltage-dependent variable capacitor commonly used to tune the oscillation frequency of both LC and ring VCOs by changing the control voltage across the varactor and thus its capacitance. The most critical parameter in designing varactors is their tuning ratio, defined as the ratio between the maximum and minimum capacitance values, C max /C min they can provide. As for inductors, another important design parameter for varactors is their quality factor Q. Although the overall quality factor of resonant LC tanks is typically dominated by the inductor Q,asmentioned earlier, careful attention needs to be paid to design varactors with maximum Q,orelse their Q may be low enough to become significant in degrading the overall Q and thus the overall performance of LC oscillators in terms of phase noise and power consumption. In the following sections, after a brief review of capacitor Q, design considerations and trade-offs of the pn-junction varactor and the accumulation-mode varactor are described and compared.
As reviewed in Section 3.7.2, inductors store and release magnetic energy. Similarly, electric energy is stored and released in capacitors in every oscillation cycle. The electric energy is given in Equation (3.66).
where ? C is the voltage across a capacitor with a capacitance value C.
Although typically much higher than that of on-chip inductors, the quality factor of capacitors used in LC oscillators may become low enough...
