On the other hand, to ensure a clock with very low jitter, a high-quality LC tank is needed. However, the quality factor of practical on-chip inductors is typically very low and frequency dependent. The low quality factor and frequency dependence is mainly due to a lossy silicon substrate and thin metal layers. A great deal of attention has been focused on improving the performance of lossy LC tanks by introducing a negative transconductance to compensate its resistive loss . By tuning this loss, and thus the factor of the LC tank, it is possible to control the amplitude of oscillation in an LC VCO implementation. The tuning results in a fast and reliable start up with an optimal bias point in terms of phase noise performance [6]. However, the bias current required by the oscillator depends on the amplitude of oscillation, the losses in the tank, and also other process and environment parameters. Consequently, if the VCO circuit is biased with a fixed current, oscillations over all conditions may not be guaranteed, nor can an optimum value to minimize power and ensure fixed amplitude of oscillation be found .

Therefore, having a stable, efficient, and cost-effective mechanism to control the resistive loss of the LC tank can also be used to tune the quality factor of on-chip LC filters . The PLL circuit locks the oscillation frequency of the VCO and thus the center frequency of the slaved filter. Assuming identical tanks for both VCO and filter, the control loop locks the oscillation amplitude to a reference signal, and feeds the same control voltage to the slaved resonator. In other words, the same concept used to tune the quality factor of the LC filters has been modified to regulate the oscillation amplitude. However, the main problem of using a VCO is its amplitude regulation during the tuning process.