voltage controlled oscillator design >Tuning of LC VCO
WITH the constant shrinking of feature sizes and increasing clock speeds in integrated circuit (IC) technology, designers are approaching a widespread use of digital systems with clock speeds in the gigahertz range. One of the major challenges in the distribution and synchronization of these gigahertz clocks is on-chip phase-locked loop (PLL) clock multipliers which rely on the oscillation amplitude of LC voltage-controlled oscillator (VCOs) . Since the oscillation amplitude of current-biased LC VCOs varies over its tuning range , an amplitude control scheme is needed to set the amplitude to a predefined level so the operation of the frequency dividers in the PLL is not compromised .
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. 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 . This concept is illustrated in Fig. 1. 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. The oscillation amplitude should be large enough so that its zero-crossing points can be detected by the loop and at the same time it should be small enough so that the negative resistor in the VCO can maintain its linear region of operation. Harmonic distortion and nonlinearities of the negative resistor can cause not only loss-control error, but also frequency-tuning error .