Linearization of oscillation frequency for integrated LC-VCO with inversion-mode varactor

Metal-oxide semiconductor (MOS) varactors are widely used in voltage-controlled oscillators (VCOs) due to the need for a tunable capacitance. Two types of varactors that have been integrated on-chip include the inversion-mode and accumulation-mode types. The inversion-mode varactor offers immunity to latch-up, but suffers from a non-linear C(V) characteristic which directly leads to a nonlinear oscillation frequency tuning curve. This paper proposes an oscillation frequency linearization technique for LC-VCO with an inversion-mode varactor. The linearity of the frequency tuning curve is improved by linearization of C(V) characteristics of the inversion-mode varactor. A new varactor configuration consisting of varactor units and resistor divider network is proposed. The single-switch integrated LC-VCO with the proposed varactor configuration is fabricated in TSMC 0.18 μm CMOS technology. The improvement of linearity of the frequency tuning curve has been verified using mathematical models and measurement results.

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