A Novel Varactorless Tuning Technique for Clapp VCO Design Using Tunable Negative Capacitor to Increase Frequency-Tuning Range

Abstract

In this paper, a varactorless frequency-tuning method for Clapp voltage-controlled oscillator (VCO) design is proposed aiming at increasing frequency-tuning range (FTR) in comparison with the conventional varactor-based counterpart. The proposed structure uses a tunable non-Foster (negative) capacitor (TNC) to sweep oscillation frequency instead of a varactor. The analytical demonstration points that total tuning dynamic (TTD) qualitatively representing FTR is traditionally limited to varactor’s tuning dynamic (TD); while the TNC-based VCO can exhibit very-high TTD, at least over TNC’s TD capacity, if the maximum negative capacitance is appropriately designed, which can tolerate the TD requirement on the tuning component for wideband operation. Circuit realization of TNC is based on a grounded negative impedance inverter (NII) with cross-coupled transistors whose gate voltage controls capacitance. A simulation script using a real discrete transistor model with the realized TNC shows that FTR can be covered with TTD of 2.46 units while the TD of the non-Foster tuning component is only 1.32 units, validating the advantageous operation as well as practical realization possibility of the proposed VCO. In addition, phase noise performance is remained below −100 dBc/Hz for every tuning voltages at 1 MHz offset frequency, while figure of merit comprising FTR effect (FOMT) can be achieved in range of 140-180, which is similar to the existing state-of-the-art VCOs.

Keywords

• JFET
• colpitt and clapp VCOs
• negative impedance inverter
• non-foster capacitor
• negative resistance