A Ka-band Voltage Controlled Oscillator with Parasitic Capacitance Canceling Technique

Abstract

In this thesis, a Ka-band push-push voltage-controlled oscillator (VCO) using a parasitic capacitance (Cp) canceling technique is presented. In wireless communication, the demand for a high speed communication system using millimeter wave bands is growing. A VCO with a high oscillation frequency (fosc) and a wide tuning range (TR) is an essential component of a high speed communication system. However, there are major limitations associated with the fosc and the TR of a VCO from the Cp of the transistors. Therefore, to enhance fosc and the TR, the Cp of the VCO is canceled with negative capacitance (NC). First of all, effects of Cp is and its extraction methods are described. fosc and TR of VCO are determined by resonance frequency of the LC tank. LC tank is composd of Cp and variable capacitance. Resonance frequency of LC tank tuned by tuning total capacitance (Ctot), however, Cp limits TR of Ctot and also creates lower bound of Ctot. Therefore Cp limits fosc and TR. Cp can be expressed differently by topology of VCO. In this thesis, Cp is analyzed in the cross-coupled VCO. Therefore, a conventional 10 GHz cross-coupled VCO is designed. Cp is mainly composed of intrinsics of transistors and constant capacitance Cconst ofthevaractors. Themathematicalanalysisof Cp isdescribed. However, the VCO is operated nonlinearly, it is difficult to extract exact intrinsic parameter of transistors and Cconst. With harmonic balance simulation, exact Cp is extracted. Extracted value of Cp is 0.35 pF. Finally,aVCOwith Cp cancelingtechniqueisproposed. The Cp iscanceled out with NC circuit. To realize NC, a negative impedance circuit (NIC) is analyzed and designed. Because of self-resonance frequency, NC is realized using an open-circuit stable NIC. For further enhancement of the fosc, the VCO is developed in a push-push operation. The proposed VCO is fabricated in a commercial 0.15 µm GaAs pHEMT process. It shows 12.4% of the TR at 31.6 GHz with 11 dBm output power. With the NIC, the center frequency (fc) of the VCO increases from 20.1 GHz to 31.6 GHz and the TR increases from 5.7% to 12.4% compared with a VCO without the NIC. The measured phase noise of the VCO is –93 dBc/Hz with the NIC and –101 dBc/Hz without the NIC at 1 MHz offset frequency. Analyzing phase noise model, Phase noise degradation is mainly caused by increased fosc, not by NIC. This shows best improvement ratio of fc and TR compared to other Cp canceling techniques.