SINUSOIDAL RC-VOLTAGE CONTROL OSCILLATORS

Abstract

This paper is devoted to investigation and comparative study of two different circuits of sinusoidal RC-voltagecontrolled oscillators (RC-VCO). The first one is the famous Wien bridge VCO with automatic amplitude control system. The second one is RC-VCO with twin T-bridge. The frequency tuning of these oscillators is provided by varactors inserted in the feedback circuits. The formulas for calculation of oscillation frequencies and amplitudes dependencies on tuning voltages are derived. The basic analytic relations for PM and AM noise power spectral densities (PSD’s) calculation in these VCO’s, taking into consideration an influence of all sources of thermal noise in both circuits, are obtained and presented. It is shown that total level of the PSD’s is determined mainly by the thermal noises of the feedback circuits’ resistors, while contribution of the automatic amplitude control circuits is negligible. Analytical results, obtained for both circuits, are confirmed by results of computer simulation using Advanced Design System package. Comparison of the RCVCO’s main quality factors was fulfilled and discussed. It is shown that the RC-VCO with twin T-bridge has PM noise PSD about 5 dBc/Hz lower than the PM noise PSD of the Wien bridge VCO with the same level of nonlinear distortions. Dependencies of oscillation frequencies and oscillation amplitudes on the control voltages providing 4 times frequencies variations were calculated for both VCO’s. An influence of temperature variations in the range from –20 °С to +50 °С on these characteristics was investigated. It turned out that in the Wien bridge VCO variations of amplitude and frequency due to these temperature variation were less than 1%. In the VCO with twin T-bridge the frequency variations were about 2% while amplitude variations were about 15%.