The Automatic Amplitude Control System for a Single-Loop RC Oscillator

Abstract

Harmonic RC oscillators have long been known and are widely used in various fields of radio engineering. They have become widely used in medical equipment, various sensors, measuring equipment, etc. Their main advantages in operating within short to long wave frequency bands are compact dimensions, simplicity and low cost of manufacturing, possibility of widely retuning the frequency, etc. In our previous articles, we examined several versions of such circuits, including single-loop RC oscillators, which are the simplest ones. It was shown that the major disadvantage of such self-oscillators in generating close-to-sinewave signals is the need to operate them with a very small self-excitation margin. An attempt to increase the self-excitation margin inevitably leads to a growth of higher harmonic components, distortion of the output signal, and failure of oscillations. As a result, additional techniques must be used to secure stable steady-state operation. One of such techniques is automatic amplitude control (AAC). With the AAC circuit switched in operation, the generator output signal amplitude is controlled, due to which a larger self-excitation margin is obtained at the initial time instant, and the required amplitude value is maintained during steady-state operation. The article presents a version of such system taking as an example a single-loop self-oscillator with the Wien bridge in the feedback circuit. As is known, the list of important issues to be taken care of in designing AAC systems includes, among other things, an analysis of their stability and the selection of time constants for the system functional units. There are many methods for analyzing the stability of an AAC system, and all of them involve the need to set up differential equations of the system. A version of addressing this objective is presented to produce the system of differential equations describing the operation of a self-oscillator fitted with the AAC system. The obtained system of equations was applied for selecting — using the Routh–Hurwitz stability criterion — the time constant for the low-pass filter (LPF) of the AAC system amplitude detector (AD). It has been shown that the selection of the time constant for the AD LPF is constrained, on the one hand, by the requirements for the amplitude detector operation mode, and on the other hand, by the need to ensure fast response of the AAC system to variations of the generator voltage amplitude. The time histories of the system response to variations of the generator voltage amplitude and the system phase-plane portrait have been obtained.