Design Methodology and Experimental Validation of a Digitally Assisted AGC System for Amplitude Stabilization in LC and RC Sinusoidal Oscillators

Stable, programmable amplitude control in self-sustained sinusoidal oscillators is complicated by the nonlinear relationship between loop gain and steady-state amplitude and by variations in circuit parameters and operating conditions. This paper presents a topology-adaptable methodology for digitally assisted automatic gain control in analog LC and RC oscillators while preserving their physical frequency-selective networks. The architecture combines topology-specific coarse and fine adjustment of an analog parameter with common measurement, calibration, temperature compensation, and embedded control functions. A user-assisted procedure characterizes the complete peak-detector path at four reference amplitudes using a second-order least-squares polynomial calculated autonomously by the microcontroller. Amplitude-dependent temperature compensation is derived from measurements at three reference levels. The methodology is demonstrated using an operational-amplifier Wien-bridge oscillator and a BJT-based Colpitts oscillator with an analog inner control loop. Measurements from 20 °C to 50 °C show relative control errors below 3% at all investigated points within the respective base-calibration intervals. Larger errors occur at measured points below the first calibration levels, where polynomial extrapolation is required. Startup and setpoint-change measurements further characterize the distinct dynamics of the two control strategies. The results confirm that the common framework is applicable to oscillators with fundamentally different amplitude-establishment mechanisms.