Mark S. Conradi
Dept. of Phys., Washington Univ., St. Louis, MO 63130-4899
A marginal oscillator is an amplifier with a resonator (electrical, mechanical, or acoustical) incorporated into the positive feedback loop. When the loop gain barely exceeds one (barely oscillating), the oscillation level is extremely sensitive to small changes in the resonator Q. Compared to traditional bridges with frequency locking, marginal oscillators are simple and inexpensive. Being regenerative, they yield useful signal levels with remarkably few stages of gain. The operating frequency automatically follows the resonator frequency. Marginal oscillators have been used to detect magnetic resonance absorption in LC resonators and to detect small impurity particles in a blood-filled ultrasonic resonator. Applications in measurement science will be discussed.