Autodyne detector for acoustic NMR

A high-sensitivity circuit for investigating direct acoustic nuclear magnetic resonance is described. The circuit contains an autodyne, an acoustic resonator, a calibrator, and a rectifier. To increase the generation stability, the circuit uses parametric negative feedback from the rectifier output to the autodyne amplifier. The autodyne uses a differential amplifier assembled on field transistors and acting as an amplifier and summator simultaneously. Besides, both halves of the amplifier have a capacitive current coupling, which reduces the noise level and increases the stability of circuit operation. The circuit described was used to determine the coefficient of resonance absorption of ultrasound by the nuclear-spin system of 75As. During the measurements the power introduced into the specimen was equal to 1 μw. The circuit can readily be adapted for investigating the ordinary NMR