Abstract: The drive signal applied to the drive winding of a magnetometer is provided by a voltage controlled oscillator. The average current flowing through the drive winding is monitored, and a signal representative of the average current is applied to the input of the voltage control oscillator. The sense windings of the magnetometer are connected to a servo-loop which causes a current to flow in the sense windings which produces a magnetic field cancelling the sensed components of the earth's magnetic field. The magnetometer uses a drive signal which saturates the core for only a short duration of time thereby reducing the power required by the magnetometer while ensuring complete saturation of the core.

Abstract: A circuit for measuring the relative phase of two periodic signals. Two analog waveforms are applied to limiting amplifiers which provide digital output signals. The output signals therefrom are applied to digital circuitry which provides an analog voltage representative of the phase difference between the two input signals over a 360 degree range. The digital circuitry compensates for any offset errors which may be introduced by the limiting amplifiers.

Abstract: An acoustic current meter in which two channels have acoustic paths oriented at right angles to each other to measure orthogonal components of a current velocity. Each channel includes two transducers aimed at an acoustic mirror so that the reflected acoustic signal from each transducer impinges upon the opposite transducer. Periodically, a burst of high frequency acoustic energy is simultaneous emitted by each transducer. A current flowing produces a relative phase shift between the received signals representative of the current velocity. The output signal produced by each of the transducers is mixed with a common reference signal to provide two, low frequency, beat frequency signals having a phase difference proportional to the current velocity. Due to the reduction in frequency, the phase shift is expanded in time with respect to the phase shift of the acoustic signals and may be easily measured.

Abstract: An active filter which can achieve extremely high attenuation of frequencies outside the passband while maintaining a very low phase shift of signals within the passband. A preferred embodiment is described in which two, second-order, multiple-feedback, active filter circuits are cascaded and the output of the second filter is fedback to the input of the first filter to realize a fourth-order filter which has very high attenuation of signals falling outside the passband while providing very low phase shifts of signals within the passband.