The United States of America as Represented by the Administrator of the

Other

United States of America as Represented by the Administrator of Nasa

John F Sutton

Mark J Hagmann

Qamar A Shams

A magnetometer configured to measure low field strength magnetic fields is provided. Certain embodiments of the magnetometer include a cylindrical coil assembly having a variable permeability core and terminals disposed at both ends. A current source circuit may be operably connected to the terminals and configured to apply a voltage controlled current across the terminals. A voltage readout circuit may be operably connected to the terminals and configured to measure a voltage across the terminals due to the applied current from the current source. An inductance of the coil assembly directly varies as an ambient magnetic field strength varies a permeability of the variable permeability core, and a voltage across the terminals varies directly with the inductance such that the measured voltage across the terminals is a direct measure of the ambient magnetic field strength.

Variable permeability magnetometer systems and methods for aerospace applications

An apparatus for measuring high frequency currents includes a non-ferrous core current probe that is coupled to a wide-band transimpedance amplifier. The current probe has a secondary winding with a winding resistance that is substantially smaller than the reactance of the winding. The sensitivity of the current probe is substantially flat over a wide band of frequencies. The apparatus is particularly useful for measuring exposure of humans to radio frequency currents.

Apparatus for measuring high frequency currents

An antenna, which may be a search coil, is connected to an operational amplifier circuit which provides negative impedances, each of which is in the order of magnitude of the positive impedances which characterize the antenna. The antenna is connected to the inverting input of the operational amplifier, a resistor is connected between the inverting input and the output of the operational amplifier, a capacitor-resistor network, in parallel, is connected between the output and the noninverting input of the operational amplifier, and a resistor is connected from the noninverting input and the circuit common. While this circuit provides a negative resistance and a negative inductance, in series, which appear, looking into the noninverting input of the operational amplifier, in parallel with the antenna, these negative impedances appear in a series loop with the antenna positive impedances, so as to algebraically add. This circuit is tuned by varying the various circuit components so that the negative impedances are very close, but somewhat less, in magnitude, to the antenna impedances. The result is to increase the sensitivity of the antenna by lowering its effective impedance. This, in turn, increases the effective area of the antenna, which may be broadband.

Active antenna

An antenna, which may be a search coil, connected to an active circuit which provides negative impedances, each of which is of the order of magnitude of the positive impedances which characterize this active antenna. In one embodiment, one coil terminal is connected to an amplifier which drives a voltage-controlled current source that, in turn, drives a feedback coil which is coupled to the original search coil. In another embodiment that additionally exhibits an advantageous signal-to-noise characteristic, both terminals of the search coil are connected to a differential amplifier that, in turn, provides the control voltage for a current source, which, as in the first embodiment, drives the feedback winding. The feedback coil is wound to provide positive feedback by additive superposition of both coil fields. The positive feedback provided by the feedback current lowers the antenna impedance which, in turn, increases the effective area of the antenna. This circuit configuration incorporates a differentiation inherent in the fundamental characteristic of a coil, which is sensitive to the rate-of-change of the magnetic field. The outstanding stability of this active antenna may be attributed to the inherent accuracy of this differentiation performed by the antenna coil, to the particular circuit configurations and to the particular form of feedback employed.

A differential, voltage-controlled current source, employing operational amplifiers as the active elements, provides an essentially symmetrical, differential, high impedance drive to a load, the drive being isolated from any circuit common or system ground. Because of the 'floating' differential drive and the identical source impedances of the two outputs, errors from common mode voltages are eliminated.

Differential current source

A synchronous demodulator includes a switch which is operated in synchronism with an incoming periodic signal and both divides and applies that signal to two signal channels. The two channels each include a network for computing and holding, for a predetermined length of time, the average signal value on that channel and applies those values, in the form of two other signals, to the inputs of a diffferential amplifier. The networks may be R-C networks. The output of the differential amplifier may or may not form the output of the synchronous detector and may or may not be filtered. The output will not include a periodic signal due to the presence of a dc offset. Additionally, the output will not contain any substantial ripple due to periodic components in the input signal. In a somewhat more complex version, containing twice the structural components of the above synchronous demodulator with a more complex switching mechanism, essentially all ripple due to periodic components in the input signal are eliminated.

Synchronous demodulator

A cathode ray oscilloscope deflection system selectively displays an integral number of cycles of a periodic or quasi-periodic input signal, regardless of the frequency of the signal. A wide band phase locked loop, synchronized to the input signal, generates pulses at a pulse repetition rate that is an integral multiple of the frequency of the input signal. A ripple counter converts the pulses into a staircase, x-axis deflection signal having a period equal to the duration of the number of cycles of the input signal selected for display and a predetermined maximum amplitude so that the x-axis of the scope is always swept the same distance regardless of the frequency of the input signal. The number of cycles displayed is determined by the repetition rate of the phase locked loop pulses relative to the frequency of the input signal. The repetition rate of the loop pulses is controlled by: (a) a first binary divider for controlling which of the fundamental or integral submultiple frequency of the input signal to which the phase locked loop is synchronized, and/or (b) a second binary divider circuit for controlling the pulse repetition rate of feedback pulses in the phase locked loop. Frequency or period of the input signal is displayed by a digital frequency or period meter that monitors the loop pulses.

Growing community of inventors

Greenbelt, MD, United States of America

John F Sutton