An improved proton magnetometer for Earth's magnetic field observation
As a precision instrument to measure the earth magnetic field, proton magnetometer is widely used in different fields such as geological survey, buried objects detection and earth field variations. Due to poor signal to noise ratio (SNR) of the system, proton magnetometer suffers from low sensitivity which directly affects the performance. In order to increase the sensitivity, we present an improved proton magnetometer. First, the effect of matching resistance on Q value is discussed to enhance SNR, and high matching resistance has been chosen to improve the Q value of the resonant circuit. Second, noise induced by pre-amplifier is investigated in order to obtain low noise signal, and we adopt the JFET with noise figure less than 0.5dB as the pre-amplifier. Third, by using band-pass filter, low-noise output signal is obtained. Fourth, the method of period measurement based on CPLD is employed to measure frequency of the square wave shaped from the output sinusoidal signal. High precision temperature compensate crystal oscillator (TCXO) has been used to improve the frequency measurement accuracy. Last, experimental data is obtained through field measurements. By calculating the standard deviation, the sensitivity of the improved proton magnetometer is 0.15nT for Earth’s magnetic field observation. Experimental results show that the new magnetometer is sensitive to earth field measurement.
Proton Precession Magnetometers, Revision 3
When I was a student in physics in the 1950s, the usage of units was confused to say the least. Most sub-branches of physics seemed to adopt a set of units which were convenient for that eld (and they still do to some extent!) and, as they used to say in Rome, scabes extremum occupet, May the itch catch the hindmost . Physics was made even more di cult than it needed to be by the necessity to convert between cgs units, mks units, esu (electrostatic units) and emu (electromagnetic units) not to mention the continual frustration brought on by the additional occurrence of British units (lb-ft-sec). Indeed, looking back on it now, it seems to me that most of the problem sets assigned to us by the prof's were concerned with converting all the disparate units into one selfconsistent set appropriate to the problem! By the time I was teaching physics in the mid 1960s some sanity had come to the eld and most textbooks, at least, were being written with a consistent set of units, most frequently the SI (Systéme International) set of units. However the usage in magnetic units has retained some of the confusion that used to be widespread in all other topics. The situation is better (much better!) but still not perfect. Part of the problem is that there is still some controversy about which is the more fundamental property of magnetism is it the magnetic eld , commonly designated by the symbol, H, or is it the magnetic induction or magnetic ux density commonly designated by the symbol, B? The two are related, in any material by the equation: B = μH
Calculation of the Signal Voltage Induced in a Toroidal Proton-Precession Magnetometer Sensor
The signal EMF induced as a function of time in a proton-precession magnetometer sensor of toroidal shape with an elliptical cross-sectional area is calculated in terms of the magnetic properties of the sensor fluid, polarizing current, orientation of the sensor with respect to the direction of the ambient magnetic field, and geometric proportions of the sensor. It is assumed that the sensor is in a uniform ambient magnetic field; the polarizing current is reduced to zero in a time that is short compared to the period of the Larmor precession of the nuclear magnetic moments in the sensor fluid; the nuclear magnetic moments do not interact; and no significant current flows in the sensor winding during the relaxation process in which the EMF is measured. In the specific case that the sensor winding fills the toroid window and that the polarizing supply is a constant voltage source, the calculations permit a simple comparison of variously proportioned sensors in terms of the signal EMF amplitude.