BUILD A PROTON PRECESSION MAGNETOMETER
An educational "backyard" project, constructed using easily obtained electronic parts. A frequency counter is used to measure the post-polarizing pulse proton precession frequency. The measured frequency is related, by a physical constant, to the magnitude of the local geomagnetic field.
For some background information and a description of a practical application for a proton magnetometer, see "The Amateur Scientist "column in the February 1968 issue of Scientific American. Construction of a dual coil magnetometer is described. Information in that article formed a basis for the details shown here.
This is a block diagram of a "listen only" version. The frequency counting circuitry is not used. Only the senor coil(s), audio amplifier and dc power source are included. A timer IC is used to provide switching contol to a relay that alternately connects the sensing coil between a polarizing current source and the input to the audio amplifier.(Click figure for larger diagram.)
This is a block diagram of a magnetometer design that adds the capability to measure the frequency of the voltage induced in the sensor coil by the precessing protons after the application of a polarizing current several seconds in duration. A four decade BCD counter dis- plays frequency to a selectable resolution of 1 or 0.1 Hz. A frequency multiplier method employs a phase locked loop to provide these resolutions using counter gate intervals much less than one second.
SENSOR CONSTRUCTION
I found the local super market to be a good source for coils forms on which to wind the magnetometer coils and contain the proton medium. Check the area where the spices are located. Particularly look for the store brand spices. I found that these use thin walled plastic containers that have encircling ridges at the bottom and just below the lid. These make a form on which a multilayer coil can be easily wound. (CLICK FIGURE FOR DETAILS )
The above referenced page shows the particular size used. There are a number of sizes available. Also found some taller ones that would provide a coil length of about 3.75 inches. A somewhat larger container would conveniently allow the use of a larger wire size. There are advantages ---lower coil resistance, providing higher coil Q and possibly higher polarizing current (if the power supply can provide it ). A higher polarizing current increases the initial amplitude of the decay signal.
The higher coil Q will sustain the ringing effect of induced by the decay signal for a longer period of time.Note that the coil inductance increases as function of the square of the number of turns while coil resistance increases as linear function of the number of turns. This suggests that the best results (high Q and tuned circuit selectivity) will be obtained using the largest number of turns and largest wire size that is practical.Also, and possibly most important, the coils will be tuned by the addition of a shunt capacitor---perhaps the most important component of all.
The coil inductance should high enough to permit the use of a reasonably valued non-polarized capacitor. A higher Q will also aid in providing a narrower tuned circuit bandwidth--important in improving the signal to noise ratio and reducing the pickup of high order power line harmonics.
AUDIO AMPLIFIER
The audio amplifier uses four bipolar transistors and one dual operational amplifier integrated circuit. The block diagram at the left shows the stage gain distribution. The operational amplifier provides a two stage active bandpass filter centered at the expected frequency of the proton precession. Maximum available gain is in excess of 130 dB. The theoretical gain vs. frequency is shown in the figure at the right. With such high gain careful construction is required to prevent oscillation
The figure at the left briefly outlines physical details. The amplifier was built on double sided copper clad PCBmaterial. Components are soldered to standoff terminals. A push-in type nylon or teflon terminal is used. Vectorboard is difficult to use for a circuit made up entirely of discrete components. The circuit board is housed in a Radio Shack molded project case. The inside of the case is lined with adhesive backed aluminum tape.
The input stage uses a 100 ohm unbypassed emitter resistor to raise the input impedance to about 12 kilohms to reduce loading on the tuned sensor coils. The tuned circuit formed by the coils and resonating capacitor present a parallel impedance of about 3000 ohms. A number of different devices were randomly selected and tried at the input stage in order to find one providing the best signal to noise ratio. The noise contribution from a 560 ohm resistor soldered across the input terminal can be detected. However, noise from the sensor coils and external pickup exceed the intrinsic amplifier noise contribution.
The following page links to the schematic of a counter implemenation that measures the precession frequency. It was intended as a educational project to attempt to provide a measurement of the magnitude of the local geomagnetic field. It is offered for informational purposes only. Others may find it of interest or may adapt it to a specific practical application. One of my objectives was economy, to use parts that were on hand or easily obtained standard components. For operation from a battery source lower power dissipation equivalent CMOS logic elements can be substituted for the TTL elements shown.
GO TO COUNTER CIRCUIT CONSTRUCTION
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Construction Notes
Rev 10 Feb 1999
