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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 PCB material.
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
LATEST REVISION: 10 June 2002