Build a Proton Precession Magnetometer

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<ul><li><p>Build A Proton Precession Magnetometer Page 1 of 14 </p><p>BUILD A PROTON PRECESSION </p><p>MAGNETOMETER </p><p>An educational "backyard" project, constructed using easily obtained electronic parts. A </p><p>frequency counter is used to measure the post-polarizing pulse proton precession frequency. The </p><p>measured frequency is related, by a physical constant, to the magnitude of the local geomagnetic </p><p>field. </p><p>Background </p><p>For some background information and a description of a practical application for a proton </p><p>magnetometer, see "The Amateur Scientist "column in the February 1968 issue of Scientific </p><p>American. Construction of a dual coil magnetometer is described. Information in that article </p><p>formed a basis for the details shown here. </p><p>I constructed a fluxgate magnetometer several years ago. It was based upon Richard Noble's </p><p>article in the September 1991 issue of Electronics World + Wireless World. With a chart </p><p>recorder, it is possible to see the dirunal changes in the east-west component of the earth's </p><p>magnetic field, after nulling out the overpowering total and north-south components. </p><p>After finding the February 1968 Scientific American article, I thought that it would be an </p><p>interesting project to try adding a frequency counter to the proton magnetometer.It would be an </p><p>interesting "backyard science" project to use it to provide a measure of the earth's total magnetic </p><p>field. The addition of a digital to analog converter can provide a output suitable for a chart </p><p>recorder. </p><p>However, a suburban backyard environment is a rather noisy one. Harmonics of the power line </p><p>frequency extend well up into the audio frequency range. These compete with the decaying </p><p>precession frequency tone. Connecting the sensor coils in differential series, sensor orientation </p><p>and instantaneous sampling of the audio signal help in contending with the noise. </p><p>From the physical sciences a quantity called the"Larmor frequency" defines the angular </p><p>momentum of protons precessing in the presence of a magnetic field. </p><p>There are currently quantum-mechanical views that explain particle precession, but a classical </p><p>explanation seems a bit easier to comprehend. A proton, a charged particle, may be thought of as </p><p>having definite "spin" about an "axis" and acts as a small magnet. An externally applied </p></li><li><p>Build A Proton Precession Magnetometer Page 2 of 14 </p><p>magnetic field does not alter the spin rate, but causes the particle to wobble at a slower rate about </p><p>an axis of precession. This axis tends to align with an external magnetic field. However in weak </p><p>magnetic fields, any alignment tends toward randomness due to thermal effects and other </p><p>molecular interactions. </p><p>The proton reacts to the perturbing effects of an externally applied magnetic force by precessing </p><p>at a rate in accordance with a precise constant called the gyromagnetic ratio. For protons this </p><p>quantity is equal to approximately 267.53 x 1E6 radians per second per Tesla or 42.58 mHz per </p><p>Tesla. </p><p>In the northern latitudes of the U.S. the total magnetic field strength is in the order of 50,000 to </p><p>55,000 nanoTesla and varies from location to location. Short period variations due to magnetic </p><p>storms may reach several hundred nanoTesla. Diurnal variations caused by solar induced </p><p>ionospheric currents are in the order of tens of nanoTesla. Presently, the long term trend of the </p><p>total field is in the order of minus 90 nanoTesla per year ( steadily decreasing). </p><p>The proton precession frequency detected by a suitable sensor in the geomagnetic field of the </p><p>earth will be at a frequency in the audio range: </p><p> Example: </p><p> 42.58 mHz / Tesla x 52500 x 1E-9 Tesla= 2235 Hz </p><p>In my northeast location the frequency readings currently average about 2271 Hz, corresponding </p><p>to a total field of about 53,300 nanoTesla. This agrees quite well with the USGS readings shown </p><p>for the Fredericksburg, VA monitoring station , 160 miles to the west. This figure also agrees </p><p>with the value obtained using the fluxgate magnetometer that was calibrated using a Helmholtz </p><p>coil. The fluxgate sensor was tipped upward from a horizontal position to nearly vertical to </p><p>obtain the maximum reading of the earth field. </p><p>I have noticed a decrease in the frequency readings of about six or seven Hertz over the past </p><p>twelve months or so since the sensors have been in place in the backyard. Originally the </p><p>frequency readings were around 2277 or 2278 Hz. This also seems to agree with the magnitude </p><p>of the predicted long term variation shown by the USGS site. </p><p>Return to main page </p></li><li><p>Build A Proton Precession Magnetometer Page 3 of 14 </p><p>PROTON PRECESSION MAGNETOMETER </p><p>This is a block diagram of a "listen only" version. The frequency counting circuitry is not used. </p><p>Only the senor coil(s) ,audio amplifier and dc power source are included. A timer IC is used to </p><p>provide switching contol to a relay that alternately connects the sensing coil between a polarizing </p><p>current source and the input to the audio amplifier.(Click figure for larger diagram.) </p></li><li><p>Build A Proton Precession Magnetometer Page 4 of 14 </p><p>This is a block diagram of a magnetometer design that adds the capability to measure the </p><p>frequency of the voltage induced in the sensor coil by the precessing protons after the application </p><p>of a polarizing current several seconds in duration. A four decade BCD counter dis- plays </p><p>frequency to a selectable resolution of 1 or 0.1 Hz. A frequency multiplier method employs a </p><p>phase locked loop to provide these resolutions using counter gate intervals much less than one </p><p>second. </p></li><li><p>Build A Proton Precession Magnetometer Page 5 of 14 </p><p>SENSOR CONSTRUCTION </p><p>I found the local super market to be a good source for coils forms on which to wind the </p><p>magnetometer coils and contain the proton medium. Check the area where the spices are located. </p><p>Particularly look for the store brand spices. I found that these use thin walled plastic containers </p><p>that have encircling ridges at the bottom and just below the lid. These make a form on which a </p><p>multilayer coil can be easily wound. (CLICK FIGURE FOR DETAILS ) </p><p>The above referenced page shows the particular size used. There are a number of sizes available. </p><p>Also found some taller ones that would provide a coil length of about 3.75 inches. A somewhat </p><p>larger container would conveniently allow the use of a larger wire size. There are advantages ---</p><p>lower coil resistance, providing higher coil Q and possibly higher polarizing current (if the </p><p>power supply can provide it ). A higher polarizing current increases the initial amplitude of the </p><p>decay signal. </p><p>The higher coil Q will sustain the ringing effect of induced by the decay signal for a longer </p><p>period of time.Note that the coil inductance increases as function of the square of the number of </p><p>turns while coil resistance increases as linear function of the number of turns. This suggests that </p><p>the best results (high Q and tuned circuit selectivity) will be obtained using the largest number of </p><p>turns and largest wire size that is practical.Also, and possibly most important, the coils will be </p><p>tuned by the addition of a shunt capacitor---perhaps the most important component of all. </p><p>The coil inductance should high enough to permit the use of a reasonably valued non-polarized </p><p>capacitor. A higher Q will also aid in providing a narrower tuned circuit bandwidth--important in </p><p>improving the signal to noise ratio and reducing the pickup of high order power line harmonics. </p></li><li><p>Build A Proton Precession Magnetometer Page 6 of 14 </p><p>Notes on Sensor Construction </p><p>1.It may be possible to place the 700 turns in four layers. However, as subsequent </p><p>layers are added it becomes more difficult to maintain close spacing. Most likely it </p><p>will take five layers. Actual turns count is not critical. If you have 700 turns before </p><p>reaching the end of the bottle, continue winding to complete the final layer. </p><p>2. Coil constructed as shown will provide an inductance of about 10 millihenries. An </p><p>approximate formula (neglects a small multilayer correction factor of about negative 5 </p><p>percent) for calculating the inductance is: </p><p> L=(r2n2)/(10(r+l)) </p><p>where: r=one half the bottle diameter in inches n= number of turns l= coil length </p><p>(inches) </p><p>3. A coil tuning capacitor for two sensor bottles as shown, connected in series, will be </p><p>about 0.25 microfarads. </p><p>4. After winding, fill the bottle with a "proton rich" fluid. Distilled water, kerosene, </p><p>methanol have been used. Common isopropyl alcohol will work. </p><p>5. Spice bottles are not designed to hold liquids. The lids may have a paper inner liner </p><p>that should be discarded. If needed to stop leaking, try making a gasket from bicycle </p><p>inner tube or similar material. </p><p>In my backyard environment, for the best signal to noise ratio, I found that two </p><p>identical coils were useful. These were connected in series and oriented for </p><p>minimizing the level of power line harmonics. An orientation with the coil axes in line </p><p>and electrically series opposing provided a degree of cancellation of common-mode </p><p>power line noise pick up. </p></li><li><p>Build A Proton Precession Magnetometer Page 7 of 14 </p><p>AUDIO AMPLIFIER </p><p>The audio amplifier uses four bipolar transistors and one dual operational amplifier integrated </p><p>circuit. The block diagram at the left shows the stage gain distribution. The operational amplifier </p><p>provides a two stage active bandpass filter centered at the expected frequency of the proton </p><p>precession. Maximum available gain is in excess of 130 dB. </p></li><li><p>Build A Proton Precession Magnetometer Page 8 of 14 </p><p>The theoretical gain vs. frequency is shown in the figure below. </p><p>With such high gain careful construction is required to prevent oscillation </p><p>The figure at the left briefly outlines physical details. The amplifier was built on double sided </p><p>copper clad PCB material. Components are soldered to standoff terminals. A push-in type nylon </p><p>or teflon terminal is used. Vectorboard is difficult to use for a circuit made up entirely of discrete </p><p>components. The circuit board is housed in a Radio Shack molded project case. The inside of the </p><p>case is lined with adhesive backed aluminum tape. </p></li><li><p>Build A Proton Precession Magnetometer Page 9 of 14 </p><p>The input stage uses a 100 ohm unbypassed emitter resistor to raise the input impedance to about </p><p>12 kilohms to reduce loading on the tuned sensor coils. The tuned circuit formed by the coils and </p><p>resonating capacitor present a parallel impedance of about 3000 ohms. A number of different </p><p>devices were randomly selected and tried at the input stage in order to find one providing the </p><p>best signal to noise ratio. The noise contribution from a 560 ohm resistor soldered across the </p><p>input terminal can be detected. However, noise from the sensor coils and external pickup exceed </p><p>the intrinsic amplifier noise contribution. </p></li><li><p>Build A Proton Precession Magnetometer Page 10 of 14 </p><p>The following page links to the schematic of a counter implemenation that measures the </p><p>precession frequency. It was intended as a educational project to attempt to provide a </p><p>measurement of the magnitude of the local geomagnetic field. It is offered for informational </p><p>purposes only. Others may find it of interest or may adapt it to a specific practical application. </p><p>One of my objectives was economy, to use parts that were on hand or easily obtained standard </p><p>components. For operation from a battery source lower power dissipation equivalent CMOS </p><p>logic elements can be substituted for the TTL elements shown. </p><p>Counter Circuit Description </p><p>The circuit shown requires twelve integrated circuits in addition to other discrete components. </p><p>Integrated circuit choice was based on economy--- that is, using parts that were on hand. There </p><p>are many alternate ICs that may be substituted for the NAND gates, counters and multivibrator. </p><p>The 4060 counter /oscillator and 4046 Phase Locked Loop IC are probably good choices in any </p><p>event, but there are other possibilities there also. If power is to be obtained from batteries, </p><p>substitution of equivalent CMOS logic ICs in place of TTL types will reduce dc current </p><p>requirements. </p></li><li><p>Build A Proton Precession Magnetometer Page 11 of 14 </p><p>(There is another circuit shown in a separate segment that is a simpler LISTEN ONLY version. It </p><p>eliminates the frequency counter and uses a timer to cycle the polarizing current to the sensor </p><p>coils on and off.) </p><p>Timing for polarizing the sensors and measuring frequency is derived from a watch crystal. </p><p>These are the tiny cylindrical units found in some digital wrist watches. They sell for about two </p><p>for a dollar at Active Electronics or a dollar each at Radio Shack. </p><p>The oscillator circuit is pretty much per CD4060/MC14060 application note. The oscillator </p><p>portion produces an output frequency of 32.768 kHz that is applied to a fourteen stage counter. </p><p>The final output of the last stage is 2 Hz or a pulse repetition rate of 0.5 seconds. This drives a 4 </p><p>stage binary counter whose last stage provides a four second high / four second low logic level. </p><p>For simplicity, the full count cycle of the 4 stage binary counter is used. If the intent is to use the </p><p>magnetometer in a portable search mode, it would probably be useful to shorten the four second </p><p>(listen) non polarizing interval to a half second. This will require the addition of at least one four </p><p>input NAND gate to decode the counter state (10 count ) and reset the counter. </p><p>Polarizing current should be applied to the sensing coils for several seconds in order to maximize </p><p>the amplitude of the precession signal. Three seconds appears to be sufficient. After removal of </p><p>the polarizing current the the relay connects the coil(s) to the input of an audio amplifier. The </p><p>output of the audio amplifier is a ringing tone at the precession frequency, whose amplitude </p><p>rapidly decreases into the background noise level. In order to obtain an accurate measurement of </p><p>the frequency, the counter should begin sampling immediately after the removal of polarizing </p><p>current. Also counting should only be done when the signal amplitude is well above the noise </p><p>level. </p><p>Measuring the audio amplifier output directly would require a 1 second counting interval to </p><p>resolve to 1 Hz at the expected relaxation frequency, and 10 seconds to resolve frequency to 0.1 </p><p>Hz. Certainly, in the last case, the signal would have long decayed below amplifier noise or local </p><p>power line harmonics. And, in a backyard environment, after one second, the signal is competing </p><p>with ac power line harmonics. </p><p>A phase locked loop is used to permit measuring the precession frequency to 1 an 0.1 Hz </p><p>resolutions using counting intervals much less th...</p></li></ul>


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