A PROTON MAGNETOMETER WITH SOLID STATE SWITCHING

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  • A PROTON MAGNETOMETER WITH SOLID STATE SWITCHING

    BY M. R. HARKNETT Portsmouth College of Technology

    SWITCHING REQUIREMENTS The switching cycle for proton magnetometers is complicated by the fact

    that in the normal circuit design the coil tuning capacitor is disconnected during the polarising and current decay period (Aitken and Tite 1962). Such switching is most easily achieved by the use of two electromechanical relays. The dis- advantages of relays are, having moving parts they are not infallible; they produce large magnetic fields; and in this application the precession signal is not consistent for each current break, due to arcing across the contacts when the current is broken.

    A solid state switch which would achieve the same switching cycle is difficult to devise but the switching requirements are greatly simplified if the tuning capacitor is permanently connected across the coil and the coil in turn permanently connected to the amplifier input. The only switching requirements then are that the polarising voltage must be connected to the coil during the polarising period and the switching circuit must present a high impedance across the coil during the precession period.

    The requirement for the coil current during the polarising period is that its value must be such as to provide a field many times greater than the earths magnetic field. The earths field is equivalent to a magnetomotive force gradient of about 0.5 oersted (40 ampere turns per metre) and in practice a current of about 1 amp in several hundred turns of wire will provide the necessary field. Such a current can easily be switched by a small transistor.

    This simplified arrangement operates in practice without loss of signal strength despite possible expectations to the contrary. It is found, however, that if the coil tuning capacitor is altered too much from its value for reasonance the fall in signal strength is much greater than can be explained by the slope of the response curve, The loss is thought to be because detuning causes the phase of the decaying field to be different from that of the precession signal which therefore becomes attenuated. However, this does not make the tuning very critical due to the relatively low Q of the coil circuit.

    DESCRIPTION OF SWITCHING CIRCUIT A suitable switching circuit which has been designed by the author and

    used in a differential proton gradiometer is given in the upper part of figure 1. Two junction F.E.T.s are employed in a multivibrator circuit to provide automatic switching, with a half period of oscillation of approximately three seconds. F.E.T.s have a very high input resistance when properly biased (loo ohm) and enable relatively low value non-electrolytic capacitors to be used in the timing circuit. The bias resistance for the F.E.T.s needs to be small at switch-on and large when the circuit is oscillating; this constant current effect can be obtained using the transistor TR5 which also provides for a good multivibralor waveform.

    Transistor TR2 has to provide sufficient base current to bottom TR6 but the base to collector voltage available for TR2 would be below the threshold voltage

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  • ARCHAEOMETRY 175

    of a silicon transistor; for this reason a germanium transistor is used for TR2. TR2 is switched on and off by the output of the multivibrator. By using this common-collector arrangement for TR6 the off coil current is reduced to a small fraction of a microampere. When TR6 is switched off it acts as a forward biased diode for the large negative going coil voltage which obtains after the first quarter cycle of the ringing signal, and the stored coil energy is dissipated in the 1 kohm resistor. The magnitude of the precession signal is of course well below the threshold level of this diode. TR1 is also switched by the multivibrator and is used to operate a panel light to indicate circuit operation.

    Large voltage transients can occur on the supply line and may damage the amplifying circuits if the impedance of the power supply is not very low at the transient frequency. To prevent such damage a zener diode, ZD1, is connected across the supply lines. Further, if the power supply is inadvertently connected to the magnetometer with the wrong polarity the zener diode will conduct heavily and thus prevent circuit damage.

    SIGNAL AMPLIFICATION General considerations: The precession signal from the coils of a proton

    magnetometer is only a few microvolts and if an amplifier with no selectivity is used to magnify the signal then the noise power, being directly proportional to bandwidth, will be such as to cause a very small signal to noise ratio.

    If the output of the amplifier is fed to earphones a very low signal to noise ratio can be tolerated since the overall ear mechanism acts as a narrow band filter and gives a large subjective improvement in signal to noise ratio. A very simple, relatively low gain amplifier (40 dB) requiring no tuning can be used in conjunction with earphones in the bleeper mode (Aitken and Tite 1962).

    In order to obtain greater sensitivity, and a meter output indication, an envelope detector can be used. Such detectors do not operate efficiently at low signal levels and do not discriminate against noise. Further amplification with frequency selectivity is therefore necessary before detection. The narrower the bandwidth of the amplifier the greater the signal to noise ratio, but if the selectivity is too great tuning becomes extremely critical and it is increasingly difficult to design a stable amplifier. In an absolute reading magnetometer a large signal to noise ratio is desirable since, for accurate frequency measurement, the longest possible measuring time before the signal falls to noise level is required.

    The differential magnetometer does not require a very high signal to noise ratio as the presence of an anomaly is detected early in the precession interval. In fact, if it were required to increase the information rate the precession period could be shortened.

    PRE- AMPLIFICATION As stated previously, the precession signal from the coils is very small

    and this signal is normally carried by a long length of radio frequency coaxial cable to the amplifiers. Although the cable is screened, interfering signals such as harmonics of the supply mains, radiation from television receivers, radio signals and engine ignition can be picked up by the cable and mask the precession signal.

    Another source of very serious interference, especially when the coils are towed is that all normal cables suffer from electrostatic microphony and signals

  • 176 ARCHAEOMETRY

    of several microvolts will be produced in a cable even by tapping it with a pencil. It is possible to obtain special low-noise graphited coaxial cable which will largely overcome this effect but the other sources of interference remain and may be intolerable at a site.

    All interference may, however, be reduced to negligible proportions and small, screened, audio frequency cable can be used if pre-amplification is employed. The disadvantages of using a pre-amplifier are that no ferrous materials should be used in its construction and capacitor switching for coil tuning is mechanically difficult. However, the change in precession frequency between sites is only 0.04 Hzfgamma and a prefixed value of capacitance can be used.

    A COMPLETE MAGNETOMETER The complete circuit diagram for a gradient magnetometer is shown in figure 1.

    The circuit employs readily available normal gradc components, except for the bottle mils.

    Integrated circuits (1.c.~) are used as the basic amplifiers, the advantages being that they cost little more than transistors and when used with negative feedback they have a very stable gain and require a minimum of extra components. The I.C.s are intended to operate from k 12v supplies but it was found that the circuit will operate satisfactorily if a decoupled resistor is placed in the I.C. common line in order to provide a negative terminal and a single supply of + 12v provided.

    The d.c. supply to each I.C. is taken across a 1Ov Zener diode to provide a stabilised supply, decoupliiig and protection against transients. No capacitor greater than 1 pF is used thus avoiding the need for electrolytics.

    All the circuits are constructed on standard printed circuit board for neatness and robustness, the I.C.s being soldered in, rather than bases being used. Use of ceramic capacitors must be avoided since they are extremely microphonic.

    The switching transistor and pre-amplifier I.C. listed have iron cans but provided the pre-amplifier is placed centrally between the coils this does not preclude their use. For more critical applications it will be preferable to use non-ferrous active components in the pre-amplifier.

    The positive d.c. supply to the pre-amplifier is camed by the screen of the coaxial cable, the inner of which carries the switching waveform. A second cable carries the amplified signal on its inner and negative d.c. supply on its screen.

    The gain of each amplifier is 40 dB at 2 kHz falling gradually either side of this value. The primary tuned coupling transformer between I.C.2 and I.C.3 provides the necessary coarse selectivity.

    The d.c. for I.C.2 is derived from the lamp supply in order that the amplifier only operates during the precession time.

    COIL DETAILS Each coil is wound with 1000 turns of 20 S.W.G. enamelled wire which

    fills a winding length of 4.7 cm to a depth of 2 cm on a former slid over a 130 cc polythene bottle 4.7 cm in diameter. If the former is fabricated from glass fibre the coil can be enclosed and flanges fashioned on it for ease of mounting. Two such coils are connected in series and arrows painted on the

  • ARCHAEOMETRY 177

    coils indicating the direction of mounting which ensures cancellation of interfering signals. The inductance of each coil is 37 mH and resistance 5 ohm. Ordinary tap water is used to fill the bottles under normal conditions.

    OPERATION In use, the coils are placed with their axes horizontal and parallel several

    feet apart. In the absence of an anomaly the needle of the microameter sweeps across to full scale and falls back progressively towards zero during the precession time (lamp on). If an anomaly is present the needle will oscillate whilst returning towards zero.

    REFERENCES

    Aitken, M. J., 196 1, Physics and Archaeology, Interscicnce Publishers Ltd. Aitken, M. J. and Tite, M. S., 1962, A Gradient Magnetometer using Proton Free-Precession.

    Mudie, J. D., 1965, The Behaviour of the Dipole Moment of Protons in a Changing Magnetic

    Waters, G. S., 1955, A measurement of the Earth's Magnetic Field by Nuclear Induction,

    Waters, G. S. and Phillips G., 1956, A New Method of Measuring the Earth's Magnetic Field,

    Waters, G. S. and Francis, P. D., 1958, A Nuclear Magnetometer, 1. Sci. Instrum., 35, 88.

    3.Sc. Instrum.. 39.

    Field, Archaeo-Physika, Bonner Jahrbucker, 15 (Bohlau-Verlag, Cologne).

    Nature, 176, 691.

    Geography Prospecting, 1.

    M