GB1009858A - Improvements in electronic filters making use of quantum transitions and in devices including such a filter, and in particular oscillators and magnetometers - Google Patents

Abstract

1, 009, 858. Magnetometer apparatus. COMMISSARIAT A L'ENERGIE ATOMIQUE. June 13,1962 [June 14, 1961], No. 22758/62. Heading G1N. [Also in Division H3] A band-pass filter for use in a magnetometer comprises a sample of material containing subatomic particles (nuclei, electrons &c.) having magnetic (i.e. spin) energy sub-levels separated by intervals corresponding to frequencies within a range comprised between audio and U.H.F. In particular, the invention makes use of a system of nuclear spins (i.e. nuclei having a gyromagnetic ratio γ = M/R where M is a non-iron magnetic moment and R a non-zero angular momentum) placed in a uniform field H and capable of energy exchange with electromagnetic radiation of frequency f = y H/2#. The linear relation between the Larmor frequence f and the field H holds up to values of H corresponding to saturation. Protons, or hydrogen nuclei, in the earths magnetic field (~ 500 millioersted) have a Larmor frequency of about 2 kHz, the width of the resonance line being proportional to 1/T 2 where T 2 is the transverse nuclear spin relaxation time e.g. (expressed in terms of H in millioersteds) 0.002 for benzene, 0.017 for pure water and 0.040 for a M/1000 aqueous solution of potassium nitrosodisulphonate. The band-pass circuit consists of two identical coils 2a, 2b connected in parallel and in opposition to input terminals 1a, 1b and coupled respectively to two identical tuned circuits 3a, 34a and 3b, 34b connected in series and in opposition between output terminals 5a, 5b. If the circuit is balanced correctly there is zero transmission between input and output terminals over a wide frequency band centred around the resonant frequency of the tuned circuits. This is made equal to the Larmor frequency f of a sample 26 (e.g. M/1000 potassium nitrosodisulphonate) placed within the coil 3a. At this frequency there is coupling between the coil and the sample and the symmetry of the quadipole is destroyed whereby a pass-band centred around f is created. In order to increase coupling between the coil and the sample, the latter is subjected by means of an auxiliary coil 28 to a saturation high-frequency field at the paramagnetic resonance frequency (~ 55 MHg). This produces polarization of the nuclear spins. A Q of about 6,000 is obtainable. When used as a filter the centre frequency of the pass-band is varied by means of the adjustable electromagnet producing a field of hundreds or thousands of oersteds within a sample of e.g. benzene. In this case, the highfrequency field is not used. The variable electromagnet is coupled to the capacitors 34a,34b whereby the frequency of the tuned circuits may be kept equal to the Larmor frequency. The magnetometer circuit of Fig. 3 uses the coil arrangement of Fig. 1 with the addition of a sample 26b of pure water placed within the coil 3b to obtain a better balance. The sample 26 consists of an aqueous solution of potassium nitrosodisulphonate and is, in this instance, immersed in the earth's field. Variable tuning of the coils is achieved by banks of capacitors 4a, 4b. The output from the coils is fed to the grids 8g,8h of a pair of cathode followers, whence, by means of the variable potentiometers 18a,18b a difference voltage is derived. This voltage is fed to the grid of a pentode 11 through a transformer 9,10 having a nickel-iron high-Á core. After further amplification, a position of the output signal is fed back to the coils 2a, 2b via a variable potentiometer 13, whereby sustained oscillations at the Larmor frequency are obtained. This is measured by a known measuring circuit 50. An AGC signal is fed via amplifier 14 to a pair of rectifiers 15p, 15n and the negative part of the signal applied as a negative bias to the grid of pentode 11. The amplitude of the oscillations may be controlled by the potentiometer 17 in the AGC circuit. An initial balance of the coils is obtained by injecting a low frequency voltage between points 20 and 37 with the AGC switched off (switch 21 open). Then, in the absence of magnetic resonance (generator 7 disconnected), the potentiometer 13 is adjusted to give a minimum voltage across the secondary 10 of the transformer.