Patent GB-958093-A (GB)
Title: Atomic frequency standard
IPC: H03L7/26
Filed: 1962-06-12
Abstract: 958,093. Automatic frequency control systems. VARIAN ASSOCIATES. June 12, 1962, No. 22559/62. Heading H3A. An oscillator output is alternately phase shifted by a bistable circuit operated by a pulse generator and applied to a cavity resonator to vary a magnetic field transverse to it and the passage of a light beam through a light absorption gas cell in the resonator, and the detected light intensity variations then compared in phase sense with the bi-stable modulator to derive an output controlling the oscillator frequency. The oscillator frequency may be a fractional sub-harmonic or harmonic of the resonator frequency and frequency multiplied and/or divided before applying to the resonator. The phase shifter may comprise a parallel resonant circuit and a series resonant circuit, both in parallel and energized from opposite halves of a bi-filar wound transformer connected to the oscillator and both having voltage variable capacitors so that a phase shift without amplitude change is produced when the bi-stable square wave output is applied to the capacitors. The cylindrical resonator comprises a narrow width central portion 29 for producing uniformity of the microwave axial magnetic field over the centre of the resonator. A coil 17 energized by a D.C. supply 18 also produces a constant weak axial magnetic field within a cylindrical magnetic shield 13. A longitudinal loop coil 31 on the outside of the gas cell and coupled to a Zeeman frequency oscillator corresponding to an atomic frequency charge #F = 0, #m =  1 produces an alternating transverse magnetic field in the gas cell favouring a change in atom population of the magnetic sub-levels of the atoms and this is used, by observing its effect on the light beam (hyper. fine resonance), for adjusting finely the steady axial magnetic field from the coil 17. The collimated light beam is produced by a Ru<SP>87</SP> lamp passing light through a filter containing R<SP>86</SP> vapour and a buffer gas (argon) to produce higher atomic energy difference light favouring the transition from S = ¢, F = 1 rather than from S = ¢, F = 2 to the higher energy level P¢ or P<SP>3</SP>/ 2  and this optical pumping increases the atom population of the S¢, F = 2 rather than the S¢, F = 1 level, since F = 2 has more magnetic sub-levels (m = 0,  1,  2) than F = 1. The microwave frequency applied to the resonator induces the transition from F = 2 to F = 1, and since the latter level is more light absorbing than the F = 2 level, the light intensity decreases atmicrowave resonance and is detected by the photo-cell 28. Light scattered by the pumping is absorbed by black paint on the inner side of the resonator. D.C. supplies 24 and 18 and heater coils respectively maintain the light source with filter, and the gas cell insulated by a vacuum cell 26, at different optimum temperatures. Other alkali metal vapours besides rubidium may be used. Maximum signal-to-noise ratio is obtained with a phase shift of 90 degrees to 165 degrees, preferably 157 degrees, and with a modulation frequency between 0.8 #f and 1.2 #f, preferably 1.0177 #f, where #f is the half-power bandwidth of the resonator. Specification 875,242 is referred to.