<span lang="EN-US">The paper presents a new lock-in amplifier with a differential input. The lock-in amplifier has a high common-mode rejection ratio of 200 dB at 1 kHz and 160 dB in the frequency range of 20 Hz to 100 kHz. A method for extending the dynamic range of comparing signals is suggested. The experimental results of metrological characteristic tests are discussed.</span>

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Abstract.A changeable source of a uniform magnetic field is necessary for test and calibration of magnetometers. Often the Helmholtz coils is used for generating magnetic uniform field, but uniform field area generating by Helmholtz coils is confined to a small volume in the centre of the coils. The calculation and modelling results of an axial 8-coil system based on Helmholtz coils is described, which enables a magnetic field with a no uniformity of not more than 0.07 % to be produced at a distance of half the radius from the centre of the coil system.