A resonator based on a segment of a transmission line is used as an intermediate circuit in a squid, with lumped reactive components at the ends, which enables one to raise the pumping frequency. Formulas are given that enable one to forecast and optimize the fluxmeter parameters, and results are presented on a device having an uncooled preamplifier at frequencies close to 40, 75, and 1000 MHz. An energy resolution of $1,6\cdot 10^{-28}$ J/Hz has been recorded at the upper pumping frequency.
Department of Physics of Oscillations, Faculty of Physics, Moscow State University, Leninskie Gory, Moscow, 119992, Russia
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