An impedance measuring device based on a cryogenic high quality factor (approx = 10~6) electrical LC resonator coupled to a dc SQUID is presented. The device was devised especially to study around 1 kHz the input impedance of a dc SQUID as well as that of a FET amplifier operating at room temeprature. The real and imaginary part of the input impedance is derived respectively from the changes of the resonator quality factor and resonance frequency. Equivalent losses added in series to the LC circuit down to 10~(-5) #OMEGA# and minimum relative changes in the reactive elements C and L of the order of 10~(-8) can be measured.
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