a dc superconductor quantum interference device is used in a flux - locked loop as a sensitive detector of magnetic flux . prior art devices of this sort had a transfer function which was frequency - limited by the transfer function of impedance matching circuitry which is used to connect the dc squid with the first preamplifier , which amplifies the dc squid signal before it is applied to a detector circuit . the present invention corrects this frequency limitation by creating a compensating circuit having a transfer function which is the inverse of that of the impedance matching circuitry , and inserting it in the system after the first preamplifier and before the detector circuit .