SQUID Sensor Emulator for Magnetocardiography System

Magnetocardiography (MCG) signal is measured with the use of a superconducting quantum interference device (SQUID) with a flux-locked loop (FLL). The FLL circuit is a feedback circuit connected to the SQUID sensor, which cancels the input magnetic field with a generated field by the feedback coil. A reduction of actual magnetic field experienced by the SQUID sensor makes the SQUID an ideal flux-to-voltage converter. In this paper, an emulator for the SQUID sensor and the feedback coil has been proposed. Instead of the magnetic coupling between the original field and the generated field by the feedback coil, an electronic circuit was used to add an emulated signal with a function generator and the output signal of the FLL. By using the emulator, electrical characteristics of FLL circuits such as bandwidth, slew rate, feedback range, and time delay can be measured and optimized without use of SQUID sensors. This is useful to save time and to reduce costs and efforts in an early stage of development of the MCG system when a magnetically shielded room or real SQUID sensors operated in a dewar filled with liquid helium may not yet be available. The emulator may also be used as a multi-channel gain calibrator for system maintenance in a later use.