Analysis of first-order feedback loop with lock-in amplifier

A dc-SQUID (Superconducting QUantum Interference Device) is usually operated in a first order feedback loop with a lock-in amplifier and this configuration is called "lock-in flux locked loop". The system response of this configuration was studied. The effect of the phase difference between the modulator and demodulator of the lock-in amplifier was quantitatively examined. The system is stable as long as the phase difference does not exceed /spl pi//2. The system response is given as a function of the phase difference. Transfer gains from two input sites to the output were calculated. Analyzes were verified to be correct by experiment.