Effects of modulation schemes on the performance of directly coupled high-T/sub c/ dc SQUID magnetometers

We have designed and fabricated the directly coupled YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// dc SQUID magnetometers with two different modulation schemes to study the crosstalk effect in the on-chip multi-channel magnetometers. In one of the modulation schemes, the modulation current was directly injected into the SQUID body and the pick-up coil. The modulation efficiency was changed by the ratio of the modulation current flowing through the SQUID to the pick-up coil. In the other schemes, the modulation flux is mutually coupled through an external coil, and the modulation efficiency was changed by the mutual inductance of the external coil and the magnetometer. The results show that the modulation efficiency was varied when the value of self-inductance or the mutual inductance of SQUIDs change. Usually, lower modulation efficiency results in a higher crosstalk. The performance of magnetometers was also influenced by different modulation schemes. Their noise spectrum was compared as evidence.