Instrumentation of a resonant gravitational radiation detector with a planar thin-film dc SQUID

Abstract

The instrumentation of a low-temperature three-mode gravitationa1 radiation antenna incorporating a low-noise dc SQUID provided by IBM is described. The feedback circuitry necessary to maintain the linearity and dynamic range of the SQUID was found to drive the resonant system due to high coupling between the input coil and the feedback coil of the SQUID. In order for this type of planar thin-film dc SQUID to be useful for gravitational radiation detectors and other applications requiring high Q input circuits, a solution to this feedback problem is needed. To this end, the nonlinear equations describing the dc SQUID with linear feedback are solved in terms of an isolated SQUID. The important feedback parameters for a high Q resonant system are found to be the slew rate of the electronics and the coupling constant ratio α2if/α2f, where α2if is the energy coupling efficiency between the feedback coil and input coil and α2f is the energy coupling efficiency between the feedback coil and the SQUID loop. Methods to reduce the effect of the feedback on the input circuit are also discussed.

Strongly coupled, low noise dc-SQUID amplifiers

“…(2.15), one can see the dominant effects of the FLL. Here, the influence of the dynamic inductance of the SQUID 1/g JΦ , which usually screens the geometric inductance L IN , is canceled out [94]. Also in this case, a resistive component is generated by the time 2.4.…”

“…A third group of publications, the papers by Folkner et al [94] and by Carroll [68] states that the SQUID is influenced by the connected impedance but it does not necessarily introduce completely new effects. According to both publications [68,94], the known small-signal parameters at low frequencies are sufficient to describe the effective input impedance and the gain of a SQUID amplifier, independent of the dominant mechanism at the Josephson frequency.…”

“…Equivalent input noise temperature constant of the FLL τ FLL . Furthermore, the (parasitic) coupling k FB,IN is directly altering the input inductance and generates resistive parts [94]. For a standard single-hole SQUID with k FB,IN > 0, the generated resistance takes negative values.…”

“…According to both publications [68,94], the known small-signal parameters at low frequencies are sufficient to describe the effective input impedance and the gain of a SQUID amplifier, independent of the dominant mechanism at the Josephson frequency. In reference [68], also the influence of noise is treated.…”

“…For a standard SQUID with one washer and both coils coupling to the same inductance, M FB,IN is typically positive. This can be changed for example by adding a small transformer between the input and the feedback loop [94]. Writing down the Kirchhoff law for the input and the output loop yields:…”