| This project concerns the theory, design, fabrication, and experimental testing of an ultrasensitive, electromechanical transducer for use on gravitational wave antennae. The transducer converts small, audio-frequency vibrations to a radio-frequency, electrical signal.; The system employs a resonant bridge circuit, driven at a "pump" frequency between 100 and 300 MHz. The circuit is capacitively coupled to a mechanical, test mass, which vibrates at approximately 1 kHz. The motion of the test mass, by changing the bridge capacitors, modulates the pump voltage to produce an excitation at the output terminals of the bridge. The output signal contains a signal at the pump frequency, with 1 kHz sidebands caused by the mechanically induced modulation. It is by observing these sidebands that a mechanical signal can be detected.; Techniques have been employed to minimize several important sources of noise. To reduce the Johnson noise in the electrical circuit, the bridge is constructed entirely of superconducting niobium and low-loss dielectrics. To allow the circuit to be superconducting and to decrease the Brownian motion of the mechanical system, the detector is cooled to liquid helium temperature. The bridge is balanced with piezoelectric tuners so that the size of the pump frequency signal, with its added phase noise, is minimized with respect to the mechanically induced sidebands.; The transducer, which has a 0.080 kg test mass, is affixed to the end of a resonant bar, gravity wave antenna with a mass of approximately 100 kg. The primary purpose of this small antenna is to evaluate the transducer, which is designed ultimately to be mounted on a full-size, 2000 kg antenna. Theory and testing of the detector indicate a noise temperature of 1.812 K using the 100 kg bar. This corresponds to a gravity wave burst sensitivity of 1.14 {dollar}times{dollar} 10{dollar}sp{lcub}-16{rcub}{dollar}, in terms of relative strain amplitude. If tuned and installed on a 2000 kg antenna, the transducer is predicted to obtain a noise temperature of 1.27 mK, which is equivalent to a burst sensitivity of 5.67 {dollar}times{dollar} 10{dollar}sp{lcub}-19{rcub}{dollar}. |
