NASA's Get Away Special (GAS) program permits experimenters to place small, self-contained, autonomously controlled experiments in the Space Shuttle's cargo bay. In some experiments the need exists to start recording data just before launch without intervention by the astronauts. This paper describes an elliptic filter whose purpose is to detect the impending launch by listening for a 600 Hz tone characteristic of the Auxiliary Power Units (APU). These are activated between five and ten minutes prior to launch. The filter comprises the key element in an APU detection circuit, and it is implemented using a pair of Generalized Impedance Converters (GIC) in cascade. The GIC configuration is highly insensitive to changes in both its passive and its active parameters, is capable of implementing a large variety of biquadratic filter functions, and so is an excellent choice for active filters. The two GIC sections represent biquadratic factors which individually implement a high-pass and a low-pass notch filter. When placed in cascade, the result is a very narrow band elliptic filter. This paper includes measurements of the sensitivity of the filter to changes in it parameters.
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