Abstract
A ‘‘prescription frequency-modulated’’ signal is a swept FM signal whose sweep rate and possibly envelope are adjusted according to some a priori prescription. An idea advanced by Birdsall and Metzger in 1994 for the ATOC long-range tomography experiment, the concept is being revisited for LOAPEX as an alternative to ‘‘m-sequences.’’ The prescription can be used to tailor the output spectrum to a preferred shape: the sweep rate is slower at frequencies where more spectral energy is needed, and faster where less is needed. The prescription can also compensate for the transducer response. In addition, the prescription can drive the envelope to predefined maximum levels at every frequency, producing a signal which is ‘‘optimal’’ with respect to drive level. This signal is in general far less stressful on transducers than standard m-sequences. Examples of prescription FMs used during the LOAPEX experiment will be presented. [Work supported by ONR.]
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