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Bulletin of the Seismological Society of America; February 1974; v. 64; no. 1; p. 103-113
© 1974 Seismological Society of America
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Gain-ranging analog or digital seismic system

E. R. KANASEWICH, W. P. SIEWERT, M. D. BURKE, C. H. MCCLOUGHAN and L. RAMSDELL

SEISMOLOGY DIVISION INSTITUTE OF EARTH AND PLANETARY PHYSICS DEPARTMENT OF PHYSICS, UNIVERSITY OF ALBERTA, EDMONTON, ALBERTA T6G 2E1 Canada

Abstract

A wide-band, gain-ranging amplifier is described that may be used for recording data with a dynamic range of 60 db in each of three different levels, 12 db apart, so that we achieve an "effective" dynamic ±160-v analog or 84-db digital, within a normal ±10-v analog system. As described, the ranging circuit reduces the gain of the amplifier by a factor of either 4 or 16 whenever the output signal approaches the maximum for the system. The wide-band response is achieved with low-noise operational amplifiers and second-order active filters. Signals with periods greater than 30 sec are amplified by 100 db and those with periods shorter than 1 sec are amplified by 70 db. The system works well in extending the useful output range of a Willmore Mark II seismometer with a natural period of 1.5 sec to over 40 sec under normal field operating conditions. When analog recording, the gain-range switching occurs when the input signal reaches ±8.1-v; when digital recording, the level is ±9.375 v. The period in a divide-by-4- or 16-state is preset by the experimentalist. The gain level is recorded on an extra channel which is also used to record absolute time.




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R. G. DANIEL
An intermediate-period field system using a short-period seismometer
Bulletin of the Seismological Society of America, October 1, 1979; 69(5): 1623 - 1626.
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