AN9515

Multiplier Improves the Dynamic Range of Echo Systems (HA-2556, HA-5177) Application Note November 1996 AN9515.1 Introduction In an echo system the returned signal amplitude is a function www.datasheet4u.com of the distance to the target, and it can be expressed mathe- matically as function of time. An echo system with a fixed gain preamp has poor dynamic range because close targets (short return times) have high signal amplitudes while distant targets (long return times) have much lower signal amplitudes. In fixed gain systems, the biggest signals establish the upper preamp gain limit based on not saturating the system, and this gain may not be high enough to process small returns properly. The solution is a preamplifier which has a gain proportional to time, such that the gain will be small for close targets and large for distant targets. The preamp still has to meet all the other normal preamp criteria such as bandwidth and noise performance, and the added time dependent gain function must not degrade the signal. The circuit shown in the figure implements the variable gain preamp with the Intersil HA2556 multiplier. This IC establishes the signal bandwidth and noise figure because it is the only component in the signal path. The equation for the multiplier gain, as shown in the accompanying figure, is given below: VX VY  R7  V OUT = ---------------  ------ + 1 = 10V X V Y 5  R8  the time it takes sound to travel approximately one foot through air. During the first portio...