ECHO_SYSTEM_DYNAMIC_RANGE_BOOSTER

Published:2009/7/17 2:10:00 Author:Jessie | From:SeekIC

Using a variable-gain preamplifier helps improve the dynamic range of echo systems. Here, Harris Semiconductor's HA2556 multiplier is used to implement the variable-gain preamplifier and establish the signal bandwidth and noise figure. An echo sys-tem with a fixed-gain preamplifier exhibits poor dynamic range because close targets (long return times) have high signal amplitudes, whereas distant targets (long return times) produce amplitudes that are much lower. One solution involves a preamplifier that has a gain that is proportional to time so that the gain witl be small for close targets and large for dislant targets. The preamplifier must meet all of the other preamplifier criteria, such as bandwidth and noise performance. Moreover, the added time-dependent gain function must not degrade the signal. This type of variable-gain preamplifier can be built with a multiplier IC, the Harris Semiconductor HA2556. This IC establishes the signal bandwidth and noise figure because it is the only component in the signal path. The HA5177 op amp and its associated circuitry make up a constant-current source with a current (I) of VD1/R2=51μA. If the switch (S1) is in the LINEAR-position with Q2's gate held high, the cur-rent source is shorted to ground by Q2 and the multiplier gain is set to zero. When the transmission of the outgoing signal is complete, Q2's gate is brought low, forcing it into a very high drain resistance state (almost an open circuit). Consequently, the HA5177 current can charge C1 in a linear manner. The voltage across 01 then ramps up from 0 to 5V in 1 ms. During the first portion of the ramp, when the returned signal is very large, the multiplier gain is small because VX, is small. As time increases, so does VX providing more gain through the multiplier as the expected echo decreases in amplitude. As a result, the output-voltage swing of the multiplier tends to stay constant for large changes in input signal. In addition, the dynamic range is improved to the amount of the ramp change, which is more than 60 dB with the values shown in the figure.



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