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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..comto the target, and it can be expressed matheof the distance
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 portion of the ramp, when the returned signal is very large, the multiplier gain is small because VX is small. As time increases VX also increases providing more gain through the multiplier as the expected echo decreases in amplitude. Thus, the output voltage swing of the multiplier tends to stay constant for large changes in input signal, and the dynamic range is improved to the amount of the ramp change, which is more than 60dB with the values shown in Figure 1. Often the returned signal is a nonlinear function and it may be desirable to linearize it. An inverse nonlinear ramp can be employed to linearize the overall function. R3, R4 and C1 generate a logarithmic ramp when S1 is in the Log position thus yielding a logarithmic gain function adequate for linearizing some transducers. Many other time-gain transfer functions can be generated by employing different types of ramps. It is important to eliminate the multiplier offsets with the adjustments [1] provided because offsets will appear in the output signal, reduce the dynamic range and contribute errors. As the circuit is configured it will sweep from a gain of 0.01, as the ramp begins, to 10 as the ramp ends. Returned signal amplitude is usually small but should not exceed 100mVP-P unless distortion can be tolerated. The circuit bandwidth can be as high as 57MHz in low gain applications, and is 5MHz as configured.
The HA-5177 and its associated circuitry comprise a constant current source whose current is VD1/R2 = I = 51A. If S1 is in the LIN position with Q2's gate held high, the current source is shorted to ground by Q2 and the multiplier gain is set to zero. When the received signal from the closest target can be present, Q2's gate is brought low forcing it into a very high drain resistance state (almost an open circuit) allowing the HA-5177 current to charge C1 in a linear manner. The voltage across C1 ramps up from 0V to 5V in 1ms which is
References
[1] Wideband Four Quadrant Voltage Output Analog Multiplier Data Sheet HA-2556, File Number 2477, Intersil Corporation, Melbourne, Florida.
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1-888-INTERSIL or 321-724-7143 | Copyright
(c) Intersil Corporation 1999
Application Note 9515
+15V -0.1F +15V R2 D1 100K www..com 5.1V 1N751A R1 200K HA-5177 U2 +15 + -15 LIN
TO CIRCUITS + 10F
-15V 0.1F
10F
TO CIRCUITS
+15
-
2N3906 Q1
R3 150K
-15V X OFFSET R10, 20K 15 16 8 VOUT R7 1K 9 R8 20.4 GAIN CONTROLLED OUTPUT
LOG S1 R4 75K VX
+15 11
-15 7
D2 5.6V 1N752A
13 C1 0.01F VY 5 HA-2556 U1
GAIN CONTROL (TTL) R5 10K
Q2 2N6901
1
12
6
10
3
4 R9, 20K Y OFFSET
SIGNAL 10K R6
-15V
FIGURE 1. MULTIPLIER IMPROVES DUAL RANGE OF ECHO SYSTEMS
All Intersil semiconductor products are manufactured, assembled and tested under ISO9000 quality systems certification.
Intersil semiconductor products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries.
For information regarding Intersil Corporation and its products, see web site www.intersil.com
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NORTH AMERICA Intersil Corporation P. O. Box 883, Mail Stop 53-204 Melbourne, FL 32902 TEL: (321) 724-7000 FAX: (321) 724-7240 EUROPE Intersil SA Mercure Center 100, Rue de la Fusee 1130 Brussels, Belgium TEL: (32) 2.724.2111 FAX: (32) 2.724.22.05 ASIA Intersil (Taiwan) Ltd. 7F-6, No. 101 Fu Hsing North Road Taipei, Taiwan Republic of China TEL: (886) 2 2716 9310 FAX: (886) 2 2715 3029
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