AN9515: Multiplier Improves the Dynamic Range of Echo Systems

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
of the distance to the target, and it can be expressed mathematically 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 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.
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:
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.
VX VY  R7

V OUT = ---------------  ------- + 1 = 10V X V Y
5  R8

The HA-5177 and its associated circuitry comprise a constant current source whose current is VD1/R2 = I = 51µA. 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
1
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.
References
[1] Wideband Four Quadrant Voltage Output Analog Multiplier
Data Sheet HA-2556, File Number 2477, Intersil
Corporation, Melbourne, Florida.
1-888-INTERSIL or 321-724-7143 | Copyright
© Intersil Corporation 1999
Application Note 9515
+15V
-15V
TO CIRCUITS
+
10µF
-0.1µF
TO CIRCUITS
0.1µF
10µF
+15V
R2
+15
D1
100K
5.1V
1N751A
+15
-
HA-5177
U2
R1
200K
2N3906
Q1
+
R3
150K
-15V
-15
LIN
LOG
S1
+15
R4
75K
11
X OFFSET
R10, 20K
-15
7
15
16
8
VX
VOUT
13
D2
5.6V
1N752A
C1
0.01µF
5
GAIN
CONTROL
(TTL)
R5
10K
Q2
2N6901
R7
1K
HA-2556
U1
VY
1
12
6
10
GAIN
CONTROLLED
OUTPUT
9
4
3
R8
20.4Ω
R9, 20K
Y OFFSET
-15V
SIGNAL
10K
R6
FIGURE 1. MULTIPLIER IMPROVES DUAL RANGE OF ECHO SYSTEMS
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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
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