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The gain of the receiving amplifier increases as a function of
time (Figure 5-6) to compensate for signal losses at further
distances and decreases its bandwidth to reduce noise pick-up.
When a signal is received by the system, a current source is
turned on if the signal is above a preset threshold level. The
current source charges a capacitor until the voltage on the
capacitor reaches 1.2 Volts. The system generates a logic level
output signal at the time of the received echo.
The time interval between the transmit signal start ( Internal
CLK or Ext. Trigger Input) and the received echo (ECHO
Output) can then be measured, multiplying this time by the
speed of sound yields the round trip distance to the object. If
the actual distance is not required, a simpler timing circuit can
be employed to determine when an object comes within some
preset zone of the sensor.
FIGURE 5-6
Note: ONLY the graphs of the first eight gain steps are
included here for clarity. Step 9-11 are identical to step 8
except that each successive step is increased in gain by 4 dB.
These graphs are generated from theoretical information, not
experimental data.
FIGURE 5-5
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