Benthos 865-A, Instructions For The Installation, Operation And Maintenance

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Model 865-A Deep Sea Acoustic Release/Transponder M-855-009
Benthos, Inc.
21
5.2.2.2 865-A Board No. 2 (Part No. C-855-132)
Board No. 2 contains the remaining receiver circuitry including the limiter amplifier, detector and the
microprocessor based command decoder. Other functions on this circuit board are:
n
Transmit frequency synthesizer.
n RELEASE acknowledgment (four extra pings).
n
RELEASE motor drive.
n 5-volt regulation.
To remove Board No. 2, power OFF, disconnect the interface connector; and remove the four corner
screws and unplug the board (see Figure 5-3).
6.0 THEORY OF OPERATION
6.1 Receiver
The receiver consists of four basic blocks:
1. Transducer
2. Preamplifier
3. Limiter-amplifier
4. Detector.
The hydrophone converts the acoustic energy into an electrical signal. At long ranges, this signal may be
very small, approximately 20 microvolts mixed with noise. The preamplifier is a very low noise amplifier
which provides approximately 52 dB of gain and some filtering. The limiter amplifier provides another
52 dB of amplification and hard limits the signal to 1.2 volts peak to peak. The total gain of the system
is more than 100,000!
The limited signal, plus noise, is then fed into a frequency detector, which is factory tuned to detect a
specific frequency between 7.0 and 15.0 kilohertz. If the correct frequency (±105 Hz) is present, then
the detector output goes true.
WARNING
: DO NOT ATTEMPT TO ADJUST THE FACTORY SET
TUNING OF THE DETECTOR CIRCUIT
.