Íslenskar landbúnaðarrannsóknir - 01.09.1978, Blaðsíða 141

HYDROACOUSTIC DATA ACQUISITION 139
the front panel will indicate if the supply
voltage drops below 11.5 VDC.
There are two time varíable gain func-
tions (TVG): A 40 log R + 2a R for single
fish analysis, and a 20 logR + 2« R for
multiaple targets and integration
analysis. The TVG have been adjusted for
operation in freshwater, with a pre-set at-
tenuation coefficient ot= 0.25 dB/100 m.
(Fig. 4). The output frequency is 10 kHz
which eliminates the need for an interface
unit when recording on magnetic tape.
SUMMARY
The development of a small, portable
echosounder for the data acquisition of
acoustic fish signals is discussed. A final
unit operates on 70 kHz with a power
output of 100 W. Power requirements are
24 W at 12 V. A wide angle transducer has
an opening of 27° and a narrow angle
transducer 12° down to the —3 dB point.
The output frequency of the receiver is
10 kHz which can be recorded directly on
a high quality cassette recorder.
Echosounder in the transducer
mounted in a plastic towing fin weighs
about 25 kg.
Fig. 4. Photograph of the oscilloscope trace of 20 log
R +2a R TVG. The horizontal divisions represent
each 20 ms and the vertical over 150 mV.
Pulsing rate is 182/mn or 91/mn de-
pending upon the range used. The voltage
gain from the transducer is 85 dB and the
linear dynamic range extends from —50 to
+9 dB//lV.
The basic weight of the sounder is 8.6 kg
and that of a narrow beam transducer
mounted in a fin 6 kg. Added to this are 5
kg of a portable oscilloscope and 5 kg of a
casette tape recorder. Thus the total
weight of the entire data acquisition sys-
tem is about 25 kg.