Íslenskar landbúnaðarrannsóknir - 01.09.1978, Side 139

HYDROACOUSTIC DATA ACQUISITION 13 7
i ^approx. 79 ms
1 ms
Fig. 2. Diagram of the oscilloscope picture of
Theoretically, it is possible to use the
sounders’ own transmitter oscallator as a
calibrator. However, in this case the
transmitter oscillator and the receiver
preamplifier were positioned on the same
printed circuit board, and created an un-
wanted indirect coupling from the oscil-
lator to the receiver input.
The major source of noise in the
echosounder was the motor system and
writing pen which, combined, gave a
reading of0.2 VoltP-P. By adding á resistor
in series with the plus line to the motor, by
increasing a capacitor, and by grounding
the negative (common) buss of all of the
circuit boards to the metal chassis, the
noise level was reduced to 75 m Volt p-p.
The practical signal to noise ratio ob-
tained was still less than 35 dB.
Some external noise became evident
during the field, work. Due to the high
receiver sensitivity, the system picked up
electrical noise from spark plugs and the
alternator, and the latter one was there-
fore disconnected during transecting. One
specific problem on Iceland was a LO-
RAN C station. This kind of station is now
under construction all over the world.
They operate in the freqiency band
8 pulses
the pulses in a burst from a LORAN station.
.80-120 kHz. The output power from these
stations could be one megawatt in peak on
the frequency 100 kHz, and 90% of the
power should bewithin 90-110 kHz. One
transmitted burst consists of 8 pulses with
1000 (ís spacing, and the time belween
each burst could be 79 ms, (Fig. 2). How-
ever, this varies from one station to
another. All these sources gave noise
pulses of 0.1-1 VoltP-P, which could be
separated visually from fish echoes.
SUBSEQUENT IMPROVEMENT
INCORPORATED IN THE EY-M
To eliminate the noise problem in the old
EY model, the receiver gain has been re-
duced 10 dB and the output power has
been increased from 20 to approximately
100 watts.
A 70 kHz narrow-beam transducer with
aNource level SL = 112.0 dB/l/u Bar. ref. 1
m eliminates the problems with operation
of 105 kHz (Fig. 3). The echosounder has
its own calibration oscillator. Operating
characteristics are not altered with a sup-
ply voltage ranging from 11.5 to 15.6
VDC. A light-emitting diode (LED) on
18