AIDJEX Bulletin #40 - Polar Science Center - University of Washington

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FIGURE 3
BLOCK DlAGRAH OF SVNW ICE STATION
Analog Electronics
Underwater acoustic ambient noise is measured
in four 113 octave bands in the overall band 3.2
to 1000 Hz, with an averaging time of 40 seconds.
The noise field is sampled by a PRL model 34
double bender hydrophone exhibiting'a flat sensitivity
from 2 to 1400 Hz. The signals are
amplified by an ultra low noise FET preamplifier
at the phone. These units are mounted in a
faired, neutrally buoyant body. The noise performance
of the amplifierfhydrophone combination
is considerably below previous Arctic minimum
noise measurements.
One hundred feet of three-conductor cable
connects the chain-tethered hydrophone to the
electronics module, passing to a post amplifier
through a 0.7 Hz, two-pole, high pass filter.
The filter prevents extremely low frequency water
current-induced self noise signals at the hydrophone
from reaching the post amplifier which
could otherwise result in system saturation. The
post amplifier drives the parallel bank of 113
octave bandwidth filters. Each filter is buffered
from the post amplifier by a pass band amplifier,
the gain of which sets the individual measurement
windows. The 113 octave filters use
three pole pairs which are stagger-tuned to give
a flat pass band response. Because of the excellent
stop band characteristics of these filters,
the effective bandwidth is equal to the half
power bandwidth.
The output of each filter drives a precision
rectifier and averager. The DC averager has an
RC time constant of 10 seconds. The full dynamic
range of this converter was measured at greater
than 50 db. The outputs of each of the four
'
averagets are switched to an analog-to-digital
converter through a COS/MOS analog multiplexer
controlled by timing and sequencer logic. The
AID converter was designed,to give a five bit
binary count which is proportional to the logarithm
of the input voltage. The quantizing increment
was selected to be 1.5 db, thereby providing
the 48 db measurement range. Conversion and clock
inputs are derived from a crystal oscillator
countdown chain. The AID has a conversion time
of 27 milliseconds.
After each conversion the contents of the converter
are placed into a parallel-in, serial-out
shift register from which data are sent to the
Random Access Memory (RAM).
Digital Electronics
Data Formatting. The data format for one synoptic
sample period is outlined in Table 1.
Running
Total
5 bits
10 bits
15 bits
20 bits
24 bits
32 bits
I Refer-
Data Step ence Range
Size Point
Word 1 5 bits 1.5db -40dbu* 48db
Word 2 5 bits 1.5db -4Odbp 48db
Word 3 5 bits 1.5db -4Odbp 48db
Word 4 5 bits 1.5db -40dbp 48db
Temp 4 bits 3O C -4OOC 48OC
Baro 8 bits 0.3906mb 950mb lOhb
31
IEEE OCEAN '75 - 41 5