Casella CEL CEL-414 - Manual (Page 10)

CEL-328/3, or to an optional external filter
set connected to the sound level meter
via the auxiliary socket, so that frequency
analysis can be performed.
Integrated RMS detectors are employed
that provide a full 120 dB measurement
range in three overlapping 60 dB dy-
anamic ranges (100 dB ranges on the
CEL-493/3). The standard Fast, Slow,
Impulse, and Peak time weightings can
be selected. The RMS output signal
passes through an analog to digital con-
verter to the built in low power microproc-
essor which drives a custom designed
liquid crystal display with both analog and
digital indication.
The digital display can be switched to
show the sound pressure level, or any of
the calculated parameters with a resolu-
tion of 0.1 dB. The analog display is a bar
graph that always gives an indication of
the instantaneous sound pressure level
with 1 dB resolution. In order to display
the whole 60 dB (or 100 dB) dynamic
range of the instrument on a 26 segment
bar, the analog display is auto-ranged in
20 dB steps with a reference datum.
Control of the display and calculation func-
tions is undertaken by the microproces-
sor. Not only is a continuous display of
the sound pressure level provided at all
times, but the detector output is also
sampled 320 times per second, and the
samples integrated by the microproces-
sor (not CEL-328/3E or CEL-414/3).
Simultaneous calculations are made to de-
termine each of the parameters available
from the instrument. The results are
stored in registers, and may be shown on
the digital display.
Special control sequences are avoided by
using simple push button controls to se-
lect any required parameter by stepping
Figure 1: Block diagram
920001
CEL-192 Microphone
30 dB
Gain
A-Wtg
Lin. or
C-Wtg
Freq
Weighting
F
Amplifier Cal
External Power Supply
Power
Supply
Batteries
Display
A / D
Converter
Micro-
processor
Low
Voltages
Internal
Controls
Battery
Condition
RAM (not
414 or
328/3C)
RMS
Detector
EPROM
Address Bus
Data Bus
Log DC Out
80
P
I
S
C-Wtg
CEL-225/3 Preamplifier
200 V Polarisation Voltage
Interface
(only B, D &
493/3)
To Ext Filter: AC Out
From Ext Filter
(not 266/328)
Type 1
10 dB Attenuator Pad
(493/3 only)
Type 2
10 Octave
Filters
CEL-230 Microphon/
Preamplifier Unit
50
20
Range
(only 266/328)
Lin, A-, or
C-Wtd AC Out
30 dB
Gain
Page 6 - General CEL Sound Level Meters Handbook