Casella CEL CEL-414 - Manual (Page 18)

The analog outputs are as follows.
1. Conditioned AC signal with typically
7.75 V RMS for FSD (8.0 V with
CEL-493/3), - recommended
impedance of any connected device,
including cable, >10 kΩ and <50 pF.
(Consult CEL about cables for use
with equipment not manufactured
by CEL.)
2. Logarithmic DC signal not exceeding
5 V, nominally 48 mV/dB, and a
minimum load impedance of 10 kΩ.
The rise time and decay time for fast
(125 ms) and slow (1 s) are correct,
however for impulse time weight-
ing, the DC signal has a rise time of
35 ms, with 0.1 ms for peak
measurement. The decay time for
both is typically <50 ms, as this is
derived from the software.
3.6Auxiliary Inputs
In addition to measurements of signals
from the microphone or vibration attach-
ment, these sound level meters will also
accept analog signals input via the pream-
plifier or auxiliary sockets. Digital control
facilities are available on the “B” and “D”
versions as detailed in Chapter 8.
The analog input facilities are as follows.
1. AC signals up to 22.5 V pk-pk with
<100 kΩ source impedance input via
a CEL-216 Line Adaptor attached to
a Type 1 preamplifier, or via a
CEL-316 Line Adaptor attached in
place of a Type 2 preamplifier.
This allows standard BNC connectors
to be plugged in to the instrument,
so that measurement and analysis
can be performed on signals
recorded by tape recorders, etc.
2. AC signals up to 21 V pk-pk with
<100 kΩ source impedance input via
pin 6 of the auxiliary socket.
This input is normally used for the
output signals from an external filter
set.
3.7Octave Filters
The CEL-266/3 and CEL-328/3 contain
ten octave filters with centre frequencies
from 31.5 Hz to 16 kHz. These are active
filters which satisfy the requirements of
IEC 225 and ANSI S1.11-1966 (R1975)
Class II, and have less than ±0.5 dB at-
tenuation at the centre frequency.
Figure 5 shows the pass characteristics
of a typical filter band.
Octave filtering becomes available as
soon as either FA or FL is selected on the
A-L-FA-FL or A-C-FA-FL switch. The in-
strument selects the 31.5 Hz filter auto-
matically, with the arrow pointing
towards 31. Other bands may be se-
lected by means of the function step but-
tons. The bar graph shows the
instantaneous sound level in the selected
frequency band, while the digital display
indicates the equivalent continuous level
for the band (L
eq ). No other measure-
ment functions are available while octave
filtering is selected. As each filter is
switched into circuit, the instrument is
paused for a short period and the L
eq reg-
ister reset. This ensures that the filter
has stabilized before each new L
eq meas-
urement is commenced.
The power consumption increases signifi-
cantly when the active filters are in use.
Therefore it is recommended that the in-
strument be operated on broad band,
with octaves selected only when re-
quired.
Page 14 - General CEL Sound Level Meters Handbook