Casella CEL CEL-368 User Manual
Page 35
As can be seen from the block diagram (Figure 2), the preamplifier input
in both instruments can be switched to pass to the wide range RMS de-
tector through a variable gain amplification stage with linear response,
or it may be passed through filters with A-weighted or C-weighted re-
sponse that meet the requirements of IEC 651 Type 1.
Linear, A- or C-weighted signals are passed to a wide range RMS detec-
tor which provides the instruments with a full 130 dB measurement
range in three overlapping 70 dB dynamic ranges. The detector has a
very rapid response time which exceeds the integration requirements of
IEC 804 Type 1. The output from the RMS detector is in logarithmic form
and passes via an analog to digital converter with a resolution of 0.1 dB
to the built in low power microprocessor. Once converted, all sub-
sequent operations are performed on digital data.
Both the analog output signal from the weighting filter in use and the
logarithmic signal from the RMS detector are also available from the 18-
line connector in the base of the instrument.
4.6
Digital Section
The microprocessor performs all calculation routines necessary, includ-
ing the calculation of the standard Fast and Slow time weightings, imple-
ments user commands, excercises control over the whole instrument,
assigns memory space according to currently set requirements, and
drives a 38 x 56 mm liquid crystal display.
The instrument memory is made up from a 64 kbyte EPROM that con-
tains control and processing programs, and a 128 kbyte non-volatile
RAM which will allow up to 60 000 data values to be stored. The most re-
cently used setup information and calibration data are also stored while
the instrument is switched OFF, and become available for immediate re-
use when the instrument is switched ON.
The instrument contains an internal clock, with accuracy better than five
seconds in 24 hours, that is used as the time base for all time and dura-
tion measurements and in the calculation of time dependent parameters.
CEL-268/368 Operator's Handbook - Page 31