Casella CEL CEL-296 - Manual (Page 12)

3.1.1 Signal Processing & Filters
The CEL-296/3 is designed to be mounted close coupled to the
bottom of a CEL sound level meter. it receives signals to be filtered
via line 4 of the connector in the top of the instrument case, and
returns them to the meter for measurement via line 6 of the same
connector.
On "Hi" range the input signal passes directly to the anti-aliasing
filter and thence to the selected octave or third octave filter unit. On
"Lo" range, 25 dB of gain is added to raise the signal level well
above the noise floor.
The input can also be switched via the "All Pass" (1.5 Hz to 45 kHz)
line directly to the output. Before the signal is fed to the two switched
capacitor filter units for frequency analysis, it passes through the
anti-aliasing filter which removes frequency components above the
band of interest. This prevents the sampling process from aliasing
them into the band for measurement.
One filter unit has a one octave bandwidth, and can be switched to
give 12 centre frequencies in one octave steps from 8 Hz to 16 kHz
The other filter unit has a third octave bandwidth, and can be
switched to give 36 centre frequencies in third octave steps from 6.3
Hz to 20 kHz.
Centre frequency selection is initiated by the (Up) and
(Down) push buttons, while the actual band centre frequency is
controlled by pulses from the microprocessor driven clock
oscillator.
The output from the filters passes through the clock residue filter
which further reduces the spurious components produced during the
sampling process. On "Hi" range, the output from the clock residue
filter is fed directly to the sound level meter for measurement, while
on "Lo" range 25 dB of attenuation is introduced first to restore the
signal level. This output signal is also available on line 4 of the
bottom connector for transmission to some other measuring or
Page 12 CEL-296/3 Operator Handbook CEL Instruments Ltd