Casella CEL CEL-160 - Manual (Page 14)

description of the F key in section 3.1, whilst
the following cable sets are available for
connecting filters to the recorder.
CableType Description
CEL-3697 CEL-196/160 Automatic Frequency
Recording
CEL-3698 SA59/CEL-160 Automatic Frequency
Recording
CEL-3699 B & K 2215/CEL-160 Frequency
Recording
CEL-3700 SA24/CEL 160 Automatic Frequency
Recording
CEL-3701 B & K1621/CEL-160 Frequency
Recording
CEL-3727 B & K1616/CEL-160 Freqency Recording
CEL-3728 B & K1618/CEL-160 Automatic
Frequency Recording
Figure 5: Standard cable sets for
frequency analysers
6.1 Stepped Serial Analysers
The CEL-160 operates with these filters
connected into its sound level meter section in
order that they filter the AC signal into
frequency bands as it is passed through this
section of the recorder. An output level for
each band is therefore obtained in sequence.
Cable sets for these filters provide plugs for
connection to the filter in and out sockets
(cable marked input to filter input socket etc)
and a control cable for connection to the
remote socket. The FILTER-A-LIN switch must
be in the filter position and the instrument
calibrated with the filter in circuit. The filter
should be set for automatic analysis as per its
own operational instructions, which in the case
of the CEL-196 means depressing its 'Reset'
button. The recorder should be set for log level
recording (10 mS time constant) at a paper
speed of 1 mm/S.
When the analysis is required the E key
should be depressed. This will cause the time
scale annotations to be printed and the filter
will then step through its various frequency
weightings and band filters. Each band is
represented by 5 mm of chart and annotated
A.B.C.D. and AP (all pass) for the broad band
settings and each individual octave band
annotated with its centre frequency. For
clarity, individual third octave bands are not
annotated but each band is clearly denoted by
a vertical line drawn from the maximum band
level for approximately 10 mm.
Most serial analysers produce switching
spikes when changing bands and these take
some time to decay. Similarly, it takes some
time for a high level in one band to decay to a
lower level that may exist in an adjacent band.
In order to ensure that these factors do not
cause errors in an analysis, the following
functions have been written into the
controlling program.
(i) The recorder always waits 50 ms
between bands.
(ii) The filter can hold the recorder from
progressing the scan until its switching
transients have decayed.
(iii) The analysis will not proceed if the signal
is decaying at a rate in excess of 1 dB/S.
The requirements of the relationship between
bandwidth and averaging time are now widely
appreciated.
Using conventional electromechanical
level recorders to drive serial analysers the
worst case had to be assumed and the scan
rate selected to suit the lowest frequency and
the narrowest bandwidth. As a result of these
constraints an analysis always took much
longer than was necessary because it was not
possible to alter the scan rate when the higher
frequencies and wider bands were reached.
With the advent of the micro- processor
control that is incorporated into the CEL-160, it
becomes possible to dynamically control the
analysis to optimise these parameters.
In selecting a serial filter program the
recorder makes basic decisions regarding
averaging time and sweep speed based upon
the filter characteristics, and these parameters
are both controlled during the analysis to
produce the optimum results in the shortest
time.
A digital time constant is introduced into
the measurement chain that is varied along
with the paper speed during the sweep.
Ideally, the product of the time constant and
bandwidth should be constant and should
therefore, for a constant percentage
bandwidth filter, change every band but
experience has shown that changes less often
are acceptable. The CEL-160 will change the
time constant every 5 third-octaves (or 2
octaves) and changes by a factor of
approximately 3 each time. Two changes are
provided giving a total change of 10 to 1.
In practice the time constant is reduced
and the paper speed increased in synchro-
nisation; the full relationsahip is given in Fig 6.
Page 14 - CEL-160 Graphic Recorder