BRUEL & KJAER 2305 - Page 9

This circuit determines the lowest frequency to which the Recorder responds
correctly. It can be set to five values 2, 10, 20, 50 and 200 Hz (cis) by the knob
LOWER LIMITING FREQUENCY . In Fig. 1.2 can be seen the response at the
five different settings . It should be noted that at the various frequency
limits marked around the knob the reading is decreased by approximately
0.3 dB only . As the stability of the complete writing system is dependent
on the combined setting of the LOWER LIMITING FREQUENCY, WRITING
SPEED and POTENTIOMETER RANGE the reader is referred to the com-
me nts under "Writing Speed", pag e 10 .
~~ SOCIS
ODets
5dB
10 100 1000
10000
Highest dev i otl ' OO a n ~=
to be found f:lr E~
any setting of ====
" Input Potentiometer"
5 100 000 2 CIS
16(807
Fig. 1.2. Typical frequency characteristics of the Level Recorder for different
settings of LOWER LIMITING FREQUENCY switch. The two dotted lines at
the high frequencies give the greatest deviation to be found for any setting Of
INPUT POTENTIOMETER.
First Limiter. To avoid overdriving (heavy charging of the condensers in
the Lower Limiting Frequency circuit) by high and very rapid fluctuations
in the input signal level, an amplitude limiter is introduced. The threshold
of the limiter is designed so that the drive system has full power when the
signal fluctuations reach its limit.
Potentiometer Range. The output signal from the first limiter network,
which is now DC, is compared with a built-in balanced DC reference
voltage, the signal difference then being used to drive the output amplifier
via a second limiter.
The difference signal obtained by comparing the rectified and limited input
signal with the buill-in reference voltage is attenuated in a step attenuator
marked POTENTIOMETER RANGE. By the different positions of the atte-
nuator the resolution of the Recorder is determined. (See also "Controls,
Terminals, and Mechanical Drives", page 22) . The resolution of the Recorder
means the degree by which the Recorder is able to record the details in
signal level variations. Fig . 1.3 illustrates the effect of the resolving power
for different settings of POTENTIOMETER RANGE. The four measurements
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