BRUEL & KJAER 1623 - Setiing the Trigger Level; Manually Tuning the Filter

4.3. SETTING THE TRIGGER LEVEL
After connecting the desired tuning signal with the TRACKING MODE pushbuttons (Ta-
chometer Input or Filter Input) the TRIGGER LEVEL knob is rotated either in the + or -
direction until the adjacent Preset window light is extinguished and the Track window
lights up and the frequency display becomes stable. The knob position is noted and is
then turned further in the same direction until the display again becomes instable or re-
verts to Preset. The knob is then turned approximately halfway back towards the previ-
ously noted position. The filter is now tuned to the frequency indicated in the display.
The nature and polarity of the tuning signal will determine whether the TRIGGER LEVEL
knob should be tuned in the + or- directions. If the peak-to-peak level of the tuning sig-
nal is high ( 1 20 V max.) and the noise level also high, it may be necessary to attenuate
the signal with a voltage divider (see Fig.4.4) in order to reduce the signal and noise le-
vels.
Max input
(to this network)
100 V peak-to-peak
90 k.Q
9k.Q
1 k.Q
o----------~.--------0
770308
Fig. 4. 4. Voltage divider network for attenuating tuning signals in or-
der to reduce the signal and noise levels. Use resistors or a
variable resistance rated at 1 I 8 W or greater. Connection
cables should be as short as possible
4.4. MANUALLY TUNING THE FILTER
Whenever the "Preset" window in the display is illuminated the filter centre frequency
is determined by the setting of the MANUAL TUNING/PRESET switch and knob (x N/D).
With the TRACKING MODE Manual pushbutton depressed the setting of the MANUAL
TUNING/ PRESET switch and knob overrides all other tuning signals. This manual tuning
facility can be used in two ways:
1.
Presetting the filter to, for example, the fundamental rotation frequency of a ma-
chine, before locking onto it with a tuning device. This reduces the filter settling
time as compared with allowing the filter to automatically align itself with the trig-
ger frequency provided by the tuning signal.
2. Frequency analysis by manually tuning the filter slowly through the frequency range
of interest and noting the signal level on the measuring instrument as a function of
frequency. An initial sweep with the 23% filter will localise the frequencies of parti-
cular interest, these can then be examined more closely using either the 1 2% or 6%
bandwidth filter.
With manual tuning, the frequency range 2Hz to 20kHz is covered in two ranges 2Hz
to 200Hz and 200Hz to 20kHz, which are selected by a range changing switch adja-
cent to the MANUAL TUNING knob. These two ranges overlap somewhat (the low freq.
range covers up
to "' 300Hz while the high range starts at "' 200Hz) which may be in-
convenient when recording a level frequency plot. This problem is overcome very simply
by performing a frequency sweep as follows:
1 . Synchronise the recorder as described in section 4. 5.
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