BRUEL & KJAER 1623 - Page 8

Mea s uring Object : __ _
DDDDDD DO DDDO D O O OODOOOOOO
0 dB
Dat e:
10 dB
20 dB
30dB
40dB
50 dB
100 200
Lower Lim . F req .:
Potentiomet er:
Zero Lev
% Bandwidth =
12% Bandwidth =
3% Bandwidth =
500 1 000 2000
5000 10000
Hz Writ ing Sp eed :
mm l s 760855
Fig.1. Typical filter characteristics of Track -
ing Filter Type 1623
The large digital display on the
front panel, showing the frequency
to which the filter is tuned , is easily
readable in daylight conditions .
Filter Tuning
Practically any well defined peri-
odic signal with a level between
20 mV (noise free) and 1 20 V peak-
to-peak and a noise level less than
20 mV peak-to-peak, is suitable
for tuning the filter. The signal can,
for example, be obtained from a ta-
chometer, photo-electric transdu-
cer, eddy current probe, accelerome-
ter, microphone, force transducer,
magnetic transducer, capac1t1ve
transducer etc . (with suitable pream-
plifier where necessary) which will
give a signal in synchronization
with some periodic motion of the
measuring object.
A manually set trigger level en-
ables the periodicity of the tuning
signal to be detected in the most su-
itable portion of the waveform ; this
adjustment together with the hyster -
esis
effect exhibited by the trigger,
makes it possible for a reliable con-
trol pulse to be obtained from al-
most any periodic waveform .
In addition to the "tachometer" in-
put , two other possibilities exist for
automatically tuning the filter: ( 1)
Where the signal to be filtered has
a
well defined periodic content it-
self, this can also provide the tun -
ing
signal. (2) In cases where a peri-
odic
component of the signal to be
filtered is not clearly defined, the fil-
ter can be tuned through any single
decade using the filter 's own output
as a tuning source. In this case, the
tracking frequency multiplier facil-
ity, of course, cannot be used. One
2
would normally expect this closed
loop
control system to be unstable,
but with the un i que digitally tuned
filter used in the 1623, this is not
the case.
Note that for ordinary frequency
analysis work the 1 623 can be auto-
matically tuned by the Sweep Unit
Type 5555.
Photo-Electric Transducers
for "Tachometer Tracking"
For analyses on rotating machin -
ery, a reliable method of tuning the
filter is by means of a photoelectric
transducer which gives a well de-
fined pulse each time a photo-diode
or transistor is triggered by an inter-
rupted light source .
The B & K Photoelectric Tachome-
ter Probe MM 0012 is a combined
infra-red light source and pickup de-
vice built into a common housing ,
which is ideal for triggering the
1 623. It is positioned up to 1 5 mm
from a convenient rotating machine
part so that the light beam is ref-
lected back from a contrasting band
attached to the part . A white paper,
or aluminium foil strip glued onto a
black background is ideal. The de-
vice is fitted with a red filter to re-
duce interference from mains-oper-
ated lights. Power for the MM
001 2 is derived directly from an
extra shield in the tachometer input
socket (BNC) of the 1 623.
Fig.2. Photoelectric Tachometer Probe MM
0012, available as an accessory for
the Tracking Filter Type 1623
Manual Tuning
By means of a tuning knob on the
front panel, the 1 623 can be manu -
ally tuned and used as an ordinary
tunable bandpass filter for fre -
quency analysis work in the field .
The tunable range from 2Hz to
20kHz is covered in this mode in
two ranges (2Hz to 200Hz and
200Hz to 20kHz) . The frequency
tuning knob is also used to preset
the filter centre frequency to the ap-
proximate starting frequency of the
tuning signal. This significantly re-
duces the time taken for the filter to
automatically lock onto the tuning
signal , especially at lower frequen -
cies.
Auxiliary Sweep Unit An alterna-
tive to manual tuning of the 1 623
for ordinary frequency analysis
work is the Frequency Sweep Unit
Type 5555 available on special order
from the B & K Systems Engineer-
ing Group . The 5555 connects to
the tachometer input of the 1 623
and facilitates automatic frequency
sweep between two preset frequen-
cies selected on the front panel of
the 5555. Start frequencies of 2,
20, 200, and 2000Hz may be
chosen while the upper frequency
may be set to 20 , 200 , 2 K, and
10kHz . Coverage up to 20kHz is
possible by using the frequency mul-
tiplier facility in the 1 623 .
It may be seen from Fig . 5 that for
a given filtered signal accuracy the
sweep speed is limited at high fre -
quencies by the writing speed of
the recorders, Types 2306 and
2309 . At low frequencies the limit -
ing factor is the 1 623 filter settling
time which in turn depends on the
filter bandwidth chosen . These val-
ues are set on the front panel of the
5555 so that changeover from one
limiting parameter to the other is au-
tomatic during sweep thus ensuring
the shortest possible analysis time .
The 5555 is equipped with a built-
in NiCd battery which may be re-
charged in-situ by battery charger
ZG 0113 which is supplied as an
accessory to the 1623 . Battery con-
dition is indicated on a meter .
Tracking Frequency Multiplier
This facility enables the filter to
be tuned to any ratio combination
between 1/ 99 and 99 / 1 times the
tuning signal frequency, thus allow-
ing any harmonic or sub- harmonic
(between these limits and within
the frequency limits of the instru-
ment) of the fundamental of the
measured signal to be examined .
This feature makes the vibration an-