BRUEL & KJAER 1623 - Tuning from the Filtered Output Signal

0,25
0,5
c
] 2,5
~
u
~ 5
~
~ 10
a:
c.
Q)
~ 25
~ 30
g
....1 50
100
250
500
100~
50 .........
N ~I" I . I . I
""
ote : a 1d for denommator settmgs D < 1 0
25G "
for D > 10, multiply Time by -RJ
10 ~
I I 110
---5~~ ~
and multiply Sweep Rate by
0
-u; I~ "~
- ~
~ 2,5 - ~ ~ -z.,
~~-1 -~~----~ - _ _'~e •• " ~
-- --+-,~"
0,5
! ........_ ~% li. 1 0"<"-? ""' Max. sweep rate with 2306
I ": ~ '""
0,25
I O"t-?
I
~~
G
"~
I
I
~
0,1
""'
I~ ~
I
I
0,05
I I I'...
2
5
10
20 50
100 200
500 1 000 2000 4000
Frequency (Hz)
760863 / 1
Fig. 4. B. Chart for determining the recommended maximum fre-
quency sweep rate and the recommended minimum fre-
quency sweep time for a filtered signal amplitude inaccu-
racy < -1 dB (additional to specified filter gain accuracy).
The
example shows a 10Hz to 50 Hz sweep with the 12%
bandwidth filter requiring a sweep time of 5 s - 1 s = 4 s.
With a logarithmic sweep the sweep rate (determined by
the lowest frequency, 10Hz) will be 5 decades/min. The
sweep rate could be progressively increased with increa-
sing frequency to 25 decades/min. at 50 Hz
4.9. TUNING THE 1623 FROM THE FILTERED OUTPUT SIGNAL
In certain circumstances, for example where an external tuning signal is not available
and where the filter input signal is unsuitable, the filter can be tuned from the filtered
output signal. This only applies to tuning within one of the decades 2Hz to 20Hz, 20Hz
to 200Hz, 200Hz to 2kHz, and 2kHz to 20kHz, and assumes that a well defined fre-
quency component can be located.
A BNC T
connector is used at the FILTER OUTPUT socket with one leg feeding the mea-
suring instrument used and the other leg connected via a BNC-BNC cable to the TA-
CHOMETER INPUT socket.
To
obtain a stable tuning frequency it will often be necessary to average over several pe-
riods
of the tuning signal. This is achieved by setting the TRACKING FREQUENCY MULTI-
PLIER knobs to, for example, 04/04.
A suitable tuning signal must be located in the filter output signal by searching with the
MANUAL TUNING/PRESET knob and the trigger level must be adjusted to a suitable le-
vel.
Setting up the filter in this way will often take some time and patience but the task
can be simplified by monitoring the tuning signal with an oscilloscope (via a BNC 'T' con-
nector at the Tachometer Input). The miniature battery operated Oscilloscope Type
4
714 is convenient for this purpose. Setting-up is more critical with narrow bandwidths,
therefore select the 23% bandwidth filter when possible.
25