Operation From An External Trigger; Triggering From Another Instrument; Triggering From Photoelectric Probe Mm 0012 - BRUEL & KJAER 4912 Instruction Manual

3.4. OPERATION FROM AN EXTERNAL TRIGGER
In both the "Tacho" and "Ext. Trig." settings of FUNCTION SELECTOR, the 4912 will op-
erate only if a suitable changing voltage is applied to the EXT. TRIG. INPUT, as described
below.
In the "Tacho" setting, the Stroboscope lamp does not flash at all - this position enables
the 491 2 to function solely as a tachometer, indicating the frequency of the EXT. TRIG.
INPUT voltage on its FREQUENCY display, in a form selected by means of FREQUENCY
SELECTOR. The indication limits are 5 to 1500 Hz (corresponding to 300 to 90000 revo-
lutions per minute). If the external trigger frequency exceeds the upper limit, the FRE-
QUENCY display will flash to indicate overload. The 491 2 will operate for approximately
15 hours from the internal batteries between charges in the "Tacho" mode.
In the "Ext. Trig." setting of FUNCTION SELECTOR, the lamp is triggered by the voltage
at EXT. TRIG. INPUT, and although the maximum flash rate is limited to 130 flashes per
second, the flashes can be synchronized with trigger frequencies up to 20000 Hz. The
FREQUENCY display indicates the trigger frequency. In this mode the lamp can also be
triggered by events occurring at irregular or sequentially varying intervals.
The 491 2 is triggered externally by a rising voltage passing a threshold set between
-0, 1 V and + 0, 1 V by means of TRIGGER LEVEL, which should be adjusted as neces-
sary to obtain satisfactory triggering. At the highest trigger frequencies the trigger range
rises to -0,2 V to+ 0,2 V.
Specific considerations concerning external triggering by other instruments and photo-
electric and magnetic probes are dealt with in the following sections.
3.4.1. Triggering from another Instrument
The 491 2 may be triggered from the AC recorder output from a measuring instrument
such as the B & K Portable Vibration Meter Type 2511, for use in dynamic balancing, for
example. It may be triggered from an excitation source such as the output of a sine gen-
erator or exciter control (B & K Types 1023, 1027 or 1047 for example) for loudspeaker
analysis or mode study vibration testing. For these applications the appropriate output
from the instrument should be connected to EXT. TRIG. INPUT on the 491 2 by means of
a standard BNC plug (JP 0035). Various ready-wired cables are available from B & K.
The EXT . TRIG. INPUT on the 4912 has an input impedance of 50 kO and is AC-coupled
by a 0,68 µF capacitor. The alternating component of the voltage applied to it should not
exceed 30 V Pk-Pk. The DC component should be kept within ± 63 V. The trigger level
may be adjusted as required between -0, 1 V and + 0, 1 V (-0,2 V to + 0,2 Vat 20 kHz)
as described above.
3.4.2. Triggering from Photoelectric Probe MM 0012
The Photoelectric Tachometer Probe MM 0012 (Fig.1, Chapter 1) provides a very conve-
nient transducer for triggering the 491 2 where the range of motion is great enough,
i.e., more than a few millimetres. The MM 0012 is supplied complete with a 3 m con-
necting cable AO 0158 which should be used to connect it to EXT. TRIG. INPUT on the
4912. This cable also carries the supply to the MM 0012, which is an active device. It
should not be connected if the 491 2 is powered from an external supply exceeding 11 V
DC as this may damage the Probe.
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