BRUEL & KJAER 4912 Instruction Manual page 8

EXT. TRIG. INPUT:
FREQUENCY:
FREQUENCY SELECTOR:
FUNCTION SELECTOR:
INTERNAL GENERATOR:
4
shows that the battery voltage is insufficient, the battery should be re-
charged (section 3.2 . 1 ).
Three-pole coaxial socket accepting standard TNC plug JP 031 5 (sup-
plied) or standard BNC plug JP 0035 to trigger the stroboscope lamp
externally or feed the tachometer. The sheath (outermost contact) is
ground, and is connected to pin 2 of POWER INPUT. The middle con-
tact (ring) carries a positive DC supply for an active tachometer probe
such as MM 0012. This supply is + 5 V to + 7 V when the 4912 is
powered from its internal batteries, and + 6 V to + 1 5 V when it is pow-
ered from an external source. The innermost contact (tip) is the trigger
input. The 4912 is triggered by a positive-going voltage at a threshold
set by TRIGGER LEVEL between -0, 1 V and + 0, 1 V (rising to -0,2 V
to + 0,2 V at 20000 Hz). Input impedance is 50 kO approx., in series
with 680 nF. Max . input 30 V Pk-Pk.
4-digit liquid crystal display indicating either EXT. TRIG. INPUT fre-
quency or INTERNAL GENERATOR frequency as selected by FUNCTION
SELECTOR. Displays hertz or revolutions per minute (r/min) in four
ranges selected by FREQUENCY SELECTOR.
4-position switch for selecting FREQUENCY display indication.
"x
10 r/min" Display range 10 to 90000 r/min.
"r/min" Display range 1 to 9000 r/min.
"Hz" Display range 1 to 1 500 Hz.
"Hz" Display range 0, 1 to 1 50,0 Hz.
Four-position switch.
"Power Off" Power is removed from the 491 2. The FREQUENCY dis-
play may continue to indicate for a few seconds after power is
switched off.
"Tacho" The 491 2 behaves solely as a tachometer, displaying the fre-
quency of the EXT . TRIG. INPUT signal. The lamp does not light. Upper
limit 1 500 Hz (signified by flashing of FREQUENCY display).
"Ext. Trigger" The 491 2 behaves as a combined stroboscope and ta-
chometer. The lamp flashes in synchronism with the EXT. TRIG. INPUT
signal, and the FREQUENCY display indicates the frequency of this sig-
nal. If this frequency exceeds 1 30 Hz, the stroboscope lamp will flash
at the highest submultiple of the trigger frequency less than 1 30 Hz,
and will continue to synchronize correctly with EXT. TRIG. INPUT fre-
quencies up to 20 000 Hz. The display indicates the signal frequency
provided this lies within the range selected with FREQUENCY SELEC-
TOR.
"Int. Gen." The 491 2 behaves as a free-running stroboscope. The
stroboscope lamp flashes at the frequency set by INTERNAL GENERA-
TOR and indicated on the FREQUENCY display in Hz or r/min as se-
lected. The frequency range available is 5 to 125 Hz.
10-turn thumb-wheel for setting the frequency of the internal genera-
tor between 5 Hz (maximum anticlockwise) and 1 25 Hz (maximum
clockwise). Operative only in "Int. Gen." position of FUNCTION SELEC-
TOR.