BNC 130 - Manual (Page 24)

24
THEORY OF OPERATION
Pulse Mode
In Pulse Mode, two conditions exist: the circuitry involved when the Baseline level is set to zero is
different from the circuits used with a nonzero Baseline. In Pulse Mode with zero Baseline, there is no
optical output between pulses (during a logical "zero" there is zero light output). The upper level is set by
the PEAK LEVEL control voltage which, in turn, determines the amount of current switched into the laser.
In Pulse Mode with a nonzero Baseline, both the upper ("one") and lover ("zero") logic levels must be
adjustable according to the Peak and Baseline settings.
Pulse Mode (Nonzero Baseline)
Pulse Mode with nonzero Baseline requires 1) producing a current that determines the optical power
radiated between pulses (the Baseline), and 2) calculating the difference between the desired Peak
power and the Baseline power, and then generating a pulse of corresponding amplitude. This pulse is
added to the already present Baseline level to produce the desired Peak level. The Baseline current is
generated and applied to the laser in the same manner as for CW operation, except that optical feedback
is not used.
The path from the BASELINE LEVEL control voltage is through Z13-7, Z14-3 (conducting), past Z14-14
and Z1 1-3 (both open), to Z8-1 (unity gain) and finally to Z7 (sheet 3). As stated above, Z7 and Q1 form
a current source, and this is what supplies the Baseline current to the laser.
The path from the PEAK LEVEL control voltage is through Z13-1 (sheet 1), Z16-1, past Z14-15 (open), to
ZS-7, through CR7, Z1-7 (sheet 3), and finally to Z2-7. As stated above, Z2-7, Q8 and Q9 form a current
source which also drives the laser. Pulse Mode differs from External Modulation in that the current is
switched by Q6 and Q7 (as defined by the level of the PREDRIVE signal) and there is no optical
feedback.
The pulse current (through Q6 and Q7) is determined for PEAK LEVEL by the precision rectifier formed
by ZS and CR7 and the two resistor dividers R31/R40 and R32/R39 (sheet 1). This circuit produces a
voltage which is the difference between the voltages at Z16-1 (the scaled version of the PEAK LEVEL
control voltage) and Z8-1 (the scaled version of the BASELINE LEVEL control voltage), yet not less than
zero. The output, at the cathode of CR7, is applied to Z1-5 (sheet 3) which provides an adjustable gain of
from xO.5 to x2. At this point, a 3 V difference between the control voltages has become 500 mV at Z1-7.
Z2-7 and R108 (47 ohm) convert this to approximately 10 mA, which is the value needed for a 0.30 mW
optical power change. Thus a 3 V difference between the PEAK LEVEL and BASELINE LEVEL control
voltages is converted to a 0.30 mW optical step. Since this step is added to the Baseline level already
present (via Z7, Q1) the Peak and Baseline levels are determined by their respective control voltages,
provided that the "gains" (voltage to current conversion factors) of the two channels are set to be equal.