BNC 130 - Manual (Page 25)

25
THEORY OF OPERATION
PREDRIVE, the timing signal from the multiplexer (Z4), is applied (from Z3-2) to the base of predriver
Q4. Q4 and Q3 are a switching pair whose current is controlled by Z2-1 and Q3. The predrive current
through Q4 and 03 increases with increasing optical output and, as the current increases, the main
drivers Q6 and Q7 receive larger switching voltages. This is achieved by utilizing a current source.
comprised of Z2-1 and 03, which tracks the EXT MOD OR PULSE AMPLITUDE control voltage at Z1-7.
A small offset is introduced into this current source so that at very small pulse levels there is sufficient
predrive current to switch Q6 and Q7.
Pulse Mode (Zero Baseline)
With Zero Baseline, only the switching transistors Q6 and Q7 (sheet 3) supply current to the laser. The
PEAK LEVEL control voltage is processed directly through Z13-1. Z16-1 and Z8-7. Switches Z11-8, Z14-
1, Z14-9 and Z14-6 are open and Z14-8 is closed. Thus Z13-1 is a x6 attenuator while Z16-1 is a x1
voltage follower. Since the switch Z14-1 is open, the BASELINE LEVEL control voltage is not presented
to Z8-3, which will now be held at ground by R24. This allows Z8-7 (through CR7) to be a xl follower of
the voltage at Z16-1
This has two implications. 1) the CW OR BASELINE LEVEL current source is turned off, which prevents
any quiescent light and 2) the PEAK LEVEL control voltage adjusts the pulsed current to values that
produce a power level between zero and 1.0 mW (the same range that the CW OR BASELINE LEVEL
current source uses with a nonzero Baseline Pulse Mode, as determined by the BASELINE LEVEL
control voltage.
External Drive (Schematic Sheet 2)
External Drive is used in the Pulse Mode to shift the source of the drive signal from the mainframe to the
front panel EXT DR connector. The circuits behave the same as under ordinary Pulse Mode operation
except that the PREDRIVE signal is now obtained (via the multiplexer Z4-5) from the External Drive
circuit. Two control signals from P2, EXT DRIVE LEVEL and EXT DRIVE POL, select the trigger
threshold and polarity for the signal presented to the EXT DR jack.
Impulse Mode
Impulse Mode is used to generate a narrow, fixed width and fixed amplitude pulse. The digital timing is
obtained from PREDRIVE (sheet 2). The leading edge of the impulse signal Z3-2 is caused by DRIVE
itself. R157 and Z3-15 provide a short time delay (less than 2 ns) that produces the trailing edge. The
result is a short, positive pulse delivered to the base of Q4 (sheet 3).
The predrivers (Q4 and Q3) and the main drivers (Q6 and Q7) have both been programmed by PEAK
LEVEL for a high (120% of full scale) current that represents the amplitude of the impulse waveform. In
order to assure fast response, the laser is biased to just above its threshold current by an auxiliary
feedback loop that holds the laser at this point regardless of duty factory variations. The optical output is
monitored by internal detector diode DSI-7, and in Impulse Mode represents the average optical power.
Both the Baseline level and the average power of the impulses themselves contribute to this signal. The
latter contribution varies with rep rate (duty factor).