BNC 130 User Manual
Page 23
23
THEORY OF OPERATION
Analog
Switches
Z14-1 H
L
H
H
H
BIAS
Z14-8 L
L
H
H
L
CW + EM
Z14-9 H
H
L
L
H
CW + EM
Z14-16 H
H
L
H
H
CW
Z11-1 H
H
H
H
L
IMPULSE
Z11-8 H
H
H
H
L
IMPULSE
In summary, the PEAK LEVEL control voltage is inverted about ground by Z13-1 and also undergoes a
gain reduction of six (-6 V from PEAK LEVEL becomes +1 V at Z13-1). The current from the monitor
diode is converted at Z16-7 to approximately 1 V for a 1 mW optical output and is applied to Z16-2 where
it will almost cancel the signal from Z13-1. If this does not happen, a large error signal appears at Z16-1
where it passes via Z14-4 and Z8-1 to the current generator, Z7 and Q1 (sheet 3)
External Modulation Mode (Schematic Sheets 1 and 3)
We have traced in detail, for CW Mode, the manner in which the PEAK LEVEL control voltage causes a
predictable current to be applied to the laser. Similar considerations show how the quiescent light level
for the External Modulation Mode is derived. The PEAK LEVEL control voltage is still used to set the
desired optical level (around which the modulation will occur). Feedback via Z16-7 is used as with CW.
Z1 1-1 is high (as for CW) but now Z14-16 is also high. This has the effect of routing the error signal
(Z16-1) through Z8-7 (a precision rectifier).
It is then applied (sheet 3) as the ETT MOD OR PULSE AMPLITUDE signal to Z1-5 (a x2 attenuator
followed by an adjustable x3 gain stage). Since PREDRIVE is held high, the laser's dc bias is now
provided by Q9 (via 06) R108 samples the current which is compared to the voltage from Z1-7 by a
bridge arrangement (R72, R73, R74, R95). Z2-7 generates the error signal applied to Q8 and then to Q9.
The actual modulation is applied from J801, via FET current amplifier Q11, to Q8 where it generates a
base voltage for Q9 which (via R112) generates a predictable signal current through R108 and Q6 that
finally drives the laser. Although this circuit appears to be direct coupled from J801, it acts as if it were ac
coupled due to the action of Z2, which monitors the total laser current (flowing through R108). The low
frequency cut off is approximately 100 Hz.