BNC 130 - Manual (Page 21)

21
THEORY OF OPERATION
Table 3-2 shows how these digital control signals affect the condition of the analog switches in each
Mode. The switches are identified by their control terminals (e.g., Z14-9); "L" and "H" indicate low and
high logic/voltage levels. By turning on and off these switches, these digital signals provide proper
routing for BASELINE LEVEL and PEAK LEVEL, the analog signals that control the amplitude at the
laser output. As an example, in the Pulse Mode with a nonzero Baseline (and not using External Drive),
the only high input to Z9 is BIAS (Z9-5). Thus Z9-10 and Z9-12 are both high and Z9-6 is low. Z14-1 is
low and this connects the BASELINE LEVEL amplitude control to Z8-3 via Z13-7 and Z14-3 Other Modes
may be similarly analyzed using this table.
Three digital control signals, EXT MOD, CW (sheet 1) and EXT DRIVE EN (sheet 2). select which drive
source is presented at the output of the ECL multiplexer, Z4 In Pulse Mode either the DRIVE signal from
the mainframe or the EXT DR signal from the module front panel can be selected. This signal, after
passing through Z3 and R162, becomes PREDRIVE, which determines when the laser is switched
between Peak and Baseline levels. In External Modulation Mode, EXT MOD selects X4 (Z4-11), which is
tied high, for the multiplexer output, causing PREDRIVE to be held high. In CW Mode, X2 is selected,
causing PREDRIVE to be held low.
Laser Module
The Laser Module is shown enclosed by dashed lines within which are six components. The laser itself is
the left hand diode whose anode is grounded and connected to the case at pins 13 and 13. The case is
connected to ground via pins 1 and 2. A 170 µH choke is provided internally for applying dc bias to the
laser. This is used in all the Pulse Modes with nonzero Baseline and in the CW Mode. The diode shown
between pins 7 and 8 is a built in monitor (detector) that is used for controlling the laser's output in the
CW, External Modulation, and Impulse Modes and is also used to detect excessive optical power. An
internal 22 ohm resistor provides wideband termination for the high speed signals from 06. A
thermoelectric cooler (TEC) is incorporated in the module and is shown connected between pins 3/4 and
3/6. Its associated temperature sensor is connected between pins 11 and 12. The temperature is
maintained at approximately 20° C by means of a feedback loop consisting of Z1 and 010. This loop
supplies enough cooling current to bring the sensor voltage to 2.9 V (Z1-3). R104 limits the maximum
current during start-up to a safe level (approximately 2 A).
CW Mode (Schematic Sheets 1 and 3)
CW operation utilizes the dc current source, Q1 and Z7. The voltage from Z8-1 (labeled CW OR
BASELINE LEVEL) is applied to one arm of the resistor bridge, R36, R37, R63 and R66. This same
voltage is forced to appear across R82 and R83 A typical calibration is for R82 and R83 to be 30 ohms
so that 20 mA/V is produced by Q1. This current flows (via R81, L1, and the internal 170 µH inductor)
through the laser to ground. Since PREDRIVE is held low the path for current through Q6 will be cut off.
Once the threshold current of the laser is exceeded, the relationship between the current in the laser and
its optical output is linear.