BNC 130 - Manual (Page 20)

20
THEORY OF OPERATION
This includes the Modes that are valid, parameter boundaries, and the type of output that the module has
(optical or electrical). It also contains the values for initializing the nonvolatile RAM.
Z4 is a 2K byte nonvolatile RAM (NVRAM). It is used to save instrument settings and power-on
conditions.
Z5 is a configurable Parallel Peripheral Interface 1C set up to allow 16 bits of output and eight bits of
input. The outputs are used to select the operating state of the Output board while the inputs monitor the
board's status.
DACs Z6 and Z7. in conjunction with op amps Z8 and Z9, generate analog signals that set the value of
the Peak, Baseline, and quiescent level of the output. They are also used to set the threshold level for
the External Drive discriminator. The variable resistors, R3-R6, are used to compensate for slight
differences in the DACs.
The voltage reference for the analog signals is CR1, a 6.2 V temperature compensated zener diode. Q1
is used to disable CR1 if the signal DAC EN is allowed to go up to +12 V. The Output board asserts this
in the event of a Mode change or if any unusual condition is detected.
The MOD DISABLE line allows the Output board to be disabled via the 6040 rear panel MODULE
DISABLE connector.
The Module Interface board has two grounds, digital and analog. The analog ground is used exclusively
by the DAC and op amp circuitry (Z6, Z7, Z8, Z9, etc.), while all other circuitry is connected to digital
ground. These are separated to prevent any noise or dc offsets by power supply return currents from
affecting the analog control voltages.
The 20-pin connectors J1 and J2 deliver the digital and analog control signals to the Output board, while
the 16-pin connector J11 delivers the power supply voltages. J3 is an expansion connector to be used in
conjunction with future modules.
LASER OUTPUT BOARD (Schematic 155-31)
Inputs and Control Signals (Schematic Sheets 1 and 2)
The Laser Output board receives three types of inputs: digital control signals from the Interface board via
P1, analog control signals from the Interface board via P2, and drive and external modulation inputs via
front or rear panel coax connectors. Four of the digital control signals, IMPULSE, EXT MOD, CW, and
BIAS, are used to control analog switches, Z14 and Z11 (sheet 1). These switches are in their low
impedance (closed) state when the control input, the terminal with inversion circle, is low (near ground).