THEORY OF OPERATION Model 360E Instruction Manual

Thermo-Electric
Cooler
Control Circuit

Sample &

Hold

Circuits

(x4) Amplifiers
Dark
Switch
Photo-
detector
Pre Amp

Phase

Lock

Loop

E-Test
Generator

Compact

Programmable

Logic Device

M/R Sensor
Segment
Sensor
E Test Control
Dark Switch
Control
X10 Clock
Phase Lock
E Test A Gate
Dark Test Gate
X1 Reference
Segment
Status LED
Phase Lock Warning
M/R
Status LED
Segment Clock
E Test B Gate
Measure Gate
Measure Dark Gate
Reference Gate
Reference Dark Gate

CO2 MEAS

CO2 Reference

56V
Bias
Variable
Gain Amp
From GFC
Wheel
From CPU
via Mother
Board
Signal
Conditioner
Signal
Conditioner
10
x10
TEC Control
PHT DRIVE

Figure 10-12: 360E Sync / Demod Block Diagram

10.3.4.2. Signal Synchronization and Demodulation

The signal emitted by the IR photo-detector goes through several stages of amplification before it can be

accurately demodulated. The first is a pre-amplification stage that raises the signal to levels readable by the rest

of the synch/demod board circuitry. The second is a variable amplification stage that is adjusted at the factory to

compensate for performance variations of mirrors, detectors, and other components of the optical bench from

instrument to instrument.

The workhorses of the sync/demod board are the four sample-and-hold circuits that capture various voltage

levels found in the amplified detector signal needed to determine the value of CO2 MEAS and CO2 REF. They

are activated by logic signals under the control of a compact programmable logic device (PLD), which in turn

responds to the output of the Segment Sensor and M/R Sensor described in Figure 10–11.

176 05232 Rev B3