Teledyne GFC-7000T - Trace CO2 Analyzer - manual - Manual (Page 221)

Theory of Operation Model GFC7000TA Carbon Dioxide Analyzer
Teledyne Analytical Instruments 203
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
CO
2
MEAS
CO
2
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 9-13: GFC 7000TA Sync / Demod Block Diagram
9.3.4.2. 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.