Teledyne GFC-7001E - Trace CO Analyzer - Manual (Page 243)

Theory of Operation Model GFC7001E Carbon Dioxide Analyzer
Teledyne Analytical Instruments 243
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 MEAS
CO 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 11-12: GFC 7001E/EM Sync/Demod Block Diagram
11.5.4.2. 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 sync/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 CO MEAS and CO 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 as shown in Figure 11-9.
The four sample and hold circuits are:
Table 11-2: Sync DEMOD Sample and Hold Circuits
Designation
Active When:
IR BEAM PASSING THROUGH
Segment Sensor Pulse is:
Measure Gate MEASUREMENT cell of GFC Wheel HIGH
Measure Dark Gate MEASUREMENT Cell of GFC Wheel LOW
Reference Gate REFERENCE cell of GFC Wheel HIGH
Reference Dark Gate REFERENCE cell of GFC Wheel LOW