Model 300E Carbon Monoxide Analyzer
Instruction Manual
THEORY OF OPERATION
P/N 04288 Rev: A
171
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 described in
Figure 10–11. The four Sample and Hold circuits are:
Active When:
Designation
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
Timing for activating the Sample and Hold circuits is provided by a Phase Lock Loop
circuit (PLL). Using the Segment Sensor output as a reference signal the PLL
generates clock signal at ten times that frequency. This faster clock signal is used
by the PLD to make the Sample and Hold Circuits capture the signal during the
center portions of the detected waveform, ignore the rising and falling edges of the
detector signal.
Sample & Hold
Active
Detector
Output
Sample & Hold
Inactive
Figure 10-12: Sample & Hold Timing