Teledyne gfc 7000ta operation manual Signal Synchronization and Demodulation

Theory of Operation Model GFC7000TA Carbon Dioxide Analyzer

56V

Bias

			Variable	Sample &	CO2 MEAS
Photo-	Pre Amp	Dark			Signal
		Switch	Gain Amp	Hold
detector
					Conditioner
				Circuits
	TEC Control
	PHT DRIVE	E-Test
		Generator			CO2 Reference

	Signal
(x4)
	Amplifiers
	Conditioner

Thermo-Electric

Cooler

Control Circuit

				E Test A Gate
				E Test B Gate
				Dark Test Gate
	Compact			Measure Gate
	Programmable			Measure Dark Gate
	Logic Device			Reference Gate
				Reference Dark Gate
M/R Sensor					Phase Lock Warning
	From GFC
Segment	Wheel
Sensor				Segment Clock
E Test Control				X1 Reference	Phase
				x10
	From CPU
		÷	10		Lock
Dark Switch	via Mother			X10 Clock
					Loop
Control	Board
						Segment
					M/R
				Phase Lock	Status LED	Status LED

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.

Teledyne Analytical Instruments 203