Detector Module - Teledyne 514 Instruction Manual

2.0 Operational Theory

2.1.4 Detector Module

See Figure 2-4. After energy has passed through the sample, it arrives at
the filter wheel where it is fed alternately through two filters (measuring and
reference) before reaching the detector.
These filters are specially selected for each application according to the
absorption characteristics of the compounds under analysis. The reference
and measuring filter waveforms occur along a baseline at approximately 16
milliseconds intervals; each reference or measuring waveform reoccurs at a
time interval of 33 mS, or one per revolution of the filter wheel.
At the detector, infrared energy is transformed into electrical pulses and
fed through an impedance-matched preamplifier (see dwg. A-14619). De-
pending upon the application, length of the cell spacer, etc., the gain of the
preamplifier may vary from 1 to 10, depending upon the energy intensity at
the detector, to achieve an AC signal output of approximately 0.1 to 1.0 volt
peak-to-peak.
Additionally, the detector, filters, and preamplifier are housed in an
electrically and thermally isolated box to provide maximum stability and
minimum noise. This box, or compartment, is normally temperature con-
trolled at 46 °C.
The negative-going video from the preamplifier is fed to the clamp
circuit (see dwg. B-14561) to establish a precise zero reference to the
baseline of the pulses. This is accomplished by applying a gate to Ql at a
time when neither filter is in the energy path. This gated signal is fed
through A2 where it subtracts itself from the composite signal at the non-
inverting input. The signal output of A3 is clamped to ground and has an
amplitude of approximately two times the input.
The gating pulse for the clamp circuit is derived from the filter posi-
tion sensor which is located in the detector compartment. The sensor emits
radiation which is reflected from the white pattern on the rear side of the
rotating filter wheel and sensed by a photo transistor. This creates a square
wave of 5 volts amplitude at TP4 which is then further processed by Q2,
A4 and A5 to generate the gating pulse for the clamp circuit as well as the
switching signals S, S', P and P', which are later used to demodulate the
composite video.
The clamped-to-ground, negative-going video is then sent to the
automatic gain control circuit (see dwg. B-14564). This circuit receives a
reference signal from the peak level detector and uses it to adjust the
current through LEDs B1 and B2. The current through LED B1 controls its
2–8
Teledyne Analytical Instruments
A Business Unit of Teledyne Electronic Technologies