Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 60)

The goal of the high current adjustment procedure is to make the movement of the absorption line symmetrical about its
center line as possible, for equal changes in the DC current. The undesired asymmetry is caused by the laser
temperature perturbation. As the laser warms up at the start of the ramp, it warms more quickly at first, and then more
slowly as it approaches equilibrium. The change in temperature causes a corresponding change in the laser’s emission
frequency that may require a larger change in DC current to move the absorption line to the left edge of the spectral
scan than the right edge. If the high current pulse adequately cancels the temperature perturbation, the laser’s emission
frequency will be determined only by the linear ramp in the current, and the same change in DC current will move the
absorption line to the left or right edge of the spectral scan. This is illustrated in Fig
. ure 4-11
Figure 4-11. High Current Adjustment Procedure
+0.6 mA
+0.3 mA
Line lock on
-0.3 mA
-0.6 mA
High current
too low High current
correct High current
too high
To set the high current, follow these steps:
1) Set the High current to an initial value (start at zero mA when in doubt)
2) Press “I” to start line lock
3) Set the modulation current as discussed in section 4.4.5
4) Select DC Current in the Dynamic parameter column
5) Repeatedly press the “-“ key to decrement the DC current in 0.1 mA steps until the center of the absorption line is
at the right edge of the spectral scan. Count the number of steps required.
6) Press “I” to start line lock again. Watch the reference detector display as the lines to come to the center of the
spectral scan.
7) Repeatedly press the “+” key to increment the DC current in 0.1 mA steps until the center of the absorption line is
at the left edge of the spectral scan. Count the number of steps required.
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