Manual, Ultra High Sensitivity Aerosol Spectrometer (UHSAS)
DOC-0210 Rev E-4
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© 2017 DROPLET MEASUREMENT TECHNOLOGIES
2.3.2
Laser Power Monitoring
The reference detector is used as a voltage reference and automatic gain control. It is also used
for monitoring laser output power, which is directly proportional to the laser cavity power. As
the laser power drifts, the instrument maintains calibration. Only large drifts in power (> 25%)
require a recalibration of the instrument, since the automatic gain control mostly compensates
for small drifts, and the particle sizing sensitivity is a sixth-root function of laser power.
2.4 Digital Electronics System
2.4.1
ADCs and Peak Height Analysis
For each of the four gain stages (2 primary, 2 secondary) there is an associated analog-to-digital
converter (ADC). The ADCs run a 16-bit conversion at 500 kHz sample rate. The chain of events is
begun as a particle traverses the laser mode and begins scattering light. The particle signal from
the highest gain stage on the primary detector (G3) feeds an analog comparator. If the signal
exceeds a preset, user-defined threshold, it generates a particle trigger. The threshold value is
independent of the particular active bin map; see the entry for
Trigger Threshold
in section 0.
Under typical operating conditions, the trigger threshold should be set to register the smallest
detectable particle (60 nm diameter).
Figure 7: Block Diagram of Analog Electronics
