TSI Instruments SIZER 3321, User Manual

Page: 46 / 101

5-6 Model 3321 Aerodynamic Particle Sizer

Spectrometer
Analog Module
Signals arriving at the analog module are sent to three different circuits for
processing: the gate and differential circuits, and the side-scatter-intensity
circuit. The output of these three circuits are then passed on to the digital
module.
Gate Circuit
The gate circuit produces a digital gate (GATE) signal, or gate window,
which represents the presence of the particle in the measurement volume.
Producing this signal is accomplished by first adding the high gain and 10
times the low gain signal together. This addresses the dynamic range of
pulse heights, roughly three orders of magnitude, for the size range of
particles (0.3 to 20

m).
This summed signal is then sent through a delay line. Two different taps
from this delay line, roughly 300 ns apart, are added together, filtered,
gained, and clamped. This creates a signal that has very little valley in it
and prevents it from going below the gate threshold. This signal is
compared to the gate threshold, typically about 100 mV and can be set by
the user through a serial command (see
Appendix C ). If it is above the
threshold, the digital gate (GATE) signal goes active. When the signal
drops back below the threshold, the GATE signal goes inactive. The time
during which the GATE signal is active is called the gate window.
Differential Circuit
The differential circuit produces a digital difference (DIFF) signal that
indicates the zero crossings of the differentiated double crested analog
signal, which occur when the slope of the signal is equal to zero. Zero
crossings, DIFF signal transitions, are positive going for a valley and
negative going for a peak. The time between the negative going zero
crossings of the differential signal (the peaks of the double crested analog
signal) is called the transit time, or time-of- flight (TOF), of the particle. This
time ranges from about 800 ns to 4.1 µs. It is this time, along with the
calibration of the instrument, that determines the aerodynamic size of the
particle.
The differential circuit produces the differential signal by taking two
different taps from the delay line used in the gate circuit, roughly 300 ns
apart and delayed from the first tap of the gate circuit by 20 ns. These two
signals are then subtracted from each other, filtered, gained, clamped, and
then filtered again. This produces a analog difference signal, both positive
and negative, that is analogous to a differential signal. This difference
signal is compared against a difference threshold, typically 0 V and is set
via a potentiometer adjustment at the factory, producing a digital difference
(DIFF) signal. If the difference signal is at or below the threshold, the DIFF
signal is inactive. When the difference signal goes above the threshold the
DIFF signal will go active giving a rising edge. When the summed signal
goes through a peak the difference signal falls below the threshold and the
DIFF signal will go inactive giving a falling edge. The DIFF signal will give a
rising edge when going through the valley, and then give a falling edge