TSI Instruments SIZER 3321, User Manual

Page: 47 / 101

Theory of Operation 5-7
when going through the second peak. It will stay inactive when the signal
falls back to or below the difference threshold.
Side-Scatter Circuit
The third circuit is the side-scatter-intensity circuit. Both double crested
analog signals from the low gain and the high gain outputs of the APD
module are filtered, gained, clamped, and input to separate peak hold
circuits, which hold the pulse height of the highest peak of each signal. An
A/D converter is then used to give the digital value of each of the side-
scatter-intensity pulse heights.
The high gain value is checked for an over range in the A/D digital value,
which indicates a large particle. If it is over range, the digital value from the
low gain A/D is output to the digital module. If it is not over range, the
digital value from the high gain A/D is output.
After the particle is processed, the peak hold circuits are cleared and
readied for the next particle.
The GATE signal, DIFF signal and the side-scatter circuit output are then
presented to the digital module to be processed and recorded.
Digital Module
The GATE and DIFF digital signals from the analog module are used to
trigger a high speed timer which has 4.0 ns resolution and a maximum
range of 4.096

s. After the gate signal goes active, the differential signal
(which indicates the first peak of the double crested analog signal) starts
the timer. The second peak then stops the timer, if the gate signal
remained active for the duration between peaks.
Two PAL ICs (Programmable Array Logic Integrated Circuits) are also on
the digital module. The first PAL is the Address PAL, which is used to
record the time-of-flight data and side-scatter intensity.
The GATE and DIFF signals are also used to create two more signals, a
gate window signal and a zero cross signal, which are provided to the
Address PAL. The Address PAL uses these two signals to classify the
particle into 4 separate events, in all cases a gate window signal must be
present and at least one zero cross must occur.
Event 1
Occurs when only one zero cross occurs during a gate window. This
happens when the particle is bright enough to exceed the threshold on the
differential circuit when it intercepts one of the beams but not bright enough
to exceed the threshold when it intercepts the other.
Detection
Threshold