TSI Instruments SIZER 3321, User Manual

Page: 45 / 101

Theory of Operation 5-5
S i g n a l P r o c e s s i n g P a t h
Signal processing is performed in the APD, Analog, and Digital modules.
APD Module
Signal processing begins at the APD module, where scattered light from
the particle is detected and converted into an analog voltage signal. This
signal, referred to as a double crested signal, consists of a pair of peaks.
Figure 5-3
Typical Example of a Double Crested Signal
Each peak represents the presence of the particle in the center of the
individual laser beams and can range in amplitude 0
–10 volts. The signal is
gained and buffered into two channels. One channel is for very small
particles and has a high gain. The other channel is for large particles and
has a low gain. Once the signals are gained up they are passed to the
analog module.