Agilent Technologies 1100 Series Reference Manual page 264

8 Theory of Operation
The photomultiplier tube needs an adjustable working voltage between -240 V and -820
V for correct operation. The PMT gains are realized by variable operating voltage of
the device. A regulated pulse width modulator is used to generate the high voltage by
switching the 24 V rail to a transformer. The feedback loop has a D/A-converter-
controlled base voltage to set the desired output voltage. The PMT output delivers a
current signal which is proportional to the detected light and to the actual working
voltage.
Reference System
A reference photodiode is located behind the cell and monitors the incoming radiation
through a diffusor window. The data are used to compensate the fluorescence data for
drift effects. The reference path provides multiple gain stages which are set
automatically according to the actual intensity.
Cell Detection
264 1100 Series FD Reference Manual