ABSTRACT
Sometimes we can manage to switch a signal source on and off directly by, for example, controlling its power supply. More generally, however, this is not possible. In some cases the source will behave poorly (or fail!) if we keep turning it on and off. Sometimes the source we are interested is a natural one (e.g. a star) which we find rather difficult to control! Therefore most PSD systems employ some form of signal Modulo.tor or Switch which periodically stops the signal from reaching the detector. When making an optical measurement this modulator can take the form of a Beam Chopper which alternately blocks and unblocks the light path
between source and detector. Figure 16.1 illustrates one common type of modulator called a Chopper Wheel This is a disc which has a series of Blades cut around its periphery. (Sometimes a series of holes are cut around the edge of the wheel to produce a similar effect.) The wheel is placed so that its edge covers the beam and is rotated during the measurement. As the chopper wheel turns, its blades pass between source and detector, alternately blocking and clearing the signal path. If we use a symmetric wheel with n blades, rotating x times per second the source signal reaching the detector will be appear as a fluctuating level, varying periodically with a ChoppingFrequency,f = nx.
