Wavelength Stepping in a Dual Beam Spectrophotometer

摘要

A dual beam spectrophotometer includes a radiation chopper 5 and a monochromator 3 which includes a diffraction grating driven by a stepper motor. The chopper 5 includes a gate pulse generator 24 which produces gate pulses GS and GR which correspond to periods during which radiation from the source 2 passes through a sample cell 19 and a reference cell 14 respectively. A stepping motor drive circuit 134 produces pulses to step the stepper motor which are synchronised with the chopping cycle by means of the GR and GS pulses. The stepper motor is arranged to step an equal number of times in each half of a chopping cycle, so that disturbances caused by stepping can be filtered out. A further condition which is preferably satisfied is that the first pulse of a sequence should start in the opposite half of the chopping cycle from that in which the first pulse started in a previous chopping cycle. At low stepping rates the motor will step twice in one chopping cycle and then pause for several cycles before again stepping twice. IMAGE