Knowledge Bank

A drive system for a spectrographic recorder can easily be made which is accurately linear with frequency. If radiation passing through the spectrometer is also passed through a Fabry-Perot interferometer, the transmitted radiation will exhibit intensity maxima, or fringes, at equal frequency intervals. If the transmitted radiation is detected and the A.C. signal is amplified, it can drive the synchronous motor which moves the chart paper. If the spectrometer scanning rate is very slow, several methods can be used to increase the motor-driving frequency: the Fabry-Perot plates can have greater separation; ultraviolet light in high order can be superimposed on the regular beam; a frequency-multiplier can be employed; a separate oscillator can be used, with servo-control system to maintain constant rate of appearance of Fabry-Perot fringes; or the synchronous motor drive can be replaced by an electromagnetic stepping-ratchet. A scaling circuit can be used to provide a marker signal for the recorder every time N more fringes have passed, N being presumably a large integer.