GB760729A - Improvements in or relating to spectroscopes - Google Patents

Abstract

760,729. Electric spectrometers. GRAESSER, Ltd., R. Dec. 16, 1954 [Dec. 16, 1953], No. 35112/53. Class 40 (3). [Also in Group XL (c)] . The resolution of a spectrometer of the type in which a radiation sensitive device such as a bolometer produces an electric output which varies in intensity with radiation transmitted by the specimen is improved by differentiating the output signal an even number of times, preferably twice. Fig. 1 indicates how the two adjacent peaks are clearly separated in the differentiated signal although obscured in the direct signal. If the logarithm of the output is also taken before differentiation the amplitude of the resolved peak will then directly indicate the density of the components present when compared with the amplitudes of the absorption maxima in the corresponding curve obtained from a known of the component. The invention is described with reference to a single beam spectrometer in which the output is chopped at 16 <2>/3 cycles/sec. As shown (Fig. 3) the output of the spectrometer is applied through a filter to remove signals due to the 50-cycle mains. A sinusoidal current is produced in resistors 1, 2 and the potential across resistor 2 is amplified and inverted by high-gain pentode 7, impedance transformed by cathode follower 8 and applied through a negative feedback loop comprising logarithmic rectifiers 3 to the junction of resistors 1, 2. A log-sine voltage derived from the input to the rectifiers is applied to phase splitter 4 rectifier 5 inverter amplifier comprising pentode 6 and triode 9 and fed to two differentiating circuits each as shown in Fig. 4. Resistor 10 may replace the rectifier to give a linear output. The differentiating circuits each consist of a condenser 12 the series impedance of which is constituted by a cathode follower 16. The output signal is tapped across an anode load. Error voltages at the cathode may be reduced to negligible levels by means of a high-gain amplifier 17. The grid of this valve samples voltage " changes " at the cathode, amplifies them and applies them inversely to the grid of the cathode follower. The Provisional Specification described an alternative logarithmic circuit (Fig. 3, Provisional Specification) consisting of a thermionic diode which has an exponential relationship between diode current and the voltage change between anode and cathode. The applied voltage is phase split at 3, rectified at 6 and fed through a 10-20 megohm resistor to the diode so that a logarithm of the applied voltage v appears at the anode. However, a steady voltage s due to electrons reaching the anode by virtue of their velocity also exists at the anode. The steady voltage is corrected for by taking the log output to the grid of a cathode follower 7 so that the cathode is at s to earth and varies in potential with the log. The cathode output potential is then fed through diodes 10 to restore the applied A.C. signal to the level - s + v.