Patent GB-1000325-A (GB)
Title: Cancellation circuit for non-dispersive infrared analyzer
IPC: G01N21/37
Filed: 1963-09-26
Abstract: 1,000,325. Infra-red analysers. BECKMAN INSTRUMENTS Inc., Sept. 26, 1963 [Oct. 4,1962], No. 37837/63. Headings G1A and G1N. In an infra-red analyser, having detector cells 12, 13, provided with capacitative transducers C 1 , C 2 , unwanted a.c. signals, transmitted, by a capacitative transducer, from a synchronous demodulator 50 to the input of an amplifier 42, are substantially reduced by the use of an R-C cancellation circuit 62, coupling a portion of the unwanted A.C. signals from the demodulator, to another input of the amplifier, in opposition to the transmitted signals. The analyser is similar to that disclosed in Specification 914, 3 81, comprising a radiant energy source 10, energized from an oscillator 18, providing pulsed energy and substantially 100% modulation. The analyser has a sample cell 11 and the detector cells 12,13 are provided with flexible diaphragms 23, 34 which each form one plate of a capacitative transducer C 1 , C 2 . The capacitors are biased with opposite polarity by a D.C. source 28. If a rectified A.C. bias is used at 28, capacitors 59, 60 and resistances 36, 37 are required, but not if a battery or a well-filtered supply is used. Absorption of energy in the detector cells results in a difference voltage signal on the line 29. This is fed through a triode connected tetrode 42, acting as a preamplifier, to amplifier 48, which may use solid-state devices, and then to the demodulator 50, which receives a reference signal from the oscillator 18. The output from the demodulator is applied to a recorder through an R-C filter 51 and is also fed-back to the transducer circuit through potentiometer 58. If the gas to be detected is CO, the first cell 12 is charged with CO and the second 13 with an inert gas, non- absorptive at the absorption wavelengths of CO, e.g. N 2 O. With the sample cell 11 empty, or with an inert gas passing through, the source is pulsed at a low rate and the apparatus adjusted for zero by the potentiometer 58. The gas to be analysed for the presence of CO is passed through the sample cell 11 and when CO is present, energy is absorbed in cell 11 rather than 12. A signal on the line 29.corresponding to the measured quantity of CO, results. The output of the demodulator rebalances the detector output, resulting in a corresponding output from the demodulator to the recording circuit. The demodulator also feeds back unwanted A.C. signals to the transducer circuit from which they are transmitted to the amplifier 42 by the capacitor diaphragms. This introduces problems of loop stability and concerning the dynamic range of the system. To cancel these signals, by the introduction of an opposing A.C. signal, a capacitance 63 and a variable resistance 64 connected to a tap 65 on potentiometer 66, are connected between the feedback line 56 and the cathode of the tube 42. The magnitude of the opposing A.C. signal is varied by adjustment of the potentiometer 66 and the frequency transmission characteristics of the R-C circuit by resistance 64. Suitable values for the components used with the tube 42 are given. Alternative preamplifiers may be used instead of tube 42, e.g. a transistor. Specification 818,102 is referred to.