Patent GB-682370-A (GB)
Title: Oscillator-frequency control by resonant modulation of gas
IPC: H03L7/04
Filed: 1951-02-27
Abstract: 682,370. Frequency and phase measurements. RADIO CORPORATION OF AMERICA. Feb. 27, 1951 [March 8, 1950], No. 4770/51. Class 37. [Also in Group XXXV] An automatic frequency control system uses the phenomenon that if an oscillator frequency is applied to a gas having at least three energy states Wa, Wb, Wc while the spectral and absorption line of frequency γac =Wc-Wa/h is being observed, where h is Planck's constant, the line splits into two frequencies γ+ac, γ-ac when the oscillator has a frequency substantially equal to the line frequency γab = Wb - Wa/h  the amount of splitting depends on the magnitude of the oscillator field, and the difference in intensity or absorption of the line components depends in magnitude and sense upon whether the oscillator frequency is greater or less than the frequency γab. In Fig. 6 an oscillator 12 to be controlled in frequency at frequency γab, or a constant difference from γab, using a constant beat frequency, applies a frequency γab to electrodes 13 in a closed chamber 10 forming a section of a wave-guide containing a gas having three energy levels Wa, Wb, Wc, e.g. carbonyl sulphide COS, at a pressure of <SP>1</SP>/ 10 th mm. of mercury and the relative absorption of the microwaves at absorption frequencies γ+ac, γ-ac from a microwave oscillator 11 swept over a narrow frequency band including γ+ac, γ-ac by a saw-tooth generator 20A is detected by a rectifier 15 in the form of a pair of pulses Ma, Mb. The pulses are applied through a delay network 35 to a gate circuit comprising multi-grid valves 36, 37 connected as a flip-flop. The first pulse Ma of each pair operates through cutting off a normally conducting valve 30 to switch over one valve 36 to a conducting state but the other valve 37 returns to conducting condition, with the other cut-off, before the second pulse Mb arrives. The pulses are thus alternately fed to oppositely acting diodes 26A, 26B of a pulse amplitude comparison network 16B producing a control voltage in magnitude and sense acting on a reactance valve 50 to correct any drift in frequency of the oscillator 12 producing inequality in absorption at frequencies γ+ac, γ-ac. Fine adjustment of the frequency controlled may be effected by adjusting the amplitude of the oscillator 12 output or A.G.C. may maintain the fine control frequency constant. With this arrangement stabilization of the microwave oscillator frequency is unnecessary but, if provided, then, Figs. 3 and 4 (not shown), the microwave oscillator may be modulated to provide constant beat frequencies equal to γ+ac, γ-ac, and constant tuned filters (Fig. 3) may act through oppositely-poled rectifiers to produce simultaneously the pulses Ma, Mb and a control voltage in an integrating network, or (Fig. 4) a single filter may be used and its tuning swept over a narrow frequency band, a common rectifier feeding the pulses Ma, Mb to a pulse amplitude comparator circuit. Alternatively, or in addition, the frequency correction may be applied manually, using an oscilloscope to compare the pulses. In a further embodiment (Fig. 9), which avoids the use of a gate circuit, a multi-vibrator 61A of frequency 2f 1  acts through a multivibrator 60A dividing the frequency to # t , to generate a sweep voltage of frequency # 1 . Differentiated pulses D from the multivibrator 61A of same sign are fed with the output of pairs of pulses M from the detector following the gas cell to a coincidence detector circuit 62A, the pulses M being fed in push-pull to the detector circuit 62A by an inverter circuit 96 with two opposite phase outputs, and the output of the detector circuit 62A acts in sense and magnitude to control the amplitude of the sweep amplifier 63 so that the spacing of consecutive pulse pairs is the same as the interval between pulses of each pair, i.e. all pulses are evenly spaced at a pulse repetition frequency 2f 1 . The result is then used that if alternate pulses M vary somewhat in amplitude the phase of the Fourier component of frequency 2f 1  of the pulse train varies proportionally and in sense with the percentage difference in amplitude of the pulses. To this end filters 70, 67 tuned to frequency 2f 1  and connected respectively to the multivibrator 61A and the output of pulses M from the inverter 96 feed sinusoidal waves to a phase comparator 16D deriving a D.C. output at 19 correcting through a reactance valve any frequency drift of the oscillator 12. A modification, Fig. 11 (not shown), uses the fact that the amplitude of the Fourier component of frequency f 1  varies in sense and magnitude proportionally with the difference in amplitude of the pulses. To this end filters tuned to a frequency f 1  are connected to the multivibrator and the output of pulses M from an inverter and feed sinusoidal waves in phase or anti-phase respectively to different grids of multi-grid valves connected to act as an amplitude and phase sense comparator, whose output, varying in sense and magnitude across a condenser, is the control voltage. The pulses M may be rectangular or of other shape.