Patent GB-675123-A (GB)
Title: Systems for stabilizing the frequency of an oscillator
IPC: H03L7/26
Filed: 1949-12-30
Abstract: 675,123. Frequency measurements. RADIO CORPORATION OF AMERICA. Dec. 30, 1949 [Dec. 31, 1948], No. 33377/49. Class 37. [Also in Group XXXV] In a system for stabilizing the frequency of an oscillator by means including an auxiliary frequencysweep oscillator, the correcting voltage is obtained from a phase comparator with two inputs, one developed by a sharply resonant circuit element (e.g. a gas cell), and the other developed by a frequency discriminator fed by the beat frequencies arising from mixing the oscillator output with the frequency-sweep oscillations. In Fig. 1, the oscillator 10 to be stabilized feeds a load 12 through a wave-guide 11, and also feeds a mixer 14 through a guide 13. An oscillator 15, whose frequency is swept over a range by a saw-tooth or other voltage source 16 of lower frequency, also feeds the mixer 14 through a guide 13, thus producing a beat output of modulated frequency. This output, amplified in a broad-band amplifier 17, is applied to a tuned frequency discriminator 18 giving an output voltage E which passes through a zero value whenever the instaneous frequency of the source 15 differs from that of the controlled source 10 by an amount equal to the tuned frequency of the discriminator 18. The output from the sweep source 15 is also applied to a gas cell 19 and rectifier 20, giving a pulse output P at the moments when the swept frequency equals the molecular resonance frequency of the cell. These pulses are amplified at 21 and are compared in phase in a mixer 22 with the output E from the discriminator 18. The mixer output, voltage, which is a measure of the phase difference and represents in sign and magnitude the frequency error of the oscillator 10, is applied through a lead 23 to correct that error, thus stabilizing the oscillator 10 at a frequency equal to the sum or difference of the resonance frequency of the cell 19 and the resonance frequency of the tuned discriminator 18. Fig. 3 shows in detail the valve circuits 24, 27, 28 for handling the pulse output P from the resonance cell 19 and rectifier 20, the inverted output from amplifier 24 being differentiated at 25, 26, clipped in the valve 27, and applied to the valve 28 which gives outputs P<SP>1</SP>, P<SP>11</SP> of opposed polarity from its anode and cathode, respectively. These outputs are applied to a phase comparer 22a similar to that described in Specification 674,936, [Group XXXV], which is also fed with the output E from the amplifier 17 and discriminator 18a, shown as of the off-tune type. Output from the phase comparer is applied, in series with a manual voltage control 58, between the control grid and cathode of a valve 55, the anode resistance 54 of which is connected between the anode and cavity of a reflex klystron 10 to adjust its frequency. Screen voltage for the valve 55 is stabilized by a resistor 57 and tube 56. When a magnetron oscillator is used it may be controlled in frequency in the manner described with reference to Fig. 2 in Specification 653,979, [Group XXXV]. For oscillators operating at lower frequencies, the gas cell may be replaced by a piezo-electric crystal. The circuit of the invention is contrasted with a similar, circuit described in Specification 658,198 by the use of the elements 17, 18, which replace the narrow-band amplifier and the saw-tooth oscillator used in the prior arrangement, and in this way give increased precision of frequency control. For giving a visual presentation of the phase relation between the voltages P, E, these voltages may be alternately applied to one input of a C.R. tube, the other input of which is supplied from the frequency-sweep source 16. A watching operator would then adjust the oscillator frequency manually to preserve the appropriate phase relation. Specification 664,014, [Group XXXV], also is referred to. Reference has been directed by the Comptroller to Specification 663,065.