Patent GB-1089074-A (GB)
Title: Automatic sweep system for frequency standard
IPC: H01S1/00
Filed: 1965-04-07
Abstract: 1,089,074. Automatic frequency control Systems; logic circuits. VARIAN ASSOCIATES. April 7, 1965 [April 24, 1964], No. 14824/65. Headings H3A and H3T. A crystal oscillator 16, Fig. 1, is stabilized at a sub-harmonic of a resonance of a caesium beam tube 10 by control signals derived from first and second harmonics of the output from the tube. The output of the oscillator 16 is modulated with a 100 c/s. signal from generator 19 and multiplied in frequency by unit 22 before being fed to the caesium tube 10. The output of the tube 10 passes through an amplifier 30 to filters 26 and 27 which select the 100 c/s. and 200 c/s. components respectively. The output of filter 26 passes to phase detectors 28 and 29 which are supplied respectively with in-phase and quadrature phase reference signals from generator 19. The output of phase detector 28 normally passes through switch 34 to integrator 17 which controls the frequency of oscillator 16. At the desired resonance the outputs of detectors 28 and 29 are zero and the output of the rectifier 33 is at its maximum. If the output of either detector 28 or 29 exceeds a maximum value or if the output of rectifier 33 is less than a minimum value, a logic circuit 32 changes over switch 34 to connect a sweep generator 35 to the oscillator control circuit 17. The logic circuit then also closes a switch 15 which supplies A.C. to vary a magnetic field influencing the tube 10; this varies the frequency of undesired resonances of the tube but does not affect the desired resonant frequency, thereby preventing the oscillator 16 from locking on to an undesired resonance. When the oscillator is sufficiently near the required frequency, the inputs to the logic circuit 32 are within required limits and switches 34 and 15 are returned to their normal position. The logic circuit preferably includes delay means to prevent transient variations in its inputs from causing switch 34 to be moved to the search position. In a modification, Fig. 4 (not shown), detector 29 is omitted and an output from filter 26 is multiplied by a second harmonic signal from generator 19 to provide one input to the logic circuit, the other input being de rived from rectifier 33 as in Fig. 1. The phase of the second harmonic signal is such as to accentuate the low voltage out of phase component deriving from the filter 26. Logic circuit. Fig. 3.-If the output of either unit 41 or 42 exceeds a predetermined value the gate 43 develops a positive signal which causes transistor 44 to conduct. A third input at terminal 55 is applied to a voltage doubler circuit, which functions as such provided the input-exceeds a value set by the Zener diode 54. When this occurs, the output of the voltage doubler reverse biases the diode 56, thereby removing the bias supplied to the base of transistor 44 and allowing this transistor to cut off if no positive bias is supplied by gate 43. When transistor 44 conducts, transistor 62 conducts, thereby energizing sweep generator 35, and transistor 63 is cut off thereby closing gate 69. When transistor 44 cuts off, transistor 62 is cut off and transistor 63 conducts. The diode 48 in the base circuit of transistor 44 gives a short time constant (0À1 seconds) when the transistor is switched off and a longer time constant (1 second) when it is switched on. In a modification, Fig. 5 (not shown) the gate 43 is omitted and a positive input is applied to the base of transistor 44 through a voltage doubler circuit (72).