Patent GB-1259152-A (GB)
IPC: H03L7/23
Filed: 1969-01-17
Abstract: 1,259,152. Frequency synthesizers. SCHLUMBERGER OVERSEAS MESSGERATEBAU UND VERTRIEB G.m.b.H. 17 Jan., 1969 [20 Jan., 1968], No. 2955/69. Headings H3A and H3F. A generator for providing a plurality of selectable frequencies comprises a basic oscillator 1 for generating a good long-term stabilized frequency, an auxiliary oscillator 15 operating in phase-locked relationship with the first oscillator for generating a good short- and long-term stabilized frequency, and a " main " oscillator 23 for producing different selectable frequencies corresponding to the selected values of frequencies, means being coupled to the outputs of the oscillators 15, 23 for mixing the frequency generated by oscillator 23 with a frequency derived from the auxiliary oscillator 15. In the embodiment, oscillators 1, 23 are of crystal type, the output from oscillator 1 being divided down in divider 13 and compared in phase-comparator 14 with the output of oscillator 15 as divided down by dividers 16, 17: the output of phase-comparator 14 is used to control the frequency of oscillator 15, via long time-constant low-pass filter 18 which provides damping. The desired frequency is obtained digitally, the lower digits being derived from ten oscillators 2 which are phase-locked to the output of divider 13. A first digit is selected by means of switch 6, the selected output from oscillators 2 being mixed with the output of multiplier 4 in mixer 5, the output of which is filtered by filter 7 and applied to divider 8. The output of divider 8 is applied to mixer 9, the second input of which comprises a second digit term derived over switch 10, and the desired frequency in the output is selected by filter 12. In place of the first digit term a continuously-variable input may be derived from variable oscillator 11. The " main " oscillator 23 comprises a set of ten oscillators tuned by varicap diode 43: the final digit is selected by means of switch 24 which is ganged with switch 25 for selecting a corresponding output from filters 26 which separate out the harmonics derived from the output of divider 19, coupled via divider 16 to the auxiliary oscillator 15. The outputs from the selected oscillator 23 and filter 26 are combined in mixer 27, to the output of which filter 28 is coupled. Filter 28 feeds one input of mixer 29, the other input of which comprises a frequency corresponding to the penultimate digit, derived via switch 31 from multipliers 30 coupled to the outputs of oscillators 2. A switch 32 ganged to the switch 31 at the same time derives a D.C. bias from source 33 which is applied to varactor diode 43 to approximately tune the corresponding oscillator 23 to the desired frequency. The output from mixer 29 is filtered by filter 34 and applied to mixer 35, the second input of which comprises the lower order digits from filter 12. The output of mixer 36 is filtered by filter 36 and applied to phasecomparator 37, the second input of which is derived from auxiliary oscillator 15 via multiplier 38: the output from phase comparator 37 is applied to varactor diode 23, via long timeconstant low-pass filter 39 which acts as a damping circuit, to accurately tune the selected oscillator 23. The desired output from the generator is obtained from the selected one of the " main " oscillators via line L3 and mixer 40, the second input of which is obtained from the auxiliary oscillator 15 via multiplier 20 and mixer 21. The second input for mixer 20 is derived from auxiliary oscillator 15 via modulator 22 which provides for control of the amplitude of the final output, which may be kept constant by means of automatic gain control derived from the output and applied to modulator 22 over A.G.C. circuit 41 and differential amplifier 42. The second input of amplifier 42 comprises a reference voltage; alternatively, switch S allows a modulating source MF to be connected to give amplitude modulation of the output.