Patent GB-1216730-A (GB)
Title: Frequency synthesiser
IPC: H03L7/22
Filed: 1968-11-05
Abstract: 1,216,730. Frequency synthesizers. C.I.T.- COMPAGNIE INDUSTRIELLE DES TELECOMMUNICATIONS. 5 Nov., 1968 [30 Nov., 1967], No. 52383/68. Addition to 1,185,812. Heading H3A. In a frequency synthesizer of the type wherein the frequency of an oscillator is swept past a plurality of harmonics of a fixed oscillator, which are counted until the desired harmonic is reached, when a phase-locked loop is completed, the range between pairs of harmonics is subdivided by means of a second oscillator which is swept to cover the frequency range between two harmonics. The second oscillator is also swept past a range of harmonics of a fixed oscillator which are counted until the desired one of these harmonics is reached, when a phase-locked loop is completed; the final output frequency is derived from a third oscillator, the frequency of which is swept over the whole frequency range and is locked when it is equal to the sum of the frequencies derived from the first and second oscillators. The desired five-figure frequency is set up on control box 600, the first three digits of which control the coarse frequency synthesizer 400, which is of the type described in the parent Specification, comprising counter 404, four oscillators 401 capable of being swept over frequency ranges, frequency dividers 402 and selecting commutator 403. A plurality of fixed harmonics 100 kcs. apart is derived from crystal oscillator Q and harmonic generator H. The coarse frequency synthesizer produces a frequency within a range with a 100 kcs. spacing on line F1. Fine frequency synthesizer.-Fine division unit 500 comprises an exploratory oscillator 501 covering the range 50-70 Mcs., the output of which is applied to modulator 502 which is followed by a narrow band filter 503 centred at 2 Mcs. supplying phase discriminator 504. Harmonics from harmonic generator H spaced 100 kcs. apart are also applied to modulator 501. Oscillator 501 also feeds a second modulator 507 whose output is connected to narrow band filter 507 centred at 25 kcs. which is followed by a divide-by-two circuit 508 which provides counting pulses to a two-decade counter 509. The anode of a diode 510 is connected via common point 511 to the output of phase-discriminator 504 and input of saw-tooth sweep generator 505. The diode has its cathode connected to the output of counter 509 and controls blocking and unblocking of synchronizing signals from phase discriminator 504 to a switching . circuit (not shown) which switches oscillator 501 from exploratory to synchronized control. A frequency divider 512 having a ratio of 1/200 receives the output of oscillator 501 and divides the frequency range of 50-70 Mcs. to the desired range of 250-360 kcs. on output line F2. Counter 509 is connected to the two lower digits of control box 600. When counting starts diode 510 clamps the output of phase discriminator 504 and saw-tooth generator 501 control frequency exploration of oscillator 501; at each coincidence with a harmonic from source H two beats are produced and issue from filter 507, but as a result of divide-by-two circuit 508 only one pulse is received by counter 509. When the harmonic posted in the last two positions of control box 600 is reached, counter 509 applies a positive voltage to the cathode of diode 510, which is back-biased and so frees the output of phase discriminator 504 to synchronize oscillator 501 and sweeping is stopped. Third synthesizer.-The third synthesizer 700 is similar to the first, in that it contains four oscillators 701, dividers 702 and commutator 703 which is operated by counter 404 in the first synthesizer. Frequency exploration is controlled by saw-tooth voltage generator 705; the output frequency from the selected divider 702 constitutes the final output and is also applied to one input of a modulator 709, the other input on which comprises the frequency on line F1. Modulator 709 feeds via 250-349 kcs. filter/ amplifier 710 into phase discriminator 711, the second input of which comprises the frequency on line F2; phase discriminator 711 provides an output signal upon detection of a coincidence which, if not blocked, is applied to a switching circuit (not shown) which switches to synchronized control that oscillator 701 which is under exploratory control. Diode 708, connected between common point 712 in the output of phase discriminator 711 and sawtooth generator 705 clamps this point at the beginning of a frequency exploration, thereby blocking the output of the phase discriminator and allowing generator 705 to exercise control. To prevent a first false synchronization occurring when the frequency produced by the third synthesizer is equal to the difference between the frequencies on lines F1,F2, a flip-flop circuit 713 with inputs a, b, c and output d is provided. The commencement of a sawtooth voltage by generator 705 provides via differentiating circuit 706 a pulse which is applied to terminal c and returns the flip-flop to zero condition, output d earthing the diode cathode, and latches the flip-flop. Upon the first, false frequency coincidence occurring, a square waveform is passed by filter 710, the positive leading edge of which has no effect on flip-flop 713. The negative trailing edge, however, applied to terminal b unlatches the flip-flop, whereby when there is applied to terminal a from filter 710 the leading positive edge of the pulse corresponding to the true synchronization, the flip-flop is triggered. It then produces on terminal d a positive voltage which biases diode 708 to cut-off, unblocking the output of phase discriminator 711 whereby synchronization of the oscillator occurs. Due to the synthesizer 500 working over the range 250- 350 kcs. the output of synthesizer 400 on line F1 is at least 250 kcs. below that of synthesizer 700, so that if the corresponding oscillator is selected in these two units, the oscillator chosen in synthesizer 700 may not be able to tune to the frequency required. If, then, the saw-tooth generator 705 reaches the end of its sweep without synchronization occurring, the differentiated pulse produced by differentiating circuit 706 at the end of the sweep triggers bi-stable circuit 707. This operates member 704 which moves the oscillator-selecting commutator arm 703a forward one step, so that this oscillator is employed on the succeeding sweep. At the end of this sweep bi-stable circuit 707 again reverses and causes member 704 to restore the commutator arm 703a to its original position.