Patent GB-1388071-A (GB)
Title: Frequency synthesizer
IPC: H03L7/18
Filed: 1973-02-06
Abstract: 1388071 Automatic frequency control; pulse discriminating PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 6 Feb 1973 [8 Feb 1972] 5835/73 Headings H3A and H3P A frequency synthesizer comprising a voltage controlled oscillator 1, an adjustable frequency converter, e.g. divider 3, and a reference source 8 has: a coarse control loop comprising a frequency discriminator 11 which produces an output signal to switch on a coarse control unit 9 when the frequency of the oscillator 1 is incorrect; and a fine control loop comprising a phase comparator 5 for producing a phase lock signal for the oscillator 1. The arrangement is characterized in that the coarse control unit 9 produces an increasing, or decreasing staircase waveform depending on the state of a two-state switch 12 set in dependence on the setting of divider 3 and is also characterized by the provision of a switching circuit 19 controlled by the output from the frequency discriminator 11 and having a first state in which the discriminator 11 is connected to the unit 9, and the output of the divider 3 is connected to the phase comparator 5, a second state in which connection between the divider 3 and the phase comparator 5 is interrupted, and a third state in which the connection between the frequency discriminator 11 and the unit 9 is interrupted. In the first state, assuming no output is being produced by discriminator 11, monostables 21, 22 are in their stable states, with a "1" output from 22 enabling gate 20 to receive any output from 11, and a "0" output from 21 enabling gate 24 to connect the divider 3 output to phase comparator 5. If the frequency of the signal from divider 3 changes, to cause discriminator 11 to produce an output, monostable 21 is switched to its unstable state inhibiting gate 24 and feeding pulses from oscillator 8 to phase comparator 5 via gate 23. The output from discriminator 11 is passed to unit 9 which comprises a counter 17 which counts up or down, depending on the state of switch 12, to produce, via D/A converter 18, a staircase waveform which changes the oscillator frequency. When the signal is within the desired frequency range, the output from 11 ceases and monostable 21 returns to its stable state triggering monostable 22 to inhibit gate 20 for a period. The oscillator may be used in a frequency shift signalling arrangement in which the keying signal operates switch 12 and changes divider 3. In a modification, Fig. 3 (not shown), the unit 9 includes a further counter circuit (29, 30, 31) for switching the range of the oscillator 1. When it is required to change frequency, switch 12 and divider 3 are altered as before and pulses from discriminator 11 pass to counter 17. If this counter 17 reaches one end of its counting range before the required frequency is reached, an output from it steps the further counter to provide a signal which changes the oscillator range and reverses the stepping direction of counter 17.  Frequency discriminator, Fig. 7.-Pulses from divider 3 are divided-by-two in bi-stable 38 to produce an output which opens gate 41 for alternate cycles of the signal from divider 3, to pass pulses from oscillator 8 to a counter 42. At the end of the count period, gate 44 is opened and the output from decoder 45, which is a pulse train at half the input rate is fed to unit 19. The decoder 45 is arranged to produce no output if the frequency from divider 3 lies within a predetermined range, corresponding to the range over which the fine control loop operates.