Patent GB-1036551-A (GB)
Title: Frequency-synthesizing apparatus
IPC: G01R23/00
Filed: 1965-04-28
Abstract: 1,036,551. Frequency synthesis; automatic phase control systems. GENERAL RADIO CO. April 28, 1965 [July 27, 1964], No. 17937/65. Headings H3A and H3F. A digitally indicating frequency generator comprises a plurality of serially-connected stepwise adjustable digit insertion units 1-7, Fig. 1, for synthesizing a multi-digit frequency, together with a continuously variable unit G; a group of successive digit insertion units can be disconnected from the remainder and replaced by the continuously variable unit 9, means being provided for beating the output of the latter with that of the disconnected group of units, so as to allow an accurate determination of the frequency to which it is set. The synthesizer is controlled by an auxiliary source 10 of standard frequencies, as described comprising a 5 Mc/s. oscillator 20 which is crystal controlled or otherwise stabilized; e.g. from an external source, using phase-lock circuitry. In addition to a 5 Mc/s. output on line 10<SP>1</SP> a 42 Mc/s. output is obtained in line 10<SP>11</SP> by means of x 5 divider 21 and x 42 multiplier 22; the output of divider 21 is also applied to x 10 divider 23 to obtain 100 kc./s. which is fed to spike generator 24 to obtain a " picket fence " frequency spectrum from which the frequencies 3, 3À1 ... 3À9 Mc/s. are selected by filters 25 and applied to line 10<SP>111</SP>. Latching push-buttons 1-7 and 9 operate double-pole, double-throw switches 11-17 and 19, respectively. With none of the pushbuttons operated the digit insertion units are connected in series and a stabilized 5 Mc/s. is applied to unit 7 from line 10<SP>1</SP>. The output from this unit comprises the 5 Mc./s. applied to its input plus a first selected decimal step of increment within the range 0À01-0À09 Mc/s. Each subsequent unit first divides the increments added by preceding units by x 10 and then adds a further selected decimal step of increment within the range 0À01-0À09 Mc/s. The final output from unit 1 is fed to a mixer/ multiplier 41 in which it is multiplied x 10 by multiplier 51 and then mixed in mixer 52 with 50 Mc/s., derived from x 10 multiplier 53 which is supplied with stabilized 5 Mc/s. from line 10<SP>1</SP>, so as to obtain via amplifier 43a resultant output frequency in the range 0-1 Mc/s. Reversal of any of the switches 1-7 by operation of the corresponding push-button causes the corresponding digit unit and all preceding units to be removed from circuit and replaced by the continuously variable unit 9; at the same time the group of disconnected units is connected via line 44 to mixer 45. Each push-button P 1 -P 7  also operates a second switch 11<SP>1</SP>-17<SP>1</SP> which extinguishes a corresponding lamp T 1 -T 7  illuminating the dial of a related digit unit and in addition the lamps of preceding units. Operation of switch 19 by push-button P 9  causes the stabilized 5 Mc/s. normally applied to the input of unit 7 to be replaced by the output of continuously variable unit 9. This unit tunes through the range 4À99 to 5À11 Mc/s. and its output also constitutes the second input of mixer 45, so that its frequency can be determined against that supplied by the disconnected group of digit insertion units, either by means of a beat output indicator 47 or by frequency monitor 48. A further push-button PCAD OFF is provided which releases all the other buttons, and by opening switch 19<SP>1</SP> extinguishes lamp L 9  illuminating the dial of continuously variable unit 9. Continuously adjustable unit 9.-A frequency within the desired range 4À99-5À11 Mc/s. is generated by continuously-variable oscillator 60, Fig. 5. Stable 5 Mc/s. on line 10 and 42 Mc/s. on line 10<SP>11</SP> are combined in mixer 54 to give 47 Mc/s., which is passed via filter 55 to a further mixer 56 in which it is combined with a frequency within the range 2À9-4À1 Mc/s. from continuously variable oscillator 57, the tuning capacitor C of which is ganged with the capacitor C<SP>1</SP> of oscillator 60 and operated by dial DG. The resultant frequency in the range 49À9- 51À1 from mixer 56 is applied over filter 58 to form one input of phase detector 5G, the second input of which is derived from oscillator 60 via x 10 multiplier 61. The phase detector output comprises an internal locking voltage which is applied over a low-pass filter and switch S<SP>1</SP> to control a voltage variable capacitor and lock the frequency of oscillator 60 to that derived from sources 10<SP>1</SP>, 10<SP>11</SP>, 57. Alternatively reversal of switch S<SP>1</SP> allows of external control via line 9<SP>1</SP>, e.g. the frequency of oscillator 60 may be continuously swept over its range. Step-wise adjustable digit insertion units 1-7. Stabilized 42 Mc/s. from line 10<SP>11</SP>, Fig. 4, and a frequency in the range 5À0-5À1 Mc/s. coming from the preceding unit over line 4, are added in mixer 62, giving a frequency in the range 47.0-47.1 Mc/s. This is passed over filter 63 to a further mixer 64 in which it is combined with a selected one of the frequencies 3À0, 3À1 ... 3À9 Mc/s., derived from digit oscillator 66, the capacitor C of which is operated by the dial of the unit. The oscillator 66 is locked to the corresponding " picket fence " frequency, derived from line 10<SP>111</SP> by comparison in phase detector 65, the output of which is amplified in amplifier 70, rectified by rectifier 71 and applied over low-pass filter E to control a voltage variable capacitor in the circuit of oscillator 66. The output of rectifier 71 is also applied to a switching circuit (Fig. 4A, not shown) which reduces the pass-band of filter E and the loop gain when capture has been effected and the beat note ceases. The output of the unit is derived from a variable oscillator G working in the range 5À0-5À9 Mc/s., which is locked to the output of mixer 64, as filtered by filter 67. Locking is performed by multiplying the oscillator frequency x 10 in multiplier 68, comparison being performed in phase detector F and the resultant beat note applied over lowpass filter H to a voltage variable capacitor in the circuit of oscillator G. The beat frequency is also applied via amplifier 70 to rectifier 71, which is coupled so as to switch filter H to a narrower band-width when the beat-note ceases. The output of rectifier 71 is also applied to a transistor T 1 -T 7 , the collector-emitter path of which is in series with the corresponding dial lamp L 1 -L 7 ; thus if malfunctioning occurs and one or other beat note persists, the transistor is biased back and the dial lamp extinguished. Use of synthesizer to indicate frequency drift.- Continuously variable unit G is switched in circuit and the unknown and drifting frequency f x  (Fig. 6, not shown) is compared in a phase detector with the synthesizer output at 43; the resultant control voltage is applied to terminal G<SP>1</SP> to lock the frequency of unit G. The output at 43 then follows and is equal to the drift frequency, the beat frequency output at 45 varying proportionally and being observed or automatically recorded.