Patent GB-543391-A (GB)
Title: An improved generator for producing a range of predetermined frequencies
IPC: H03J5/24
Filed: 1940-06-17
Abstract: 543,391. Automatic frequency control systems. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. June 17, 1940, No. 10457. Convention date, June 16, 1939. [Class 38 (iv)] [Also in Groups XL and XXXVI]  A generator for producing oscillations of predetermined frequency comprises a main oscillator 7 tunable over a range of frequencies, an "interpolation oscillator" 25 variable over a small range of frequencies, means for obtaining beat currents of a frequency dependent on the frequencies of both oscillators, and means responsive to such beat currents for shifting the frequency of the main oscillator in dependence on the tuning of the " interpolation oscillator." The main oscillator is calibrated in equal steps of frequency and the interpolation oscillator is adjustable over a range of frequency equal to one such step. In the arrangement described, oscillations of frequency f1 (which may vary from 100 k.c. to 334 k.c.) are increased in frequency to 10f1 by a frequency-multiplier 11, and then to 100 f1 by multipliers 13, 15, the outputs of 7, 11, 15 being connected to contacts of a switch 17 gauged to a switch 21 in an amplifier 19 having a number of coils for the several bands of frequencies and a single variable condenser gauged to the condensers in units 7, 11, 13, 15. The stable interpolation oscillator 25 produces currents of frequency f2 (which may vary over the small range from 300 k.c. to 290 k.c.), which together with part of the output of multiplier 11, pass to a mixer 23 producing upper side band frequencies equal to 10 f1 + f2. A third standard frequency oscillator and multivibrator 31 supplies standard frequencies f3 (of any multiple of 10 k.c.) through a filter 33 (to which reduces the intensity of the fundamental) to a second mixer 29 to which is fed the output from mixer 23. The lower side band (10f1 + f2 - f3) from mixer 29 is passed to an amplifier 35 and the value of f3 is so chosen that this lower side band has a frequency of approximately 100 k.c. The output from 35 and a frequency of exactly 100 k.c., from the standard oscillator 31 are applied to a phase detector 37, which controls the bias on a frequency control unit 43 determining the frequency of the oscillator 7. Assuming that it is desired to produce a frequency of 200:5 k.c., selector switch 17 is placed in the position corresponding to range 100-334 k.c., the interpolation oscillator is set accurately to 300 k.c., (corresponding to 0 on the dial 27) and the main oscillator is set approximately to 200 k.c., on dial 9. In order to adjust the frequency of the main oscillator to exactly 200 k.c., switch 45 is closed and the output of mixer 23 (10 f1 + f2) beats with the appropriate harmonic f3 (=10n k.c.) of the standard oscillator 31 to produce currents which through the phase detector 37 influence the frequency-control unit 43 to cause the main oscillator to generate exactly 200 k.c. The interpolation oscillator dial is now set at 0.5 (on the 0-1 scale), which decreases the frequency f2 by 5, and since the intermediate frequency (10f1 + f2 - f3) must remain constant owing to the automatic frequency control, and since f3 cannot change, 10 f1 must necessarily increase by 5 k.c., that is f1 increases by 0.5, so that the frequency generated by the main oscillator is now 200.5 k.c. as required. The final amplifier 19 can also function in the second and fourth positions of the switch 21 as a frequency-tripler, five inductances suitable for the bands f1, 3 f1, 10 f1, 30 f1, and 100 f1, being provided and the selector switch 21 having five corresponding positions (Fig. 5, not shown). Mixer.-The mixer 23 may be a triode or pentode 51, Fig. 2, the input frequencies 10 f1 and f2 being applied to the cathode and control grid respectively, and the output circuit tuned to 10 f1 + f2 by condenser 53 ganged to the other variable condensers of the system. In the mixer 29 the input circuit, to which the frequency f3 is applied, is tuned instead of the output circuit. Phase-detector.-The currents from the amplifier 35 and standard oscillator 31 are led to transformers 59, 61, Fig. 3, the secondaries of which are connected to rectifiers 67, 69, thereby producing opposing voltages across resistances 71, 75, so that if the two currents are not in phase a biasing potential is developed which is applied to the frequency control unit. Frequency control unit.-The tank circuit of the oscillator 7 is connected to a valve 81, Fig. 4. the anode of which is earthed through a resistance-condenser combination 91, 93, the junction of which is connected to the control grid. A voltage in quadrature with that of the tank circuit is thus impressed on the control grid and is amplified in accordance with the bias applied from the phase detector, this being equivalent to a shunt reactance across the tank circuit, thereby controlling its frequency.