Patent GB-810153-A (GB)
Title: Improvements in or relating to automatic frequency controlling systems
IPC: H03J7/04
Filed: 1956-11-26
Abstract: 810,153. Automatic frequency control systems. TELEFUNKEN G.m.b.H. Nov. 26, 1956 [Dec. 30, 1955], No. 36074/56. Class 38 (4). The invention relates to automatic frequency control systems of the type in which the frequency of an oscillation generator may be coarsely adjusted to be near any desired one of a plurality of reference frequencies and a control voltage is derived from the frequency difference existing between the generator frequency and reference frequency to automatically control the fine tuning, e.g. by a reactance valve. As shown, a reference frequency generator 1 produces a square wave which is rich in harmonics. The generator 2 to be controlled is tunable over a wide range by a variable condenser 3. The outputs from the generators 1 and 2 are fed to a mixer 6 whose output is fed to a filter 7 selecting the desired difference frequency, which is rectified at 8 and controls a reactance valve 5 in a frequency-determining circuit of the generator 2. When the frequency from the generator 2 passes through the particular reference frequency to which its frequency is to be adjusted, the difference frequency from the mixer 6 becomes zero and a direct voltage is produced of value dependent on the relative phase of the generated and reference oscillations and this automatically holds the generator in tune with the selected frequency. This portion of the circuit is known, but has the disadvantage that if the manual tuning control is brought near a frequency which is the arithmetic mean of two adjacent reference frequencies the automatic tuning control will tune the generator to that undesired frequency. In the present invention this is prevented by superimposing an auxiliary alternating voltage on the control voltage whereby it is given a frequency sweep. In Fig. 3, nfn is the frequency to which the generator 2 is to be adjusted (n + 1)fn the next higher reference frequency, #f the frequency range over which the reactance valve can operate and (n + ¢)fn the frequency with which synchronization is to be prevented. If the generator frequency has a value X outside the operating zones of the reactance valve a coarse tuning motor 4 is operated through contact 9<SP>11</SP> of relay 9 which is energized via a high-pass filter 10 and rectifier 11 and the frequency of the generator 2 is moved in the direction of the arrow, the frequency of the generator also being swept through a band B by an auxiliary alternating voltage Uz applied via contact 9<SP>1</SP> of relay 9 to the reactance valve 5. When the capacitor controlled by the timing motor reaches its limiting value at a frequency between (n + ¢)fn and (n + 1 )fn its rotation is continued in the same direction and brings the tuning Back in the direction nfn. The difference frequency between nfn and the generator frequency becomes steadily smaller until finally it comes below the critical frequency of the high-pass filter 10 and the relay 9 releases to cut off the supply of auxiliary voltage Uz and stop the motor 4. The reactance valve then takes charge and effects fine tuning to the frequency nfn. Separate relays may be provided for controlling the auxiliary voltage and the motor.