Patent GB-1191095-A (GB)
Title: Improvements in or relating to Automatic Frequency Control Circuits
IPC: G01S7/36
Filed: 1969-02-05
Abstract: 1,191,095. Automatic frequency control systems. ALBISWERK ZURICH A.G. 5 Feb., 1969 [22 March, 1968], No. 6105/69. Headings H3A and H3Q. The invention relates to an arrangement for the automatic control of the frequency f 0  of an oscillator 1, Fig. 1, relative to the frequency f B  of a reference source 2, in which a search generator 9 shifts the frequency of the oscillator continuously through a predetermined frequency range until, on reaching the pull-in range of a frequency-discriminator 4 supplied with the difference of the two frequencies-as shown, by mixer 3-the search generator is switched out and control of the oscillator is taken over by the output signal from the frequency discriminator. The oscillator may, for example, comprise the local oscillator of a radar receiver, and the frequency f B  be that of the transmitter. In such an arrangement there are two oscillator frequencies for and f 02  spaced equally on either side of the reference frequency fB, Fig. 2, only one of which is correct. Setting to the incorrect point is prevented by the invention, according to which the output of the frequency discriminator 4 is connected to the two inputs of a bi-stable trigger circuit 8 in each case via a separate AND gate 6, 7 respectively. The input signal to the frequency discriminator 4 is also fed to a marker 5 which produces an output voltage when its input signal is within the pull-in range of the frequency-discriminator 4: a control circuit 10 connected to the oscillator 1, frequencydiscriminator 4, marker 5, bi-stable trigger circuit 8 and search generator 9 continuously changes the frequency of the oscillator in a predetermined direction on the occurrence of a discriminator output signal of predetermined polarity, and with a change in the polarity of the discriminator output signal in the presence of a marker output voltage, effects switching of the oscillator control to the discriminator output signal. In the embodiment, oscillator 1 is continuously tuned by means of motor 13 which also drives limit switches 14, reversing the direction of rotation when the ends of the frequency range are reached. Bi-stable trigger 6 supplies an output to relay 11, energization of which causes contacts 11-2 to reverse and connect motor 13 to a fixed potential source U and causes contacts 11-1 to connect marker 5 to relay 12. If marker 5 then energizes relay 12, holding contacts 12-1 close and contacts 12-2 reverse to connect motor 13 to the output of discriminator 4. Thus if, e.g., the initial oscillator frequency is at the point A, Fig. 2, which shows the discriminator output characteristics, the correct oscillator frequency is f 02 , relay 11 is not energized and the limit switches are set to cause the motor 13 to reduce frequency, this will fall until point B is reached, beyond which the discriminator produces a negative output which opens gate 7. Bi-stable trigger 8 then reverses and relay 11 is energized, connecting motor 13 to source U. The motor then drives so as to increase the frequency; at point C the discriminator 4 again produces a negative output which leaves the state of bi-stable trigger 8 unchanged. As soon as frequency f 02  is passed, however, the discriminator 4 produces a positive output, closing gate 7 and opening gate 6, to reverse the bi-stable trigger 8. Relay 11 is then de-energized and contacts 11-1, 11-2 reverse; relay 12, however, remains energized by holding contacts 12-1 and, contacts 12-2 being reversed, control of motor 13 is taken over by frequency-discriminator 4.