Patent GB-958383-A (GB)
Title: Frequency scanning radio receiver
Inventor: SANDERS ROYDEN COE
IPC: H03J7/32
Filed: 1960-11-16
Abstract: 958,383. Radio receiving circuits; frequency changing circuits; automatic frequency control systems. RAYTHEON CO. Nov. 16, 1960, No. 39432/60. Headings H3A, H3F and H3Q. In a frequency-scanning radio receiver in which the sweep is interrupted upon the interception of a signal, the means interrupting the sweep at the same time energizes further means which superimpose a frequency modulation on the received energy. The energy so modulated is amplified and demodulated and the demodulation product is compared with the original frequency modulation in a phase detector; if the received energy comprises a coherent signal of substantial duration a holding voltage is produced, but sweeping is caused to resume if the signal to noise ratio falls below a particular value. The invention is particularly applicable to a radar system of the C.W. type (Fig. 5, not shown). In an embodiment, signals from terminal 16, previously reduced to a first intermediate frequency, are fed to a balanced modulator 15 which is driven in push-pull from a local oscillator 10 via a phase-inverter valve 14. The local oscillator frequency is swept over the tuning range by means of a reactance valve modulator 12 whose reactance is determined by the grid bias, which is derived from a sawtooth sweep generator 11. The sweep generator 11 comprises condenser 20 which is charged through resistors 39, 37 and gas-discharge tube 36, from locking thyratron circuit 31 and is discharged over thyratron 21. The output of the balanced modulator 15 is fed to a narrow-band I.F. amplifier 18 feeding discriminator 19 the output of which across resistors 22, 23 is connected via line 24 to the charging capacitor 20 and via line 26 to the reactance tube circuit 12. The voltage at resistor 22 is also fed to a selective cathode-follower 28 with two outputs, that on line 29 being fed to a balanced phase detector 49 and that on line 30 being applied as a control signal to fire locking thyratron circuit 31. The grid potential of the selective cathode follower 28 comprises the sawtooth sweep voltage derived from condenser 20 over lead 24, with superimposed the output from the discriminator 19 which comprises an S-shaped response if a signal is encountered (Fig. 3, not shown). The latter, if positive-going produces a sharp positive " spike " in the tuned cathode circuit 130, 131 which fires the locking thyratron circuit 31; ringing of the tuned circuit is prevented by diode 138. The thyratron circuit 31 is normally held non-conductive by a cathode bias derived from resistors 115, 315. When the looking thyratron circuit 31 fires it extinguishes gas discharge tube 36 so isolating condenser 20; it also energizes relay 35 which reverses contacts 41, grounding the screen grid of the thyratron 21 and preventing it firing again. Energization of relay 35 also reverses contacts 42, short-circuiting resistor 44 to feed energy from a low-frequency oscillator 46 to the reactance valve 12 whereby the local oscillator 10 is frequency-modulated. The signal energy thus modulated is, after detection in discriminator 19, passed by selective cathodefollower 28 to be compared in balanced phase detector 49 with the low-frequency oscillations, which are fed to the phase detector 49 in pushpull over lead 47 and phase inverter 48. If a true signal is present, a control voltage appears on line 51, the phase-detector being unbalanced to a selected degree by means of resistor 164, and is applied to recycle circuit 52. This includes a thyratron 53 which normally becomes conductive within a prescribed time interval after looking thyratron 33 becomes conductive and then cuts off the thyratron 33 and causes sweep to recommence. The appearance of a control voltage on line 51 prevents thyratron 53 firing and so inhibits resumption of the sweep. Reversal of contacts 42 also causes operation of a delay action relay 216 which performs any desired control function on the receiver. When the receiver is locked on a signal, automatic frequency control (A.F.C.) is applied from the discriminator 19 to the reactance valve 12 via leads 24, 26, diodes 184, 185 being normally non-conductive and of high resistance due to the bias derived from resistance 181 in the cathode circuit of the reactance valve. In the case of a large shift in frequency, the bias on these diodes changes sufficiently for one or other of the diodes to become conductive, thus greatly reducing the time-constant for charging of the condenser 20 from the A.F.C. circuit. The I.F. amplifier 18 is provided with A.G.C. by means of diode 298 and the A.G.C. voltage is also applied over lines 57, 30 to control the threshold bias on locking thyratron 33. Another feature of the invention is that resistor 39 is provided in the sweep generator circuit, the current in which ceases when the sweep is interrupted, causing a slight reduction in the control voltage applied to the reactance tube modulator 12 and so compensating for any overshoot in tuning. To limit the sweeping range, a circuit 85 tuned to the highest frequency desired is included in the grid circuit of thyratron 21 and is fed via line 94 from the anode of the local oscillator 10; when the highest frequency desired is reached, an A.C. voltage appears in circuit 85 the positive peaks of which fire the thyratron 21, which is biased back by means of resistors 90, 101.