Patent GB-712763-A (GB)
Title: Improvements in or relating to panoramic radio receivers
IPC: H03J7/32
Filed: 1951-08-17
Abstract: 712,763. Radio receiving systems; frequency modulation. GENERAL ELECTRIC CO., Ltd., and FOOT, J. B. L. Aug. 18, 1952 [Aug. 17, 1951], No.. 19487/51. Class 40 (5) In a panoramic receiver, to reduce second channel interference, the received band of signals is first reduced in frequency by a local oscillator, preferably of higher frequency than the received signals and the I.F, signal band beat in a second mixer with oscillations from a second oscillator varied over a frequency band coinciding with all or part of the LF. pass band, the resulting beats at or near zero frequency with signals received producing deflections of the cathode-ray tube trace. The second mixer can be a balanced ring modulator 13, thus producing two low frequency outputs in succession for each signal one on either side of coincidence of the signal frequency with the instantaneous frequency of the'sweep oscillator, with no output between, which after amplification in low frequency amplifying and output stages 17, 18, 22 are passed to a rectifier 24 and smoothing circuit 67 so that one pulse signal only is passed to the vertical deflection coils 25 of the oscilloscope. The second oscillator 15 is a phase shift oscillator comprising an amplifier valve 27 and three phase shift cathode followers 28, 29, 30 all cascade connected in a closed loop, and its frequency, e.g. 60 Mc/s, varied over, e.g. 10 Mc/s, by a sawtooth voltage applied to the grids of the cathode follower valves 28, 29, 30 superimposed on an adjustable mean frequency determining voltage applied from a potentiometer 65. The output of the sweep oscillator 15 is amplified by wide band amplifier stages 37, 38 before application' to the modulator 13. The sawtooth generator frequency modulating the oscillator 15 comprises a' Miller transitron saw- 'tooth generator 41 locked in frequency, e.g. 25 c.p.s., by a valve 44 to the grid of which a modulating voltage of frequency 50 c.p.s. from the heater voltage is applied. The output of the generator 41 is applied to an amplifier 47 feeding the horizontal deflection coils 40 of the oscilloscope and a potentiometer 50 in series with the coils which determine the maximum sweep of the oscillator 15. A further potentiometer 51, across the potentiometer 50 adjusts the sawtooth voltage amplitude which, after phase inversion by a valve 54 is applied across the potentiometer 69 to frequency modulate the oscillator 15. The oscillator frequency sweep is thus synchronized from 'the deflection coils 40 with the horizontal scan of the oscilloscope and is rendered more linear by reason of the condenser 68, the frequency and bias changes of the oscillator 28-30 being not otherwise exactly proportional. The position of 'the 10 Mc/s display band is altered by adjusting the inductance of the first local oscillator 9. The frequency sweep may be reduced to 2 Mc/s and .4 Mc/s respectively by a switch 56 ganged with switches 20, 21 and 66 which introduce a low pass filter 60 or 61 between the stages 18, 22 to reduce the bandwidth of the signal trace, e.g. to 20 Kc/s and 9 Kc/s and different values of smoothing condenser 67. Further, in one position of the switch 66, no condenser 67 is connected so that a double trace is reproduced for each signal. The low frequency response of the circuit between the modulator 13 and rectifier 26 is preferably reduced for the lower frequencies by choice of the transformers 16, 23 and circuit components. Amplitude compression of the signal trace may be effected by switching a rectifier 58 in circuit across the vertical deflection coils 25. A telephone jack 26 enables the modulation on any signal trace to be reproduced. To achieve this the signal required is brought mid-way of the scan and the horizontal scan stopped by adjusting the potentiometer 53. The condensers 67 are then disconnected by the switch 66 and the first local oscillator 9 altered slightly in frequency by a trimmer capacitor so that a second LF. is produced by the modulator 13 and is finally detected by the rectifier 24. A crystal oscillator 62 with crystal of natural frequency, e.g. 2 Mc/s, may be connected to the R.F. stage 3 so that its harmonics around 60 Mc/s pass to the I.F. amplifier and serve to produce calibration traces on the oscilloscope. Specification 712,773 is referred to.