Patent GB-752158-A (GB)
Title: Improvements in or relating to superheterodyne receivers for use in frequency-shift telegraphy with automatic frequency correction
IPC: H04L27/14
Filed: 1954-06-25
Abstract: 752,158. Automatic frequency correction. PHILIPS ELECTRICAL INDUSTRIES, Ltd. June 25, 1954 [June 29, 1953], No. 18689/54. Class 38 (4). [Also in Groups XXXVI and XL (c)] In a superheterodyne receiver for frequency-shift telegraphy, with automatic tuning correction acting on a local oscillator, the signals are applied to two biased cut-off frequency detectors responsive respectively to excursions of the work and rest frequencies beyond the nominal range and connected in push-pull to the corrector circuit. In one embodiment, the normal receiver 1 ... 5 feeds a second frequency changer, I.F. amplifier and frequency detector circuit 7, fed by a controlled local oscillator 6, and its output appears as a push-pull voltage across the resistors 8, 9. The telegraphy pulses across resistor 8 are normally fed through switch 10 to a pentode amplifier 12 controlling a doubletriode trigger circuit 13, 14, 15, the anodes of which are connected in push-pull to the windings of a polarized relay 16, operating a recording stylus 18. For frequency control the pulses across resistors 8, 9 are applied in push-pull through switch 10 to the control grids of triodes 27, 28 normally cut off by the bias across the common cathode resistor 29 connected through resistor 30 to the H.T. supply. If the mean frequency detected in circuit 7 becomes abnormally near the rest frequency, the work frequency pulse voltages across resistor 8 rise and triode 27 passes anode current to energize a relay 31. If the frequency moves abnormally in the other direction, the rest frequency pulse voltages across resistor 9 rise, valve 28 responds and relay 33 is energized. The contacts 32, 34, respectively, of these slow-acting relays close energizing circuits for coils 20, 21 of the frequency controller 19 to adjust the frequency of oscillator 6 in opposite directions. The device 19 comprises a liquid-filled glass tube 22 containing a metal piston 23, with a central softiron ring 24, the position of which is adjustable by the magnetic field of windings 20, 21. The piston forms the intermediate electrode of a variable capacitor, having fixed electrodes 25, 26 on the tube 22, and is connected in parallel with the frequency-determining circuit of the oscillator 6. Upon continuous reception of the rest frequency during a signal pause, a slowacting pause relay 36, in the anode circuit of triode 13, operates to connect the cathode of valve 27, through its contact 37, with a tapping 38 on the resistor 29. The valve 27 then has a response frequency adjoining the rest frequency and automatic frequency correction continues during the signal pause. The switch 10 is provided to invert the polarity of the outputs from resistors 8, 9 upon any interchange of the rest and work frequencies at the transmitter end. In another embodiment, the output voltage from the main frequency detector is unipolar and is used to control a similar frequency-correcting device through differently biased double. triode trigger circuits, Fig. 3 (not shown).