Patent GB-821767-A (GB)
Title: Improvements in or relating to receivers for use in electric signalling systems
Inventor: TYLER DAVID CHARLES
IPC: H04L27/14
Filed: 1956-05-29
Abstract: 821,767. Code telegraphy. GENERAL ELECTRIC CO. Ltd. May 27, 1957 [May 29, 1956], No. 16589/56. Class 40 (3). [Also in Group XL (c)] In a telegraph system having a receiver responsive to electric signals conveying information by relatively rapid transitions of an electric variable to one or other of a small number of discrete states, e.g. magnitude of a direct current or voltage, amplitude or frequency of a carrier wave, and in which the operation of a receiving instrument is controlled by a junction transistor switching arrangement arranged to be operated in response to the received signals, the receiver includes a generator adapted to produce at least one output voltage of substantially rectangular waveform in response to the application of the received signals, and the output voltage(s) having transitions corresponding to, but taking place appreciably more rapidly than those occurring between the discrete states of the electric variable in the received signals is/are utilized to operate the switching arrangement. In the receiving circuit for a frequency-shift telegraph system, the received mark and space signals are applied at input, terminals 1, 2 to resonant circuits 3a, 4a, 5a and 3b, 4b, 5b tuned to the mark and space frequencies respectively so that the voltages across resistors 5a, 5b amplified by transistors 7a, 7b and rectified by elements 10a, 11a and 10b, 11b are applied to capacitors 12a, 12b provided with an equalizing resistor 16 and having their junction point biased by potentiometer resistors 13, 14, 15 to a negative potential having a value greater than the substantially equal amplitudes of the voltages developed across the capacitors. The voltages at opposite ends of the potentiometer 16 which are of quasirectangular waveform and correspond to the transitions of the received signals are applied through the windings 17a, 17b of a transformer to the points A, B of a pair of oscillator circuits 18a, 18b comprising an oscillator system, as described in Specification 821,768, [Group XL (c)]. The oscillator circuits are similar, and .the circuit 18a includes two P-N-P germanium junction transistors 19a, 20a operated in pushpull with feed-back provided by means of a transformer having its primary winding, 21a tuned by capacitor 22a, connected between the collector electrodes of the transistors 19a, 20a and its secondary winding 23a connected between the base electrodes of the transistors. A self-biasing arrangement rectifying the voltage across the secondary winding 23a is constituted by rectifiers 24a and 25a, a capacitor 26a and a resistor 27a and with the circuit 18a in operation, is arranged to produce across resistor 27a a voltage somewhat less than the amplitude of the voltage transitions at the point A so that the base electrodes of the transistors 19a, 20a are biased negatively with respect to earth by an amount equal to the sum of the voltages at A and across the resistor 27a. The emitter electrodes of the transistors 19a, 19b, 20a, 20b are all connected to one end of an earthed resistor 28. Assuming that the reception of the mark frequency renders the point A more negative (e.g. 5.5 volts negative) than the point B (5.0 volts negative), and that a small bias voltage of 0.3 volts is developed across the resistor 27a, the base electrodes of transistors 19a, 20a will be at - 5.8 volts, and due to the flow of current from the emitter electrodes of the transistors 19a, 20a the emitter electrodes will be slightly less negative than the base electrodes, e.g. 5.7 volts negative, and this voltage at the emitter electrodes of transistors 19b, 20b will hold circuit 18b inoperative. During the initial portion of the next transition when A changes slowly to 5 volts negative and B to 5.5 volts negative, the oscillator circuit 18a continues to operate at substantially constant level with circuit 18b inoperative and the mean potential of the base and emitter electrodes of transistors 19a, 20a become less negative at substantially the same rate as the potential of the point A. When the potential at B becomes more negative than that at A it will be sufficiently more negative than that of the emitter electrodes of the' transistors 19b, 20b for the operation of the oscillator circuit 18b which is reinforced by the bias voltage across resistor 27b. At the same time the base and emitter electrodes of the transistors 19b, 20b rapidly become more negative and likewise the emitter electrodes of the transistors 19a, 20a so that the oscillator circuit 18a becomes inoperative. The transition times of the circuits 18a, 18b can be reduced to a very small value, e.g. 300 Ásecs. If the receiver is operative in the absence of received signals, the potentials at the points A, B are equal, and the circuit 18a or 18b last in operation during signal reception remains operative. Also when the receiver is switched on, but signals are not being received, one of the circuits 18a or 18b will start into operation causing a bias voltage to appear on the base electrodes of the transistors of the operative oscillator circuit, thus driving more negative the emitter electrodes of all the transistors so that the other circuit is held inoperative. The junction transistor switching arrangement for operating the receiving device of a teleprinter is of the form described in Specification 821,765, [Group XL (c)], and comprises two switching circuits, such as are disclosed in Specification 821,766, [Group XL (c)], and each of which incorporates three transistors 30a, 31a, 32a and 30b, 31b, 32b. The switching circuits are generally similar and connected as shown with resistor 35a having a value half that of 36a and resistor 37a having twice that of 38a with the emitter and collector of 30a, 32b respectively connected to the ungrounded end of the load 29. The feed-back transformers of oscillator circuits 18a, 18b have windings 39a, 40b and 39b, 40a respectively, the output of 39a when circuit 18a is operative being rectified at 41a and charging a capacitor 42a to a value such that the connection of its electrodes to the base and emitter of transistor 30a holds it in bottomed condition. The output from the winding 40a where the circuit 18b is operative is rectified at 45a and charges a capacitor 46a so that its output applied to the base and emitter electrodes of transistor 30a hold it in the cut-off condition. Similarly with the circuit 18b operative the transistor 30b is bottomed and circuit 18a operative holds transistor 30b cut-off so that in effect the mark elements render transistors 30a, 30b bottomed and cut-off respectively whilst the space elements reverse the conditions of the transistors 30a, 30b. The transistor 30a will have a very low impedance in its bottomed condition and in consequence transistors 31a, 32a will also have a low impedance and are in bottomed condition by choice of the values of resistors 33a, 35a, 36a, 37a so that only a small voltage is developed between the emitter electrode of the transistor 30a and the collector electrode of transistor 32a, and the unearthed end of the load 29 will be at a relatively large negative potential. The high impedance between the emitter and collector electrodes of transistor 30b in its cut-off condition holds transistors 31b, 32b cut-off and the values of the resistors 35b ... 38b are such as to avoid appreciable reduction of the impedance between terminal 34b and the unearthed end of the load 29. The collector electrode of transistor 32b will, therefore, be at the potential of the unearthed end of the load 29 and the relatively large potential drop is shared equally by the transistors 30b, 31b, 32b since the base electrodes of transistors 31b, 32b are maintained at potentials with respect to the emitter electrode of the transistor 30b which are respectively one-third and two-thirds of the potential of the collector electrode of transistor 32b with respect to the emitter electrode of the transistor 30b, and that since practically no emitter current flows in the transistors 31b, 32b the emitter electrodes of these transistors, and consequently the collector electrodes of transistors 30b and 31b will be maintained respectively at approximately the same potentials as the base electrodes of the transistors 31b, 32b. In response to a spare element the transistors 30a, 30b are cut-off and bottomed respectively so that the unearthed end of the load 29 is at a substantial positive potential.