US2235021A - Carrier frequency telegraph system - Google Patents

Description

March 18, 1941. Q JUNGA yCMRIER FREQUENCY TELEGRAPH SYSTEM Filed July 22, 1937 2 Sheets-Sheet l March 18, 1941. G, JUNGA 2,235,921

CARRIER FREQUENCY TELEGRAPH SYSTEM Filed July 22, 1957 2 Sheets-Sheet 2 AAAAAAAAA Patented Mar. 18, 1941 UNITED STATES CARRIER FREQUENCY TELEGRAPH SYSTEM Georg Junga, Berlin-Lichtenberg, Germany, as-

signor to Siemens & Halske, Aktiengesellschaft, Siemensstadt, near Berlin, Germany, a corporation of Germany Application July 22, 1937, Serial No. 154,976 In Germany July 24, 1936 10 Claims.

The invention relates to carrier-frequency telegraph systems, and more particularly to methods and means for rendering the reception of signals in such systems free from distortions due to amplitude variations of the incoming signals. Such amplitude variations tend to affect not only the amplitude of the .signals ultimately reproduced in thereceiver, but also the length of these reproduced signals.

It has been suggested to reduce these distortions by providing the receiver system with an automatic level control, and it is known to provide the grid or plate circuit of the receiving three-electrode tube of a telegraph system with a resistance and condenser combination which eiects such a. level control. These circuit arrangements, although maintaining the amplitudes of the telegraph signals constant, have the disadvantage that they affect the envelope curve and thus the length of the signals in the sense of a. distortion. This distortion can be kept within desired limits only in the case of relatively small level variations.

The main object of the invention is to eliminate this disadvantage of carrier-frequency telegraph` receivers. To this end, the invention provides for impressing on the receiver relay a voltage corresponding to the length which the incoming signals have at a predetermined fixed level. This eect is obtained by level control means which are effective before and between the intervals during which the relay performs its switching operations. The operating and releasing points of the relay lie always at the same place of the envelope curve and thus, independently of the level, have the same time distance so that the length of the signals remains unchanged a-nd is independent of level variations.

The just-mentioned level regulation is obtained in the following manner. The incoming signals are impressed on the grid circuit of a receiving tube so that the grid voltage is varied in accordance with the signals. In this grid circuit, a con- -trol voltage is locally produced, thus obtaining a superposed grid control which displaces the working range of the triode along its characteristic. The magnitude of this control voltage is made equal or approximately equal'to half of that portion of the amplitude or intensity of the incoming signals which exceeds a given base level. Since this excess amplitude, in general, is not constant,.the magnitude of the control voltage is dependent upon the intensity or volume variations of the arriving signals.

Further, according to the invention, the periods lduring which the above-mentioned control voltage is effective are long as compared with one signal impulse in the case of closed circuit operation, or as compared with the spacing between the impulses if open circuit operation is employed.

That amount of the excess amplitudes which is in excess of the control voltage involves a time constant which is of about the order of the time within which the amplitude of the incoming signais increases to its full value or decays to Zero. In alternating current telegraph systems, this time is chiefly determined by the building-up time of the lters. Due lto this time constant, the vadjustment of the amplitude to its normal value (Ego) becomes eiiective only after the signal has been transmitted without distortion to the receiving relay and has already become ineffective before the receiving relay makes the next step.

The invention is particularly suitable for closed circuit operation but is also applicable with open circuit operation.

The above-characterized invention will be better understood from the drawings in which Fig. 1 is a diagram showing the voltage and current curves in the grid and anode circuits of a triode in a carrier-frequency system according to the invention.

Figs. 2, 3 and e exemplify three different circuit arrangements with the aid of which the invention may be realized in practice and in accordance with Fig. 1,

While Figs. 5 and 6 are diagrams of explanatory nature for elucidating the matter illustrated in Figs. 1 and 2.

In the arrangements shown in Figs. 2, 3 and 4, U signifies an input transformer, the secondary winding s of which is connected in the grid circuit g of a three-electrode tube R. The anode circuit a of this triode is connected to a receiving relay ER which controls the -telegraph circuit b. The terminals -G, O, |A are to be connected to a voltage source furnishing the negative grid voltage, the Zero potential and the positive anode voltage respectively. The carrier-frequency modulated according to the telegraph signals to be transmitted is supplied to the primary winding n of transformer U.

If in arrangements of the above type the signal intensity were constant so that the grid voltage caused by the signals maintains a constant level Ego, the length of the signals and the silent intervals `transmitted through the triode and indicated by the telegraph relay would maintain a correct value. In fact, however, the intensity of the iincoming signals and hence the grid voltage controlled thereby are not constant. This actual grid voltage, therefore, may be considered as being composed of the above-mentioned level voltage Ego plus an excess amplitude Egu, so that the total grid voltage Eg=Ego-|-Egu. This rise of the level above the value E'go would result, as stated before, in length distortions of the signals indicated by the telegraph receiver. In order to avoid such distortions, the grid circuit of the triode shown in Fig. 2 -contains an impedance combination R1, R2, C1, C2, which, according to the invention, produces an automatic level control with the result that a correct length of the signals and intervals is maintained irrespective of variations in the volume or intensity of the incoming signals. The embodiments of Figs. 3 and 4 show other impedance combinations of similar function. The operation of these level controlling combinations and the requirements to be fulfilled with respect to the dimensioning of the individual combination elements, will become apparent from the following consideration.

The upper portion of the coordinate system of Fig. 1, having the zero point 0, shows the time curve of the anode current Ia, while the lower nals.

portion of the diagram shows the time curves vof incoming signals.

To the left are plotted the voltages Eg in the grid -circuit prior to the level regulation and to the right upwards the anode currents as function of the grid voltages of the level regulation. It shall be assumed, for the sake of simplicity, that the voltages arising in the grid circuit are direct voltages and that the telegraph signals appear in the grid -circuit already as rectified sig- It is of course obvious that the level control Would function similarlyT if the telegraph signals arrived in the form of keyed alternating current signals. In this case Ithe lines representing -direct voltages would represent the envelope curve of an alternating current curve which, in

this case, would have to be supplemented symmetrically to the other side. 'Ihe oblique line A/B represents diagrammatically the valve characteristicwith its lower bend A, and OC represents the grid current curve. First it may be assumed that a vol-tage Ego is applied to the grid so that the valve is modulated up to the lower bend A and that no anode current ows when no signal Voltage is present.

In the case of closed circuit operation, during the inoperative interval, a telegraph voltage is permanently eii'ective at the grid. This voltage, at the lowest amplitude level, is equal in magnitude but of opposite direction as compared with g the biasing voltage Ego and permits the maximum anode current to ow. This state continues from time point I to poin-t 2 and from I to 2 of the curve of the signal characteristic as shown in a solid line. The telegraph signal the curve of which is assumed for the sake of simplicity to be of trapezoid shape extending through the points 2, 4, 5, 6, 1, may commence at the time indicated by point 2. Consequently, in the anode circuit a curve according to points 2', 3', 4', 5', 6', 1' results. The responding limit of the receiving relay may lie on the line 3-6'. In the case of a correct level control it is necessary that the lengths of identical telegraph signals are also equal even if the level is different, this length being in this case equal to the distance 3*-6 or 3 6 assumed that the receiving relay is accurately adjusted to, and sufficiently maintains, the responding voltage level 3-6, a requirement which can easily be complied with with the modern, highly sensitive telegraph relays. Now when the incoming signal in the grid circuit of the tube arrives not at the level Ego but at a higher level result. The greater length of lthe dis-tance I8-l9 compared with 3--6 represents a distortion because of the then longer duration of the period during which the relay responds to the telegraph signal.

Here the invention comes in. By the means descrbedhereinafter, a control voltage is locally produced which is equal or approximately equal to half the excess amplitude. By virtue of this control voltage, the difference between excess amplitude and control voltage is made to disappear before `the receiving relay responds. It is thus achieved that the'length of the signal approaches more nearly the ideal case, which it attains theoretically for the case in which the control voltage Ego is equal to half the excess amplitude voltage Ecu. More particularly, with closed circuit operation, in the grid circuit of the tube a permanent grid voltage Eg=Ego+Egv is established as has been shown in Fig. 1 by the line shown dotted throughout, which passes through the points |5-l6. Thereby the telegraph signal which would originally have passed through the points 8 9, is displaced so that it actually passes through the points I0, 3, Il, I2, 5, i3. A geometric construction, which will be explained in the following, shows that all signals for which the relationship Eg-:Ego-l-Egv is fullled, through the points 3 and 6. As a result, all signals received have equal lengths, even if the receiving amplitudes are different. The geometrical consideration which proves that under the conditions above mentioned the curve of the signal voltage always passes through points 3 and 6 regardless vof the amplitude of the voltage, shall now be given with reference to Fig. 5. Fig. 5 shows a trapezoid ABCD. A second trapezoid ABEF is erected on the same base AB, the line EF having the same length as line CE. Dotted lines EC and FD measure the difference in the heights of the two trapezoids. By bisecting this difference, the points E and F are obtained. The line EF now forms the basis for drawing another trapezoid ABEF which is made equal to the trapezoid In the construction thus obtained, `the slanting sides AD and BC of the original trapezoid ABCD and the slanting sides A'F and BE' of and H respectively. This results from the following. Due to the premises, AB and AB' are parallels, and since trapezoids ABEF and A'BE'F are congruent, lines AF and AF must also be parallel. In the triangle ADF, the side DF is bisected at point F, also according to the premises. Since line AF, going through midpoint F', is parallel to side AF of triangle ADF, the third side AC must be bisected by AF. That, further, line AF is bisected by line ACv follows from the facts that each of the two triangles AGA' and FGD has its -three sides parallel to the respective sides of the other triangle, and that the angle AGA is equal to the angle F'GD so that both triangles are congruent.

' The foregoing proves that all trapezoids ABEF of any possible Aheight pass through the points G and H if displaced half the distance between their tcp side EF and that of the original trapezoid CD. The present invention, dealing with signal voltages of approximately trapezoidal curve shape, utilizes this geometrical law by applying a displacement voltage Ego which is equal,

or approximately equal, to half the excess moduin which case Egc=1/2Egu, must pass 30 produced in the receiver have always the desired length regardless of the value of the excess voltage.

Reverting now to Fig. 2, it will be seen that `the impedance combination R1, Rz, C1, C2, there shown, represents the simplest circuit diagram for realizing the operating conditions established with reference to Fig. 1, i. e. for producing the above-mentioned additional control voltage Ego. R1 and R2 designate a resistance arrangement `connected in the grid circuit of the triode R in parallel to condenser C1. The resistance arrangement is tapped in the middle so that R1=R2. From thisk center tapping the resistance R2 is bridged by the condenser Cz. The grid, in known manner, a-cts upon the anode circuit of the valve Rand thus upon the polarized receiving relay ER, a definite position of which is in known manner effected by applying a magnetic bias 2i? through a second winding. It is necessary, for

an advantageous control, that the time constant of the resistance-condenser combination Rz and Cz is large v`compared with the time constant of the resistance condenser combination (R14-R2) CI.

The functioning of the cir-cuit arrangement follows from a consideration of Fig. 1. When the signal commences, the full grid vol-tage Ego-i-Egu, in consequence of the small time constant of the vcombination (Rl-l-R2)C1, drops from point I4 to point I5 and thus to the voltage Ego-l-Egv which is maintained by the condenser C2. This voltage remains in existence during the duration of the signal because of the large time constant of the combination C2-R2. Only after the relay has responded does the voltage rise again'along the curve I 6-I'I in consequence of the charge on the condenser CI, the vol-tage then reassuming the full grid voltage value.

The diagram shown in Fig. 6 serves to further indicate the relation of arrangement of Fig. 2 to the trapezoidal voltage curves shown in Fig. l. The elements S, g, C1, Cz, R1, and Rz, in Fig. 6, are identical with the similarly designated elements in Fig. 2. 'I'he legends in Fig. 6 identify the voltages occurring in the arrangement by referring to the voltage curves shown in Fig. 1. Accordingly, the voltage across the secondary S- of the input transformer corresponds to the original curve 2, 8, 9, 'I of Fig. 1. The voltage having the curve I4, I5, I6, I1 lies across resistor R1. The time constant of the resistance combination C1 R1 is very small so that the voltage decreases quickly from I4 to I5. Across the end of the regulating network, i. e. between the grid conductor g` and the pole -G, appears the regulated 'output voltage corresponding to curve 2, I0, 3, II, I2, .6, I3, 1.

Fig. 3 shows another advantageous circuit arrangement for practising the method according to the invention. In this circuit arrangement, the resistance RI-l-R2 which lies in parallel with the condenser, is divided at the ratio pared with the circuit arrangement according to Fig. 2, the circuit arrangement according-to Fig. 3 is of advantage in that, in the case of closed circuit operation, it is sooner in operating condition and that, in the case of variations in intensity of the incoming signals, the desired level is sooner established. This follows from the fact that the time for charging the condenser C2 is about half the time constant required for discharging the condenser. Lastly, this circuit arrangement has the advantage that in the case of continuously transmitted signal combinations of one polarity, the most favorable displacement voltage is unilaterally displaced to a substantially smaller extent than with the circuit arrangement according to Fig. 2.

In the circuit arrangement according to Fig. 4,

.the condenser C2 forms the terminating link of a chain network, the attenuation of which is so dimensioned that \/0.5 times the excess voltage Egu is effective at the displacement condenser Cz, when charged. When discharging through the chain network, this voltage is reduced again to \/0.5 times the amount so that in the grid circuit there is again effective, as control voltage, a voltage equal to 0.5 the excess amplitude. In the grid Vcircuit lies the condenser C1 for the normal level control, in parallel with which lies the resistance of the chain network. The insertion of the condensers C3 and C4 has the purpose to still better balance the eiect of unfavorably dimensioned telegraph signals. This is of advantage in cases where telegraph signals are transmitted continuously which comprise for example 4 -limpulses and l impulse or, Q

1 impulse and 4 -limpulses, an operating condition which without the additional condensers may lead to a shifting of the permanently adjusted control voltage Egt from its normal position.

I claim as my invention:

l. In a communication system having a receiving terminal provided with a. circuit for compensating for the variations in amplitude level of the transmitted signals in a signal transmission circuit, the method of regulating the amplitude of the -incoming signals which consists in producing a locally generated control voltage approximately equal to one half of the excess amplitudes, and impressing said control voltage on said receiving terminal so as to thereby effect a distortion-free signal reception.

2. In a carrier-frequency telegraph system having a receiving grid-controlled tube connected in a transmission circuit for compensating for the Variations in amplitude level of the transmitted signals, the method of regulating the amplitude of the incoming signals which consists in kproducing a locally generated control voltage approximately equal to a magnitude having a given fixed relation to the excess amplitudes, and impressing said control voltage on said receiving terminal so as to thereby effect a distortion-free signal reception.

3. In a carrier-frequency telegraph system for closed circuit operation having a receiving terminal provided with a transmission circuit and a grid-controlled receiving tube in said circuit for compensating for the Variations in amplitude level of the transmitted signals, the method of regulating the amplitude of the incoming signals which consists in producing a locally generated control voltage approximately equal to one half of the excess amplitudes, and Iimpressing said control voltage on said receiving terminal, and

maintaining after a signal has commenced said control voltage effective during a period which is long compared with the durationvof one pulse ofthe signals.

4. In a carrier-frequency telegraph system for open circuit operation having a receiving terminal provided with a transmission circuit and a grid-controlled receiving tube in said circuit for compensating for the variations in amplitude level of the transmitted signals, the method of regulating the amplitude of the incoming signals which consists in producing va locally generated control -voltage approximately equal to one half of the vexcess amplitudes, and impressing said control voltage on said -receiving terminal, and maintaining after a signal has ceased said control voltage eiective during a period which is long compared with one spacing of the longest duration between two pulses of the telegraph signals employed.

5. A carrier-frequency receiver system having a transmission circuit for compensating for the variations in amplitude level of the transmitted signals, a receiver relay connected withsaid circuit, -a grid-controlled tube having its anode cir- Icuit connected so as to form part of said transmission circuit, a grid circuit for impressing on the grid of said tube a bias voltage in accordance with the incoming signals, and reactive means connected with said grid circuit for locally producing a control voltage approximately equal to one half of the excess amplitudes, said reactive means having a time constant in the order of magnitude of the duration of the individual pulses of said signals to maintain said control voltage effective during a period having at least the duration of one pulse of the signals.

6. vA carrier-frequency receiver system having a transmission cir-cuit for compensating for the variations in lamplitude level of the transmitted signals, a receiver relay connected with said circuit, a grid-controlled tube having its anode circuit connected so as to form part of said transmission cincuit, a grid circuit for impressing on the grid of said tube a bias voltage in accordance `with the incoming signals, and an impedance combination connected in said grid circuit to effect a distortion-free signal transmission by locally producinga control voltage, said impedance combination including two series-connected resistors of equal resistance forming a voltage divider, and a condenser connected with one of said resistors so as to be charged in the intervals between subsequent signals with a voltage approximately equal to one half of the excess amplitudes, whereby said condenser after a signal has Iceased acts upon the grid of said tube with its full charging potential.

7. A carrier-frequency receiver system having a transmission circuit for compensating for the variations in amplitude level of the transmitted signals, a receiver relayconnected with Said circuit, a grid-controlled tube having its anode cir- Icuit connected so as to form part of said transmission circuit, a grid circuit for impressing on the grid of said tube a bias voltage in accord-ance with the incoming signals, and an impedance combination connected in said grid circuit to effect a distortion-free signal transmission by locally producing a control voltage, said impedance combination including a voltage dividing resistance network and at least one condenser connected in said network so as to be |charged between subsequent signals with \/0.5 times the excess amplitudes of the signals received, whereby said condenser afterr cessation of a signal acts upon the grid of said tube with \/0.5 times its charging potential.

8. A lcarried-frequency receiver system having a transmission circuit for compensating for the variations in amplitude level of the transmitted signals, a receiver relay connected with said circuit, a grid-controlled tube having its anode circuit connected so as to form part of said transmission circuit, a grid circuit for impressing on the grid of said tube a bias voltage in accordance with the incoming signals, and means arranged in said grid circuit for imposing in response to the signal voltage on the grid of said tube a control voltage approximately equal to one half of the excess amplitudes of the signals received, said means having a time constant designed to render said control voltage effective during a period beginning after and ending before the variation of the grid voltage caused by a received signal passes through the responding value of said relay.

9. A carrier-frequency receiver system having a transmission circuit for compensating for the variations in amplitude level of the transmittedl signals, a receiver relay connected with said circuit, a grid-controlled tube having its anode circuit connected so as to form part of said tran-smission circuit, a grid circuit for impressing on the grid of said tube a bias voltage in accordance with theincoming signals, 4and an impedance combination connected in said grid circuit for locally producing a control voltage approximately equal to one half of the excess voltage amplitudes, said impedance combination comprising two series-connected resistances of equal magnitudes, a condenser connected in parallel to said two resistances, and a second condenser con-v nected in parallel to only one of said resistances so as to be charged in the intervals between subsequent signals with about one half of said excess voltage amplitudes, the time constant of the part combination consisting of said latter condenser and the one Iresistance parallel connected thereto being greater than the time constant of the part combination consisting of said two series-connected resistances and said first condenser.

10. A carrier-frequency receiver system having a transmission circuit for compensating for theI variations in -amplitude level of the transmitted signals, a receiver relay connected with said circuit, a grid-controlled tube having its anode circuit connected so as to form part of said transmission circuit, a grid| circuit for impressing on the grid of said tube a bias voltage in accordance with the incoming signals, and a chain network connected in said lgrid circuit for preventing signal distortions by producing a local control voltage, said chain network containing one or more T members composed of resistances and linked together with one another and with Said grid circuit by capacitors and having another capacitor forming the termination of said chain, the total attenuation of said chain being designed to effect a charge of said latter capacitor between subsequent signals with \/0.5 times the excess voltage amplitudes of the signals received.

GEORG J UNGA.

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