US2291369A

US2291369A - Polar carrier telegraph system

Info

Publication number: US2291369A
Application number: US420028A
Authority: US
Inventors: John E Boughtwood
Original assignee: Western Union Telegraph Co
Current assignee: Western Union Telegraph Co
Priority date: 1941-11-21
Filing date: 1941-11-21
Publication date: 1942-07-28
Anticipated expiration: 1959-07-28

Description

July 28 1942 .'l. E. BouGH-rwoon P'OLR CARRIER TELEGRAPH SYSTEM 3 Sheets-Sheet l Filed Nov. 2l, 1941 Chitty/mug July 28, 1942- .1. E. BouGHTwooD 2,291,369 POLAR CARRIER TELEGRAPH SYSTEM Filed Nov. 21, 1941 3 Sheets-Shea?I 2 3 sheetssheej s i mw S RSS s@ mmf bw..

July 23 l942 J. E. BOUGHTWOOD POLAR CARRIERJTELEGRAPH SYSTEM Filed Nov. 2l, 1941 Patented July 28, 1942 UNITED STATE POLAR CARRIER TELEGRAPH sysraiu Application November 21, 1941, Serial No. 420,028

21 Claims.

This invention relates to systems of transmission in which carrier currents are employed for the transmission of telegraph signals and more particularly to improvements in polar carrier frequency systems employing separate frequencies for the marking and "spacing signals. This application is a continuation in part of my prior application Serial Number 307,717, filed December 5, 1939.

Telegraph transmission by the carrierfrequency method is usually accomplished by opening and closing the circuit of a carrier frequency generator at the transmitting end to send the spacing and marking signals respectively. At the receiving end the rectified carrier current operates a relay to produce the marking signal while the absence of current denotes a spacing signal. On occasion this relation between opening and closing of the circuit and marking and spacing may be reversed.

While a high degree of transmission eiciency may be secured by means of this elemental system, the signals are subject to bias as a result of variations in the level of the received signal. As a means of avoiding this diiliculty it has been proposed to secure the equivalent of .polar telegraph signals by employing separate frequencies for the marking and spacing signals, respectively, thus the receiving relay is operated positively by an incoming carrier subjectedl to the same line 30 condition during both the marking and the spacing periods. Various arrangements have been developed for accomplishing this system of transmission.

The objects ofthe invention are as follows: 35

To eliminate bias heretofore existing in the signais of polar carrier systems; to increase the ei'- iiciency of polar telegraph systems; to modulate the frequency of a carrier current in accordance with a telegraph signal of preferred shape; to 40 cause the slow and smooth translation of the frequency generated by an oscillator between two values corresponding to marking and spacing telegraph signals; to gradually introduce inductancey and capacity into the frequency determining circuit of an oscillator; to provide a regenerative limiting amplier; to provide a differentiating circuit for coupling a discriminating circuit for polar telegraph signals to a balanced amplifier'. Other objects will appear from the 50 following detailed description.

The polar carrier channel disclosed herein while incorporating the usual features of such systems also embodies novel features for controland spacing frequencies whereby the carrier signal possesses a single set of characteristic frequency modulation products the essentials of which are encompassed within the 180 cycle band width. l

As a consequence of these improvements it has been possible to operate a polar carrier channel within the 180 cycle band -which is available when carrier frequencies are spaced at 300 cycle intervals, at the same lhigh speeds which may be attained with single frequency working under favorable conditions. It is thus possible to introduce the improved features of my invention into existing carrier systems without loss of message capacity and with the gain of the advantages above outlined.

The invention is illustrated schematically in the accompanying drawings in which Figures 1 and 2 show respectively transmitting and receiving circuits; Figures 3, 5 and 6 show other lmprovements in transmitting circuits; Figures 4 and 7 illustrate further arrangements of receiving circuits; Figures 8 and 9 are illustrative diagrams explanatory of certain operations oi the receiver shown in Fig. '7. l

The transmitting circuit comprises an oscillator illustrated as of the dynatron type although any oscillator capable of rapid frequency change may be employed. In variable oscillator type transmitters it has been customary to employ a single contact relay which when open, .permitted the oscillator to generate a spacing frequency and when closed so altered the frequency determining circuit as to cause the oscillator to produce a different frequency for marking It 'can be seen that with the single contact circuit the relay travel time is added to the spacing period to produce a spacing bias, which must be eliminated by relay adjustment at the receiving end. The transmitter of the type disclosed herein overcomes that difliculty by adjusting the oscillator to generate normally a frequency located midway between the marking and spacing frequency and then positively shifting the oscillator frequency to the marking and spacing values.

Referring to Fig. 1, the normal frequency of the oscillator circuit of the tube I is determined by the values of the condenser and the inductance of the frequency determining circuit 6.

The frequency of the oscillator is then shifted from the normal or intermediate frequency to marking or spacing frequency as the armature or tongue 5 of the transmitting relay operates between its two contacts. Thus when the armaling the mode of transition between the marking ture engages its spacing contact, the oscillator tuning circuit 8, is shunted by the capacity 1, thereby causing the oscillator to operate at spacing frequency; and when the armature is shifted to its marking contact, the tuning circuit I is shunted by the inductance lI, thereby causing the oscillator tooperate at marking frequency, With this system the generated frequency shifts from marking through an intermediate frequency to spacing and vice versa to produce signals of equal length. 'I'his feature has been found to contribute prominently to the attainment of high speeds while retaining close spacing of the two signaling frequencies. Not only then are -the marking and spacing periods symmetrical, but the transition of frequency from the one to the other is accomplished in two half steps thus achieving in some degree a controlled and gradual change between the two frequencies. However, signals from such a transmitter when received and demodulated into the original telegraph signals still show'a slight fortuitofus signal loss. A probable reason for this loss is as follows:

It is well known in frequency modulated tiinsmission systems that when the modulation m=' deviation frequency signal frequency is in the neighborhood of unity, the side band components are limited to a very small number with the energy principally concentrated in the rst component which is situated a distance from the carrier equal to the signaling frequency. Consequently, if a high speed telegraph signal, approaching the deviation frequency in value, contains only its. fundamental component, the side bands will be as Just defined with the 'more distant components virtually absent. On the other hand, if a square top telegraph signal were used with its infinite series of components there would be numerous additional side band frequencies distantly spaced from the carrier. only would a large portion of this side band energy be outside the available transmission band andv thus be suppressed in the ytransmitting filter, but the energy relationships between the carrier and the side band frequencies which are contained within the available band would not be such as to provide the best signal shape after demodulation. liowever, where the signal is confined to substantially a single frequency component not only is most of the energy preserved and transmitted but the essential relationships between side bands and carrier are retained to provide a signal which upon demodulation substantially duplicates the original.

'I'he signaling frequency waves pass through the usual channel amplifier and sending tuner to thesendIng bus for the various channels and from thence via the sending line amplifier to 1 line in the manner well understood.

In order that the full benefit of polar operationAmay be secured, the proper type of receiving equipment must be utilized. Figure 2 shows schematically one form of receiving terminal. 'Ihe signals of the carrier system after amplification, are selected by a channel tuner in the conventional manner and passed to the translating circuit via a limiting device I0, of the push-pull type, and harmonic suppressing filter i2. I'he translation circuit Il, comprises two series resonant circuits I5, tuned to the lower and higher signaling frequencies respectively, and sufficiently damped to eliminate amplitude distortion. The voltage across each circuit being Not a function of the instantaneous carrier frequeney, there is produced an amplitude'envelope proportional to the' frequency envelope of the modulated signaL Thus when the carrier voltage across one tuned Vcircuit is a maximum, the voltage across the other is a minimum and vice versa;

These voltages are separately amplified at Ii, detected at I1 and applied differentially to the receiving relay 20'.' Thus the receiving relay vis actuated by polar D. C. telegraph signals, similar in form and character to those operating the transmitting relay at the distant station.

The double translation circuit shown and the method of transmission employed, permit the entire system to be operated on a polar basis with the advantages pertaining thereto. In addition, the limiting device i0, permits the receiving relay 20. to operate at its most efficient current level regardless of the received carrier level. Bias free operation is still possible on carrier levels below the limiter range, since in any polar system both marking and spacing signals are similarly reduced' in level by line changes or other fortuitous circumstances.

Another type of transmitter shown in Fig. 3,

employs a different method for varying the oscillator frequency. The oscillator may be identical with that of Figure 1, but instead of employing a supplemental inductance and capacity to obtain the required frequency deviation, these reactances are secured synthetically through the functioning of. a pair of vacuum tubes. It is well known that a thermionic tube may be made to act as a reactance if its grid voltage is derived from and is in phase quadrature with its plate voltage. AThe magnitude of this reactance is a function of tube gain and can be controlled by the grid bias voltage. Thus by alternately shunting the oscillator tunedv circuit with a pair of vacuum tubes having properly coupled plate and grid circuits, it is possible to have one tube act as a capacity and the other as an inductance to shift the frequency of the oscillator to thereby produce marking and spacing signals.

The functioning of this transmitter will now be described in greater detail. The cathode-anode circuits of the two tubes A and B are connected effectively in parallel with the frequency determining tuned circuit 6, of the oscillator l. It is apparent that output voltages from the oscillator are applied to the grids of these two tubes.

through the two identical phase shifting circuits Ra and C1 and that the output voltage from the tubes A and B, 'operating in parallel, is returned to the oscillator tuned circuit. The phase shifting networks are so adjusted that the grid voltf ages of the two tubes A and B, while 180 apart, are each displaced substantially from the voltage across the oscillator tuned circuit 8;, Under these circumstances tube A which re-` ceives the leading grid voltage, functions as an inductance, While tube B, which receives the lagging voltage, appears as a capacity. Depending then upon which tube is functioning, the fre-,

this positive potential and thus restore the gridcathode potential to an appropriate operating value. The tubes A and B therefore function alternately in response to the operation of the marking and spacing contacts respectively of the transmitting relay 25, to vary the period of the tuned circuit 6. In consequence, marking and spacing frequencies respectively are generated and transmitted to line.

Figure illustrates another modification of the transmitter wherein the supplementary inductance L1 and capacity C1 employed to change the frequency of the ,oscillation circuit of the tube I, are connected to the main frequency determining circuit 6 via rectifier elements Rif1 and Rif1 whose conductivity may be controlled by the application of biasing potentials.

When the transmitting relay is connected to negative battery to send a spacing signal, the rectifier Rtfl is biased thereby so that it is effectively open circuited and hence inductance L1 has no effect upon the oscillator frequency. Simultaneously, however, the rectifier Rtf2 is rendered conductive so that the capacity C1 forms a shunt to the tuned circuit 6, thus causing a spacing (low) frequency signal to be transmitted. The inductance La shunted by condenser C3, provides a path to ground for the required polarizing current through the rectifier Rtf".

Likewise when the transmitting relay is connected to positive battery for the purpose of sending a marking signal, the rectifier element Rtf2 is biased to effectively open the circuit therethrough, so that thecapacity C1 is ineffective. At this time the polarizing current from-positive battery via a path through L1 and L in series, causes the rectifier Rtfl to become conductive so that the supplementary inductance L1 effectively shunts the tuned circuit 6, whereby a "marking (high) frequency is transmitted.

In this transmitter provision is made for causing the transition from the marking to the spacing frequency to be carried out smoothly and slowly. This is effected by means of a shaping network L4C4 located in the sending leg which causes the controlling potential applied to the rectifiers to rise gradually and smoothly. The sharp corners of a square top signal are eliminated so that the modulated signal is largely limited to its fundamental component, the higher harmonics being absent. Thus a true frequency modulation is provided. The carrier signal possesses a single set of characteristic frequency modulation products the essentials of which may be encompassed within the 180 cycle band width designed to operate with carrier frequencies spaced 300 cycles apart. It is therefore possible to substitute such a channel for an existing amplitude modulated channel, utilizing the same frequency space and filters, and to operate at the same or an even higher speed while gaining the advantages of polar carrier operation and frequency modulation described herein.

Anothermodification of my transmitter is shown in Fig. 6. 'I'he oscillator l is illustrated as of a particular type but other types of -oscillators capable of rapid frequency change will serve as well.

'Ihis oscillator is coupled by any suitable form of coupling to one or more amplifying stages as needed for applying the modulated carrier current telegraph signals to line. The oscillator is adjusted by means of the anti-resonant Icircuit 6 to generate a carrier frequency located at the center of the assigned channel band. To produce the marking and spacing signals, the frequency of the oscillator is varied by like amounts upward and downward from the center frequency by alternately connecting in parallel with the tuned circuit ii, auxiliary reactance elements L1 and C1. This switching operation is accomplished by means of the rectifier devices S and M under control of the telegraph signals. The inductance L1 is chosen with respect to the inductance of circuit 6, so that the inductance of the two in parallel causes an increase in the natural period of the oscillating system equal to the desired frequency deviation. Likewise the condenser C1 and the inductance Lz in combination, are similarly chosen with respect to the condenser of circuit 6 to produce a like decrease in frequency. The inductance L2 furnishes a direct current by-pass around the condenser C1 while the large condensers C2 provide a carrier frequency by-pass to ground.

The function of the elements contained within the area Z is to alter the received direct current telegraph signal to the shape which has been found particularly desirable in the production of frequency modulated carrier telegraph signals and to control the conductivity of the marking and spacing rectifiers S and M alternately in accordance with the shape of these telegraph signals. The square top telegraph reversals received over the line N from any suitable form of transmitter of polar signals `are given a smoothly rounded form by the inductance L3 and the condenser Ca in combination and flow thence to ground through the resistance a. The potential developed across this resistance is applied equally to the two rectifiers S and M through the rather high value resistances b and c, respectively. If, for example, a spacing or positive signal is being received, the resistance of the rectifier S is reduced from infinity down to substantially zero to gradually introduce inductance in parallel with the tuned circuit 5 up to the full value of theinductance coil L1 at a rate determined by the smoothly rounded form of the telegraph signals. The generated carrier frequency will then have shifted upward in exact accordance with this change in tuning by an amount equal to the predetermined deviation frequency. Similarly. application of a marking signal will cause the gradual introduction of the effective capacity of the C1Lz combination in parallel with the tuned circuit up to the full value of the combination to cause a like downward shifting of the generated carrier frequency. It should be noted that the signal potential which renders one rectifier conductive to introduce the desired reactance element, at the same time renders the other rectifier non-conductive at the same time-rate to remove the other reactance element. This simultaneous introduction and withdrawal of the mutually opposing reactance elements in parallel with the circuit 6 causes relatively slow and smooth transition of the oscillator frequency between the marking and spacing values as determined by the shape of the modulating signals. It should be understood that although there will be some loss in efficiency, theI action of the two rectifiers need not be simultaneous but that the introduction of the one may follow the withdrawal of the other. The function of resistances b and c is to prevent short circuiting of the resistance a, through the rectifier which is at the moment c-onducting and'thus reducing to zero the opposing potential which maintains the other rectier in the non-conducting condition.

The conducting function of the two rectiers in alternately connecting the auxiliary reactances into the tuned circuit may best be understood by considering them as variable resistance devices upon which is impressed the continuous carrier current voltage generated by the oscillator and appearing across the terminals of circuit 6 and having a frequency determined by the circuit 6. Assume, as before, that a positive potential is applied over the telegraph line to cause a current to flow through the resistance b, the inductance L1 and the rectifier S, and thence to ground via the circuit 6. Under vthis condition rectifier S will have low resistance and will pass a direct l current upon which is superimposed the carrier current generated by the oscillator. The inductance Li is now effectively in parallel with the inductance of tuned circuit 6 and will cause an increase in the generated frequency. 'I'he direct current component should be adjusted to exceed in value the highest possible peak value of the alternating carrier current so that the carrier current may flow freely through the rectifier without clipping of the peaks, except briefly during the period of reversal. By thus providing an excess conductivity for the rectiers, ordinary variations in the amplitude of the received telegraph current will not influence the frequency deviation.4 While the rectifier S is conducting, the rectifier M is non-conducting by virtue of the equal positive potential applied to its cathode, thus opening the circuit of the CiLz combination. In like manner, to send a marking signal an energizing negative potential is applied to the rectifier M to render it conductive and connect the combination C1L2 in parallel with the-tuned circuit 6 to cause a decrease in the generated frequency. At the same time the negative potential imparts a very high resistance to rectier S and thus effectively removes inductance Li from the circuit.

A particular property of the transmitter above described is the slow and smooth transition between the marking and spacing frequencies. This is contributed to by several factors, including the shaping elements In and Ca and the low decrement of the tuned circuit 6. It is by Virtue of this slow transition that a true frequency modulation with a single orderly set of characteristic side bands of preferred disposition is obtained. As a consequence, the signal energy is contained within arange of useful frequencies sufiiciently small to be contained within a narrow channel space, and extraneous frequencies which might cause interference in adjacent channels are substantially absent.

A small amount of amplitude modulation of the carrier signal envelope occurs at the moment of reversal, due to partial conductivity of the two rectiers at this instant when the polarizing potential is in the neighborhood of zero. This effect is slight and since it is a reverse type of tirely independent of variations in the exciting potentials received over the telegraph line.

Figure 4 illustrates a circuit arrangement at the receiving end which greatly aids in the superior performance oiv my telegraph system. In this receiver direct detection is accomplished by means of the discriminating circuits and the differential polar lrelay and is not dependent upon a synchronized demodulating frequency, as in certain prior systems and which is always a source of trouble.

The translation circuit lcomprises two parallel the low impedance of the translation circuits at` the higher frequencies and they thus appear only to a negligible extent. As a result the harmonic suppressing filter indicated at I2 in Figure 2, can be eliminated.

The output voltages of the tuned circuits are p detected at 30, the marking and spacing signals flowing separately through their associated redistortion to that which occurs in the transmitsistance ll, across which the potential is of con-V stant maximum value, reversing in sign as the carrier is modulated by the transmitter and is applied differentially to the grids of the push-pull amplifier 3l. 'I'he resistances 36 and the condensers 35 serve to establish the potentials of the gridsv of the amplifier tubes with respect to ground. The amplified voltage is applied differentially to the receiving relay 32. A biasing cir cuit is indicated at 39 to compensate for minor asymmetries of the transmitting and receiving systems.

'I'he above described receiver illustrated in Fig.

4 employs a single stage amplifier which functions as a combined amplier and limiter. .This receiver is satisfactory wherev the signals are received at a substantial level or when a common receiving amplifier for all channels precedes the channel receivers. In Figure rI I have shown a modification of the receiver which employs a regenerative limiting amplifier of high gain in receiving terminals Ifor frequency modulated carrier telegraph channels which are intended to operate on lines or circuits where the received levels are very low. The arrangement disclosed in Figure '1 effectively combines the two functions of amplifying and limitingin a single two-stage amplifier in such a manner that very high gain is secured along with perfectly effective limiting of the output current as the input level varies over a very wide range.

The limiting amplifier is embodied ina twostage resistance coupled thermionic amplifier employing preferably for the rst stage a triode and for the second stage a screen grid tube. A type of tube in which both the stages areineluded within a single glass or metal envelope has been found to operate very satisfactorily while making for economy in consumption of space and power. In the first stage, tube V1, includes in series with its grid a resistance Rz of comparatively high value shunted by a small condenser C1. With such a resistance of suitable value the amplifying action of the tube remains approximately linear up to a certain limiting value of input voltage. but above this value the positive half waves of the input signal will cause a spacecurrent to ow in the grid-cathode circuit to produce ascisse an IR drop across the resistance R2 in such direction as to make the grid more negative, thus reducing the amplification of the tube. As the input level increases the grid potential is progressively depressed in a negative direction until a balance is reached between the energizing input potential and the paralyzing bias potential. At

this point, which is governed by the value of the resistance, the output level tends to become constant. 1

A single stage limiter operating in this fashion might provide a gain of 20 db. 'I'hat is, it would lift an input signal say from a level of -20 db. to zero db. with effective limiting at the zero level. However, if the input level dropped below -20 db., theoutput level would not reach the desired constant limited value. Accordingly, in previously designed limiting ampliers it has been necessary to provide pre-amplification vsufficient to insure this overloading of the limiting stages. The present device comprises an organization of two tubes constituting a unitary limiter and high gain amplifier which will provide a constant output working level of adequate value for operating the discriminating or other detecting circuits while receiving extremely low input levels. Very high gain is secured by means of regeneration between the output and input stages so that even very low input levels may be amplied above the level where limiting action occurs. This regeneration is accomplished -by means of the resistance R which isy common to the cathodes and grids of both tubes. The current from the cathode of tube Va flows through this resistanceto produce a potential drop thereacross which is in phase with and so augments the input voltage to tube V1. By this regenerative action the gain of the amplifier as a whole is tremendously increased over the mere aggregate gain of the two tubes taken separately. It is possible, therefore, to extend the lower operating limits of the amplifier to exceedingly low levels while maintaining a constant output at a relatively high level.

y The combination of the two tubes Vi and Vz without regeneration provides a limiting action as indicated in Curve A of Fig. 8. At relatively low levels the amplifying action of tubes V1 and V2 is substantially linear but at a certain desired higher level dependent upon choice of values for the resistance R2 and the condenser C1 the output tends to become constant. By virtue ofthe added amplification caused by the regenerative action of the tube V2, however, the amplier will reach to a much lower level as indicated by the Curve B while still producing a saturated output. Through the lower portion of this curve the action is again substantially linear and the feedback voltage is proportional to the signal input or output levels. However, the signal input voltage although very small is augmented by the feedback voltage to quickly reach a point where limiting action takes place, and since the output is constant the feedback remains constant. A substantially stable condition now obtains, the feedback furnishing the greater part of the energizing potential while the signaling potential remains just large enough to maintain control and prevent oscillation. In case the action of tube V1 alone is insuflicient to give a perfectly constant output level a supplemental limiting action may be obtained by proper choice of the values of the resistance R3 and the condenserCz in the grid circuit of tube Vn. By this means the dotted portion of the curve of Fig. 8 may be rendered horizontal as indicated. v

It can be seen that the cathode current of each tube in flowing through the resistance R impresses a degenerative potential upon its own grid. These degenerative effects, however, are so much smaller than th'e regenerative effect produced by the vcathode current of tube V2 upon the grid of the tube Vi that they are entirely overcome. The amount of regeneration secured is dependent upon the value of the resistance R. This resistance should be large enough to provide a large degree of feedback, approximately 90% of the voltage on the grid of tube V1, but the feedback should not be so large that any tendency toward oscillation would not be eiec tively suppressed with the signal at the lowest received level 'to be expected.

This amplifier while exceedingly simple and brief in its .design provides a very large gain for the two stages yet with perfect stability and with constant limitation of output level at a value sufciently high for the operation of the discriminating and detecting circuits. It functions very satisfactorily, therefore, in the reception of incoming signals at exceedingly low levels in frequency modulated telegraph channels of the type described.

After amplification the frequency modulated telegraph signals are applied to the translating circuit, comprising the two anti-resonant circuits 28 and the rectiers 30 which discriminate between the marking and spacing frequencies. The two resonant circuits 28 are separately tuned, one to the marking frequency and one to the spacing frequency, their outputs being applied separately to the two rectifiers 30. Resistances 29 provide damping for the tuned circuits for the prevention of amplitude distortion effects which tend to accompany sharp tuning. 'I'he circuits 23 are designed to have low impedance to the harmonics of the carrier frequencies which are normally present as a consequence of the limiting or distorting action of the preceding amplifier so that these harmonics are effectively by-passed. This expedient renders unnecessary the use of the low pass lter i2 illustrated in Fis'. 2. After rectication the marking and spacing signals flow separately through their associated resistance 34 the potential across which is applied differentially to the grids of the push-pull amplifier 3|. The resistances 34 perform the uniquel function of removing the large direct current component of the demodulated signal voltage which occurs asa consequence of the imperfect separating action of the discriminating circuits 28. Since these tuned circuits are rather broadly tuned and the marking and spacing carrier frequencies are spaced relatively close to each other, the momentary received current is not confined to the appropriate rectifier but an appreciable portion flows also through the inactive rectifier and in effect opposes the useful signal current. This spurious current is continuously present in both of the resistances 34 but flows in opposite directions so that no potential from this cause appears across the outer terminals of the

resistnations

31, 31 have been inserted to correct the small amount of characteristic distortion which commonly occurs in carrier telegraph channels of res ricted band width. 'Ihe eiect of these device may be supplemented if desired by the similar combination 38 shown in shunt to the relay 32.

Referring to Figure 9, the current in the two rectifiers is represented by the two upper curves in which-the quantities +I and -I represent the continuously flowing direct current upon which is superimposed the useful spacing and marking signals. If the Junction of the two resistances 34 was grounded and the potentials occurring across their outer terminals were applied directly to the amplifier, this direct current component would be impressed upon the grids and a large but useless direct current would be present in'the coils of the relay in the plate circuit. To avoid the necessity for providing oversize vacuum tubes to handle this large current and also the excess heating of the relay, the resistances 34 were provided to perform a differentiation between the outputs of the two rectii'lerswhereby the D. C. component is suppressed, leaving only the marking and spacing telegraph signals illustrated in the nal curve of Figure 9.

I have illustrated and described several circuit arrangements of transmitters and receivers embodying the features of my polar telegraph system but it will be evident to engineers that various modifications may be made to suit varying conditions within the scope of my invention defined in the accompanying claims.

I claim: l. In a carrier wave transmission channel, the

method of transmitting bias free signals which comprises introducing auxiliary inductive reactance and auxiliary capacity reactance alternatively into the frequency determining circuit of an oscillatory source which normally operates at a definite frequency in amount sufficient to invariably change its frequency to an equal degree above or below its normal frequency.

2. In a carrier wave transmission channel wherein the frequency of a carrier wavegenerated by an oscillator is varied'in accordance with direct current polar telegraph signals, a frequency determining tuned circuit in association with the oscillator, and means for shifting the oscillator frequency to the marking or to the spacing frequency comprising means controlled fying and relay circuits for indicating said signals, symmetrically associated with said resonant by the polar signals to shunt the tuned circuit l with an inductance or with a capacity dependentupon the polarity of the signal.

3. A carrier wavev channel in which the frequencylof a carrier wave is varied in opposite directions to produce marking and spacing signals respectively, an oscillator having a seriesconnected frequency determining circuit normally tuned to a frequency intermediate to the frequencies of the marking and spacing signals, frequency determining elements having equal and opposite reactance characteristics connected to said circuit, means to impress polar signals upon said circuit and means to render one or the other element ineffective uponthe application of positive or negative polarities respectively.

4. In a carrier wave channel as set forth in claim 3, a network operating to shape Vthe square topped polar signals to provide a smooth transition from marking to spacing frequency and vice versa.

5. In a -carrier wave transmission cnannel wherein the carrier wave generated by an oscilhaving a center tapped secondary, separate reso-l nant circuits tuned respectively to the marking and spacing frequencies connected across each half of said secondary, and damping means in each resonant circuit to eliminate amplitude distortion eii'ects. l

6. In a carrier wave transmission channel wherein the carrier wave generated by an oscillator is varied by marking and spacing telegraph signals to produce corresponding marking and spacing frequencies, a translating circuit com` prising an amplifying tube which tends to generate harmonics, two damped parallel resonant circuits connected in series in the output circuit of said tube and tuned to the marking and spacing frequencies respectively for distinguishing said marking and spacing telegraph signals, said parallel resonant circuits having relatively low impedance at high frequencies for shunting the harmonics generated in said amplifying tube.

7. A receiving system for distinguishing and indicating two slightly separated frequencies representing marking and spacing telegraph signals comprising an amplifying tube having current limiting and harmonic producing properties, two damped parallel resonant circuits having low impedance at high frequencies connected in series with the output of said tube for distinguishing said marking and spacing signals and for shunting said harmonics, and separate recticircuits respectively.

8. In a carrier current telegraph transmitter, an oscillator having a natural frequency determined by a frequency determining element, two auxiliary reactances connectively associated with said element, and means responsive to unidirec- .ional marking and spacing signal currents for alternately connecting said reactances to said element to thereby produce marking and spacing carrier telegraph signals by increasing or decreasing said oscillator frequency.

9. In a carrier current telegraph transmitter, an oscillator having a natural frequency determined by a frequency determining element, two auxiliary reactances connectively associated with said element, and high impedance devices whose conductivity is responsive to polarized signals connected in circuit respectively with said reactances for alternately connecting said reactances to said element to produce marking and spacing carrier telegraph signals by increasing or decreasing said oscillator frequency.

10. In a carrier wave telegraph channel for permitting operation on a polar basis throughout its extent with unbiased signals, a signal producing transmitting'means comprising an oscillator having a natural frequency determined by a frequency determining element, two auxiliary reactances connectively associated with saidelement, and high impedance devices whose conductivity is responsive to polarized signals connected in circuit respectively with said reactances for alternatelyrjoining said reactances to said element to produce marking and spacing carrier telegraph signals by increasing or decreasing said oscillator frequency, and receivinr. means for maintaining the unbiased signals free from harmonics .and substantially at the most eiiicient current levels regardless of the received carrier level, comprising an amplifying tube having current limiting and harmonic producing properties, two damped parallel resonant circuits having low Aimpedance at high frequencies connected in series with the output of said tube for distinguishing said marking and spacing signals and shunting said harmonics, and separate rectifying and relay circuits for indicating said signals, symmetrically associated with said resonant circuits respectively.

11. A telegraph system comprising a transmitting device for producing marking and spacing signals permuted in accordance with each character or other information to be transmitted, means including an oscillator andan associated oscillating circuit tuned to normally generate an alternating current having a predetermined frequency in the carrier spectrum, a plurality of impedance means selectively connectable to said tuned circuit, a plurality of means connected to said impedance means respectively for disabling the impedance means to prevent them from changing the tuning of the tuned circuit, and means controlled by said transmitting device for controlling said disabling means to cause said impedance means to be operatively connected alternately to said tuned circuit to cause said oscillator to generate marking and spacing signaling currents of two different frequencies respectivelyspaced equidistantly above and below said normally generated frequency, said signaling frequencies being unvarying irrespective of the speed at which the marking and spacing signals are transmitted, receiving means responsive tosaid signaling frequencies, means in the receiving means for translating the signaling frequencies into unidirectional signal currents, and means comprising a differential circuit responsive to said translated signal currents for 'producing marking and spacing signals corresponding to the signals produced by said transmitting device.

12. A telegraph system comprising a transmitting device for producing marking and spacing telegraph signals representative of each character or other item of information to be transmitted, means including an oscillator and an associated oscillating circuit tuned for normally generating an alternating current having a predetermined frequency in the carrier spectrum, an inductive reactance device, a capacitive reactance device, and means controlled by said transmitting device for selectively connecting said in-A ductive and capacitive reactance devices to said tuned circuit to cause said oscillator to generate marking and spacing signaling currents of two different frequencies respectively spaced equidistantly above and below said normally generated frequency, said signaling frequencies being unvarying irrespective of the speed at which the marking and spacing signals are transmitted, receiving means responsive to -said signaling frequencies, means in the receiving means for translating the signaling frequencies into unidirectional signal currents, and means comprising a differential circuit responsive to said translated signal currents for producing marking and spacing signals corresponding to the signals profduced by said transmitting device.

13. A telegraph system comprisingl a transmitting device for producing marking and spacing telegraph signals representative of each character orother item of information to be transmitted, means including an oscillator and an associated oscillating circuit tuned to normally generate an alternating current having a predetermined frequency in the carrier spectrum, a plurality of impedance means each operatively connected to said tuned circuit to change the constants thereof for causing the oscillator to generate marking and spacing signaling currents of two different frequencies respectively above and below said normally generated frequency, a plurality of potential-controlled unidirectional current conducting devices respectively in circuit with said impedance means and each arranged'to operatively connect its associated impedanceA means with said tuned circuit to produce said marking and spacing signaling currents of different frequencies, and means controlled by said transmitting device for selectively applying biasing potentials respectively to said unidirectional current conducting devices, whereby to send a marking signal one of said unidirectional currents conducting devices only will permit its associated impedance means to be operatively connected to the tuned circuit and to send a spacing signal, the other of said unidirectional currents conducting devices only will .permit its associated impedance means to be operatively connected to said tuned circuit.

14. A telegraph system comprising a trans'- mitting device for producing marking and spacing telegraph signals representative of each character or other item of information to be transmitted, means including an oscillator comprising a thermionic tube and a tuned circuit including an inductive reactance and a capacitive reactance connected to the control electrode of said thermionic tube for normally generating an alternating current having a predetermined frequency in the carrier spectrum, a plurality of impedance means each operatively connective to said tuned circuit to change the constants thereof for causing the oscillator to generate marking and spacing signaling currents of two dierent frequencies respectively above and below said normally generated frequency, a potential-controlled rectifier connected in series with one of said impedance means, another potential-controlled rectifier connected in series with another of said lmpedance means, said rectiers being oppositely poled in said series circuits so that when either of said series circuits is operatively connected to said tuned circuit, the other of said series circuits is disabled relative to said tuned circuit, and means controlled by said transmitting device for selectively applying opposing biasing potentials respectively to said rectiers, whereby to send a marking signal one of said rectifiers only will permit its associated series circuit to be operatively connected to the tuned circuit and to send a spacing signal the other of saidrectilers only will permit the series circuit with which it is associated to be operatively connected to said tuned circuit.

, 15. A telegraph system comprising a transmitting device for producing marking and spacing telegraph signals representative of each character or other item of information to be transmitted, means including an oscillator comprising a thermionic tube and a tuned circuit including a first inductive reactance and a first capacitive reactance connected to the control electrode of said thermionic tube for normally generating an a1- ternating current having` a predetermined frequency in the carrier spectrum, a second inductive reactance and a second capacitive reactance each connectable in shunt with said tuned' circuit for causing the oscillator to generate marking and spacing signaling currents of two different frequencies respectively above and below said normally generated frequency, a potential-controlled rectifier connected in series with said second inductive reactance and another potentialcontrolled rectifier connected in series with said second capacitive reactance, said rectiiiers being opposltely poled in said series circuits so that when either of said series circuits is operatively connected in shunt with said tuned circuit, `the other of said series circuits is open relative to said tuned circuit, and means controlled by said transmitting device for selectively applying opposing biasing potentials respectively to said rectiiiers whereby to send a marking signal one of said rectiflers only will permit its associated series circuit to be operatively connected in shunt with the other of said rectiiiers only will permit the series circuit with which it is associated to be opera-..

tively connected in' shunt with said tuned circuit.

16. A telegraph system comprising a transmitting device for producing marking and spacing telegraph signals representative of each character or other item of information to be transmitted, means including an oscillator and an associated oscillating circuit tuned for normally generating' tively connect its associated impedance meanswith said tuned circuit to produce said marking and spacing signaling currents of diiferent frequencies, means including a biasing circuit controlled by said transmitting device for selectively applying biasing' potentials respectively to said unidirectional current conducting devices whereby to send a marking signal one of said unidirectional current conducting devices only will permit its associated impedance means to be operatively connected to.the tuned circuit and to send a spacing signal the other of said unidirectional current conducting devices only will permit its associated impedance means to be,l operatively connected to said tuned circuit, and impedance means in said biasing circuit for insuring smooth transition from either of the signaling frequencies to the other.

17. A telegraph system comprising 'a transmitting device for producing marking and spacing telegraph signals representative of each character or other item of information to be trans'- mitted, means including an oscillator andan associated oscillating circuit tuned for normally generating an alternating current having a predetermined frequency in the carrier spectrum, means'controlled by said transmitting device for selectively changing the constants of said tuned circuit to cause the oscillator to generate marking and spacing signaling currents of two different frequencies respectively spaced equidistantly above and below said normally generated frequency,` receiving means comprising a high impedance thermionic tube responsive to said signaling frequencies for amplifying the signals, the character of said tube being such as to cause the tuned circuit and to permit a spacing signal the signaling currents in the plate circuit of the tube' to be maintained at a-substantially constant level, two resonant circuits connected in series with the plate circuit of said tube and tuned respectively to the marking and spacing frequencies, each of said resonant circuits having a low impedance whereby distortion produced in the amplifying tube is by-passed, detecting devices associated with said tuned circuits for producing unidirectional potentials representative of said original marking and spacing signals, a balanced am- 'plifier connected to said detecting devices, and

a dinerential relay connected to said amplifier for reproducing the original marking and spacingsignals.

18. A telegraph system comprising a transmitting device for producing marking and spacing telegraph signals representative of each character or other item o! infomation to be transmitted, means including an oscillator and an associated oscillating circuit tuned for normally generating an alternating current having a predetermined frequency in the carrier spectrum, a plurality of impedance means eaoh,operatively connectable to said tuned circuit to change the constants thereof for causing the oscillator to generate marking and spacing signaling currents of two different frequencies respectively above and below said normally generated frequency, a plurality of potential-controlledy unidirectional current conducting devices respectively in circuit with said impedance means and each arranged to operatively connect its associated impedance means with said tuned circuit to produce said marking and spacing .signaling currents of diiferent frequencies, means controlled by said transmitting device for selectively applying biasing potentials respectively to said unidirectional current conducting devices whereby to send a marking signal one of said unidirectional current conducting devices only will permit its associated impedance means to be operatively connected to the tuned circuit and to send a spacing signal the other of said unidirectional current conducting devices only will permit its associated impedance means to be operatively connected to said tuned circuit, receiving means comprising a high impedance thermionic tube responsive to said signaling frequencies for amplifying the signals, the character of said tube being such as to cause the signaling currents in the plate circuit of the tube to be maintained at a substantially constant level, two resonant circuits connected in series with the plate circuit of said tube and tuned respectively to the marking and spacing frequencies, each of said resonant circuits having a low impedance to by-pass vdistortion produced in the amplifying detecting nals, an oscillator adapted to generate a median frequency under control of a frequency determining element, means connected in parallel with said frequency determining element for shifting the frequency of said oscillator equidistantly upward or downward from said median frequency to represent marking and spacing signals, comprising a rst vacuum tube and a second vacuum tube, both of said tubes having cathode, anode, and control grid elements, the grids of said tubes being coupled to their plate circuits through 90 phase shifting networks to produce a leading current in the grid circuit of said first vacuum tube and a lagging current in the grid circuit of said second vacuum tube, and means for alternately paralyzing said tubes in accordance with marking and spacing telegraph signals to cause said rst tube to increase the frequency of said oscillator to send a marking signal and said second tube to decrease the frequency of said oscillator to send a spacing signal.

20. In a carrier wave transmission channel a transmitter comprising an oscillator adapted to generate a carrier current whose frequency is determined by a frequency determining circuit, auxiliary inductance and capacity reactances arranged to be alternately associated with said frequency determining circuit by means of oppositely polarized conducting devices for the purpose of shifting the frequency of said oscillator upward ordownward to represent marking or spacing signals respectively, said' polarized devices being connected to a transmitter of polar telegraph signals via an impedance network whereby equal potentials are applied to the two oppositely polarized devices to render them alternately conducting and non-'conducting in paired sequence.

21. 1n a,A carrier wave transmission channel for two element signals a transmitter comprising an oscillator adapted to generate a carrier current of median frequency as determined by` a. frequency determining circuit, auxiliary inductance and capacity reactances arranged to be alternately associated with said frequency determining circuit by means of oppositely polarized conducting devices for the puropse of shifting the frequency of said oscillator upward or downward from said median frequency to represent said two element signals; said polarized devices being connected to a source of direct current signals vla an impedance network whereby equal potentials are applied to the two polarized devices to render them alternately conducting and non-conducting in paired sequences for the purpose of cyclically shifting the oscillator between the two signaling frequencies.

JOHN E. BOUGHTWOOD.