US2832825A - Neutral to polar current conversion circuit - Google Patents

Description

April 29, 1958 J. s. HARRIS 2,832,825

NEUTRAL TO POLAR CURRENT CONVERSION CIRCUIT Original Filed April 29, 1952 5 Sheets-Shea#l 1 rime/wrm? mmf/www 2m' i Rini/rif? I :4a/mmf l mmf/Wma# WE l A if? MEM/m' I Kim; 7W/i 5W/K l j C l M2M/wf 1 I `l im wif i i fw/2 a2/ifi .aw/iai I '2f/www wfi/i Mmm/yf! i l .Dfw/c2' i j l l i 7 Z/ I l A I T l E:

JAMES S. HARRIS April 29, 1958 J. s. HARRIS 2,832,825

` NEUTRAL To POLAR CURRENT CONVERSION CIRCUIT Original Filed April 29, 1952 5 Sheets-Sheet 2 JAMES S. HARRIS reim/wrm@ I I I fwf afl/f! i j fv- 2 C I l l I /wm/ wir! I www! .faz/iff l www: wif-wai I E INI/ENTOR.

April 29,V 1958 J. S. HARRS NEUTRAL TO POLAR CURRENT CONVERSION CIRCUIT Original Filed April 29, 195? 3 Sheets-Sheet 3 1N l. ENTOR. JAMES S. HARRIS United States Patent O NEUTRAL TO POLAR CURRENT CONVERSION CIRCUIT James Stallings Harris, Old Greenwich, Conn., assignor to Radio Corporation of America, a corporation of Delaware Divided and this a licatio Se t b 17 nai No.61o,1ss pp n P em er 1956 Se Claims. (Cl. 178-70) The invention relates to telegraph signalling circuits. It particularly pertains to circuit arrangements for converting signal currents appearing in neutral form into signal currents of polar form. This application is a division of my copending application Ser. No. 284,863, filed April 29, 1952, now Patent No. 2,770,667, granted November 13, 1956.

Fundamentally in any telegraph signalling circuit intelligenceis transferred by means of switching the transmission line input terminals to eflect a change in the current `conditions of the circuit. A familiar example of such a circuit is a D.C. telegraph circuit employing printing telegraph equipment in which the signal is actually detected by a magnetically operated armature in the receiving printer. In order for the receiving equipment to properly interpret the intelligence transmitted, it is essential that the position of `the armature of the receiving printer accurately follow the keying device at the remote transmitter. That is to say, the duration between transitions of the armature should be the same as the duration between transitions of the contacts of the keyer. Assuming that the wave filter characteristics of the transmission line are satisfactory, the adjustment of the circuit which will satisfy the requirement as mentioned above is largely a matter of adjusting the power supply voltage and total circuit resistance to provide a specified lcurrent in the armature coil during steady-state marking condition. In usual practice the power supply voltage is fixed so that the adjustment is actually an adjustment of resistance in .the circuit. Because the transmission line impedance is usually fixed, such adjustment may be made only at the ends of the transmission line.

As is well known to those familiar with the telegraph art, such a circuit works satisfactorily only so long as the transmission line impedance actually is a fixed quantity, and any changes in this impedance require compensation at one or both of the terminal ends of the line if operating tolerances are to be maintained.

In the case of long transmission lines the current at lthe load end of the circuit Will vary due to changes in both humidity and temperature. Such changes do occur relatively slowly as compared with the speed of changes due to the intelligence keying impressed upon the line, but the circuits require periodic readjustment to maintain adequate tolerances.

In order to eliminate the requirement for these readjustments due to changes in line characteristics, which may be considered as changes in line attenuation, it is almost universal practice to use a different circuit on long lines. This circuit is known as the two-current or polar circuit.

In the two-current circuit the mark and space currents are identical in magnitude but opposite in polarity. Changes in line attenuation, therefore, affect the magnitudes of the mark and space currents to the same degree, but have no effect on the polarity component. The prior art signal detection device in such circuits may be, and usually is, a simple polar relay which responds to a combination of current magnitude and polarity, the

2 response being effected at a current magnitude determined -by the physical characteristics of the particular relay; In a properly operating circuit, the magnitude of'current required for proper operation of the relay in either direction is usually the same. With such a signal detection device it is only necessary to assure that under the maximum anticipated line attenuation the current supplied to the device will not exceed the safe maximum which may be applied to the device without damage or misfunction.

There `are a large number of existing neutral signal type lines however that `do not lend themselves for polar operation and which would require extensive alterations in order to convert them for polar` operation. Obviously such alterations are expensive from both pecuniary and circuit time standpoints.

It is an object of the invention to convert neutral or single signalling current to a form approximating the performance of polar or two signalling currents in an improved manner.

It is another object of the invention to provide an improved control circuit arrangement for converting neutral or single current signals to polar or two current signals.

A further object of the invention is to provide an improved tone signal converter and monitoring circuit adaptable for use in a multiplex receiving installation and including a control circuit arrangement for `converting neutral or single current signals to polar or two current signals.

The objects of the invention are attained in a circuit arrangement comprising three resistive elements, a `capacitive element and a switching element. One of the resistive elements is arranged between the neutral signal input terminals. The other resistive elements are connected in series across the capacitive element to form a potential storing and `dividing network which is connected in series with the switching element across the input terminals. The series arrangement between the junction of the series connected resistive elements and the ,switching element is connected to the output terminals of the novel circuit arrangement. In accordance with the basic principles of the invention the switching element is arranged to close when, and only when, the signal voltage across the input terminals equals or attempts to exceed the voltage across the capacitive element.

A current conversion circuit according to the invention may be used to operate a polar relay `directly yor through the intermediary of an electronic amplifier. Alternately a differential amplifier may be driven by the current conversion circuit to operate any desired utilization device. Another mode of operation may employ a conversion circuit according to the invention for triggering a monostable or bistable reciproconductive circuit, sometimes respectively referred to as flip-flop and locking circuits.

By changing the relative values of the series connected resistive elements zero current space-current mar signals, proportional current mark-space signals, and double current or opposite polarity mark-space signals may be accommodated. Such changes can also be made for the purpose of reducing distortion present in a received signal or conversely for the purpose of introducing a desired distortion o1- weighting of the signal.

The invention will be illustrated in connection with specific embodiments thereof given by way of example only and described with reference to the accompanying drawing forming a part of the specification and in which:

Figs. l and 2 are diagrammatic representations of telegraph signalling circuits known to the art;

Fig. 3 is a schematic diagram of a circuit arrangement basic to the invention;

Figs. 4 and 5 are functional diagrams useful in ex- Patented Apr. 29, 1958 plaining the operation of the circuit arrangement shown in Fig. 3;

Figs. 6-10 are schematic diagrams of alternate embodiments of the circuit arrangement according to the invention; and

Fig. ll is a schematic diagram of a tone signal converter and monitoring circuit incorporating the current conversion circuit of the invention.

Referring to Fig. 1, there is shown an equivalent circult diagram of a known single-current electric signalling circuit comprising a transmission line to which a signal detection device 17 and the equivalent load impedance 19 are connected. Intelligence is transferred to the detection device 17 located at the receiver by means of a keying device 21 located at the transmitter which switches the input terminals of the transmission line 15 to effect a change in the current conditions of the circuit. A current source 23 and the equivalent mark source impedance 25 are connected to the line 15 by the keying device 21. It is obvious that the corresponding space source impedance 27 may be equivalent to an open circuit in which case the keying device 21 may consist of only two contacts in a make-and-break circuit. In order for the receiving equipment to properly interpret the intelligence transmitted, it is essential that the signal detection device accurately follow the movable contacts of the keying device 21. That is to say the duration between transitions of the armature at a telegraph printer, for example, should be the same as the duration between transitions of the contacts of the keying device 21. Assuming that the wave lter `characteristics of the transmission line 15 are satisfactory, the adjustment of the circcuit which satisfies the requirement as mentioned above is largely a matter of adjusting the voltage of the source 23 and total circuit resistance to provide a specified current through the detection device 17 and the load impedance 19 during steady-state marking condition. In usual practice the source voltage is xed so that in practice the adjustment is actually an adjustment of resistance in the circuit and is made only at the ends of the transmission line.

Such a circuit will be satisfactory as long as the transmission line impedance actually is aiixed quantity. Any changes in this impedance require compensation at one or both of the terminals in order to maintain the operating tolerances.

In the case of long transmission lines, the two-current or polar circuit shown in Fig. 2 is employed at the transmitter, the remainder of the circuit being as shown in Fig. l.

In this type of circuit the mark and space currents obtained from the sources 23 and 29 are usually identical in magnitude but opposite in polarity and the source impedances 25 and 27 are likewise identical so that changes in line attenuation alect the magnitudes of the mark and space currents to the same degree.

The double-current circuit is also valuable under conditions where the signal as received may include eX- traneous noise or interference components of a magnitude suiiicient to cause false operation of a single current device, as they can be made ineffective in a two-current circuit, provided they are within the limitations of allowable maximum current.

The preceding information, repeated here only for background purposes, is well known and more complete discussions may be found in the literature of the communications art.

Further, while the preceding discussion has assumed a continuous D.-C. circuit between the keying device at the transmitter and the signal detection device at the receiver, it is to be understood that the circuit theory of the embodiment to be described is not affected if carrier methods are employed on the transmission line 15 with the understanding that the actual signal applied at the nal utilization point (for example, the printer magnet) is usually a single current signal, and that single frequency d on-off A.C. or keyed tone signals on the transmission line correspond to single current circuitry, While two frequency or frequency shift signals on the transmission line correspond to two-current or polar circuitry. It should also be recognized that where the transmission line includes a radio circuit the changes in line attenuation may be quite rapid.

Referring to Fig. 3 there is shown in schematic form a basic circuit arrangement according to the invention, comprising neutral' signal input terminals 31-32, a resistive clement R3, a shunt network 35, a switching element S and polar output terminals 37-38. The resistance element R3 may be an actual resistor or may be found in a resistive path of the circuit to which the input terminals 31-32 are connected. The shunt network 35 comprises a capacitor Crand two resistance elements Rt and R2 connected in series across the capacitor C. The resistance elements Rland R2 may be separate resistors, a single tapped resistor or a potentiometer having a variable tapping arm. The switching elements may he any known device which will present a very low impedance at the operating frequency between the lower end of the shunt network 35 and the terminals 32 and 3S. In accordance with the invention, the switching elements should close when, and only when, the potential applied across the input terminals 31-32 equals or attempts to exceed the voltage across the network capacitor C. This feature will be discussed further as the description progresses.

The circuit supplying power to the neutral input terminals .3l-32 should preferably be relatively low in impedance as compared to the sum of resistances of the network resistors R1 and R2 and should also preferably supply current-and voltage-during the idle condition. That is, mark is the steady state idle condition, while the space or no current condition appears during keying. The time constant of the network comprising the capacitor C and the resistors R1, R2 should be long with the respect to the anticipated maximum continuous spacing interval.

Under these conditions, the schematic representation of Figs. 4 and 5 illustrate the manner in which the circuit functions. The capacitor C is charged to the steady state mark voltage Es through the switching element S and the relatively low effective resistance of the source re.-

sistor Rs and the resistance element R3, while the discharge path is the series connection of the resistors R1 and R2. In the space condition the resistance element R3 serves to reduce the potential between the neutral input terminals 31-32 to zero when the current source 18 is removed. As shown in the two Figs. 4 and 5, the output voltages for the two conditions are as follows:

Thus the voltage between the output terminals 37--38 reverses in polarity for the two conditions. The total magnitude of change in the output voltage is equal to the magnitude of the input voltage under mark condition and the relative magnitude of the mark and space voltages is determined by the relative resistances of the two resistors R1 and R2. Thus if the resistance values of the resistors R1 and R2 are equal, the output voltage will be a balanced polar voltage, while if the values are unequal, the output will be an unbalanced polar voltage with the degree of voltage unbalance equal to the degree of resistance unbalance.

While the actual transmission line between terminals still remains a neutral signal line, the circuit arrangement according to the invention provides the equivalent of an entirely polar signal line within practical tolerances. This is so because the polar signal detector device is affected only by differences in polarity and not by differences in amplitude of the keyed signal.

The switching element S has been shown to be required to be closed when the voltage across the neutral input terminals 31-32 equals the voltage across the capacitor C, and open when the capacitor voltage exceeds the input voltage. A fast operating relay connected across the input terminals 31-32 could be used to approximately satisfy this requirement, while a unilateral impedance element such as a diode, vacuum or otherwise, with a high back resistance, makes an almost ideal electronic switch. A multielement switching tube could also be used if desirable, of course. For example, a triode tube might be used in a multiplex circuit where a plurality of such triodes were triggered in succession or determined by the timing device of a time division multiplex system.

A practical signal repeating circuit can be made using the basic neutral-to-polar circuitry of Fig. 3 with a diode switch 61 and with a sensitive polar relay 63 as an amplitude selection device as shown in Fig. 6.

In this circuit, it is obvious that during spacing condition the operating power for the polar relay 63 is supplied from the energy stored in the capacitor C. In some cases this requirement for energy supply will make the desirable ratio between charging and discharging time constants of the shunt circuit 35 difficult to obtain, and in such `cases the use of an electronic circuit, which requires little or no energy from the conversion circuit, but functions on voltage differences, is indicated. This electronic circuit may in turn drive an electromechanical polar relay, or alternatively, with any required D.C. power amplication, may drive a single current relay, or the nal utilization device.

The electronic circuit may be in the form of the well known diiTerential-amplifer circuit, or it may be of the bistable reciproconductive circuit form.

As employed herein, the term reciproconductive circuit is construed to include all two-tube regenerative devices in which conduction alternates in one or the other tube in response to applied triggering potential. The term multivibrator is sometimes applied to this circuit and the term locking circuit is sometimes applied to the bistable reciproconductive circuit in which two triggers are required to switch from one stable state to the other and return.

In certain instances it may be a single-ended stabilized circuit such as is `commonly used in vacuum tube voltmeter circuits. Two separate stabilized single-ended arnpliiiers may be employed in connection with polar relays, either electromechanical or electronic, with one amplier connected to one Winding of the polar relay to provide bias or correction voltage and the other amplier connected to the other Winding to provide signal voltage.

The basic circuit may be used where the source twocondition signal is zerocurrent for space and current for mark as outlined in the explanation of the basic circuit of Fig. 3, with the resistances of the resistors Rl and R2 equal (or replaced by a center tapped single resistor) to convert the neutral signal to a balanced polar signal andthus accurately repeat the transmitted signal without regard to the absolute magnitude of the mark current, within, of course, the operating limit of the circuit.

The basic circuit also may be used wherethe space" condition is not zero but of the same polarity and is proportional to the mark signal by making the resistances of the resistors of Ri and R2 proportional to the sum and diierence ratios of mark and space signals, and this signal will be converted to a balanced polar signal.

It may be used where the space condition is not zero but is of opposite polarity and fractionally proportional to the mar signal by making the resistances of the resistors R2 and R1 proportional to the sum and difference ratios of the mar and space signals, and this signal will be converted to a balanced polar signal.

In any of the previous instances, if it is desired to repeat the signal with an added characteristic distortion, or to eliminate a characteristic distortion from the original signal by introducing .an equal but opposite characteristic distortion, this may be accomplished by changing the resistance proportions of the resistors R1 and R2. If an appropriate signal delay network, for example, the source and transmission line impedance, is introduced in the circuit ahead of the conversion circuit, and the tap between the resistors R1 and R2 is made variable by using a potentiometer type of resistor with or without series fixed resistors, then the characteristic distortion introduced may be varied at will and calibrated if desired.

While the basic neutral-to-polar circuit is shown as a metallic circuit, with no reference to ground, it is to be clearly understood that the circuit may be a single-wire and ground return circuit or may have a ground connection (or ground reference connection) to either end of the power source.

Further, the switching element S may be connected to either the grounded or ungrounded leg of the transmission line, and either polarity. of the source 18 may be grounded, provided any diode or multi-element tube used as the switch S is properly polarized.

In the basic circuit the operating range, or the signal level variation, over which the circuit will function is limited at one extreme by the minimum sensitivity of the devices or circuits following the neutral-to-polar circuit, and at the other extreme by the maximum allowable, practicable or desirable signal available. If in practice it is desirable to provide a minimum sensitivity below which the signal output will assuredly be space then a minimum biasing voltage arrangement may be inserted in the neutral-to-polar conversion circuit as shown by way of example only as a battery 71 in Fig. 7. Ihis arrangement will not cause any distortion to a signal that exceeds the bias voltage of the battery 71.

If a diode employing a hot cathode or other hot cathode multi-element tube is employed as the switch and the eiiects of contact potential cause undesirable output signal distortion at minimum operable signal input levels, the effects may be eliminated or minimized by interposing a source of corrective voltage as shown by way of example as a battery 81 in Fig. 8. Compensation for both contact potential and establishment of minimum signal bias may be combined, as shown by way of example by batteries 81 and 7l in Fig. 9. It is to be understood, of course, that any source of poetntial such as a generator, an A. C. rectifier or the like may be substituted for the batteries shown,

In a D.C. or on-oii` single tone-frequency A.C. circuit employing more than two conditions, with each condition or combination of conditions providing diierent items of information, and where each condition bears a delinite proportional relationship tothe maximum mark and space conditions, a multiplicity of the circuits according to the invention may be employed at the load end of the transmission line with appropriate cross connections and/ or couplings to reproduce the information transmitted.

In a circuit utilizing frequency shift A.C. as the signal carrier over the transmission line and where due to practical conditions the absolute magnitude and/or polarity of the demodulated signal cannot be utilized to determine the intelligence being transmitted but where the limits of amplitude of excursion can be established according to the invention, the neutral-to-polar conversion circuit may be adapted to deiinitely interpret the information.

In a system employing a multiplicity ot' signal sources and/ or loads with standardized impedance and/ or attenuation characteristics, and with associated transmission lines which may have dierent and/or varying characteristics, and where cross connections may be established between lany pair of signal sources and loads through available transmission lines, the use of the circuits described allows connection changes without the requirement of readjustment of sources or loads when such changes introduce a change in transmission line characteristics.

Where the basic circuit, such as is shown in Fig. 3, is followed by a bistable reciproconductive or locking circuit and reversals are not being received during idling periods, there is the distinct possibility that t i' conductive circuit Will be triggered to the marking condition and there remain. This disadvantage can be readily avoided by rectifying the input signal, applying the rectified signal to a resistance-capacity circuit of time constant long with respect to the maximum expected spacing between signal elements and, thereafter, to a pulse amplier tube which is so coupled to the reciproconductive circuit to restore it to the spacing condition.

A preferred arrangement avoiding this diiiculty is shown in Fig. 10, wherein a pair of electron discharge devices S3 and 35 are connected in a differential amplier circuit 87. The grid of the tube 83 is connected to the arm 91 of a potentiometer 93 which serves as the series connected resistors R1 and R2 of the basic circuit. A

semi-conductor type rectier 61 is shown replacing the diode previously shown. The germanium crystal diode has been used in this circuit with excellent results.

The bias battery 81 and potentiometer 97 shown provide sufficient bias voltage to give initial imbalance of the ditlerential circuit 87 in the absence of signal. When a signal of sufficient strength to cause operation of the diode 61 appears, the unbalance disappears in so far as the response to a 'keyed signal is concerned. in this manner squelch action in the absence ot a signal is provided while giving proper action when a signal is present without unduly complicating the circuit. A change of symmetry of the circuits without disturbing the balance is obtained by providing the potentiometers 93 and 97 with identical tapers and gauging them for simultaneous adjustment. Direct output is obtained from the anode of the tube 85. lf desired an indirect or inverted signal train may be taken from the anode of the other tube S3. When deriving the output signal from the tube 35, a constant current tube circuit is preferably used for the cathode impedance element instead of the resistor S9 where the mark-space cross-over voltage is close to the negative limit of amplitude variation.

ln Fig. l1 there is shown an arrangement used in connection with an existing electromechanical multiplex receiving installation where the signal is transmitted as an on-ot tone signal with a tone signal demodulator 101 at the load end of the transmission line. The neutral-topolar conversion circuit follows the demodulator 101 and a smoothing lter comprising an inductor 103 and a capacitor 105 'is interposed in the circuit. No change in the theory of operation is involved provided the varying attenuation characteristics of the transmission line are considered to react on the output power of the demodulator in the same manner as if the demolulator power were constant and a similar attenuator were inserted in the transmission line between the demodulator and the conversion circuit.

A poteniometer 109 in the input circuit is used to adjust the signal level to obtain optimum results from the conversion circuit and the differential amplifier 111. Two outputs are derived for use in the multiplex apparatus. The rst is a signal output amplilied by a triode vacuum tube 115 for application to the message handling device and the other is a correction voltage obtained at the anode of another amplifier tube 119 for application to an automatic phasing circuit in which a locally generated signal is automatically adjusted in phase relationship to the incoming signal. The amplifier tube 119 acts as an isolation device to prevent any voltage changes in the phase adjusting circuitry from adversely affecting the signal voltage obtained at the signal amplifier tube 11S.

The differential amplifier 111 is so connected that the mark-space crossover voltage lies substantially midway of the normal signal limits. Any large o-vershoot in signal amplitude will then have no effect on the operation of the circuit and no constant current tube circuit is necessary. in either of the arrangements shown in Figs. 10 and 1l, a marginal current relay may be used without any constant current tube circuitry.

in order to determine the proper setting of the level potentiometer 109, a signal level monitor circuit 121 is provided. The monitor circuit 121 comprises a pair of electron discharge structures 12S- 124 each having a neon indicator lamp 1277 128 in the anode-cathode path of the tube. A semi-conductor rectifier 131 and filter comprising a resistor 133 and a capacitor 135 apply a direct voltage proportional to the peak signal level to the grid of the signal voltage tube 123. The monitor circuit 121 is a dilerential amplifier so designed that a fixed reference voltage derived by means of resistors 151 and 152 is applied to the grid of the reference tube 124. The peak signal voltage applied to the signal tube is then compared to the reference voltage. The cathode resistors 157-159 are so chosen that a change of approximately 6 db in signal level is required for complete transfer of current from one of tubes 123-124 to the other. Hence when the signal level is within i3 db of the desired level both neon lamps 127 and 128 will glow. The cathode follower action of tubes 123 and 124 renders the variation between individual neon lamps of commercial availability of substantially no importance. If the level is too high only the neon lamp 127 will glow. lf the level is too low only the neon lamp 128 will glow. When the signal level is between the desired limits both lamps will glow. Because the relative brightness of commercially available neon tubes is discernible to the eye, the signal level can be readily adjusted to lie very nearly midway between the levels. Thus the receiving station operator can tell at a glance whether the proper level of signal is being applied to the receiving apparatus.

An example of an adaptation of the basic circuitry according to the invention to an electronic multiplex receiver will be found in copending U. S. application Serial No. 227,344, tiled May 21, 1951, of E. R. Shenk, A. E. Canfora and P. E. Volz, now Patent 2,716,158, granted August 23, 1955.

In an embodiment of the invention actually constructed and tested along the lines of the arrangement shown in Fig. l1, the following component parts values were employed.

Resistors Reference No.: Value R1 Meg-- 1.3 R2 Meg-- 1.3 R3 Kil 3.9 109 Kil 100 133 Kil 43 151 Meg-- 1.3 152 Kil 180 156 Kil 51 V157 Kil 510 158 Kil 510 159 Kil 18 160 Meg 2.3 161 Kil 180 162 Kil 180 163 Kil 180 165 Meg 1 167 Kil 270 168 Meg/ 2.4 169 Meg-- 1.5 171 Meg 1.3 4172 Meg 2.2

9 Resistors-Continued Reference No.:

Value 173 Meg 1.3

174 Meg 2.2

175 Kil 200 179 Kil 200 Capacitors Reference No.: Value C mfd-- 0.25

135 rnfd 0.25

Tubes Reference No.: Type No. 110 l/z 6SN7 115 1/2 6SN7 119 1/2 6SN7 127 NESI 183 lo 6SL7 Diodes Reference No.: Type No. 63 1N67 W'hat is claimed is:

1. In a telegraph signalling circuit, an on-of neutral tone signal input circuit, a tone signal demodulator coupled to said signal input circuit, a capacitor and a unidirectional impedance element connected in series between said tone signal demodulator and a point of fixed reference potential, a first resistor connected across said capacitor, first and second electronic switching devices each having at least an input electrode and an output electrode, means for connecting the input electrode of said first device to a point intermediate the ends of said first resistor, means for connecting the output electrode of said first device to the input electrode of said second device, and a second resistor having a point intermediate the ends thereof connected to said point of fixed reference potential coupling the input electrode of said second device to the output electrode thereof.

2. ln a telegraph signalling circuit, an on-off neutral tone signal input circuit, a tone signal demodulator coupled to said signal input circuit, a capacitor and a unidirectional impedance element connected in series between said tone signal demodulator and a point of fixed reference potential, a first pair of resistors connected in series across said capacitor, first and second electronic switching devices each having at least an output electrode and an input electrode, means for connecting the input electrode of said first device to the junction of said first pair of resistors, means for obtaining an output signal from the output electrode of said first device, a resistor connected between the output electrode of said first device and the input electrode of said second device, and a second pair of resistors connected in series coupling the input electrode of said second device to the output electrode of the same device, the junction of said second pair of resistors being connected to said point of fixed reference potential.

3. In a telegraph signalling circuit, an on-off neutral Vtone signal input circuit, a tone signal demodulator coupled to said signal input circuit, a capacitor and a unidirectional impedance element connected in series between said 'tone signal demodulator and a point of fixed reference having a point intermediate the ends thereof connected to said point of fixed reference potential coupling the control grid of said second device to the anode thereof.

4. In a telegraph signalling circuit, an on-ofr` neutral tone signal input circuit, a tone signal demodulator coupled to said signal input circuit, a capacitor and a rectifier connected in series between said tone signal demodulator and a point of fixed reference potential, a first pair of resistors connected in series across said capacitor, first and second electron disch-arge devices each having at least a cathode, control grid and an anode, means for connecting the control grid of said first devi-ce to the junction of said first pair of resistors, a resistor connected between the anode of said first device and the control grid of said second device, and a second pair of resistors connected in series between the control grid of said second device and the anode thereof, the junction of said second pair of resistors being connected to said point of fixed reference potential.

5. In a telegraph signalling circuit, an on-of neutral tone signal input circuit, a tone signal demodulator coupled to said signal input circuit, a capacitor and a unidirectional impedance element connected in series between said tone signal demodulator and a point of fixed reference potential, a first resistor connected across said capacitor, said first resistor having a tapping connection `afiixed thereto intermediate the ends thereof, a differential amplifier including'first and second electron discharge ldevices each having at least a cathode, a grid and an anode, a common cathode resistor, individual anode resistors, means for connecting the grid of said first device to said tapping connection, a second resistor connected between the anode of said first device and the Igrid of said second device, and a third resistor having a point intermediate the ends thereof connected to said point of fixed reference potential coupling the grid of said second device to the anode thereof.

6. In a telegraph signalling circuit, an on-of neutral tone signal input circuit, a tone signal demodulator coupled to said signal input circuit, a capacitor and a rectifier connected in series between said tone signal demodulator and a point of fixed reference potential, a pair of resistors connected in series across said capacitor, a differential amplifier including a pair of electron discharge devices each having at least a cathode, a grid and an anode, a common cathode resistor, individual anode resistors, means to connect the junction of said pair of resistors to the grid of one of said devices and means to obtain an output signal from the anode of said one device, a resistor connected between the anode of said one device and the grid of the other device, and a pair of resistors coupling the grid lof said other device to the anode of the same device, the junction of said last-mentioned pair of resistors being connected to said point of fixed reference potential.

7. In a telegraph signalling circuit, an on-ofi neutral tone signal input circuit, a potentiometer connected across said input circuit, a tone signal demodulator coupled to said potentiometer, a capacitor and a rectifier connected in series between said tone signal demodulator and a point of fixed reference potential, a pair of resistors connected in series across said capacitor, a differential amplifier including a pair of electron discharge `devices each having at least a cathode, a grid and an anode, a common cathode resistor, individual `anode resistors, means to couple the junction of said pair of resistors to the grid of one of said devices and means to obtain an output signal from the anode of said one device, a resistor connected between the anode of said one device and the grid of the other device, and a pair of resistors coupling the grid of said other device to the` anode of the same device, the junction of said last-mentioned pair of resistors being connected to said point of fixed reference potential.

8. In a telegraph signalling circuit, an on-of neutral tone signal input circuit, a potentiometer connected across said signal Vinput circuit, a tone signal demodulator coupled totsaidpotentiometer, -a first resistor connected betweenv theioutputterminal of said tone signal-demodulator and a point of fixed referencepotential across which is produced -a'keyed direct current neutral signal, a capacitor and 2a unidirectional impedance element connected in'series across said first resistor, a second resistor connected across said capacitor, said second resistor having -a tapping'connection aliixed 'thereto intermediate the ends thereof, a differential amplifier including first and second 'electronic switching devices-each having at least an input electrode and an output electrode, means to con neet the linput electrode of said irst device to said tapping connection, a third resistor connected between the output electrode of said first rdevice and therinput electrode of said second device, and a fourth resistor having a point vintermediate the ends thereof connected to said point of fixed reference potential coupling the input electrode of said second device to the output electrode thereof.

9. ln -a'telegraph signalling circuit, an on-off neutral tone signal input circuit, a potentiometer connected across said input circuit, a tone signal demodulator coupled to said potentiometer, a resistive element connected -between the output terminal of said tone signal demodulator and a point of iixed reference potential across which is produced a keyed direct current neutral signal, a capacitor and a rectifier connected in series across said resistive element, a pair of resistors connected in series across said capacitor, a differential amplifier including a pair of electron discharge devices each having at least a cathode, a grid and an anode, a common cathode resistor, individual anode resistors, means to couple the grid and cathode of one of said devices be tween Ythe junction of said pair of resistors and said point of fixed reference potential, means to obtain an output signal from the anode of said one device, a resistor connected between the anode of said one device and the grid of the other device, and a pair of resistors coupling the grid'of said other device to the anode of the same 40 device, the junction of said last-mentioned pair of resistors being connected to said point of fixed reference potential.

10. In a telegraph signalling circuit, an on-oi neutral tone signal input circuit, a potentiometer connected across said input circuit, a tone signal demodulator coupled to said potentiometer, a resistive element connected between the output Yterminal of said tone signal demodulator and )a point of fixed reference potential across which is produced a keyed direct current neutral signal, a. capacitor and a rectier connected in series across said resistive element, a pair of resistors connected in vseries across said capacitor, a ditferential ampliier having signal and correction voltage outputs, said differential amplier including a pair of regeneratively coupled electron discharge devices each having at least a cathode, a grid and an anode, a common cathode resistor, individual anode resistors, means to couple the grid of one of said devices to the junction of said pair of resistors, a resistor connected between the anode of said one device and the grid of the other device, and a pair of'resistors coupling the grid of said second device to the anode of the same device, the junction of said last-mentioned pair of resistors being connected to said point of fixed reference potential, a signal level monitor circuit coupled to the output of said tone signal demodulator, said monitor circuit including a pair of electron discharge structures each having at least cathode, control and anode electrodes, neon lamps individually connected to the anode electrodes of said structures, a unilateral impedance element coupling the output of said tone signal demodulator to one of said control electrodes, means to apply al reference voltage to the other of said control electrodes, and means to bias said structures differently to cause said neon lamps to light at different levels of output from said tone signal demodulator, whereby said potentiometer can be adjusted to provide -proper operating signal when both of said neon lamps light at the same time.

` References lCited in thele of this patent UNITED STATES PATENTS

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