US2422309A - Distortion correcting impulse repeater - Google Patents

Description

June 17, 1947. H. J. M cREARY 2,422,309

DISTORTION CORRECTING IMPULSE REPEATER Filed March 10, 1945 INVENTOR. HAROLD J. M CREARY ATTORNEY- Patented June 17, 1947 UNITED STATES PATENT OFFICE 2,422,30

DISTORTIONYCORRECTING IMPULSE REPEATER Harold J. McCrear-y, Lombard, Ill, assignor to Automatic Electric Laboratories, 1110., Chicago,

111., a corporation of Delaware Application March 10, 1945, Serial No. 581,984

9 Claims.

The present invention relates to impulse repeaters for use in signaling systems such as auto- When impulses are transmitted over long lines such as are sometimes encountered in telephone limits for reliable operation of the selecting apparatus.

The principal object of the present invention is to provide an improved vention and adapted to be controlled by impulses transmitted over a conventional telephone line;

. igure 2 is a schematic diagram of a modification of Figure 1 in which an Eccles-Jordan trigger circuit is substituted for the pair of gas discharge tubes employed in Figure 1.

Briefly described, the invention consists in utithe secondary winding, nected in series across the secondary winding in series with two condensers. A pair of gaseous discharge tubes have their cathodes connected to the junction between the two resistors and each has a control electrode with an impedance that is common to the pair of Acondenser is connected directly between the anodes of the tubes so as to cause either tube to be deionized when the alternate tube fires.

is also connected in The invention will now be described in detail with reference to the accompanying drawing. Referring to Fig. 1, there is shown a subscriber substation set having an impulsing device and connected over line 2 to a source of potential in series with relay 3. Condenser 4 and resistor 5 are serially connected in multiple with thelower winding of relay 3. The lower portion of resistor 5 is connected between the grid and cathode of vacuum tube 5. The anode circuit of tube 6 is coupled by means of transformer (to the control grids of the gaseous discharge tubes 8 and 9. An impulse repeating relayl0-is connected in series with the anode circuit of tube Hl. Contacts H may be used to repeat impulses corresponding to those produced by the impulsing device I to switching apparatus such as would normally be controlled-by the line relay 3.-

The line 2 has been represented as an H network of resistance elements. Actually the series resistance of the line is uniformly distributed, but the shunt resistance may be either distributed or lumped as itmay be produced by a fault at one or more points-in the line or may be unavoidable distributed leakage resistanc The line-willalso have distributed capacity. The substation set includes the usual telephone ringer 12 which is connected across line 2 in series with condenser 13. Although only one substation sethas been shown it should be understood that as many as twenty sets may be similarly connected across the line. They may be distributed alon the line or a number may be connected at the same point.

Typical values for such a telephone line are 0 to 1200 ohms series resistance-and 5000 ohms to infinite shunt resistance. The ringers, such as 12, may each have a resistance of 1600 ohms and the condensers, such as l3, may each have a capacitance of one niicrofarad. The relay 3 usually has a resistance of 200 ohms per winding and the battery voltage usually has a nominal value of 48 volts. Due to the charging anddisharging of the ringer condensers during impulsing the current flowing through relay 3 is greatly distorted. The. actual wave shape depends upon the line constants and the number and distribution of the ringers. oftentimes oscillations in the line current occur each time that the contacts of the impulsing device open. At the instant these contacts open the ringer condensers commence to charge in series with the ringers, the series resistance of the line, the windings of relay 3, and the battery. The flux in the core of relay 3 com mences to collapse causing a voltage to be induced in the windings of relay 3 which tends to maintain the current flowing in the line. As a result of this self induced voltage in relay 3 the potential across line 2 at the relay may become higher than the battery voltage. When this occurs the ringer condenser continues ,to charge until it reaches a potential equal to thatacross the line at relay 3 at which time the condenser commences to discharge causing a reversal in line current to take, place and causing a flux in the opposite direction to normal to be built up in the core of relay 3. The ringer condenser continues to discharge until it again reaches a potential equal to that across line 2 at relay 3, which potential will at this time be less than the battery potential. The line current then again reverses and the same cycle of events is repeated but with reduced amplitude. Ordinarily, the amount of damping present in such an impulsing circuit is high enough to so reduce the amplitude of the first reversal of line current as to prevent reoperation of the impulse responding relay. However, the present impulse repeater is intended to be responsive to the rate of change in the line current rather than the magnitude of the current and it therefore becomes necessary to eliminate such oscillations. This is accomplished by means of rectifiers l4 and 15, each of which is-connected across one of the windings of relay 3 so as to be normally non-conductive. Now when the contacts of the impulsing device i open, the two windings of relay 3 are in effect short-circuitecl, thus preventing the potential across the line? at relay 3from exceeding the battery due to the collapse of flux'in the core of relay 3. As the ringer condenser is prevented from charging to a potential higher than the battery voltage no reversal in line current takes place. The short-circuiting of relay-3 renders it slow-to-release and may prevent it from responding to the impulses completely. Since the function of the impulsing contacts normally controlled by relay 3 are now taken over I! the slow release action of relay 3 is no disadvantage and in fact it will be advantageous in caseswhen a slow to release relay, such as the holding relay employed in telephone systems, must be held operated by contacts of relay 3 during impulsing. Due to the above described action of the rectifiers the line current always decays exponentially when the impulsing contacts open. The line current also rises exponentially when the impulsing contacts close. If all of the line current flowed through relay 3 the voltage across relay A would be a maximum at the instant the impulsing contacts opened and would decay exponentially. This voltage would also be opposite in polarity to the normal voltage across the relay. Since the rectifiers prevent such a reversal of polarity of the voltage across relay A, the'voltage will actually drop to substantially Zero at the instant the impulsing contacts open and may later rise slowly according to an exponential law to the steady state open circuit value which is determined by the value of the line shunt resistance. When the impulsing contacts close, the voltage across each of the windings of relay A will instantly rise to onehalf of the battery voltage and will decay exponentiallyto the steady state closed circuit value which is determined by the line resistance. It is thus apparent that rapid changes in the potential across either winding of relay 3 take place at the instants of make and break of the impulsing contacts. These changes in potential across the lower winding of relay 3 also appear across resistor 5. The condenser 4 serves as a blocking condenser to prevent the steady voltage drop whichnormally appears across relay '3 from affecting the amplifying tube 5. The cathode of tube 6 is connected to the negative terminal of the battery through resistor IT to provide the proper bias for operation of the tube as a linear amplifier. The cathoderesistor i1 .is bypassed by condenser [8 to prevent degeneration of the steep wave fronts which must be amplified by the tube. A portion of the signal voltage developed across resistor 5 by operation of the impulsing device I is applied to the grid of tube '6 through resistor I9. The purpose of resistor I9 is to prevent excessive grid current from flowing in case the adjustable tap on resistor 5 is set high enough to cause the grid to at times be driven positive. The screen grid of tube 6 is connected to an adjustable tap on resistor 20 which is bridged across the battery. The screen grid is also bypassed to the cathode by condenser 2I. The anode of tube 6 is connected to the positive terminal of the battery, which is usually grounded, through resistor 22. The primary winding of transformer I is connected across resistor 22 in series with condenser 23. Due to the high plate resistance of pentode type tubes the plate current of tube 6 will have substantially the same wave shape as the signal voltage. The voltage induced in the secondar winding of transformer I will be proportional to the rate of change of current in the primary and thus will consist of sharply peaked impulses which occur at the instants of make and break of the impulsing contacts. The polarity of these impulses Will also be opposite for makes and breaks of the impulsing circuit. The secondary of transformer 1 is connected to resistors 24 and 25 in series through condensers 26 and 21. The values of these condensers and resistors are chosen so as to provide a Very low time constant whereby the magnitude of the displacement current flowing through them in response to a peaked voltage impulse in the secondary of transformer I will be large but the displacement current which flows due to other signals, such as voice currents, will be small. The junction point between resistors 24 and 25 is connected to the negative battery terminal and the other ends of resistors 24 and 2-5 are connected to the control electrodes of tubes 8 and 9, respectively, through resistors 28 and 29. The cathodes of tubes 8 and 9 are connected to an adjustable tap on resistor 30 which is bridged across the battery. The voltage drop across the top section of resistor 30 thus biases the control electrodes of tubes 8 and 9 negatively with respect to the cathodes. This section of resistor 30 is bypassed by condenser 35. The anode of tube 8 is connected through resistor 3| and impedance 32 to ground or positive battery. The anode of tube 9 is connected through relay I 0, resistor 33 and impedance 32 to ground or positive battery. Impedance 32' may conveniently take the form of a relay similar to relay I0 but without springs. One of the tubes 8 or 9, which are of the gaseous discharge type, is always ionized. In the drawing it is assumed that tube 9 is the one that is ionized when the impulslng circuit is closed.

When the impulsing circuit is opened the resulting displacement current flowing in resistors 24 and 25 tends to make the control electrode of tube 8 positive and the control electrode of tube 9 more negative. Consequently tube 8 becomes ionized. During the time that tube 8 was deionized condenser 34 became charged to a potential equal to the voltage drop produced across relay I0 andresistor 33 by the anode current of tube 9. 'When tube 8 becomes ionized it produces a voltage drop in resistor 3I which lower the anode potential of tube 9 momentarily since the potential across condenser 34 cannot change instantaneously. Tube 9 thus becomes deionized and condenser 34 discharges and recharges in the reverse direction to a potential equal to the voltage drop produced in resistor 3| .by the anode current of tube 8. The impedance element 32 assists in lowering the anode potential of tube 8 when tube 9 fires since it is common to the two tubes. The circuit is operative without impedance 32 but more reliable operation is obtained when it is used.

When the impulsing circuit is closed the resulting displacement current flowing in resistors 24 and 25 tends to make the control electrode of tube 9 positive and the control electrode of tube 8 more negative. Consequently tube 9 becomes ionized and brings about the deionization of tube 8 in a similar manner to that previously described for the ionization of tube 8 and the deionization of tube 9.

It is now apparent that tube 9 will be conductive during the time that the contacts of the impulsing device I are closed and that tube 9 will be non-conductive during the time that the contacts of the impulsing device I are open. Since the repeater is controlled by the rate of change of line current rather than the magnitude of the current the on and off conduction periods will be substantially the same as the break and make periods of the impulsing contacts regardless of the distortion introduced by the line or impedances bridged across the line. The anode current of tube 9 may be used to control a stepping switch or other selective switching apparatus directly or may control a relay as illustrated. Since the anode current of tube 9 is a square wave the relay III will accurately reproduce impulses, corresponding to the make and break periods of the impulsing device I, at its contacts I I.

In an actual test it was found that no perceptible variation in the impulse ratio of the contacts II could be observed on an impulse ratio meter when the characteristics of the line 2 and the number of ringers, such as I2, were variedover the maximum range encountered in telephone practice. In this test the following circuit values were employed.

Tube 6 6A0? Tubes 8 and 9 2050 Condenser I8 10 mf. Condenser 2| 10 mi. Condenser 23 1 mf. Condensers 26 and 21' .04 Inf. Condenser 35 10 mi. Condenser 34 5 mi. Transformer 7 2 to 1 step up turns ratio. Relay I0 30 ohms Impedance 32 200 ohms Resistor 5 50,000 ohms Resistor I9 100,000 ohms Resistors I1, 3|, and 33 300 ohms Resistor 20 1,000 ohms Resistors 22, 24, 25, 28, and 29 10,000 ohms Resistor 30 500 ohms Referring now to Fig. 2, there is shown a conventional Eocles-Jordan trigger circuit which is adapted to control relay I0 in accordance with the impulses induced in the secondary winding of transformer 1 by the anode current of tube 6 in a similar manner as described for the gaseous discharge tubes 8 and 9 of Fig. 1. Each of the tubes 36 and 31, which are of the high vacuum type, has a separate plate load impedance. The plate load impedance for tube 31 being relay I0 and that for tube 36 being a similar impedance 38 which may be a relay similar to relay I0 but without springs. The plates of each of the tubes are coupled to the grids of the alternate tube through resistors 39 and 40 which are bypassed by small condensers H and 42, respectively. The connections between the plates and grids of the tubes tend to make each of the grids positive with respect to the cathodes. This positive bias on the grids, equal to the voltage drops in resistors 24 and 25, is opposed by the voltage drop in the upper section of resistor 30. The plate currents of tubes 36 and 31 cannot remain equal because the circuit is unstable. Any slight increase in the grid potential of one of the tubes will cause an increase in its plate current. The voltage drop in its plate load increases and thus reduces the potential on the grid of the other tube. The plate current of the other tube then decreases causing a decrease in the voltage drop in its plate load thus further increasing the grid voltage of the first tube. This action continues until the plate current of the first tube reaches a maximum value and the plate current of the other tube is cutoiT. The action is accelerated by the condensers 4| and 42 because they increase the instantaneous change in voltage impressed on the grid of one tube by a change in plate current of the other tube. f the circuit constants are properly chosen the grid potential on the non-conducting tube can be made greater than the cut-off value so that minute changes in its grid voltage will not result in any flow of plate current and thus operation as a multivibrator is prevented. However, an increase in the grid potential which is sufficient to produce a flow of plate current in the tube formerly cut off will resuit in the cumulative action described above which continues until the tube which was formerly conductive is cut off. For the purpose of the present invention, the transfer may be considered as occurring instantaneously. Assuming that tube 3: is conducting, which is the normal condition when the impulsing circuit is closed, the voltage impulse induced in the secondary of transformer I by the change in plate current of tube 6 in response to the contacts causes the grid voltage of tube 35 to increase and that of tube 31 to decrease. Tube 3 thereupon becomes conductive and tube 31 becomes cut-oiT. This condition will be sustained until a further impulse of reversed polarity is induced in the secondary of transformer l in response to the closing of the impulsing contacts. The plate current of tube 31', being substantially a square wave, causes relay H) to accurately repeat impulses corresponding to the make and break periods of the impulsing device I. It should be noted that the values of resistors 40 and 24 are so high as to draw an inappreciable current through relay l and thus do not interfere with the release of the relay. Suitable values. for the circuit components of Fig. 2 are indicated below.

Tubes 36 and 3? 6N7 Condensers 26 and 21 .002 mi. Condensers 4i and 42 50 mmf. Resistors 24, 25, 30, and ii 250,000 ohms Resistor 38 1,000 ohms Relay l0 30,000 ohms Impedance 38 30,000 Ohms Although the invention has been illustrated in its most simplified form, it should be obvious that numerous modifications are possible. For example, instead of the impulse repeater being associated with a single line, it may be made available:

opening of the impulsing 8 to any one of a number different characteristics. In this case the independence of the repeated impulses from the line characteristics becomes an extremely desirable attribute of the repeater. Although only a single subscriber substation set connected at the end of the line has been illustrated, it should be apparent that several such sets located at different points along the line may be used. Of course it is understood that only one set may be used at a time, the impulse transmitter of each set being disconnected when not in use by the usual hookswitch. The ringers of each of the sets will always be connected to the line. The distortion in the line current will be different for each location of the impulsing device, but the operation of relay III will be unaffected by such variations.

What is claimed is:

1. In a signaling system, an impulsing circuit including an interrupting device, a line, an impedance, and a source of potential, a pair of gaseous discharge tubes each having an anode, a cathode, and a control electrode, means responsive to a decrease in the voltage across the impedance for initiating a discharge between the cathode and control electrode of one of said tubes and responsive to an increase in the voltage across the impedance for initiating a discharge between the control electrode and cathode of the other of said tubes whereby one tube is fired at the instant that the interrupting device opens said impulsing circuit and the other tube is fired at the instant that the interrupting device closes said impulsing circuit, and means in the anode circuits of said tubes for extinguishing either tube in response to the firing of the alternate tube.

2. A signaling system as claimed in claim 1 in which said first means comprises a amplifier hav ing input and output circuits, the input circuit or said amplifier being coupled to the impedance and the output circuit of said amplifier being coupled to the contol electrodes of said gaseous discharge tubes in push-pull relationship.

3. In an impulse repeater; a circuit over which impulses are at times transmitted; a pair of gaseous discharge tubes each having an anode, a cathode, and a control electrode; means coupling the control electrodes of said tubes to said circuit so as to initiate a discharge in one tube at the beginning of an impulse transmitted thereover and to initiate a discharge in the other tube at the end of the impulse; a circuit between the anode and cathode of one of said tubes including a resistor, an impedance, and a source of direct current; a circuit between the anode and cathodeofr" the other of said tubes including a second resistor, said impedance, and said source of direct current; a condenser connected between the anodes of said tubes; said condenser, impedance, and resistors cooperating to extinguish either tube in response to the ionization of the alternate tube and an impulse repeating relay having a winding connected in series with the anode circuit of one of said tubes.

4. In an impulse repeater, a transformer having two windings, means for causing a periodic current representing impulses to flow through one of said windings, a series circuit comprising a condenser and a pair of resistors connected across the other of said windings, the induced voltage in said other winding being substantially proportional to the rate of change in current in said one winding and the current flowing in said series circuit varying in accordance with the rate of change in said induced voltage, a pair of therof lines each having mionic tubes each having an anode, a cathode, and a control electrode, a connection between the cathodes of said tubes and a junction between said resistors, connections between the control electrodes of each said tubes and opposite ones of said resistors, said connections causing one of said tubes to become conductive in response to an increase in the rate of change in current in said one Winding and causing the other of said tubes to become conductive in response to a decrease in the rate of change in current in said one winding, means interconnecting said tubes for rendering a conductive one of said tubes non-conductive in response to the other tube becoming conductive, and an impulse repeating relay controlled by the anode current of one of said tubes.

5. In an automatic telephone system, a subscriber substation set including an impulse transmitter, a relay, a source of potential for supplying transmission battery to said substation set, a line connecting said substation set to said source of potential in series with said relay, asymmetrical conductance means shunting said relay for suppressing oscillations otherwise produced in said line in response to the operation of the impulse transmitter, a second relay, and thermionic tube means controlled by the transient voltages produced across said first relay in response to the operation of the impulse transmitter for operating said second relay in accordance with the impulses transmitted.

6. In a signaling system, an impulsing circuit including an interrupting device, a line, and a source of potential, a pair of thermionic tubes each having an anode, a cathode, and a control electrode, means coupling 1e control electrodes and cathodes of said tubes to said line in pushpull relationship whereby one of said tubes is rendered conductive at the instant that the interrupting device opens said impulsing circuit and the other of said tubes is rendered conductive at the instant that the interrupting device closes said impulsing circuit, means interconnecting said tubes for renderin either one of the tubes non-conductive in response to the other tube becoming conductive, and an impulse repeating relay controlled by the anode current of one of said tubes.

7. In an automatic telephone system, a subscriber substation set including an impulse transmitter, an inductance coil, a source of potential for supplying transmission battery to said substation set, a line connecting said substation set to said source of potential in series with said response to the operation of the impulse transmitter, a relay, and

trolled by the transient voltages produced across said inductance coil in response to the operation of the impulse transmitter for operating said relay in accordance with the impulses transmitted.

8. In an automatic telephone system, a subscriber substation set including an impulse transmitter, an inductance coil, a source of potential for supplying transmission battery to said substation set, a line connecting said substation set to said source of potential in series with said inductance coil, a pair of thermionic tubes each having an anode, a cathode, and a control electrode, means coupling the control electrodes and cathodes of said tubes to said inductance coil in push-pull relationship whereby one of said tubes is rendered conductive at the instant that the impulse transmiter opens said line and the other of said tubes is rendered conductive at the instant that the impulse transmitter closes said line, means interconnecting said tubes for rendering either one of the tubes non-conductive in response to the other tube becoming conductive, and an impulse repeating relay controlled by the anode current of one of said tubes.

9. In an automatic telephone system, a subscriber substation set including an impulse transmitter, an inductance coil, a source of potential for supplying transmission battery to said substation set, a line connecting said substation set to said source of potential in series with said inductance coil, asymmetrical conductance means shunting said inductance coil for suppressing oscillations otherwise produced in said line in response to the operation of the impulse transmitter, a pair of thermionic tubes each having an anode, a cathode, and a control electrode, means coupling the control electrodes and cathodes of said tubes to said inductance coil in push-pull relationship whereby one of said tubes is rendered conductive at the instant that the impulse transmitter opens said line and the other of said tubes is rendered conductive at the instant that the impulse transmitter closes said line, means interconnecting said tubes for rendering either one of the tubes non-conductive in response to the other tube becoming conductive, and an impulse repeating relay controlled by the anode current of one of said tubes.

HAROLD J. MCCREARY.

REFERENCES CITED The following references are of record in the file of this patent:

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