US2332142A - Long dash interrupter - Google Patents

Description

Oct. 19, 1943. E. HAJos LONG DASH INTERRUPTER Filed Jan. 4, 1941 2 Sheets-Sheet l r @III Ybbww ma,

ATTORNEY Patented Oct. 19, 1943 LONG DASH INTERRUPTER Eugene Hajos, Jamaica Estates, Long Island, N. Y., assignor to Radio Corportion of America, a corporation of Delaware Application January 4, 1941, Serial No. 373,071

5 Claims.

The present invention concerns an arrangement for interrupting unwanted' long dashes accidentally left on transmitter keying circuits.

In the remote keying of radiol transmitters by automatic sending devices, trouble is frequently experienced due to'long' dashes left oncontrol lines. The auto occasionally sticks during transmission, or isy stopped at the termination of transmission-,squarely on a dash. Since one attendant is calledupon; to supervise many circuits, both; at theftranic-center and at the transmitting station, this condition at times remains` unnoticed fcr'manyv minutes, especially when the attendant is occupied. withl another circuit. Besides.- radiating. unnecessary signals and wasting power, longv dashes: consume, unproductively, a valuable part of the useful lifeV ofthetubes in a transmitter. In addition, service is` interrupted when one or morey tubes. are renderedlowemission by a long.' dash.

Thel foregoing dicuities areovercomc by the present inventionwhich provides means for opening the local4 keying circuit of the transmitter whenever. a dash occurs which' exceeds a predetermined: time. limit. This means includes a relay and: a timing element whcsesetting determines; theY time of operationof the relay. In accordance with the invention, the keying circuit is automatically restored-to the-normaly operating conditionl as soon as the. long dash has been removed fromtheccntrollineby the remote operator.

A better understanding cfthe invention may be hadV by referringtothe following description which is accompanied by drawings, wherein Fig. 1 illustrates the' preferred circuit arrangement of. the invcntion,.and Figs.,2 and\3'show graphs giving relationswhich a-idf inthe construction of thecircuit of Fig. 1 for certain desired condi.- tions.

Referring to Fig. l of the drawings, there is shownv schematically' a standard` keying circuit employed in a short wave transmitter which has been modified by the addition of certain elements containedi in box I0. Only those circuit elements have been shown which it is believed are necessary for an. understanding ofy the principles underlying; the invention. The keyed transmitter is indicated: only by the keyed radio frequency stage. I2 whose. output; is suitably coupled through leadgII: to anantenna,.no.t. shown. Transmitter stage; IZiskeyed by a pair offkeyer vacuum tubes I4',A I4 acting in; parallel, said. keyer tubes in tum being controlled by a polarized keying relayF 5;. Itzshouldbe noted;.that the mar contact of relay 5 is connected to the grids of the keying tubes I4, I4 while the armature of relay 5is connected to a negative potential of approximately' 200 volts. The winding of relay 5 is connected by means of a land line or channel 6 with a distant keying circuit located at a remote telegraph station. rhe details of the transmitter represented by stage l2 and the keying unit I4, I4 are well known in the art and form no part, per se, of the present invention.

The circuit elements contained in box il) and which go tc make up the system of the invention include a vacuum tube 3 in whose output is an ordinary telephone relay 4 and in whose input is a timing circuit constituted by a capacitor I and a potentiometer 2 connected in shunt to the capacitor. The grid of tube 3 is suitably tapped to a point on the potentiometer 2 intermediate its ends. One terminal of the timing circuit I, 2 is grounded while its other terminal is connected to the space contact of the polarized relay 5. The armature of relay 4 is connected'to the marking contact of polarized relay 5, While the contact of relay 4 is connected to the grids of keying tubes It, I4. In this Way, the armature and contact of relay 4' are serially arranged in the connection extending between the mark contact of relay 5 and the keying unit.

In the operation of the system of Fig. l. keyed telegraph signals appearing on line 6 will operate polarized relay 5 to cause its armature to alternately engage the mark and space contacts. Let us assume that themark contact is engaged when a dash is sent over line 6 While the space contact is engaged when spacing current appears on line S. The same 20D volt negative potential on the armature of polarized relay 5 which applies negative potential to the grids of keying unit I4, I4 when the armature engages the mark contact will, it will be seen, apply negative potenial to the timing circuit I, 2 when the armature engages the space Contact. Whenever space current flows in the keying line, capacitor I will be charged to a potential of 200 volts (negative relative to ground) A portion of this potential appears on the grid of vacuum tube 3, its value being dependent cn the adjustment of the tap on potentiometer 2.

During normal keying conditions, capacitor I will be charged to a potential averaging only slightly less than 200 volts. Potentiometer` 2 is adjusted to provide a negative potential on the grid cf'tube 3, in excess of the value required to stop the plate current. Thus, under normal conditions of keying, no current will flow in the coil of the telephone relay 4, and the contacts of the latter will therefore be closed, providing normal continuity for the local keying circuit.

However, when a long dash occurs on the line 6, thus maintaining the armature of the keying relay 5 on the mark contact, capacitor I will gradually discharge through potentiometer 2. After an interval of time, dependent on the adjustment of the potentiometer, the negative potential on the grid of tube 3 will drop sufficiently to permit plate current t0 flow, thereby actuating relay 4 and thus opening the local keying circuit.

When normal keying is resumed on the control line 6, or a space put thereon, the capacitor I will again be charged to the aforementioned negative potential; which stops the plate current through tube 3. The armature of relay 4 being thereby released, continuity is again restored in the local keying circuit between the grids of tubes I4, I4 and the mark Contact of relay 5.

The following is a study of the quantitative relations inherent in the circuit of Fig 2. The voltage across a capacitor shunted by a resistance, at any time t, after removal of the charging source, is given by:

Emme-RC (i) where Ec equals voltage after a time t, Eo equals initial (or source) voltage, e equals 2.7183 t equals time, in seconds, after removal of the source, R equals resistance shunting the capacitor (in megohms), and C equals capacitance (in microfarads) Assuming the charging source to be 200 volts, and adopting suitable values for R and C, such as 2.5 megohms and 4 microfarads, respectively. graph II of Fig. 3 shows the relation between the voltage Ec and the time t, as evaluated by the above formula. A consideration of this graph will show that, even during relatively low keying speeds, the voltage Ec cannot fall appreciably below 200 volts. Thus, at ten words per minute, the average duration of the marking interval is approximately 1/8 of a second. During this interval Ec can drop, at most. only 3 volts; as may be seen from graph II. Thus, for the practical purpose in hand, it may be assumed that the voltage Ec remains substantially at 200 volts during normal keying conditions.

The action of the interrupterudevice depends also upon the operating characteristics of vacuum tube 3 and that of relay 4. A relay that will give positive operation 0n 4 milliamperes, direct current, can easily be purchased in the market. One such relay sold in the industry has a coil resistance of 200 ohms.

Regarding vacuum tube 3, selection may be made from a multitude of receiving circuit tubes, although the R. C. A. 56 is thought to have very suitable operating characteristics. Fig. 2, graph I, shows the lp/Eg characteristic of this tube at a plate voltage of 120. This plate voltage was chosen because it limits the plate current to maximum of 16 milliamperes, even though the bias on the grid should drop to zero. This maximum current, though ilcwing through the relay coil indenitely, will cause but negligible heating.

Referring to Fig. 2, it is seen that Ip will be 4 milliamperes when the negative bias on the grid of the tube drops to 5 volts. Although the static characteristic has been employed in this determination, it is to be noted that the direct current plate resistance oi the tube at this point of the curve is approximately 30,000 ohms, compared with a relay resistance of 2,000 ohms. The small drop in plate voltage caused by the introduction of the relay does not affect the operation to any noticeable extent.

It will now be of interest to study the relationship between the potentiometer setting and the timing of the circuit. As before mentioned, relay 4 will operate when the plate current reaches a value of 4 milliamperes, a condition realized the moment the grid potential drops to -5 volts. Now the grid potential at any time is given by the following relationship:

Ec T2,500,000 (2) where Ec equals voltage across capacitor I, Eg equals voltage on the grid, r equals resistance included between potentiometer slider and ground. (Total resistance across capacitor equals 2,500,000 ohms.) Setting Eg equal to 5, and transposing, we now get:

12,500,000 Tn Ec olnr s (3) The procedure for setting the device may best be considered by means of an example. Suppose it is desired that the device interrupt long dashes of ten seconds duration, and no less; then consulting Fig. 3, graph II, we flnd that Ec drops to '74 volts when a dash has been on for ten seconds. Substituting this value of Ec in Equation 3 gives us a value of 169,000 ohms for r. This, then, is the permanent setting of the potentiometer for the interruption of ten-second dashes.

To facilitate the making of settings, without the necessity for any calculations by the operator, graph III of Fig. 3 has been given to show at a glance the required setting of the potentiometer, in terms of 1', for any value of time from zero to thirty seconds. While circuit insulation resistance, and the deviation of individual tubes and relays from their average characterlstics, have not been considered in the preparation of this curve of Fig. 3, it is nevertheless sufficiently accurate for practical use. Circuit insulation re-` sistances of the order of 15 to 20 megohms are easily realizable, employing ordinary 600 volt switchboard Wire and inexpensive condensers. Time settings may also be made by the cut-andtry method; a procedure which may be required in cases Where the keying bias of a transmitter dilers appreciably from 200 volts.

An advantage of the invention lies in the fact that once a suitable value of timing has been decided upon, and the potentiometer set accordingly, the circuit of the invention requires no further attention on the part of the operator, with the exception of infrequent tube renewals. Furthermore, the device can cause no interference with the local keying of the transmitter by the attendant, during tuning adjustment, as relay 5 ls then out of service.

In one embodiment tried out in practice, a Western Electric 209 FA keying relay 5 was employed and carefully observed to detect any deleterious sparking on the space contact, lncident to the operation of the timing circuit. During normal keying, absolutely no sparking could be detected. This is because, for this condition, the space contact and tongue o! the relay do not diier materially in potential and therefore little current is handled by these contacts. After a long dash, however, the capacitor I is in the discharged state and, upon restorationv of space current'` on. they line by the remote operator, current flows from tongue to space contact of the.A relay to recharge the capacitor. On@ these occasions a barely perceptible spark sometimes occurred; However, it was always of lesser magnitude than the sparking noticeable on the keying (or mark) contact of the relay. Since the charging of the capacitor is practically instantaneous, the space contact is never called upon to break any current; accordingly sparking never occurred on the break at this contact. No provisions for spark suppression need therefore be made.

What is claimed is:

l. In combination, a telegraph line, a polarized relay connected to said line and responsive to marking and spacing currents sent over said line, said relay having an armature and oppositely disposed marking and spacing contacts, a transmitter for sending out marking impulses interspersed with spacing intervals, an electron discharge device circuit for keying said transmitter, a connection from said discharge device circuit to the marking contact of said polarized relay, a connection from the armature of said relay to a source of potential, a condenser connected to the spacing contact of said relay for storing an electric charge during spacing intervals only, and means responsive to a diminution of said charge on said condenser below a predetermined value for breaking the connection between said electron ischarge device circuit and the marking contact of said polarized relay.

2. In combination, a telegraph line, a polarized relay connected to said line and responsive to marking and spacing currents sent over said line, said relay having an armature and oppositely disposed marking and spacing contacts, a transmitter for sending out marking impulses interspersed with spacing intervals, a keyer for said transmitter, a. connection from said keyer to the marking contact of said polarized relay, a connection from the armature of said polarized relay to the negative terminal of a source of unidirectional potential, a time constant circuit composed of a condenser connected to the spacing contact of said polarized relay for storing an electric charge during spacing intervals only and a resistor in shunt to said condenser, an electron discharge device having its control electrode connected to a point on said resistor intermediate the ends thereof, said point being chosen to provide a negative bias on the control electrode of said device which prevents the ilo-w of current through the device at normal signaling speeds by virtue of the negative potential supplied to said time constant circuit during spacing intervals, each spacing interval being sufcient to charge said time constant circuit to a value which prevents the flow of current through said last electron discharge device, and a relay in the output circuit of said device controlling the continuity of the connection between said keyer and the marking contact oi said polarized relay, said last electron discharge device being responsive to the dissipation of the electric charge on said condenser below a predetermined value caused by the absence of spacing current on said telegraph line for a predetermined interval to produce a ilow of current in said output circuit ior interrupting the connection between said keyer and said polarized relay.

3. In a radio communication system, in combination, a telegraph station, a remote station having a radio transmitter associated therewith,

a linelextending; between said stations, an electromagnetic relayat: said remote station operativelyassociated'with. said. line and responsive to markingand spacing currents: sent over said line,

f said, relay` having,y an; armature and oppositely disposed marking and spacing contacts, a connection from said armature, to the negative terminal of a source of potential, a keyer connected to said marking contact and controlled by said relay for keying said radio transmitter in accordance with the signal characters received over said line, a vacuum tube circuit including a vacuum tube having a grid, an anode and a cathode, a time constant arrangement composed of a condenser shunted by a resistor connected between said cathode and said spacing contact, a connection from the control electrode of said vacuum tube to said resistor, and a connection from said anode to a circuit for controlling the continuity of the connection from said keyer to said relay, each engagement of said armature with said spacing contact being sufficient to charge said time constant circuit to a value necessary to prevent the flow of current in the anode circuit of said vacuum tube, whereby only a continuous marking current over said line for a predetermined period of time Will cause the new of current through said vacuum tube circuit with a resulting interruption between said keyer and relay.

4. In combination, a telegraph line, an electromagnetic keying relay connected to said line and responsive to marking and spacing currents sent over said line, said relay having an armature and oppositely disposed marking and spacing contacts, a radio transmitter for sending out marking impulses interspersed with spacing intervals, an electron discharge device keyer for said transmitter, a connection from the grid of said keyer device to the marking contact of said keying relay, a connection from the armature of said keying relay to the negative terminal of a source of unidirectional potential, a time constant circuit composed of a condenser and a resistor in shunt to said condenser connected to the spacing contact 0i said keying relay for storing an electric charge during spacing intervals only, a vacuum tube having its control electrode connected to a point on said resistor so chosen as to provide a negative bias on the control electrode of said tube which prevents the ilow oi current through' the tube at normal signaling speeds by virtue of the negative potential supplied to said time constant circuit during spacing intervals, each spacing interval being suilicient to charge said time constant Circuit to a value which prevents the flow of current through said last tube, and an electromagnetic relay inA the output circuit of said tube controlling the continuity of the connection between said keyer and the marking contact of said keying relay, .said tube being responsive to the dissipation of the electric charge on said condenser below a predetermined value caused by the absence of spacing currents on said telegraph line for a predetermined interval to produce a ow of current in said output circuit for interrupting the connection between said electron discharge device keyer and said keying relay.

5. In combination, a telegraph line, a polarized relay connected to said line and responsive to marking and spacing currents sent over said line, said relay having an armature and oppositely disposed marking and spacing contacts, a transmitter for sending out marking impulses interspersed with spacing intervals, a keyer circuit for keying said transmitter, a connection from said keyer circuit to the marking contact of said polarized relay, a connection from the armature of said relay to a. source of potential, a condenser connected to the spacing contact of said relay for storing an electric charge during spacing intervals only, and means responsive to a diminution of said charge on said condenser below a predetermined value for breaking the connection between said keyer circuit and the marking contact of said polarized relay.

EUGENE HAJ OS.

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