Reference / Paper · 1938
Bell Laboratories Record, June 1938 (The Crossbar Switch in the 755 PBX; The 755 PBX)
The one-switch 755A crossbar PBX: lines on the verticals, links and trunks on the horizontals, relay control with no marker.
Reference / Paper · 1938
The one-switch 755A crossbar PBX: lines on the verticals, links and trunks on the horizontals, relay control with no marker.
ELL LABORATORIES RCORt) JU_NE 1938 VOLUME XVI NUMBER X Studying the dynamic characteristics of carbon transmitters www.americanradiohistory.com The Crossbar Switch in the 755 PBX By J. N. REYNOLDS Apparatus Engineer THE switching problem at a quired for talking, and there must be private branch exchange is provision for ringing the called exsimilar to that at a small tension, for signalling the operator, central office. There are lines running to the various extensions, and trunks to the central office, corresponding to the subscriber lines and trunks to other offices at a central office. An extension, of course, may either place a call or receive a call, so that the direction of the call between two extensions or between an extension and the central office may proceed in either of two directions. To meet the exigencies of this situation, it is customary in one way or another to wire each extension to two sets of terminals in multiple, one of which will be used for outgoing, and the other for incoming calls. Then, by some linking circuit, provision must be made for connecting the calling terminals of the extension placing the call to the receiving terminals of the extension being called. A diagram of this general switching scheme is shown in Figure i. To simplify the illustration, only four extensions are shown and two links. Obviously there would never be needed more than half as many links as there are extensions and trunks combined, because two extensions, or an extension and a trunk, are involved in each call. Usually the number of links will be less than this, because all of the extensions and trunks will not be in use at any one time. It is not sufficient to connect the two extensions with a simple wire connection. A battery supply is re332 and for additional supervisory uses; and the proper performance of these and other functions necessitates that a certain amount of equipment be associated with the extensions for each call. Some of this equipment is most conveniently connected into the circuit by way of the link circuit. This is indicated by the boxes marked "equipment" in Figure i, and the link is divided by the equipment into two sections: an originating section, which connects to the calling extensions, and a completing section, which connects to the called extension. In the present manual boards these links are usually cords and the terminations of the extensions are jacks. In the step -by -step PBX, the cords and jacks are replaced by a form of switch in which the lines end in fixed terminals, and the links end in brushes, which are moved over the fixed terminals until the desired one is reached. In their many varied forms both of these systems have worked satisfactorily for many years, but a new piece of switching apparatus has been developed in recent years that for many applications seems to offer greater advantages. This new unit is known as the crossbar switch.* The arrangement for a four-line group using the crossbar switch is shown in Figure 2. The lines, when *RECORD, July, 1937, p 338. June 1938 this switch is employed, are multipled where each line represents a number to sets of terminals for each side of of wires, and each contact, indicated each link. Both lines and links by a circle, represents an equal terminate in relay-type contact number of spring contacts. The vertisprings, and a set of springs on a cal lines represent ten possible exline faces a corresponding set on a tension lines -each line having conlink. All that is required to establish tacts for connection to ten horizontal a connection is to press two of these circuits, which may be links for sets of springs into contact. The closing of one set will connect the EXTENSION LINES :2 3 :4 LINK calling line to the originating side EQUIPMENT ORIGINATING of the link, and the closing of another set will connect the completing side of the link to the line called. ORIGINATING In the crossbar switch these sets of spring contacts are arranged at the intersection of a rectangular lattice. This resemblance to a lattice is enhanced by the appearance of the switch from the front, shown in Fig. a- Schematic representation of method Figure 3, where the vertical and of establishing a connection between two subscribers with the crossbar system horizontal bars that make the lattice are used to close the contacts. It is these bars that give the name "cross- establishing connections between the bar" to the switch. The ends of the local extensions or trunks to a central spring contacts may be seen behind office. The upper six horizontal rows the bars. Such a switch has loo sets serve as trunks for the originating and completing ends of three link EXTENSION LINES circuits, and the lower four rows are 3 ;4 2 I for trunks. These various sets of contacts are closed by the operation of horizontal ó d o o lb and vertical bars in a way that has COMPLETING ORIGINATING LINK already been described in the RECORD. EQUIPMENT Each vertical bar is operated by an A electromagnet, the ten magnets being located in a row along the bottom of Fig. 1-Schematic representation of method the switch as may be seen in Figure 3. of establishing a connection between two Each horizontal bar is controlled by subscribers with previously existing systems two magnets -one to rotate the bar in each direction. Three of these of contacts, corresponding to the pairs of magnets are located at one intersection points of ten vertical end of the switch and two at the other, as may be seen in the same and ten horizontal rows. The interconnection of these con- illustration. Each vertical and hortacts in the actual switch used for the izontal bar also operates a set of 7 55A PBX* is as shown in Figure 4, ofd normal contacts, which are employed for a variety of purposes such *Page 336, this issue. +I rune 1938 33.) www.americanradiohistory.com normal, but the vertical bar will remain op- erated. When the horizontal bar returns to normal all of its selecting fingers are also returned to normal except the one opposite the calling line, which is held by the pressure of the vertical bar corresponding to that particular line. These fingers have been made very flexible so that they may readily be held in an operated position without retarding the return of the horizontal bar and yet will reFig. 3 -The crossbar switch as used in the 75511 PBX turn to their normal position when released. After the calling line has dialed as indicating whether or not a bar is operated. These off-normal contacts the number wanted, the same horfor three of the horizontal bars may izontal bar is again operated, but be seen at the left of Figure 3. Those this time in the downward, or comfor the other two horizontal bars pleting, position. The vertical bar are at the right -hand end of the corresponding to the line called is switch, while those supplied for the now operated, and thus connects vertical bars are placed at the top, the completing side of the link to the and may also be seen in Figure 3. line called. Following this the horThe general scheme of operation may be Ea INK M ENT o q followed from Figure 4. Y I When line 3, for example, places a local call, the horizontal bar of an idle link is rotated up and immediately afterward the vertical bar corresponding to line 3 is Operated, which connects the calling line to the originating side of the link. The horizontal bar then returns to 334 I d S TWO ADDITIONAL LINKS TRUNK EQUIPMENT U a ft U a a 0 ;I I I I !s I I b I TWO ADDITIONAL TRUNKS a 12 a :3 a :4 L a d jI I :5 :6 :7 6 :9 EXTENSION LINES Fig. 4- Schematic diagram showing the general scheme of interconnecting contacts in the crossbar switch lune 1938 izon tal bar will again return to normal, but two of its fingers will remain held: one by the vertical bar of the calling line and one by that of the line called, thus establishing the connection. On establishing a connection to a trunk only a single set of operations is required. The horizontal bar corresponding to the trunk selected operates first, and then the vertical bar corresponding to the calling line. This connects the line to the trunk, completing the central -office circuit. In the 755A PBX two of these crossbar units may be employed to give a capacity of twenty lines. The connections and operations are so arranged that the switches act as a single switch with twenty vertical bars. The crossbar switch is concerned primarily with completing the actual switching connection, and other devices, largely relays of the u and Y types, are employed for the many other switching functions as described in the article already referred to. This model was devised by C. E. Fay, A. L. Samuel, and W. Shockley to illustrate the mathematical relationships between current and potential in the space between grid and plate of a vacuum tube The 755 PBX By C. R. GRAY Central Office Switching Development TO apply the crossbar switch and the U -type relay to private branch exchange service, a new dial PBX has been developed for residences and smaller business houses. This 755A dial PBX will replace the 750A* and by reason of its four trunks and twenty station lines it will have a wider field of use. The power requirement is much less than that of the 75oA; and it is expected, because of the improved design of the new switches and relays, that the maintenance effort will be reduced. The basic function of the new PBX is to establish connections between central office trunks and its local stations, or between two or more of these stations. The actual talking *RECORD, February, 1930, p. 278. 336 connection is made at the crossbar switch, but for the complete functioning of the PBX six control circuits are employed: the line circuit; the trunk circuit; the link circuit; the common control and cut -off circuit; the common timing, tone, and ringing circuit; and the "link -allotter" circuit. There is one each of the three latter circuits for each PBX regardless of the number of lines or trunks; but there is one line circuit for each line, and one trunk circuit for each trunk. The link is the circuit employed for making a local connection between two lines, and from one to three may be employed as desired. Each of the crossbar switches will accommodate ten lines, and where ten lines or fewer are to be used, only June 1938 corresponding to the trunk or link, and a holding magnet corresponding to the local station. A complete link circuit includes two of the selecting magnets one to make connection to the calling station, which is called the originating magnet, and one to make connection to the station called; which is called the completing magnet. A local call illustrated schematically in Figure i, as originating at station No. 36, is made by pressing the "local" key, and lifting the handset off the mounting. The line circuit then operates to signal the common will be provided in a separate unit. control and cut -off circuit that a call Each local station is connected to has started. The common control a set of contacts in a vertical column circuit then determines from the link of the crossbar switch, while the allotter circuit that an idle link is trunks and links are connected to sets available, and operates the start relay of contacts in the horizontal rows. A in the link circuit. The link originatconnection between a trunk or a link ing magnet is then operated, and and a local station is thus established immediately afterward the common by operating the selecting magnet control circuit operates the holding one switch will be installed, but for more than ten lines and up to the full capacity of twenty lines, two will be employed. This PBX is designed to employ a handset on which are mounted the dial and the key buttons used to set up the various trunk and local calls. One of these buttons is used for local calls, one for holding, and the others for making connections to the four trunks. Where desired, however, the usual types of telephone sets may be used in place of the combined handset and when this is done, the buttons CROSSBAR SWITCH 20 TO 29 CROSSBAR SWITCH 30 TO 39 LINES 20 TO 39 ORIGINATINGEND COMPLETING END SELECTING MAGNETS 20 30 2 39 --1 3 {f o 2 1 1 I Q iO I I Y Y 4 o J J 7Cr 21 LINK 1 o I en N N c7 O I 1 31 Z Z I I II x 1 i 11 I I I J L LINK ALLOTTER 0 HOLDING MAGNETS I g o 0 0 0 o L q?? J HOLDING MAGNETS A TO OTHER LINES LINE CIRCUIT CALLED STATION oo LOCAL LINE CIRCUIT CALLING STATION NO.36 LOCAL 000 Door KEY 0 o f i.-____ COMMON CONTROL AND CUT -OFF CALL ALLOTTER CHAIN COMMON TIMING TONE AND RINGING Fig. 1-Block schematic illustrating the setting-up of a local call Yune 1938 337 magnet associated with the calling line, which closes the crossbar contacts to connect the calling line to the link. The common control circuit then releases the link originating magnet, and restores itself to normal, ready for another call. Dial tone is received through the link by the calling station, which then dials. Pulsing and register relays in the link circuit record the two digits dialed, and at the completion of dialing the link tests the called line and returns a busy signal if it is busy. If the called line is found to be idle, the link signals the common control circuit which operates the completing magnet of the link and the hold magnet of the called line. The link completing magnet is then released and the common control circuit again returns to normal. The link then rings on the called line, and when the station answers provides a talking path between the two stations. The common control circuit is used only for a very short period at a time, and may thus serve two or more calls during approximately the same interval. If it is in use on one call when needed on another, the delay while waiting for it to become idle is only a fraction of a second and is unnoticeable. The establishment of a trunk call is illustrated by the block schematic of Figure 2. To make such a call, the subscriber presses the trunk key corresponding to the trunk over which he wishes to make the call, and then lifts the handset. The line circuit immediately operates to signal the common control circuit, which then operates the selecting magnet corresponding to that trunk, and immediately afterward operates the holding magnet of the calling line. The station is CROSSBAR SWITCH 20 TO 29 LINES 20 TO 29 then connected direct-SELECTING MAGNETS ly to the trunk as 20 29 9 ` shown in the diagram. Dial tone will be returned by the central vl office, and the prog0 ress of the call will ( then proceed in the usual manner. All ap- - TO CENTRAL OFFICE TRUNK I 0 L__ i HOLDING MAGNETS LINE CIRCUIT CALLING STATION 0 NO.250 tKEY (TRUNK NO.1) COMMON CONTROL AND CUT -OFF CALL ALLOTTER CHAIN COMMON TIMING TONE AND RINGING Fig. 338 2 -Block schematic illustrating the setting-up of a trunk call through the 755/1 PBX paratus is returned to normal when the handset is replaced on the mounting. Inward trunk calls are indicated to the stations by the lighting of a lamp or the operation of an audible signal, either of which occurs when the central office rings on the trunk. Any of the key stations may an7une 1938 swer by pressing the key button associated with that trunk, and lifting the handset. The line circuit, the common control circuit, and the crossbar switch function as on outgoing calls, and the station is connected to the trunk. If the call is for some station other than the one answering, the hold button is pressed, which then connects a holding bridge across the trunk and releases the trunk key. The station wanted is then called as for local calls, and informed that there is a call for it. This station then operates its trunk key, which releases the local connection and the hold on the trunk, and connects its own handset to the trunk the station that originally answered being then free to originate or receive other calls. The equipment, shown in Figure 3, is arranged on a unit Fig. 3-Installation of the 75521 PBX for the basis to make it possible to 195 Broadway Corporation equip only the combination of lines, trunks, and links required for for a maximum capacity. The apa particular installation. The same paratus on each unit is wired, adframework and equipment arrange- justed, and tested as the unit is ment is used for all installations so stocked, so that after a unit is inthat units may be added as required stalled in a PBX, it is necessary only by traffic growth. Station line circuits to connect the inter -unit leads and are arranged in four -circuit units with to perform an operational test. Three classes of trunk service are either two or four circuits equipped, available for the stations. "Non so that station line growth is in steps allows the station Trunk lock -out" service of two circuits at a time. a trunk whether it is on to connect to circuits are arranged two- circuit -out" busy or not; "lock service allows one or two units with either circuits connect only to idle may be the to that trunks station equipped so added one at a time. Each link is a trunks; and "restricted" service preseparate unit, and links may be vents a station from making outadded one at a time. The PBX is going trunk calls, but allows it to equipped initially with a local cable answer or transfer incoming calls, or including all the wiring between pick up transferred trunk calls. A units, switches, and terminal strips station may be arranged to have the - Tune 1938 339 same class of service on all trunks or a different class on each trunk. This allows several stations to have preferential service on different trunks. The class of service is changed merely by strapping on the terminal strips at the ends of the line units. Restricted stations may not be provided with the key- button handset, but trunk service may be given these stations through the aid of some key station designated as the control station. This station sets up the incoming or outgoing trunk call for the restricted station with an auxiliary key, and the station is then free to make other calls. By use of a spare push button, a station may be enabled to pick up calls directed to another, and may thus perform secretarial functions. A lamp may be installed at the secretarial position to indicate calls incoming to the supervised position. On an incoming call the lamp flashes and after it is answered and the line is busy, it lights steadily, or if preferred may be arranged to go out. This provides an indication as to whether the supervised position has answered, and thus whether or not she should come in on the line. Other features available with this new PBX are line hunting and con- ference service. The former provides for automatically routing calls to another line if the one called is busy. By this means heavy traffic to one number may be routed to another so that important calls will not be lost or delayed. The conference feature permits three lines to be connected together. To set up such a connection, the originating station calls one of the parties, tells him that a conference is to be held, and asks him to hang up until his line rings again. Without disconnecting, the originating party then calls the third party, and ringing will occur on both lines. When ringing ceases, indicating that the third party has answered, the second party lifts his handset, and the connection is established. Ringing for the new PBX is usually supplied over feeder wires from the central office, but tones are generated at the PBX by a buzzing relay, as with the 75o PBX. Batteries for the PBX are located in a compartment in the base of the cabinet, and are ordinarily charged over cable pairs from the central office, but the charging is automatically controlled by relays at the PBX. Where conditions make it desirable a local ringing machine may be employed and a rectifier for charging the batteries. An Inductance and Capacitance Bridge By S. J. ZAMMATARO Transmission Apparatus Development BRIDGE designed for miscel- but are more stable with frequency, laneous uses in the laboratory and more compact. It was decided should be capable of measuring also to use a shunt arrangement of both inductive and capacitive im- the standard capacitance and resistpedances. It should be able, moreover, ance because such an arrangement to measure both types of impedances with equal convenience and accuracy whether their Q values are high or low. Such a bridge has recently been developed for the frequency range from 3o to io,000 cycles to be used in measuring impedances of lines and apparatus for program transmission. Before the detailed design of the bridge was begun, it was decided to employ capacitance standards for measuring the inductive as well as capacitive impedances, because condenser standards are not only less expensive than inductance standards, Tune 1938 lends itself to simpler shielding. With the choice of the type and arrangement of the working standards thus settled, it was apparent that the most direct manner of measuring capacitive impedance was by the familiar comparison bridge with equal ratio arms. This arrangement is shown schematically in Figure i. As is customary in such bridges, a resistance R3 in the diagram used to shunt any high unknown resistance components of the unknown capacitance so that the resistance of the arm CD can be measured by the resistance -is - 341 standard Rs. Such a bridge determines standard must be converted into the unknown impedance in terms of its parallel components, which is preferred for capacitive impedances. equivalent inductances by computations. Finally, the determination yields the parallel components of the unknown, while for inductive impedances the series components in general are of major interest. If in the original arrangement of Figure I, however, a switch were used to interchange the AB and AD arms, and to change the position of R3, the comparison circuit would be changed to a Maxwell circuit, as shown in Figure 3. This circuit offers the same convenience and simplicity of operation in measuring inductive impedances that the comparison circuit does for capacitive impedances. When the bridge is balanced, the relations are such that the series components of an inductive impedance in the arm CD are proportional to the parallel components of a capacitive impedance in the AB arm. In other words, a positive impedance is related to a positive admittance. The proportionality factor is the product of RI and R2, for which reason the bridge is classed as the product -arm type in contrast to Fig. 1-Schematic of parallel comparison bridge with equal ratio arms The simplest change required to connect such a bridge so that it could measure inductive impedances would be to provide a switch for transferring the condenser standard from the AD to the CD arm of the bridge. This would give the parallel resonance arrangement shown schematically in Figure 2. Such a method of measuring inductive impedance, however, is not so direct and convenient as is the previous arrangement for measuring capacitive impedance. In the first place, frequency enters into the B determination, with the result that it must be more accu- Lx (PARALLEL)= w-¿CS rately known and controlled than would otherwise be necessary. Also, the readings of the condenser 342 Rx(PARALLEL)=\RS K= Fig. R3/ R1 2 2- Schematic of parallel resonance bridge Tune 1938 the ratio -arm type, where the proportionality factor is the ratio of the arms R1 and R2. It is apparent that the product R1R2 may be made to give a simple relationship between the inductance and capacitance units, so that the bridge becomes essentially direct reading. Thus by making the product DETECTOR equal to Io', one microhenry of the unknown inductance corresponds to one micro microfarad of the standard capacitance. Fig. 4- Complete shielding arrangement for the comparison The resistance compobridge used to measure inductive impedances nent of the unknown, as already stated, is given in terms of to that of R, in the comparison cirthe conductance of the AB arm. The cuit, where its function is to keep the resistance R4 is inserted in series with conductance in the CD arm above a the unknown to keep the total re- definite minimum. By use of these sistance of the CD arm above a given two arrangements of the bridge arms, minimum that can be measured therefore, the same testing procedure within the range of the standard R,. may be followed whether the unknown This use of resistance R4 is analogous impedance is inductive or capacitive, since direct reading is provided for the reactance component, and substitution for the resistance component. Although the product -arm bridge reduces inductance measurements to a simplicity on a par with capacitance R5 measurements, it results in a more difficult design. The symmetry of the network of a comparison bridge permits simple disposition of shielding; RQ and the compensation of the residual impedance or admittance of corresponding elements can easily be Lx=KCs preserved over a wide range of condiRx = - R4 tions. In addition, the equality of the Rs WHERE K= RiR2 ratio arms, both in magnitude and D Fig. 3- Schematic arrangement of Maxwell phase, is relatively easy to secure and maintain. The product -arm bridge, on or product -arm type of bridge June 1938 343 the other hand, imposes a more elaboThe elements designated by lowerrate shielding scheme; and the prod- case letters are adjustments incorpouct adjustment of the fixed arms re- rated after the bridge is assembled, quires a more painstaking technique. and have required more than the The complete shielding arrange- usual share of attention because of ments for the two types of circuit are the dual function of the bridge. shown in Figures 4 and 5. Two With the comparison bridge, for exdouble- shielded transformers are used ample, there is no important restricin tandem across the supply corners tion of the absolute values of the fixed arms R1 and R2. The main adjustment I requirement is that their difference in magnitude and phase be zero. With the prod uct -arm bridge, on the other hand, the prod uct of these arms must be a specified value, c and the sum of their phase angles must be R4 zero. To satisfy both sets of requirements, R1 and R2 must be closely adjusted, and the residual capacitance and inductance of each -including the portion contributed by Fig. 5- Complete shielding for the product-arm bridge the shielding -must be compensated to give of the product -arm circuit instead of zero phase angle in the separate arms. a single transformer with triple shield- It is important, moreover, to miniing, to secure a more economical con- mize the residual reactance that needs struction. In designing a composite to be compensated so that the effecbridge that could function as either tive resistance of the arms will retype, it was necessary to strike a main sufficiently constant over the judicious compromise between the given frequency range. theoretically complete shielding The phase angle compensation of shown in Figures 4 and 5 and the less the arms R1 and R2 is indicated in perfect shielding that would permit a Figure 6 by the inductances 1, and ready interchange of connections and 12. Inductance is compensates for the still yield the desired accuracy. The residual capacitance across the CD modified shielding finally adopted for arm of the product -arm circuit, but it the composite bridge is shown in is switched out of the circuit for the Figure 6 which corresponds to the ar- comparison connection. Instead, the rangement of the circuit with the key CD arm residual capacitance is padded thrown for inductance measurements. by c3 to balance the standard arm AD I I ' I 344 June 1938 Fig. 7 -The capacitance standard comprises three units when the condenser standard is set on zero. With the product -arm circuit, on the other hand, the zero residual capacitance of the standard cs is compensated by inductance 14 so that the range of inductance will extend down to zero. The key K3 serves to discon- s 6 nect the D corner of the bridge from ground when the ground -potential condition is to be controlled by the unknown. Keys Ki and K2, operated together, effect the interchange of circuits with no other manipulation. The various adjustments have been made with sufficient accuracy to insure satisfactory performance over the frequency OSCIL- range from 30 to Io,000 cycles. The values of the fixed arms Ri and 7/2 ohms R2 are made io each, so that with the ATOR product -arm circuit, DETECTOR one micro -microfarad 14 R4 in the capacitance standard cs measures an inductance of ten microhenries in the arm CD. Since the standard has a total range of ten microfarads, inducL Fig. 6- Shielding adopted for new composite circuit when arranged as a product -arm bridge 7nne 1938 tances up to one hundred henries can be measured. The range for capacitance meas345 urements, of course, is up to ten microfarads -the range of the capacitance standard. These reactance ranges can be measured for all values of Q normally encountered. In the model constructed, the adjustable capacitance and resistance standards to be available for other uses. The condenser standard, shown in Figure 7, consists of three shielded units: an air condenser, a threedecade box of mica condensers, and a single- decade box of paper condensers, which is used for the largest steps. The resistance standard is a six decade box adjustable up to 10,000 ohms in o.oi-ohm steps. The bridge unit proper is assembled on an aluminum panel and covered with an aluminum box, which serves as an overall ground shield. The appearance of the panel from the rear with cover removed is shown in Figure 8. This new bridge will Fig. 8 -Rear view of bridge with cover removed find wide application in the laboratory for misstandards are made up as separate cellaneous impedance testing. It will units which may be readily connected prove especially useful in making freto the rest of the bridge circuit by quency measurements of the impedcoaxial plugs and jacks. This ar- ance of networks in which the sign of rangement increases the portability the impedance may change many of the equipment, and allows the times over the test band. The Morris Liebmann Memorial Prize has been awarded by the Institute of Radio Engineers to Dr. G. C. Southworth of the Radio Research Department `for his theoretical and experimental investigations of the propagation of ultra-high-frequency waves through confined dielectric channels and the development of a technique for the generation and measurement of such waves." Making Broadcast Synchronization Easy By J. C. BAYLES Broadcast Development Department N recent years there has been some interest in operating a number of radio transmitters in "synchronism" when they are all broadcasting the same program. The usual objective of such operation is to make it practicable to permit the full -time operation of stations otherwise restricted to part time, or to permit the operation of additional transmitters on frequencies already assigned, so as to increase program coverage. The technical difficulty of frequency control is only one of a number of factors which must be adequately taken into account, but this element in the overall problem is met by the synchronizing apparatus developed by the Laboratories. This apparatus can readily be applied to almost any type or size of radio transmitter. "Synchronization" as applied to broadcast stations does not mean that the transmitters involved are operating at exactly the same carrier frequency, but that the variation from exact synchronization is very small. For satisfactory operation it has June 1938 been found desirable to maintain the frequency difference between two stations of a system to within less than 0.1 cycle per sec. In developing the Western Electric 28oA synchronizing panel, the Laboratories have met this requirement, and have produced a compact bay of equipment, completely a -c operated and self-contained. The method of synchronization used in this equipment is that of continuously correcting the frequency of all transmitters so as to maintain a 347 selected relation to a basic reference frequency transmitted to all stations from a very stable common source. Among the advantages of this system is that in case of an interruption of this reference frequency there is no interruption of the program, since the oscillators of Western Electric transmitters will maintain adequate synchronism for short periods without correction being necessary. The method of synchronization may be followed in outline from the block schematic of Figure I. In brief, the scheme is to derive from the reference frequency, usually 4 kc, a harmonic that is 5o kc higher or lower than the station carrier frequency. This frequency and a portion of the station carrier are passed through a modulator, at the output of which will appear a frequency of 5o kc plus or minus the deviation of the carrier from its assigned value. If this deviation of the carrier is called 0, the output of the modulator is thus (5o ±0) kc. A 5o -kc current, also derived from the reference frequency, is passed through a phase- shifting network providing two outputs -one in phase with the reference frequency and one shifted 90 degrees from it. These three frequencies -the 5o kc "in phase," the 5o kc at an angle of 90 degrees, and the (5o ±0) kc-are then fed to a second modulator, which provides a two -phase current at frequency A. This current supplies power to a polyphase control unit which operates a shunting condenser for the transmitter oscillator. The direction MODULATOR NO.2 CONTROL UNIT 50 KILOCYCLES 90- DEGREE ANGLE +B 50 KILOCYCLES 0- DEGREE ANGLE 2- Fig. Simplified schematic of modulator No. 2, which produces a two-phase current offrequency 0 of rotation of the control unit is such as to decrease the frequency of the oscillator when it is too high, and to increase it when it is too low. In greater detail, it will be noted from the diagram that the 4-kc reference frequency is first passed through a selective amplifier, and then through a ferro- magnetic frequency III I VARISTOR RECTI FIER ¡'' I SELECTOR 5OKC MODULATOR 500 -1500 N0.1 KC _I AMPLIFIER F*A F ±OKC RASTN MITTER _1 f50KCtb. 4 KÇr AMPLIFIER 4KC 4 KC FREQUENCY CONVERTER 10 KC HARMONIC PRODUCER KC (2IOn-I) - AMPLIFIER 50 KC 50 KC Ftpl AMPLIFIER TRANSMITTER 50 KC OSCILLATOR SOKC L9 PHASESHIFTING NETWORK 50 KC MODULATOR SO KC Le_ to NO.2 s CONTROL UNIT Fig. i -Block schematic of the 28oí1 synchronizing system 348 7une 1935 converter that provides a io-kc frequency. This is then amplified and passed through a ferro- magnetic harmonic producer that gives all odd harmonics of io kc. Two branches lead off from the harmonic producer: one going to a 5o-kc amplifier, which selects and amplifies a 5o-kc voltage for supplying the phase-shifting network, and the other passing through the selector that picks the harmonic 5o kc above or below the assigned carrier frequency. When the carrier is an odd harmonic of io kc, a varistorused as a rectifier, and shown dotted on the diagram -is inserted ahead of the selector to provide the even harmonics by a process involving frequency- doubling. The output of the selector is thus always 5o kc above or below the assigned carrier, and it is supplied to modulator No. i together with an amplified portion of the actual carrier. The output of this modulator isas already noted a frequency of 5o kc ±0, and after amplification is -at supplied to modulator No. 2 along with the two 5o -kc components from the phase- shifting network. A simplified schematic of this modulator, which produces the two -phase current of frequency A, is shown in Figure 2. This circuit balances out the 5o kc, and only the component at frequency A remains, which because of the two circuits that are supplied by the quadrature components, appears as a two -phase current. Equipment for the 280A synchronizing panel is divided into three units: the harmonic producer unit, which includes the reference -frequency amplifier, the frequency converter, and the harmonic producer; the selector modulator unit, which includes the selector, the phase-shifting network, and both of the modulators; and a power- supply unit. The two -phase control unit for adjusting the oscillator of the transmitter is located in the transmitter. Power -about Zoo watts -is drawn entirely from the a -c mains at 115 volts and 6o cycles. Short- Circuiting Relay Protectors By D. T. OSGOOD Protection Development Department BELL System circuits are fre- effective protection, but if they are quently of open -wire construc- called upon to carry heavy current for tion, and extraneous voltages more than a very short time, they may are occasionally impressed upon them permanently ground the conductors by lightning, by contact with power with which they are associated. Perwires, or by induction from paralleling manent grounds are much more power lines during times of ground likely to occur from high -voltage faults on the power system. To limit power surges than from the very short these voltages to safe values, so as to lived surges of lightning. In either protect telephone equipment and to case, however, a service interruption guard against other hazards, carbon will result, even in central offices block protectors are connected be- where personnel are available to retween the telephone conductors and place the grounded protectors. Where ground. These protectors ground the protectors are installed either on a conductors with extreme rapidity line remote from a central office, or in whenever excessive voltages appear. small offices where plant forces are not Such carbon blocks provide very in continuous attendance, the loss in 350 June 1938 circuit time from permanent grounding may become excessive. To minimize this difficulty, relays were designed which would operate promptly to short -circuit the protector for the duration of the disturbance. With such an arrangement, the protectors would discharge the moment the voltage rose above the critical value, and the immediate action of the short-circuiting relays would prevent the protector blocks from fusing together and, as a result, creating a permanent ground. One form of this arrangement, which is known as the multi -grounding relay protector, is described in an accompanying article. This was developed for installation in central offices or in the outside plant on lines where excessive voltages to ground from induction might occur frequently. A telephone line equipped with these Fig. 2- Test set for adjusting short-circuiting relay protectors protectors is shown in the photograph at the head of this article. Another form is called the unit - This device consists of a relay, shown type relay protector. It is designed for in Figure i, which carries the protectwo wires; and is applicable where tor blocks at one end. After one of the only a few circuits are to be protected, blocks breaks down, the discharge or where it is not desired to ground all current passes to ground through the wires'at a given point upon the opera- winding of the relay. The armature tion of one or more carbon protectors. operates; and the contacts directly short -circuit the blocks. This relay is rugged in construction and its contacts are designed to withstand at least 150 amperes without fusing. The magnetic circuit of the relay saturates at a relatively small value of current, so that the voltage across the relay will remain low even for high discharge currents. To prevent the relay from being held up by direct current from a grounde