Reference / Paper · 1949
Bell Laboratories Record, March 1949 (No. 5 Crossbar)
Introduces the No. 5 crossbar system (context for what replaced SxS).
Reference / Paper · 1949
Introduces the No. 5 crossbar system (context for what replaced SxS).
R E C O R D BELL LABORATORIES VOL. XXVII NO. 3 MARCH 1949 Io. 5 R. C. DAVIS Switching Development The post -war crossbar One of the important post -war switching projects of the Laboratories has been the development of the No. 5 crossbar system. This new switching system has a more extensive field of application than any previously developed. It meets the requirements of areas on the outskirts of large cities where at the present time calls to the metropolitan offices are handled manually because suitable equipment to permit direct dialing has not heretofore been available. It is quite possible that the No. 5 crossbar system may ultimately be used in the large cities themselves, in which case it would take the place of the No. 1 crossbar system. The No. 5 system is capable of operating directly with all present local, tandem, and toll-switching systems. In addition, it is designed to serve as a tandem or toll-center switching office for moderate proportions of its traffic where this is advantageous. It is particularly well suited to the broad requirements of nation -wide operator toll dialing, and is arranged for automatic message accounting, which permits extended subscriber dialing of toll calls. In addition, there is sufficient flexibility in the system to enable it to be used in areas ranging from four -digit numbering plan operation to full eleven -digit operation. This permits it to be used in isolated communities where an office code is not required for local calls and yet to serve eleven -digit calls which require a three -digit national area code, a three -digit office code, and five digits for the numericals and station letter of the called number. The No. 5 system takes advantage of some ten years of operating experience with the original No. 1 crossbar system and of the advances in theory and techniques that inevitably follow continuous research in telephone switching. As a result, cold cathode tubes together with improved relay designs and more extensive use of contact protection are employed, and these are expected to contribute to longer life, low maintenance, and better performance of the equipment. These and other features permit No. 5 crossbar offices to be operated on an unattended basis to a greater degree than heretofore without affecting the quality of service given. Such occasional failures of the equipment which may occur are detected by continuous monitoring of calls and by other self- checking devices, and these failures are automatically recorded on trouble record cards. Many of its switching and apparatus units differ radically from those of the No. 1 sys- 85 March 1949 www.americanradiohistory.com tem, but of particular economic importance frames, has 160 outlets to which trunks and is the fact that the number of different types registers can be connected. When more than of switching frames and units used in the twenty line -link frames are required in an No. 5 system has been reduced. office, a supplementary bay is added to each Since offices vary widely in size, one of trunk -link frame, thereby increasing the the objectives of the No. 5 system has been number of junctors for connecting to lineto secure a switching plan having the de- link frames from 200 to 400. In this case, sired flexibility which at the same time each junctor from a line-link frame connects would permit it to serve both small and to two trunk-link frames. This arrangement large offices economically. As a result, a is necessary to provide a minimum of ten new switching plan has been devised which junctors from each line -link frame to each permits the No. 5 system to handle effec- trunk -link frame so that satisfactory calltively offices varying in size from somewhat carrying capacity is insured. less than two thousand lines to more than One of the major cost items in any comten thousand lines. The switching plan mon- control system is the "sender," which used is shown in Figure 1. There are only in the No. 1 crossbar system records the two types of switching frames: the line -link number dialed by the subscriber and conand the trunk-link. This compares with trols the out -pulsing signals needed to comfour types of frames used in the No. 1 plete the connection automatically in the crossbar system: the line -link, district -link, office serving the called subscriber. Since office -link and incoming -link frames. Like the No. 5 crossbar system will be used to those of the No. 1 crossbar system, the No. a large extent in towns having only one 5 frames use a primary and secondary central office, most of the calls will be to switching network. All connections on other lines in the same office and thus do these frames are established by the "com- not require pulses to be sent to any other mon control" marker circuit. office. A simpler control unit called the The line -link frame of the No. 5 system subscriber register is therefore employed has half as many secondary switch outlets for recording the number dialed, and the as the No. 1 system, and yet it will serve marker is used to complete all calls termias much traffic as the equivalent frame of nating in the same office. Senders are rethe No. 1 system. This is made possible by quired only on outgoing calls to other ofthe new switching plan which uses a differ- fices, and are provided only in sufficient ent method of switching the calls. The quantity to handle this traffic. basic line -link frame of the No. 5 system When a subscriber places a call, a comhas a capacity of 290 lines, but by the use mon marker locates the calling line and of 100 and 200 -line supplementary bays, as connects it via the line -link and trunk -link many as 690 lines may be served by a single frames to an idle subscriber register, which line group as in the No. 1 system. A maxi- gives dial tone and records the number mum of forty line -link frame groups can be dialed. The marker establishes this conaccommodated by a No. 5 crossbar unit nection in less than half a second, and is which compares with an eighty line -link then dismissed to serve other calls. After frame capacity for the No. 1 crossbar unit. the register has recorded the call, it seizes Each line -link frame has one hundred out- a marker. Operating in the usual manner, lets known as junctors which are used the marker finds a suitable idle trunk, to connect the line -link frame to all causes the initial connection to the subtrunk-link frames -approximately an equal scriber register to be released, and estabnumber of junctors connecting to each lishes the connection from the calling line trunk-link frame. to the trunk, after which the marker is disThe trunk -link frame provides the means missed. This action also occurs within for connecting the line -link frames to all of about half a second. If the call is to another the outgoing and incoming trunks as well subscriber of the same office, an infra -office as to all of the subscriber dial registers. trunk will be selected. Such trunks have This frame, in addition to having a capacity two connections to the trunk -link frame of 200 junctors to all of the line -link one for connecting to the called subscriber - 86 Bell Laboratories Record www.americanradiohistory.com and the other to the calling subscriber. After the connection to the trunk is completed, ringing the called line is done by the trunk. If the call is to a subscriber in another office requiring pulses to establish the connection, an inter-office outgoing trunk is used. This is connected by a directaccess crossbar switch to a sender having the desired type of signaling. The marker imparts the necessary signaling information to the sender, and then releases. From this point on, the sender takes over control of the call. For incoming calls, the incoming trunk seizes a register which records the called number as a result of signals transmitted to it from the distant office. Immediately after, a marker is seized momentarily to establish the connection to the called line. Besides providing for the ordinary local, outgoing, and incoming calls, the No. 5 system is arranged to serve as a tandem office or a toll switching center to switch calls through it to other distant offices. Tandem and toll trunks have connections to both line -link and trunk-link frames. When a call on such a trunk is passing through the office, the connection on the line -link frame will be used, and the call will be carried through the office in the same general manner as for a local subscriber making an outgoing call. For a call served by a tandem or toll trunk that is to be completed to a subscriber in the No. 5 office, the trunk connection to the trunk-link frame will be used, and the call will be handled as for an incoming call. To give greater assurance that toll calls will be com- pleted without encountering busy paths through the line -link and trunk -link frames, each toll trunk connects to two line -link frames as well as to a trunk-link frame. The No. 5 system uses only one type of marker and only one trouble recorder instead of the originating and terminating markers and their respective trouble indi- LINE -LINK GROUP 290 TO 690 LINES MAIN DISTRIBUTING FRAME SUB LINE BASIC LINE - LINK FRAME SUPPLEMENTARY LINE - LINK BAYS 100 LINE 200 LINE TRUNK - LINK FRAME 180 TRUNKS JUNGTOR GROUPING FRAME INTRA OFFICE TRUNKS SWITCH BAYS: TRUNK JUNCTOR SWITCH BAYS: LINE JUNCTOR 0 oIO IO 100 -POINT 200 -POINT SWITCHES 3 -WIRE SWITCHES 3 -WIRE I I 10 10 l0 10 260 JUNC- 200-PT 3-WIRE 100 200 -POINT 200- POINT JUNC- 3- WIRE TORS 3 -WIRE SWITCHES SWS ^ 200-PT 6-WIRE SWITCHES SWS H - CONCENTRATING RELAYS -INCOMING TRUNKS IN- COMING OUTGOING TRUNKS TRUNKS OUT- GOING NUMBER GROUP CONNECTOR I PER 1000 DIRECTORY NUMBERS 1 SUBSCRIBER REGISTER Ft SENDER SENDER LINKS INCOMING REGISTER ti CONNECTORS LMARKER Fig. 1- Simplified diagram of a No. 5 crossbar central office showing only one each of the line -link and trunk-line frames and of the various units of common control equipment 87 March 1949 www.americanradiohistory.com cators of the No. 1 system. This is another example where fewer different types of equipment permit larger scale production of the type used. Another is the use of only one junctor- grouping frame instead of the three of the No. 1 system: district junctor, office junctor, and line junctor. Many other new and interesting features will be included in the No. 5 system. It may be arranged, for example, to automatically test the insulation resistance of lines during wet weather, and automatically record all lines having less than the critical insulation resistance. In addition, a line link group can serve as many as thirty different classes of subscribers. Complete flexibility in the assignment of ringing codes to directory numbers is available without restriction for all types of ringing including individual, two -party, four -party, eight, and ten -party lines. Automatic alternate routing up to three possible routes is provided. The fundamental signaling plan between No. 5 offices is multi- frequency pulsing using two out of five frequencies. This plan is self -checking and avoids wrong numbers due to signaling errors. The first installation of the No. 5 crossbar system is at Media, Pa., a suburb of Philadelphia, and was placed in service on July 11, 1948. This is a typical metropolitan fringe office of about 4,000 lines. It is equipped with automatic message accounting facilities, and will interconnect directly with step -by -step offices and with the No. 4 toll crossbar office* in Philadelphia. Through the latter office, it may also interconnect with panel and crossbar offices. *RECORD, November, 1943, page 101; and October, 1945, page 368. THE AUTHOR: R. C. DAVIS was associated with the Indiana Bell Telephone Company in their Plant and Engineering Departments from 1912 until 1921 when he came to these Laboratories. For the next four years he participated in the development of circuits for manual switchboards, and in 1925 became a supervisor of one of the manual circuit development groups. From 1927 to 1942 he had charge of various groups engaged in the development of circuits for panel and crossbar dial systems, and much of the early crossbar development was done under his supervision. During World War II he had charge of the group which developed the aircraft Crew Trainers for the Navy. In 1945 he was placed in charge of the group developing the No. 5 crossbar circuits, and in 1947 was appointed Switching Development Engineer in charge of switching apparatus studies, automatic message accounting center, and step -bystep circuit development. 88 Bell Laboratories Record www.americanradiohistory.com The Type -A Transistor R. M. RYDER, Electronic Apparatus Development When used as a solid state triode amplifier, the Transistor exists in several forms. One form may be thought of as a varistor of the point -contact type* with a second point placed very close by in order to control the reverse characteristic of the first point. Figure 1 is a diagrammatic illustration. One point, called the collector, is biased strongly in the low -conducting direction. The other point, called the emitter, is biased slightly in the highly conducting direction. The third electrode, called the base, is a large non -rectifying contact to the body of the semiconductor. By virtue of the interaction between the points, the Transistor has transfer properties that can be used for amplification and other effects commonly thought of as associated with electron tubes. Since the first announcement of the invention of the Transistor,f a form known as the Type A has been developed. Arranged in a small cylinder slightly under a half inch long and less than a quarter inch in diameter, it is shown with a 6AK5 tube above and in a cut -away view in Figure 2. The base is connected to the cylindrical shell, while the two points come out on leads. To differentiate these two leads, that from the collector is bent twenty degrees from alignment with the other as indicated in the upper part of the diagram. A convenient schematic symbol for use in wiring diagrams is shown at the right of the cross section. The base in this latter diagram is *RECORD, December, 1948, page 485. f RECORD. August, 1948, page 321. indicated by a horizontal line with a vertical connection to it from beneath, and the emitter and the collector by two short lines making contact with it at equal angles from above. The arrow indicates the emitter, and shows the direction of easy current flow. Although shown in Figure 2 with the base horizontal, the symbol may be used in any convenient orientation. In the application of Transistors, it is helpful to keep in mind the well-developed methods used for electron tubes. Two kinds of data are of interest in both cases. The static characteristics, detailed plots of electrode voltages and currents, are useful for large -signal applications which may involve the entire operating range of the device. On the other hand, for small -signal applications they are cumbersome. Instead, one uses small -signal parameters analogous to transconductance or plate resistance for low -level amplifier problems, such as those involving noise, feedback, and the onset of oscillations. Because of the inherent differences between electron tubes and Transistors, the methods of presenting the characteristics for the latter are not quite the same as those used for electron tubes. Tubes, for example, usually are operated with fixed bias voltages, and the static characteristics are plotted with voltage as the independent variable and current as the dependent variable. With Transistors, on the other hand, which may oscillate when measured in that way, it is more convenient to use constant current sources as power supplies and to 89 March 1949 www.americanradiohistory.com Fig. 1 -Two electrodes make point contacts to one surface of a germanium plate. A third electrode makes contact to the other side of the germanium give the static characteristics with current as the independent variable and voltage as the dependent variable. Electrode voltages are measured relative to the base and, in general, vary with the current in both the collector and the emitter. In analytic form, the static characteristics may be expressed as: VE = f1(IE, Ie) (1) Ve = f2(IE, Ic) where the subscripts E and c represent emitter and collector. It is convenient to express the static characteristics by means of four sets of curves, which show each of the voltages as a function of each of the currents, with the other currents held at a fixed value. Four such sets of static characteristics are plotted in Figure 3. Although the static characteristics give a complete picture of the low- frequency behavior of Transistors, for small-signal applications it is much easier to use the characteristics in differential form, or in other words, to find an equivalent circuit valid for small signals. With small letters to represent differential currents and voltages, these small -signal relations, obtained by differentiating the static equations, are: sf1 VE - ()If + ve = iEsf2 IC sf, Sicc (2) 8f., + ie SIe orE Since the static characteristics give the relationship of voltages to currents, their derivatives are impedances; that is, the differential coefficients in the equations for the small -signal characteristics are merely the open -circuit impedances of the Transistor. Thus equations (2) may be rewritten as: VE Ve SCHEMATIC DIAGRAM E - 11 IE+/112 Ie 21 if + (3) 2 Ie These equations are represented by the equivalent circuit (a) of Figure 4. Amplification may be possible if 321 is sufficiently greater thaw 312. The exact values of the impedances for a particular unit depend upon the d-c biasing currents, just as electron tube admittances depend on the biasing voltages. At low frequencies, where the impedances reduce to resistances, the symbol is used for them. Many other equivalent circuits may be set up. All give exactly the same results in circuit computations; the choice between them is a matter of convenience. The one having most use at the present time is that of (b) of Figure 4, having three resistors arranged in a T and only one voltage generator. It has been found that at low frequencies, the elements of this equivalent circuit are all positive resistances, and vary ,9 Fig. 2 -Two views of the Type-A Transistor. The schematic diagram at the right is used to represent the Transistor in circuit drawings 90 Bell Laboratories Record www.americanradiohistory.com One way to arrange a Transistor as an amplifier is to ground the base, use the emitter as input electrode, and take the output from the collector as indicated in Figure 6. Working from a 500 -ohm generator fairly slowly with d -c bias conditions. The variation with bias is illustrated for a particular unit in Figure 5. A preliminary data sheet for the Type -A Transistor is given in Table I. olir, 0.8 0.4 03 NE O 20 A 0 o. 9 o. I o. y ,n11111 if FEED BACK CHARACTERISTIC SLOPE =°I2 INPUT CHARACTERISTIC SLOPE =MN '1 fl -0.8 I I =O o , turn -10 - 20 r 'I,,_ 11111 lii HIE -40 FORWARD CHARACTERISTIC SLOPE =0221 OUTPUT CHARACTERISTIC SLOPE =M22 50 - 60 -0.5 ir 0 05 1.0 le IN MILLIAMPERES 1.5 2.0 -4 -3 -I -2 Ic IN MILLIAMPERES Fig. 3 -Four sets of static characteristics for the Type-A Transistor 91 March 1949 www.americanradiohistory.com 100 X1o4 TABLE I- PRELIMINARY DATA SHEET FOR A 80 60 50 TYPE-A TRANSISTOR 40 Maximum Ratings 30 Not to be exceeded in continuous operation. Voltages are measured relative to the base. Collector Voltage 70 V Collector Dissipation 0.2 watt r, 20 - rn, 10 8 Typical Operating Conditions Emitter Current 0.6 ma Emitter Voltage 0.7 V Collector Current -2 ma 6 5 JI fI o - 40 V Collector Voltage 3 2 Average Equivalent Circuit Parameters Emitter Resistance re Base Resistance rb Collector Resistance re Mutual Resistance ref 4 240 ohms 290 ohms 19,000 ohms 34,000 ohms 1.0 0.8 0.6 0.5 0.4 rb 0.3 re Grounded Base Operation 0.2 Class A, working from 500 -ohm generator into 20,000 -ohm load. Operating Power Gain 17 db approx. Power Output 5 mw approx. o 01 02 03 04 05 06 07 08 Ie IN MILLIAMPERES 0.9 1.0 II Fig. 5-Variations in parameters with bias. Ie= -1.5 ma LC (a) ___ VOLTAGE GENERATORS 2 L _ 121 if SIGNAL TRANSISTOR LOAD GENERATOR EQUIVALENT CIRCUIT fe Fig. 4 -Two equivalent circuits for a Transistor l E-- Fig. 6 -A Transistor as a grounded -base amplifier 92 Bell Laboratories Record www.americanradiohistory.com output impedance and with no change of signal polarity in transmission, is somewhat reminiscent of the grounded -grid electron tube amplifier. In such an analogy, the 280 ohms Input Impedance Z1, emitter becomes equivalent to the cathode, the collector to the plate, and the base to 9,400 ohms Output Impedance Z22 the grid. Although this comparison is not 17 db Operating Power Gain. exact, still it is suggestive and fruitful. On 5 milliwatts Power Output investigation of the analogs of the other 65 db Noise Figure* common triode tube connections: grounded The figure given for power output is the cathode and grounded plate (or cathode approximate limit of operating power, follower ), it turns out that broadly the which still gives good waveform as esti- analogy holds. The grounded emitter conmated from an oscilloscope. The noise fig- nection is similar to the grounded cathode ure given is the value per unit band width in having relatively high impedance input at 1,000 cycles, in db above thermal noise. and output, high gain, and a reversal of It varies approximately inversely with fre- polarity in transmission. The grounded -colquency. As for frequency response, in a lector connection acts like a cathode foIgrounded -base video amplifier, the unit has lower in having high input and low output a cutoff frequency of about 5 megacycles. resistance and in having no phase reversal The grounded -base Transistor amplifier, in transmission. This tube analogy, even with comparatively low input and high though inexact, serves as a guide by which the very extensive electron tube technology *For definition and discussion of Noise Figure, may be carried over to the various applicasee H. T. Friis, Proceedings of the I.R.E., 1944, tions of the Transistor. page 419. into a 20,000 -ohm load under Class A conditions, the typical Transistor of Table I would have the following properties: . . THE AUTHOR: ROBERT M. RYDEn graduated from Yale in 1937 with a B.S. degree in physics, followed three years later by a Ph.D. degree in physics from the same university. He joined the Laboratories in July, 1940, to work on microwave amplifier circuits, and during most of the war was a member of a group engaged in studying the signal -to -noise performance of radars. In 1945 he transferred to the Electronic Development Department to work on microwave oscillator and amplifier tubes for radar and radio relay applications. Last year he joined the group engaged in the development of Transistors. March 1949 93 www.americanradiohistory.com Transmission features of the V3 repeaters In view of an expected demand of considerably over 15,000 voice-frequency repeaters a year, it seemed worth while to take advantage of experience in producing miniature apparatus to develop a less expensive voice -frequency amplifier. With the development of the Vl repeater* some years ago, the size of these amplifiers was radically reduced, but the V3 repeater, now in production, has an amplifier only one -sixth the size of that of the Vl. Its appearance and many of its structural features have already been described in the RECOxn. f The circuit of the amplifier for the V3, shown in Figure 1, provides a gain, reaching a maximum of 36 db, that is substantially flat over the voice band as indicated in Figure 2. With the standard 130 volt plate battery, an output of 20 db above one milliwatt can be obtained before overloading takes place. This is sufficient power carrying ability for any application in the telephone plant. Feedback is applied from the output to the input from a winding on the output transformer, coil F in Figure 1, through a cathode resistor in much the same manner as in the Vl. The total amount of feedback is about 14 db, and is independent of the gain. This is 6 db more than that of the VI, and permits better stabilization of the gain against tube and battery variations. Since the V3 is to replace the V1, its characteristics had to be made essentially the same. The basic feedback provided in the V3 gives the nominal output impedance of 600 ohms of the Vl, but in the latter amplifier the output impedance rises at the lower frequencies. This characteristic has been essentially duplicated in the V3 by a *RECORD, September, 1941, pages 20 and 24; June, 1943, page 352. (RECORD, February, 1949, page 45. R. L. CASE Transmission Development resistor and capacitor network in the grid circuit evident in Figure 1. The nominal input impedance of 600 ohms is independent of feedback, and is made up of the input transformer with a 1,200-ohm resistor bridged across its primary winding and a gain control potentiometer bridged across its secondary. This gain potentiometer is logarithmically tapered and gives continuous control over a range of 40 db. With the OUT PI PIN JACK; *WINDING ON OUTPUT TRANSFORMER Fig. 1- Circuit of the amplifier of the VS repeater V1, on the other hand, the gain dial had a range of only a few db, and for greater adjustment, soldered taps had to be changed on the input transformer. The gain and impedance characteristics of the V3 are so close to those of the VI that the existing equalizers and line equipment can be used without change. A number of features have been included in the new amplifier to simplify its maintenance, and some of the new maintenance techniques and apparatus provided will be described in a forthcoming issue. With both the V1 and V3, cathode activity was determined by measuring the voltage across the cathode resistor. With the V1, however, 94 Bell Laboratories Record www.americanradiohistory.com the activity was determined by making two measurements with different heater currents. With the V3, on the other hand, a single reading is made when the amplifier is first installed, and the value is recorded on a small roughened area on the front of the amplifier. Subsequent determinations are then compared with this original one, and when the voltage has decreased a specified amount, the tube is replaced. A connection from the high-voltage end of this cathode resistor is brought out to a small pin -type jack on the front of the amplifier, and to make a measurement, the point of one of the meter leads is inserted in this jack, and the other lead to ground. Two additional pin-type jacks on the front of the amplifier connect to an auxiliary winding on the output transformer, and are used for monitoring. These monitor+I.0 1+0.5+0.5 V3 REPEATER (NORMAL)- 0 ,.-' I 0.5 . REPEATER (AVERAGE) VI - s= ',,, ' 1.0 j \ i 1.5 / / 2.5 -3. 100 200 4000 2000 1000 600 FREQUENCY IN CYCLES PER SECOND 300 Fig. 2- Gain -frequency characteristics 10,000 ing jacks may also be used for measuring the output level with a test set that forms part of the equipment for maintenance. The plug -in feature of the amplifier not only permits a defective amplifier to be replaced rapidly, but facilitates testing of JW o In U LU 1 z 2 -- LS) 3 - - -- VI REPEATER ? V3 REPEATER V1 REPEATER (POTENTIOMETER MAXIMUM) (POTENTIOMETER ¢ MINIMUM I INII Min DB) Z 4 W CC LL LL ó -5 12 13 15 18 OUTPUT IN 14 POWER 22 20 2 19 18 17 DECIBELS (ABOVE I MILLIWATT) Fig. 3 -Gain- output characteristics the amplifiers at a central location. When an amplifier is removed, moreover, the socket provides a convenient point of access for transmission tests on the incoming and outgoing circuits. With the use of the 408A miniature tube, the plate power drain is the same as that of the V1, but the heater drain is reduced to one-third. Although designed principally for voice -frequency repeaters, the V3 should be attractive for any use requiring voice frequency amplifiers such as for carrier terminals, for four-wire switching, and as blocking amplifiers for signaling circuits. R. L. CASE received the A.B. and B.S. degrees from Denison University in 1921, and joined the Laboratories in July of that year. In 1926 he received his M.A. degree from Columbia. During his early work at West Street, he engaged in the transmission design of two and fourwire telephone repeaters, echo suppressors, and the terminal equipment for the first transatlantic radio circuits. In 1929 he was placed in charge of a group responsible for the design of repeaters, and later of amplifiers and associated equipment for voice -frequency program broadcasting facilities. In World War H he was responsible for voice and carrier wire -line facilities for the Armed Forces. Since then he has been concerned with the transmission design of radio control terminals and voice frequency repeaters of which the V3 amplifier is a recent development. THE AUTHOR 95 March 1949 www.americanradiohistory.com 1 Testing repeaters with circulated pulses A. C. BECK Radio Research Television, facsimile, frequency modula- sion circuit under test is inserted in a loop tion and pulse modulation place new and including a delay line, an attenuator, and more stringent distortion requirements on an auxiliary amplifier indicated by the box communication systems. In general, dis- marked "gated amplifier." The test pulse tortion may arise both over the transmis- is introduced into the loop at the input to sion path itself and in the equipment at the the equipment under test, and a sample of repeaters and terminals. With radio relay the output is removed from the loop and systems, however, inherent distortion over connected to the viewing circuit. The major the transmission path is essentially zero. portion of the output is fed through the For such systems, therefore, the design delay line, attenuator, and gated amplifier effort is directed toward reducing the dis- back into the input. The delay in the loop tortion introduced by the amplifiers and circuit is made greater than the duration modulators. of the test pulse, and the gain around the In the repeaters designed for the radio - loop is made unity. Under these conditions, relay system between New York and Bos- a single pulse inserted in the lodp will cirton,* the distortion caused by one repeater culate around the loop indefinitely, and the is so small that it is difficult to measure. viewing oscilloscope will show a succession Many of these repeaters may be used in of pulses spaced along the sweep. Each tandem, however, and before completing successive pulse on the oscilloscope shows the design, it was necessary to know the original pulse after another trip around whether or not the distortion in a long the loop, and, therefore, another trip chain of repeaters would build up to ob- through the apparatus under test. jectionable values. To build a large number of repeaters and measure the cumulative distortion in a ATTENUATOR DELAY LINE single circuit would have been an expensive and awkward procedure. It was made unnecessary, however, by a suggestion of G. W. Gilman that the same signal be sucTRANSMISSION HGATED CIRCUIT AMPLIFIER r UNDER cessively passed through a single repeater TEST a great many times, and distortion measured after any desired number of such SIGNAL GATING VIEWING INSERTION BRANCH transits. This method was adopted and sucBRANCH BRANCH cessfully carried out in developing comt ponents for the Boston -New York radio Ii relay system, and its further application PULSE & seems promising. GATE The essential features of the circuit set GENERATOR VIEWING SWEEP EQUIPMENT up at Holmdel for measurements of this SYNCHRONIZATION type are shown in Figure 1. The transmisFig. 1- Simplified block diagram of circuit set up at *RECORD, December, 1947, page 437; March, Holmdel for measuring distortion by circulating pulses 1948, page 97; and May, 1948, page 193. 96 Bell Laboratories Record www.americanradiohistory.com Since the pulse of signal lasts only about one microsecond, it is necessary to send a succession of pulses that superimpose on the oscilloscope to secure a continuously visible trace. The spacing of the successive input pulses is such that each may travel around the loop more than a hundred times before the arrival of the next new pulse. At the time a new pulse is started in the loop, the remainder of the previous one must be entirely erased. This is accomplished by the gated amplifier, which is periodically blocked for a short period just before the new pulse is applied. Timing is under control of the pulse and gate generator which generates and times the signal pulses applied, generates a gating pulse that holds the gate open until just before a new pulse is applied, and generates another pulse to start the sweep of the oscilloscope. A more complete diagram of the circuit employed is shown in Figure 2. A relaxation oscillator operating at about 3,000 damped oscillations per second controls the sequence of operation. In the gate generator it starts a pulse of adjustable length that opens the gate long enough to permit the desired number of trips to be made by the test pulse; in the pulse generator it generates a one-microsecond pulse that amplitude- modulates the output of the signal oscillator to form a one-microsecond pulse of oscillator frequency for application to the test circuit; and in the sweep synchronizing-pulse generator it forms a pulse to start the sweep of the oscilloscope at the proper point. By adjustment of a delay circuit, the sweep may be made to start after any number of trips of the test pulse, and the speed of the sweep may be made such as to display only one or any number of the pulses, each of which represents the signal after a different number of trips. One of the obvious requirements of such a circuit is that the auxiliary elements -the delay line, the attenuator and the gated amplifier -must not introduce sufficient distortion to produce appreciable change in the circulating pulse. So far as the amplifier is concerned, this requirement can be met by sacrificing gain and by other refinements that, although giving an uneconomical amplifier for a commercial system, achieve the desired objective. Two types of delay line have been used: one is a coaxial line that is used for frequencies in the vicinity of 65 megacycles, and the other a waveguide line that is used for frequencies of the order of 4,000 megacycles. A distortionless line for a video band is very difficult and has not been built. In existing radio relay systems, transHATTENUATOR t DELAY LINE I RANSM ISS ION CIRCUIT GATED AMPLIFIER I UNDER TEST SIGNAL - GATING VIEWING BRANCH INSERTION BRANCH BRANCH GATE GENERATOR RELAXATION OSCILLATOR PULSE AMPL TUDE GENERATOR MODULATOR DETECTOR i' SIGNAL 1 OSCILLATOR SWEEP SYNCH PULSE GENERATOR - VARIABLE DELAY - SWEEP GENERATOR VIEWING EQUIPMENT Fig. 2 -Block diagram of circuit used for measuring distortion by circulating pulses mission is at a frequency in the neighborhood of 4,000 megacycles, but at each repeater station this frequency is demodulated to 65 megacycles for amplification and the greater part of the amplification is done at this intermediate frequency. For testing 4,000- megacycle amplifiers, the circuit shown in Figure 2 using the wave guide delay line is satisfactory. For testing I -F amplifiers, the same circuit is used with 65- megacycle components and the coaxial delay line. Figure 3 shows the appearance of a typical pulse after it has made 1, 3, 10, 30, and 80 trips around the loop. This illustration also shows the original pulse, while with the circuit of Figure 2 the pulse could be shown only after at least one trip through the amplifier under test. In this 97 March 1949 www.americanradiohistory.com diagram, the signal is inserted at a low level point in the loop, so that relatively small pulse voltage is required. The viewing branch is shown connected to a high level point, so that less amplification is necessary to get a satisfactory oscilloscope deflection. If enough high -quality amplification is provided in the viewing branch, both branches can be connected to the loop at the same point, and the test pulse can be monitored directly. Only the I -F amplifier was under test when the oscillograms of Figure 3 were made, but the complete repeater may be tested with the same circuit. Instead of merely inserting the 65-megacycle amplifier in the test circuit, this amplifier, the 65 to 4,000-megacycle modulator, the 4,000 megacycle amplifier, and the 4,000 to 65megacycle demodulator may all be connected in tandem and tested as a unit. The same 65- megacycle signal pulse could then be used, but it would undergo distortion in all four units. The waveguide delay line of the coaxial delay line can be used for this. A similar method was suggested by W. M. Goodall to make the test circuit applicable to the video band. A modulator following the video equipment being tested would raise the band to 65 megacycles for passage to the delay line and the gated amplifier, while a demodulator following the gated amplifier would restore the signal to the video range. Tests of non -linearity or compression have been made by using a stepped test pulse having two or more levels, and observing the relative change in levels after a number of trips through the equipment under test. Observations of signal-to -noise ratios as SIGNAL PULSE ONE TRIP THREE TRiPS TEN TRIPS THIRTY TRIPS EIGHTY TRIPS Fig. 3-Appearance of signal pulse as originally sent and after 1, 3, 10, 30 and 80 trips the noise builds up after many trips have been made by operating at levels where the noise can be observed on the viewing scope. This system is used for FM testing by shifting the frequency to a different value, determined by the desired deviation ratio, and back again during the test pulse. This signal then circulates around the loop the desired number of times. An FM detector is used in the viewing branch to determine the response of the circuit under test. THE AUTHOR: A. C. BECK, after receiving the E.E. degree from Rensselaer Polytechnic Institute in 1927, remained there as instructor in mathematics for the following winter. At the end of the term in 1928, he joined the technical staff of these Laboratories. With the Radio Research Department he has since been engaged in the development and design of short-wave and microwave antennas. During the war he was chiefly concerned with radar antennas and the waveguide structures and fittings associated with them. At the present time he is working on microwave antennas. 98 Bell Laboratories Record www.americanradiohistory.com r' BOSTON BUFFALO MILWAUKEE DETROIT p CHICAGO SOUTH BEND TOLEDO 11 NEW YORK CLEVELAND "'TT'. 1 1 PITTSBURGH PHILADELPHIA 1 DANVILLE /I _ 1 TERRE HAUTE . li t -, e LOUIS I 1 .-- I -. i/ / i l .`` // BALTIMORE i `se' I / WASHINGTON / / I ST. / -Z I // RICHMOND TELEVISION ROUTES . : 1,a / COAXIAL RADIO RELAY JAN.12,1949 H. A. LEWIS Transmission Development Operating control of television networks To meet the rapid expansion of commercial television broadcasting since the war, the Bell System has been called upon to provide many new transmission facilities to carry video program material. Because of the wide frequency bands employed for television, and of the need for associating audio with the video circuits, m