US2462896A

US2462896A - Line finder synchronizer

Info

Publication number: US2462896A
Application number: US628610A
Authority: US
Inventors: David H Ransom
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC; Federal Telephone and Radio Corp
Priority date: 1945-11-14
Filing date: 1945-11-14
Publication date: 1949-03-01
Anticipated expiration: 1966-03-01
Also published as: CH285634A; GB650764A

Description

March 1, 1949. D. H. RANSOM 2,462,896

LINE FINDER SYNCHRONIZER Filed Nov. 14, 1945 7 Sheets-Sheet 1 FIG. I.

COMMON EQUIPMENT TO OTHER LINKS DIAL PULSE 24: 1

SECOND LINK CIRCUIT LIN E RE S RCUIT RCUIT 'garras/ne;

IT- i March 1, 1949. D, H, RANSOM 2,499,999

LINE FINDER SYNCHRONI ZER Filed Nov. 14, 1945 7 sheets-sheet 2 MASTER OSCILLATO BUSY PULSE I SHAFER FREQUENCY DIVIDER IO KC BY y March 1, i949. D. H. RANSOM LINE FINDER SYNCHRONIZER 7 Sheets-Sheet 5 Filed Nov. 14, 1945 March l, 17949. Q H, RANSOM 2,462,896

LINE FNDER SYNCHRONIZER Filed Nov. 14, 1945 7 sheets-:sheet 5 i IN VEN TOR.

LAST REGISTER 1 15V ATTORNEY March l, 1949.

Filed NOV. 14, 1945 D. H. RANSOM LINE FINDER SYNGHRONIZER /37 /36 V34 L38 3M I 146 l l I f I I I lI I 45 I "LE I 1 I 3A l PULSE -r- L I- I l sIIAPING I AMPLIFIER I I i I PuL sa I I Aim T I I IER I 47 IB- sYN. courrgql I| 333 l I I I j B+ I /a I I I GAIN I Pl I coN-rRoL I5I I I I I T Low PASS I I FILTER a I molo AMPLIFI Y I I58 I I I -E- I/MQI l Ll-II l I L f T I IHM-- l sI-:coNol B- COUNTER I FIG@ LAST GATEH:` 3B LAST COUNT I I I IN VEN TOR. D/I l//D H. PAA/60M March 1, 1949. v H RANSOM LINE FINDER SYNCHRONIZER 7 Sheets-Sheet '7 Filed Nov. 14, 1945 IN V EN TOR.

LAST LINK |NTERMED- IATE LINK Patented Mar. l, 19d-9 LINE FINDER SYN CHRONIZER Application November 14, 1945, Serial No. 628,610

(Cl. l79--18) 10 Claims. l

This invention relates to synchronizing circuits and more particularly to the circuits especially useful in synchronizing line finders.

In a copending applic-irtion of E. M. Delcraine (5l), Serial No. 628,613, filed Nov. 14, 1945, a telephone system is disclosed wherein the tele-- phone lines are all brought into common electronic distributor. Villen any line initiates a call, voltages are established on the corresponding distributor electrode which voltages are transferred through to a plurality of link circuits each provided with line finders. One of the line finders operates to make connection with the calling line and transmit the signals on to line selectors which in turn select the called line so that the communication is completed. In the system as disclosed in the above-identified copending application, the line finders are each provided with an oscillator which may operate at a frequency slightly lower than the rotary or repetition frequency of the distributor. The calling line provides series of pulses which, after processing, may be of constant amplitude. These pulses together with pulses derived from the oscillating circuit are applied to a gate tube which will pass energy only When the pulse from the oscillator and the incoming calling pulses coincide. These passed pulses then serve to synchronize the oscillator with the rotary distributor so that this channel is continuously selected to the exclusion of any other calling line.

According to my invention, l provide means whereby the local osci ator in the line finder circuit operates at a multiple frequency of the distributor rotation. Also, the distributor preferably is supplied with its driving voltage from a source which is a multiple frequency of the distributor frequency and is frequency divided for the purpose of securing more stable operation. The incoming signal then serves to synchronize the local oscillator at the multiple fre-- quency instead of at the actual distributor frequency. By this arrangement, an improved synchronization can be obtained since even if the ,local oscillator is a part of a cycle out of step with the multiple frequency source, it will only throw off the timing of this synchronizing circuit by a small fraction of the total distributor cycle. This local oscillator frequency is divided so that it will produce control pulses for the line finder gate slightly lower freq v icy than the distributor operation until such as incoming signals are received and synchronizing is effected.

As an alternative feature, in accordance with my invention, the high frequency source which serves to control thevdistributor may be used directly to synchronize the local oscillator upon receipt of a called signal. In this forrn, ythe incoming pulses serve to operate a second gate circuit which in turn Will permit application of the master oscillator voltage to the local-,os-

illa`tor of the link circuit for the purpose of synchronization. v

It is an objectJ of my invention to provide a multiple frequency oscillator together with a frequency divider as a lovver frequency sourcaand means for synchronizing thernultiple' frequency oscillator to control said lower frequency source.

It is anotherobject of my invention to `provide a high frequency master source together with a local source operating-at a slightly different frequency together with frequency dividers for energy from both of the sources andmeans responsive to signals for synchronizing the master oscillator and the local oscillator to bring? the divided frequency of the two sources'into' Syn'- chronism. l

It is a further object of my invention to provide a synchronizing system for a line :under of the type in which the line nder is synchronized with the frequency of incoming pulses of a' channel by synchronizing' a sourceof'energy at a substantially multiple` frequency of the pulse repetition frequency and dividing' vthis frequency for synchronizing purposes.

It is a still further object of my invention', in the line finder of the-type generally set fortli in the objects as above, to provide a means for syncl'ironizing the local oscillator with the' master oscillator by means of energy 'directly therefrom, the application of which is controlled in accordance with the received signals of said channel.

In a system incorporating the features of my invention, the signal or speech currents' in the various lines or other channels` may be replaced at the exchange by a series of narrovv'pulses of amplitude correspondingto the amplitude of the original-current at the corresponding tinie. rihe pulses are produced at sufficient rapidity so that they denne substantially the signal envelope. In this manner by allotting'diferent timepositicns to each line, the signal or voice currents' Within the exchange may be distributed over acoinmon channel each signal being repeated by a series of pulses displaced in time in accordance with the distributor time position.` This distribution may be readily accomplished by means of a cathode ray tube serving as a distributor which Will sequentially scan the lines connected to' predetermined terminals and respond if there is a signalling voltage on the line. rIhe channels may be separated by time selection and may be applied through time displacement means and a low-pass filter which serves to reproduce the audio envelope to the same or another distributor also coupled to the lines. The incoming signals may serve to adiust the time displacement means so that they will represent the time difference between the time position of the calling line and the selected called line. The time displacement means may be an actual delay line of some form or an equivalent circuit which, while not producing an actual delay of the signals, will effectively serve to store the energy and release it after a predetermined interval equal to the desired delay. In this manner, the interconnection of any one line with any other line of the system may be accomplished. Upon making this interconnection, the communication signals may pass through the same delay means between the interconnected lines. Furthermore, since the scanning cycle covers each of the lines connected to the distributor, as many simultaneous connections may be made as there are time displacement trunking channels within the exchange.

Preferably, means are provided responsive to the interconnection of the lines to tie up these lines so that they cannot be selected by another subscriber attempting to get the connection. If desired, any conventional type of busy signal may be applied to the subscribers line when this condition exists so that he will know that he must Wait an interval for the line to become free so that he can make the desired connection.

I provide synchronizing means for the line finder in which a frequency higher than the scanning frequency is generated and frequency division is provided to reduce it substantially to the scanning frequency. Thus, a more stable frequency with less phase displacement may be obtained as well as other advantages which are outlined below.

While I have broadly outlined certain objects and features of my invention, a better understanding of my invention and the objects and features thereof may be had from the particular description of an embodiment and certain modications thereof made with reference to the accompanying drawings, in which:

Fig. 1 is a block diagram illustrating the general circuit set up;

Figs. 2 and 3 are sectional circuit diagrams and views respectively, of a distributor tube used in my system;

Figs. 4 to 8 inclusive, constitute a circuit diagram of a link exchange in accordance with my invention;

Fig. 4 illustrating the common equipment,

Fig. 5 showing the pulse forming equipment,

Fig. 6 the line finder equipment,

Fig. 7 the dial register equipment, and

Fig. 8 the line selecting equipment;

Fig. 9 is a diagram illustrating how Figs. 4 to 8 inclusive, should be arranged to illustrate the complete circuit; and

Fig. 10 is an alternative form of circuit including both a common equipment and a line finder circuit which may be substituted as a whole for Figs. 4 and 6 in accordance with my invention.

In an example of a telephone exchange incorporating my system as outlined above, the system may be divided into three parts as shown in Fig. 1: rst, all the subscribers lines, twenty for example, assigned numerals l to 2li, each of these lines having a subscriber subset equipment such as 2l; second, the equipment common to al1 line circuits, hereafter referred to as common equipment 22; and third, a group of link circuits one of which is needed for each simultaneous call. Each of the link circuits may be further subdivided into line finder circuit 23, dial pulse forming circuit 24, dial register circuit 25 and line selecting circuit 2t. These several major components are interconnected by wires 21-38 inclusive, as shown in Fig. 1. For the sake of simplicity in the description only one-way conversation is illustrated.

As shown, all lines l to 2B terminate in common equipment 22. This equipment 22 performs a scanning function, preferably by means of a suitable tube having an electronic beam which sweeps each of the lines in turn.

When one of these lines has a potential indicative of a calling condition, the common equipment 22 applies signals over

wires

21 and 28 to all the link circuits in parallel and specifically to the line finder circuit 23 of the first link (chosen for discussion). This line finder 23 operates to find the calling line and transfer the signals over wire E3 to the dial pulse forming circuit 24.

When dialing ensues, this circuit 24 produces dial pulses which are counted and stored in dial register circuit 25. The dial pulse register 25 then serves to control the line selector circuit 25 which may comprise a delay line or other time displacement apparatus.

The incoming speech signals are then transferred from common equipment 22 over wire 28, line iinder circuit 23, wire 33, line selector circuit 210 and thence over wire 36 back to the common equipment 22, from whence they are applied to the selected outgoing line, The part of Fig. 1 comprising line lnder 23, dial pulse forming circuit 24, dial register 25 and line selector circuit 2@ may be considered together as a link circuit. For certain embodiments of the system, a synchronizing frequency may be fed from common equipment 22 over lead 29 to line selector circuit 26 and line finder circuit 23 respectively. The

five leads 21, 28, 29, Se and 31 from common equipment 22 may also be multiplied to other link circuits of the system as shown.

The distributor function of common equipment 22 may be performed by a rotating distributor in the form of a cathode ray tube as illustrated in detail in Figs. 2 and 3. The distributor tube is indicated generally at 39 and may comprise a cathode 4S, the usual grid 4I, focus and anode electrode 42, horizontal deflector plates 43 and vertical deflector plates 44. Two-phase distributor currents from a suitable sweep control may be applied over

leads

45, 4S, 41 and 48 to the horizontal and vertical delector plates respectively, so as to produce a cyclic rotation of the electron beam. At the target end of tube 39 are provided twenty coupling targets 49 to 68 respectively, which are coupled with the individual lines l to 2i) inclusive. These targets may comprise secondary electron emissive elements associated with a common anode 69 to provide dynodes all having a common output. A mask or screen 1t may be provided, if desired, having apertures therein so that the electron beam will impinge on each dynode only when the beam is aligned therewith thus preventing possible secondary emission from others. The output of the distributor tube 39 is connected from anode 69 over lead 1l, then signal isolating circuits hereafter described to

leads

21 and 28 which go to the line finder circuit as shown in Fig. 1. The

output from the line selecting circuit 26 may be applied as indicated over line S5 to the grid 4| serving to modulate the beam in accordance with the selected signal energy. Thus, referring to Fig. 1, the output from lead 'H may be applied after suitable delay (produced in line selecting equipment 25 as hereafter described) over lead 3d to grid lil to provide the desired communication channel between the chosen pair of lines.

The common equipment 22 is illustrated in Fig. 4. For illustrative purposes a base frequency of 10,000 cycles per second has been selected as the scanning rate of the rotating distributor. This frequency is sufficiently high to reproduce voice frequencies with adequate fidelity for transmission of speech. For the twenty-line system the base frequency is derived from a 200 kilocycle stable oscillator 'i2 preferably crystal controlled. This higher frequency is utilized since it is generally easier to build a more stable oscillator at the higher frequencies than at the lower 10,000 cycle frequency which is to be used. Further'- more, in certain of the modications illustrated, the 200 kilocycle wave may be utilized for other control purposes. The sinusoidal frequency generated in master oscillator l2 is reduced to the base frequency of ten kilocycles in frequency divider T3.

The output of frequency divider 'i3 is applied over 90 phase shifter 'i8 to the vertical and horizontal sets of deiiecting plates i3 and i4 of distributor tube 39 herein diagrammatically illustrated. This will serve to rotate the beam at a frequency of 10,000 revolutions per second so that each of the dynodes #39 to 58, illustrated in Figs. 2 and 3 and in this figure, will be scanned once every 10,000ths of a second. Incoming lines i, 5 and 29 are shown connected to the respective dynodes 4S, 53 and 68.

At 2| is illustrated a typical subscriber subset (shown connected to line 5) for use in the system according to my invention. Such a subset will be connected to each of the incoming lines I to 29 inclusive. The Voice transmitter 'l5 is connected in series with dial I5 and the normally open switch hook '11. The receiver 'E8 is bridged permanently across the line, since, for simplicity of illustration, no separate ringing equipment has been illustrated. Accordingly, the signal for summoning a called subscriber may be applied as a special tone which will be reproduced in receiver 'i8 to call the listener to the phone.

As in the usual equipment, switch hook ll is normally open. However, upon initiating a call, the switch becomes closed, completing a circuit in the calling line loop over low-pass filter i9 and the associated lines at the sub-set, applying a negative potential from battery 80 to the asso ciated dynode 53. Normally the dynode electrodes 49 to 88 are at the same potential as anode 69 so no current Hows. This negative potential will produce a difference in potential and cause secondary emission current to flow from the dynodes upon impingement of the beam of tube 39 thereon, producing a negative output pulse in output line ll. The pulses are preferably signal modulated to a depth of only 25 to 50 per cent so that there will always be sufficient amplitude to furnish energy to establish and maintain connections regardless of modulating signals. The negative pulses resulting from operation of the selected dynode 53 are fed to the grid of inverter tube 8l. The anode circuit of tube 8l is coupled to the grid of clipper tube 82 which serves to clip these pulses at a predetermined level to pass only the modulated portions of the incoming pulses. Thus, the output of this tube, representing the speech signals, may be substantially 100 per cent modulated. These clipped pulses are then applied to a cathode follower tube and from there to all of the link circuits over the cathode follower output lead 28. A second output is taken across the cathode resistance of inverter tube 8|, these pulses being applied to a clipper tube it@ which serves to clip the pulses to a constant level eliminating modulation effects therefrom. The anode circuit of tube 84 is coupled to the grid of a cathode follower tube 85 which serves to apply pulses 86 through common feed resistor 8l' over wire 2l to the grid of signal finder gate tube 88 (shown in Fig. 6) of line finder 23 (shown in Figs. 6 and 1) in the rst link circuit (now under consideration) and in parallel to the grids of the corresponding line iinder gate tubes in all other links. The pulse St after passing through resistor 8l may be called 89, so that the pulse actually arriving at the grid of tube 83 and of the other similar tubes is pulse 09. Under the conditions now assumed, when none of the grids of the line finder gate tubes is rawing grid current, pulse 89 is nearly as strong as pulse 86; but under other conditions it may be much weaker than 86. In the absence of any signals on the cathode of this signal finder gate tube 88, the above traced pulse 89 on its grid is insufficient to cause the flow of plate current, because the bias applied to the grid is suiiciently far below cutoff.

A; line finder circuit incorporating my invention is illustrated in Fig. 6. A lock-in oscillator 09 which incidentally performs a frequency division and is controlled through the medium of master oscillator l2 is provided. The lock-in oscillator 90 operates at a frequency slightly less than the two hundred kilocycles, its 50 kc. output being fed through a clipper differentiator circuit 0| to a kilocycle synchronized multivibrator 92. Before synchronization the 50 kc. and l0 kc. contents also will differ by corresponding intervals from these frequencies as in the oscillator 90.

The output of multivibrator 92 is applied through the differentiating network 93 and 9G to tube 95 which serves to form and amplify the pulses 96. Tube 95 is normally biased beyond cut-olf but the leading edge of each square wave output from multivibrator 92 is of sufficient strength to first drive the grid positive on a portion of the square wave. A negative pulse 96 of approximately ve microseconds is produced in the plate circuit. A cathode follower tube 9T passes the signal or control pulse 8e to the cathode of line finder tube $8. When the signal 89 on the grid of tube 88 coincides with the above-described selecting pulse 98, the tube S3 conducts and passes a pulse 99 to four places, namely to diodes |0| and |82 and over wire 32 to the line selecting circuit and to tube |09.

This pulse 99 is rectified in tube |02 and fed to an integrating network |03. The negative potential from the integrator is amplified in tube 0d reducing the potential in cathode resistor |05 which is common to tube |04 and tube |06. The reduction of this potential renders tube |84 conductive. Thus, this tube IM now commences to pass the sine Wave fram master oscillator 12, which is continuously applied to the grid thereof over line 29. This amplified wave is then passed through phase corrector circuit |01 serving to lock-in oscillator 9|! with the master oscillator 12. Accordingly, progression of selection is now stopped so that the pulses 89 will pass through tube 88 and over network Hi8 and tube H19 to open line finder gate tube II@ at the correct instants, thus causing the latter to pass the desired signal pulses from Wire 28 to Wire 33.

Simultaneously, the application of pulses 99 to vdiode lill charges. circuit lll and actuates tube I I2 which in turn actuates tube 95 to prevent enygagement of other line nders as is more fully Aset forth in the aforementioned copending application.

A combination of a pulse register circuit and associated line selection circuit is shown in Figs. 7 and 8 respectively. Also, a form of pulse forining circuit is illustrated in Fig. 5. The circuits of Figs. 4 3 arranged as shown in Fig. 9 illustrate a complete system incorporating my invention.

Turning first to Fig. 5, the incoming pulses I lilo on line 33 with their dialing breaks are passed through network U3, amplifier tube lll, transformer H5, tube lill, integrating netvvork ll'l and dial gate tube l it to an output line 35.

Dial gate tube il@ in Fig. 5 is normally biased to conduction by the voltage on its suppressor from the dial gate control lli-3, l2@ (with tube Il@ normally conducting). Pulses passed by transformer H5 are also applied over integrating network IEE, tubes |22 and |23, arranged in a time constant circuit, and over rectifier ld to the grid of tube i9. The time constant of the circuit is such that tube E23 remains operated for a period corresponding to the interval of the dial pulsing. Tube l lll cuts oi at the beginning of a series of dial pulses (sending out an ineffective positive pulse) and reoperates at the end, sending out a negative pulse to the dial gate control lid, |20. This negative pulse from tube E23 cuts off tube I I9 causing tube l2@ to conduct. This biases tube IIS to cut-oir" locking out the dial gate so that transients and voice modulations or even additional dialing will not disturb the registers (described later). At the same time, a control voltage is sent out over lead 38 biasing the control grid of the output gate tube |52 (Fig. 8) for conduction.

Turning now to Figs. 7 and 8, the output line 135 is coupled to a plurality of trigger circuits IE5, |26, |21 and |23. As many of these trigger circuits are provided as there are subscribers lines. Normally, the right hand tubes of each pair (except ior the preliminary one |25) are cut-01T, biasing the respective gate tubes i253, i3d and i3d to cut-01T on their associated suppressor grids. When a series of dial pulses has been received, it is intended that one of the right hand tubes of the trigger circuits will be left conducting, permitting its gate tube to pass the signal.

The operation of the register may be described as follows: The first trigger circuit |25 has no associated gate and tube |32 is normally conducting. A rst pulse over wire 35 from the pulse forming circuit cuts off tube l32 of trigger circuit IZ causing tube '533 to conduct. When tube |33 conducts, a negative pulse is sent from its plate circuit to the grid oi the normally conducting tube I3?.` of trigger circuit lr6 .cutting oli this tube and causing tube I 35 to conduct. This operation of tube |35 serves to open the rst gate tube |29` The second negative pulse from the pulse forming circuit cuts oir tube 235 transferring conduction to tube |34. A negative pulse from the plate of tube I 3d cuts off tube |35 restoring the rst gate to blocked condition which transfers conduction to tube H31 in the next ,register |21. The cut-01T of tube |34 restores the 8 rst gate tube |29 to blocked condition Whileth conduction of tube |31 unblocks the second gate tube |30. This cycle is repeated in succeeding registers until the last dial pulse has been sent.

Thus, at the end of dialing only the gate corresponding to the number of dial pulses is left unblocked. Assuming for convenience that line 3 is calling line tl, the second gate i3d Will be unblocked. As a consequence then, any pulses which may arrive on the grids of the gate tubes (e. g. over lines |38, H33, etc., described later) Will be passed only by gate i3@ to the common output Wire lflll as Will be described in more detail in connection With Fig. 8.

After the calling subscriber has hung up, the release circuit for restoring the registers |25-|28 to normal Will be operated. This circuit is controlled so that tube lill is normally cut-off. When the line nder 23 linds a line and locks-in, a signal is applied over line :il and tube 42 so that the tube lill conducts. The resulting negative pulse from tube Ml is passed on to tube |43 over condenser I lill but is not eiective since tube |43 is already cut oi. However, when the line finder releases, tube lfll is returned to its normal cut-oli condition, passing a positive pulse to tube |43. ll'his is amplied as a negative pulse in the plate circuit or" tube M3 serving to reset all of the register trigger circuits lZii-ll to normal.

The manner in which the speech signals are delivered to the selected called (e. g. line 5) will now be considered in detail in connection With Figs. 7 and 3. In this embodiment the speech signals from the calling line are stored and then later released. To determine the proper instant for releasing these stored signals, the equipment counts the pulses which niark the time channels intervening between the calling and called line channels.

in order that this counting may start at the instant corresponding to the calling lines time channel, a synchronizing pulse from the line finder gate tube over line 32 is passed through the shaping amplier M5 to the synchronizing counter Mld. Thereafter, pulses produced in shaping amplifier lli? under control of oscillator l2 carry forward the operation of the system through counters such as Idd, Iii@ and l5@ in a manner similar to that described for the register circuits ltd- Mit Only three counters are shown corresponding to the three described gates |29, 53|) and lill, and these counters and gates cooperate to control the instant of release of speech energy to the called line. As the counters operate successively, they apply potentials to Wires i33, |39, etc. But if only gate i3d is open, only the potential on Wire le@ will pass to common Wire |451, thus properly timing the release of speech signals to the called line as Will shortly appear.

The incoming energy from the line lnder selector gate tube 355 is applied by means of line 33 to a low-pass filter and audio amplifier Ml which serves as a storage means for the incoming speech signals. These speech signals from the output of .filter ibi are continuously applied to the control grid of gate tube it. However, no energy can pass until the completion of dialing when the cut-off bias is removed from control grid of gate tube M52 by the gate control Iig, lit and further until such time as the proper gate pulses are applied from Wire Edil to cut-ofi gate control tube |53 Which in turn removes the bias on the Suppresser grid of this tube i152. The latter bias will be removed at the proper time relation depending upon which one of the gate tubes |29, |30 is open 9 at that time. The gate tube |52 will, therefore, be opened at the proper instant in accordance with the incoming line signalso that the energy from storage lter |51 will be passed only at the proper time for application to the outgoing line over lead 36.

The foregoing description covers a complete system incorporating one form of my invention. However, an alternative structure for use in the system using my invention is illustrated in Fig. 10, which shows an alternative arrangement of line finder and common equipment. According to this arrangement, the same master oscillator l2, frequency divider 'I3 and phaser lll for controlling the sweep of the beam in tube 39 is used. A slightly modied form of coupling circuit for dividing the signal and synchronizing pulses is shown differing somewhat from that illustrated in Fig. 4. The output negative pulse from distributor 39 is fed over line 'H to an inverter |5|l and then into two cathode followers |55, |55. The tube |56 passes the speech signal to line 28 extending to all the links. This signal has not had its modulation depth increased since this function is performed in the link circuits in this form. The control signal is clipped to constant amplitude in a slightly different manner with a clipping circuit comprising duo diode |58' which limits the amplitude of the signal to the grid of the cathode follower |55. This cathode follower feeds i through a series resistance 8l to the grids of all the link circuit tubes 8S, as explained before.

These tubes are normally biased suilciently beyond cut-olf so ythat signals SQ alone on the input electrode will produce no change in the output and as before, coincidence with signals derived from the local oscillator is necessary to produce any response. Instead of providing a lock-in oscillator and clipper, I provide in this system a local oscillator |58 operating at two hundred hilocyeles -i-O-.1%. 89 from the tube |55 are applied to the grid of signal gate tube 88 while the output from oscillator |58 is applied through two frequency dividing multivibrators |59 and Hill to provide the desired pulses which operate through tubes 95 and 9T to apply a selecting signal 98 to the cathode of .this same tube Sli. The relationship between

pulses

89 and 98 will progress as previously described until such time as a selecting pulse 9|! on the cathode of tube 8S is applied simultaneously with a control pulse SS to the grid thereof. Thus, tube 88 passes a pulse S9 through to the grid of tube l! of a delay flip-flop circuit comprising tubes ||5| and |52 thus triggering this flip-flop circuit to its abnormal condition with tube |62 conducting, sending to shaper |53 an abrupt voltage rise. This delay flip-nop circuit has a period of action adjusted by the constants of the grid circuit of tube When it spontaneously returns to normal, the voltage to the shaper |53 drops back abruptly thus completing a long positive pulse to the Shaper. The pulse shaper serves to differentiate this pulse and suppress the leading portion, the ltrailing portion of which has a desired delay. This trailing portion is then amplified and applied to oscillator |58 to synchronize it with the master oscillator 72. The halting of the relative drift of these two oscillators stops the pulse progression of pulse 98 with respect to 39 and serves to lock the line finder to the selected line as previously described. Upon locking into step, the pulses 9S from tube 88 are rectied in the gain of tube 95 and hence the amplitude of The output pulses l@ the pulses Q5 and then llt which are applied to the cathode of tube d8. Because of the fact that a higher frequency is used for the local oscillator, a more stable operation and precise lock-in can be obtained.

The busy gate tubes it! and operate to impose upon the pulses 89 an upper limit somewhat lower than the limit imposed by clipper |51. This new limit being high to hold a previously engaged line finder but low enough to prevent engaging a new one. In performing this function, tubes ltd and |65 act in a manner similar to duo diode clipper lill. At the instant of arrival of a positive pulse from busy pulse shaper |66 upon the grid of tube EE/i it becomes highly conductive and thus acts as a diode to prevent Wire 2l from rising above the potential of its cathode. Tube leb acts as a reverse clipper to discharge the negative potential which would remain at the end of such pulses.

The modification shown in Fig. lo may be used in place of Figs. 4 and 6 in combination with the circuit arrangement disclosed in Figs. 5, 7 and 8. Furthermore, the synchronizing features shown in connection with line finders in the two modiiications illustrated may be used in other types of circuits than the complete telephone or exchange systems shown herein, as will be apparent to those skilled in the art. The use of multiple frequencies for this synchronizing purpose of the type described in particular constitutes one of the principal features of my invention. It should therefore be clearly understood that the specific description of the system and the modifications thereof given herein are merely by Way of example and are not to be considered as limitations on the scope of my invention as set forth in the objects thereof and in the accompanying claims.

I claim:

1. A selector circuit for selecting a pulse train, the pulses of which have a predetermined repetition frequency, comprising a source of energy of a frequency slightly displaced from a multiple frequency of said repetition frequency, means for deriving from said energy selector pulses having a repetition frequency slightly lower than said repetition frequency, receiver means for receiving said pulse train and said selector pulses, means in said receiver means for selecting pulses .of said pulse train when said pulses and said selector pulses coincide therein, and means responsive to said selected pulses for tuning said energy source substantially to said multiple frequency whereby said circuit will continue to select said pulse train.

2. A method of selecting a pulse train, the pulses of which have a predetermined repetition frequency, comprising providing energy at a frequency slightly displaced from a multiple frequency of said repetition frequency, deriving from said energy selector pulses having a repetition frequency slightly lower than said repetition frequency. selecting pulses of said pulse train when said pulses and said selector pulses coincide, tuning said energy source substantially to said multiple frequency in response to said selected pulses.

3. A selector system for selecting one channel of a plurality of channels of pulse trains of a predetermined pulse repetition frequency, comprising a local oscillator operating at a frequency slightly lower than a multiple frequency of said repetition frequency, frequency dividerv means for dividing the frequency from said -local oscillator to provide control pulses at a repetition -Y rate slightly lower than said predetermined repetition rate, gate circuit means normally biased beyond cutoff so that said pulses normally will not pass, means for applying said signal pulses and said control pulses to said gate circuit, means to condition said circuit to pass said channel pulses upon simultaneous application of the pulses thereto, Aand means responsive to said pulses passed by said gate circuit to bring said local oscillator to said multiple frequency whereby said signal pulses will be continuously selected.

4. In a selector system for selectingl one channel of a plurality of channels of pulse channels of a predetermined repetition frequency, the method comprising generating oscillations at a frequency slightly lower than a multiple frequency of said repetition frequency, dividing the frequency of said generated oscillations to provide control pulses at a repetition rate slightly lower than said predetermined repetition rate, mixing said signal pulses and said control pulses to pass said channel pulses upon simultaneous occurrence of said signal and control pulses, bringing said local oscillator t said multiple frequency in response to said passed pulses whereby said signal pulses will be continuously selected.

5. A selector system for selecting one channel wherein each of a plurality of `channels of pulse channels of a predetermined repetition fre.- quency, comprising a master oscillator operating at a multiple of said repetition frequency, a local oscillator operating at a frequency slightly lower than said multiple frequency, frequency divider means for dividing the frequency from said local oscillator to provide control pulses at a repou tition' rate slightly lower than said predetermined repetition rate, gate circuit means normally biased 'beyond cutoff` so that said pulses normally will not pass, means for applying said signal pulses and said control pulses to said gate cir cuit to condition said circuit to pass said channel pulses upon simultaneous application of the pulses thereto, and means responsive to said pulses passed by said gate circuit for passing energy from said master oscillator to said local oscillator to bring said local oscillator to said mul" tiple frequency whereby said signal pulses will be continuously selected.

6. In a communication system of the type wherein each of a plurality of channels is represented by interleaved signal pulses of a predetermined repetition rate; means for assuring selection of one channel Vto the exclusion of others of said channels comprising a selector circuit comprising a gate means normally conditioned so that said signal pulses normally will not pass, means for applying said signal pulses to said gate circuit', a local oscillator operating at a frequency slightly lower than a predetermined multiple of Ithe repetition rate, frequency divider means for dividing the frequency from said local oscillator to provide control pulses at a repetition rate slightly low-er than said predetermined repetition rate, means for applying s-aid control pulses to said gate means to condition said gate to select and passpulses, and means responsive to said pulses passed by said gate circuit to bring s'aid local oscillator to said predetermined multiple of the repetition' rate whereby said signal pulses will be continuously selected.

7. In a telephone exchange system of the type wherein each 'of a. plurality of lines is coupled to separa-te distributor terminals and means is provided to scan said terminals successively and cyclically at a predetermined frequency to provide signal pulses of a predetermined repetition rate in response to the initiation of a call on a line, means for assuring selection of a calling line to the exclusion of others of said lines comprising a source of energy at a multiple of said predetermined frequency, eans responsive to energy from said source to provide said predetermined frequency for said scanning, and a line finder circuit comprising a line finder gate tube, normally biased beyond cutoff so that said pulses normally will not pass, means for applying said signal pulses to said gate tube, a local source of energy normally at a frequency slightly lower than the frequency of said first-named source, means responsive to energy from said local source to provide control pulses at a repetition rate slightly lower than said predetermined repetition rate, means for applying said control pulses to said line nder gate tube to lower the bias thereon so that said signal pulses will be selected and passed upon simultaneous application Aof said signal and control pulses thereto, and means responsive to said pulses passed by said gate tube for applying energy from the first-named source to said local source to synchronize it with said first source whereby said signal pulses will be continuously selected,

8. In a telephone exchange system of the type wherein each of a plurality of lines is coupled to separate distributor terminals and means is provided to scan said terminals successively and cyclically .at a predetermined f `equency to provide signal pulses of a predetermined repetition rate in response to the initiation of a call -on a line, means for assuring selection of a calling line to the exclusion of others of said lines comprising a line finder circuit comprising a line finder gate tube normally biased beyond cutoff so that said pulses normally will not pass, means for applying said signal pulses to said gate tube, a local oscile lator operating at a frequency slightly lower than a multiple of the frequency of said repetition rate, frequency divider means for dividing the frequency from said local oscillator to provide control pulses at a repetition rate slightly lower than said predetermined repetition rate, means for ap plying said control pulses to said line lfinder gate tube to lower the bias thereon so that said signal pulses will be selected and passed upon simul taneous application of said signal and control pulses thereto, and means responsive to said pulses passed by said gate tube to bring said local oscillator to the multiple frequency whereby said signal pulses will be continuously selected.

9. In a telephone exchange system of the type wherein each of a plurality of lines is coupled to separate distributor terminals and means is provided to scan said terminals successively and cyclically at a predetermined frequency to provide signal pulses of a predetermined repetition rate in response to the initiation of a call on a line, means for assuring selection of a calling line to the exclusion of others of said lines comprising a master oscillator operating at a multiple of said predetermined frequency, frequency divider means for providing said predetermined frequency for said scanning, a line finder circuit comprising a line finder gate tube normally biased beyond cutoff so that said pulses normally will not pass, means for applying said signal pulses to said gate tube, a local oscillator operating at a frequency slightly lower than the frequency of said master oscillator, frequency divider means for dividing the frequency from said local oscillator to provide control pulses at a repetition rate slightly lower than said predetermined repetition rate, means for applying said control pulses to said line nder gate tube to lower the bias thereon so that said signal pulses will be selected and passed upon simultaneous application lof said signal and control pulses thereto, and means responsive to said pulses passed by said gate tube to synchronize said local oscillator with said master oscillator whereby said signal pulses will be continuously selected 10. In a telephone exchange system of the type wherein each of a plurality of lines is coupled to separate distributor terminals and means is provided to scan said terminals successively and cyclically at a predetermined frequency to provide signal pulses of a predetermined repetition rate in response to the initiation of a call on a line, means for assuring selection of a calling line to the exclusion of others of said lines -comprising a master oscillator operating at a multiple of said predetermined frequency, frequency divider means for providing said predetermined frequency for said scanning, a line nder circuit comprising a line finder gate tube normally biased beyond cutoff so that said pulses normally will not pass, means for applying said signal pulses to said gate tube, a local oscillator operating at a frequency slightly lower than the frequency of said master oscillator, frequency divider means for dividing the frequency from said local oscillator to provide control pulses at a repetition rate slightly lower than said predetermined repetition rate, means for applying said control pulses to said line finder gate tube to lower the bias thereon so that said signal pulses Will be selected and passed upon simultaneous application of said signal and control pulses thereto, and means responsive to said pulses passed by said gate tube for applying energy from said master oscillator to said local oscillator to synchronize it with said master oscillator whereby said signal pulses will be continuously selected.

DAVID H. RANSOM.

No references cited.