US1501691A - Selective transmission of electric waves - Google Patents

Description

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HARLD E. VEN NES. 0F WESTMGUNT, QUEBEC, CANADA, ASSIGNGR T0 WESTERN ELEG- TRIO CCMIPANY, INCORPORATED, OF NEW vYORK, N. Y., A CORPORATION F N'EW YORK.

SELECTIVE TRANSMSSION GIF ELECTRIC I Application filed July 26, 1922.

residing at Westmount, in the Province or 5 Quebec, Dominion of Canada, have invented certain new and useful In'iprovements in the Selective Transmission of Electric `Waves, of which the following is a `full, clear, conoise, and exact description.

This invention relates to the selective transmission of electric waves and particularly to the prevention of interference due to stray or leakage currents in suc-h a system.

This invention provides an improved arrangement or associating electric wave or alternating currenttransmitting and receiving circuits with a transmission medium or line. In a more limited aspect it provides an improved arrangement for associating carrier wave transmitting and receiving circuits With a transmission medium or line.

For a general description ot carrier systems, reference may be had .to a paper entitled Carrier current telephony and telegraphy, byvMessrs'. Colpitts and Blackwell, published in the Journal of the American Institute of Electrical Engineers, for April, May and June, 1921. In the systems therein described, the carrier terminal apparatus is connected to the carrier line through a balanced transformer, so that the transmitting and receiving circuits are conjugately connected to the line with respect to one another, a balancing network being used to balance the impedance of the line. A more simple arrangement consists in directly connecting the transmitting and receiving circuits to the transmission line, and preferably to employ distinctive 'frequencies for transmissions in opposite directions. Such an arrangement, however, is often trouble some due to stray or leakage currents arising from the unbalanced arrangement of the terminal circuits.

An object of this invention is to provide a simple terminal arrangement for electric wave transmission systems, which is free from noise and interference due tostray or leakage currents in the system, but. which still makesuse of a simple connection between the terminal apparatus and the transmission medium or line. Another object is to provide such a simple arrangement for Serial N9. 5??,493.

carrier current signaling systems. Other objects of the invention will become apparent as the description proceeds.

The term transmission medium as used in tht; specification is vto be understood to embrace a transmission circuit including both metallic conductors and other media for the transfer of electrical energj7 between two points. The electrical midpoint of such medium or ot' a transmission line refers to the positions at which the potential is midway between the desired driving potentials. For example, if a. certain driving otential is applied between the line wires o a transmission line, the electrical midpoint of the line would be at the point where the p0- tential drop from one wire to said midpomt is equal to the potential drop from said midpoint to the other wire.

In the preferred embodiment of the in vention, a shielded transformer is connected between a common carrier receiving circuit and a carrier transmission line, while a common transmitting ciicuit is connected to the electrical, midpoint ot' the line side of the transforn'ier. A plurality7 of carrier receiving branches are connected to the common receiving circuit on the one hand, and a plurality ot' carrier transmitting branches, to the transmitting circuit on the other. An electrostatic shield which is maintained at ground potential is located between the line winding and the station Winding of the transformer. The invention, however, is applicable to a terminal station comprising only one transmitting and one receiving branch.

Referring now to the drawing which shows the preferred embodiment of the invention, a carrier transmission line ML is connected to a carrier transmitting circuit TC, and a carrier receiving circuit RC b means of a two winding transformer 5.

high pass filter HPF is inserted between'the "transformer 5 and the line ML to allow the passage of carrier currents, but to substantially prevent the passage of' currents having essential frequencies in thevoice range or below. This filter is of the'generel ty e described in the patent to GeA. Campbell, No. 1,227,113, May 22, 1917.

An ordinary telephcneli-ne TL may also be connected to the transmission line ML'.

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low pces lter LPF of the general type 'described in the ahove mentioned Campbell patent is inserted in the line TL to allow the passage of currents of essential voice frequencies and lower frequencies and 'to prevent the liow of carrier currents to the line TL. A

A transmitting branch comprising e source O of carrier frequency Waves, modulator M, and a transmitting band filter SBF,

is connected to the transmitting circuit TC.

A receiving brunch comprising e .receiving band filter BBF, deinoduletor l), deniodula-v tor amplifier A., and a low pass voice fre quency filter VF, is connected to the receiv ing circuit RG. 'A low frequency line L is` associated with the transmitting and receiving branches by ineens of a balanced treinsformer 6. A low frequency network LN balances the line L, so that the transmitting and receiving channels are conjugateiy connected toy the line L with respect to one another.

The oscillator O, modulator M, detector D, and detector amplifier A, ere preferably of the well lniowii vacuum tube types, 'out any other suitable type of apparatus may he substituted.' therefor. The band ltcrs SBF and BBF are of the generul type described in the Cumplxill patent hereinbefore :inen-v tioned, differing from euch other only in the constants of the inductence and capacity elements, the velues of which depend upon the bend of frequencies which it is desired to transmit.

Other transmitting und receiving branches I may be connected to the transmitting circuit TC, and to the receiving circuit RFC, as indicated by blocks 7, 8 and 9, l() respectively. In the preferred system, each bend iilter passes a distinctivebund of frequencies, and

the transmitting frequencies as e. group occi'ipy a diiferent range from the receiving frequencies as a group. For exemple at a giventerniinal station carrier frequencies of 6, 9 and 12 kilocycles might be used forA transmitting and carrier frequencies of l5,

y 18 and 21 kilocy'cles, for receiving.

At the distant station the group of higher frequencieswould be used for transmitting and' the group-of lower frequencies, 'for receiving. The end sections of these band filters, adjacent to the transmission line ML are of low impedance to all frequencies, except those which the individua-l tilteris designed to pass to' or from the line ML as the cese maybe. These end sections of all ofthe transmitting filters are connected iii series with each other and with the transmitting circuit TC. Likewise the end sectionsof all of the receiving band filters are connected in series with each other and with the receivcircuit RC.

the preferred system, a grouping filter GF is inserted in the receiving circuit RC,

exclude the range of frequencies used for transmission. Likewise, a grouping filter TGF (shown in dotted lines, since it is not actually used inf the preferred system) may be inserted in the transmitting circuit TC to freely transmit the frequencies employed for' transmitting, and to substantially exclude the frequencies employed for reception at this station. vBotlrthe filter GF and the filter TGF maybe omitted, if the band filters in the several transmitting and receiving branches provide satisfactory discrimination between the transmitting and receiving frequencies. These filters are likewise of the general type described in the aforementioned Campbell patent. If the grouping filters GF and TGF are both omitted, it is obvious .that the grouped arrangement of transmit ting 'anclreceiving frequencies need not be used. The so-callcd staggered frequency arrangement may be found preferable.

`,The transformer 5, has been described as a two winding transformer. By this is meent that in respect to its transforming ac tion, it .may be treated as having only two windings, one called a station winding to which the receiving circuit RC is connected, end the other a line winding to which the transmission line MIL is connected. In fact, however, tholinc wind n t leest is composed of two electrically i ntical windi. so 4that the transmitting. circuit TC may 'connected in series therewith, undgto the eleci ground potential'. This grounded shield furnishes a short circuit path to ground for any unbalance currents originating in the line ML or any other of the circuits connected to the line winding of transformer 5. Such unbalance currents would otherwise find their way into the receiving circuit RC, due to the capacity between the line and station windings, and would cause interference and noise,

In a complete carrier system another station similar to theorie described and shown in, the drawing, would be provided at the distant end of the transmission line ML. This station would be so arranged that the transmitting branches'at the station de, scribed, would cooperate with lthe branches at the Adistant station, and t transmitting branches at the distant station would coo crate with the receiving branches at the station shown. vThe meth tion will not be described, since it is ohvious from the Colpitts and Blackwell paper hereinbefore mentioned.

It should be noted that since the end section of. the grouping filter GF, if such a filter is used, or the end sections of the receiving band filters if iro grouping lter GF is employed, vare of low impedance to the transmitted frequencies, and therefore actas a short circuit for the station 4Winding of the transformerv 5, and consequently the transformer 5 o'ers small impedance to the transmitted frequencies. This follows from Well known transformer theory. The self inductance of the primary or line Winding is largely neutralized by the magnetic effect of the short circuit current in the secondary or station Winding, so that the primary impedance is greatly reduced over what it would he with an open circuited secondary. Also, the end section of the transmitting grouping iilter TGF or the end sections of the transmitting hand filters, if the filter TGF is not employed, are of low impedance to the received' frequencies, and therefore function as a direct low impedance connection between the electrical midpoint terminals of the line Winding for these frequencies. Thus a balanced terminal circuit isobtained `with very little decrease in transmission eiliciency, over what would he'ohtained were the transmitting and receiving circuits directly connected to the transmission liiiell/[L Thev invention has heen described in its embodiment in a multiplex carrier telephone system, hut its use is not limited to that speciiic system.

What is claimed is:

1.' An alternating current system cornprising a transmission line, a transmitting circuit for impressing alternating current upon said line, another circuit for receiving alternating current from said line, and a transformer having a line. lWinding and a station Winding, said receiving circuit being connected to said station Winding, said line Winding being connected to said line, and said transmittingcircuit being connect ed in series with said line Winding at its electrical midpcnt.-

2. A carrier terminal system comprising a transmitting circuit, a receiving circuit, a ,transmission line, and a two winding transformer having one of its windings connected to the receiving circuit, the other connect ed to the transmission vline and the transmitting circuit connected in series with the line Windingat its electrical midpoint.

3. A carrier terminal system comprising a transmittin ',circuit, a receiving circuit, a transmission ine, a source of lcarrier current of operaot characteristic frequency for said trans`- mltting circuit, a select1ve circuit adapted to pass this characteristic frequency connected between said source of carrier current andv said transmitting circuit, a detector for carrier current of a different frequency, a se. lective circuit adapted to pass current of said different frequency connected between said receiving circuit and said detector, anda tworwinding transformer having one of its windings connected to the receiving cir-` transformer having a line winding and av station Winding, said receiving circuit -.be-

ing connected to said station Winding, said;l

line Winding being connected to said trans-` mission line and said transmitting circuit bei connected in series with said line win ing at its electrical midpoint.

5. ln a multiplex carrier telephone terminal system, a transmission line, a trans!- mitting circuit, a plurality of transmitting branches each comprising means to modulate a characteristic carrier frequency Wave 1n accordance with voice frequencies and a hand filter adapted to transmit Waves of the resulting hand of frequencies, means to connect the end sections of said hand filters in series with each other and with said transmitting circuit, a receiving circuit, a pluraiity of receiving branches each compris ing meanseto demodulate Waves of a characteristic hand of frequencies and a band filter adapted to transmit Waves of said characteristic cand'cfv frequencies, means to connect the end sections of said last named band filters in series with each other and said re 'ceiving circuit, and a transformer having a line winding and a stat-ion Winding, said receiving circuit being connected to said station Winding, said'line Winding bein connected to said transmission line, an said transmitting circuit 'heilig connected in series with said line winding at its electrical midpoint.

6. ln a multiplex carrier terminal system, a transmission line, a transmitting circuit, a

plurality of transmitting branches cach coinprising a hand filter adapted to pass a characteristic handof frequencies and hav ing an end section of low impedance to all frequencies except those which it is adapted to transmit, means to connect said and sec,u tions in series with each other and with the transmitting circuit, a receiving circuit, a plurality of receiving branches each 'c0m llo of said. last named filters in series with each other and with the receiving circuit, and. s transformer having e line winding and a station winding, seid receiving circuit being connected to seid station winding, seid line Winclingy being connected to seid transmission. line and said transmitting circuit being connected in Ser-ies with said line Winding at its electrica-l midpoint.

7. In e multiplex carrier terminal system, a transmission line, e transmitting circuit, a plurzility of transmitting branches each comprising-a band filter adapted to ess a characteristic hand of frequenciesen hsving an end section of low impedance to all frequencies except thosevvhich it Ais adapted to transmit, means to connect said end sections in series with each other and with the inerte-i transmitting circuit, ,a receiving circuit, e

plurality oit receiving branches .a hcom- 25. i prising a band lter adapted to pets. a cliery acteristie band oit frequencies and having v end sections of low imedence .to .all reqnencies exceptv those which it is adapted to transmit, means to connect,4 said lest recited `30 end sections in series with each other end with the receivin circuit, a transformer' having e line Wining and e station Wintiing, said receiving circuit being connected to seid ste-tion Winding., said linewinding 35 being connected to said transmission line and seid transmitting circuit being con` nected` in series 4with said line Windinget its electrical midpoint, and an electrostatic shield between the windings of said trans- 40 former, said shield being maintained at! ground potentiai Y 1n Witness whereof, I hereunto subscribe my neme this 18th dey of July A. D, 1922.

l HARALD J. VENNES.

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