US2105305A - High-frequency electrical transmission system - Google Patents

Description

. Jan. 11, 1938. c, WHITE 2,105,305

HIGH FREQUENCY ELECTRICAL TRANSMISSIQN SYSTEM Filed Aug. 50. 1935 Patented Jan. ll, 1938 HIGH-FREQUENCY ELECTRICAL TRANS- MISSION SYSTEM Eric Lawrence CaslingWhite, Hillingdon, England, assignor to Electric & Musical Industries Limited, Middlesex, England, a company of Great Britain Application August 30, 1935, Serial No. 38,529 In Great Britain August 31. 1934 7 Claims.

The present invention relates to the transmission of high-frequency electric currents, such as 10 two wires of an open wire line. Disturbing potentials may occur between the two conductors in parallel and earth, owingto conduction currents in the earth or by induction from neighbouring conductors carrying other currents, e. g. power 15 lines.

In order to obtain only the potential difference which is required, the receiving apparatus may be connected only to the two conductors, and insulated from earth, but it is often essential to Q earth some part of the apparatus, e. g. when the receiving apparatus is a wireless transmitter. It is not possible to earth one of the conductors, even the sheath of a concentric cable, since the interfering voltage would then be partly transg ferred to give a. potential difference between the two conductors.

In telephone practice this difliculty has been overcome by connecting the primary of a transformer to the line, insulated from earth, and con- 9 necting the secondary to the receiving apparatus,

any part of which can then be earthed. However, it is not feasible to use transformers in television work, owing to the very wide frequency band to be covered with no appreciable phase dis- 35 placement.

An object of the present invention is to provide means for reducing or eliminating the undesired potentials in a terminal station of the transmission line, while permitting a part of the apparatus at this station to be connected to earth.

According to the present invention there is provided a high frequency electrical transmission system comprising a transmission line having two conductors coupled to a terminal station without 1 the use of a transformer wherein the terminal station includes a thermionic valve or valves having a part of the electrical circuit associated therewith connected to earth, and is of such a nature that whilst the signal potential differences 60 existing between the line conductors are applied to and transmitted by the valves without appreciable phase displacement or with almost exact phase reversal, nevertheless interfering potential diiferenoes existing between the line conductors, in parallel, and earth are prevented or largely prevented from influencing the output of the valve or valves. The terminal station may be so arranged as to prevent or largely prevent the interfering signals from being applied to the input valve of the terminal station. Alternatively 5 valves of the terminal station may be so connected to the two line conductors and to earth that the interfering signals are applied to the valve system in such a manner as to be substantially eliminated thereby. 10

Thus the line conductors may be connected together by a terminating resistance across which are developed the signal potentials to be applied to a valve the cathode of which is connected to earth through an impedance which forms part of the output circuit of the valve, and which is of such a high value relative to the terminating and line resistances that only a low potential is developed across the terminating resistance by the interfering potential differences, while the value of the impedance is low enough relative to the anode slope resistance of the valve to prevent appreciable variations in the anode current, due to the interfering potentials developed across the impedance. 5

Alternatively the line conductors may be connected to the control grids of two valves whose cathodes are connected to earth, one of these valves being associated with a phase-reversing device the output circuit of which is in parallel with the output circuit of the other valve, and. the arrangement being such that interfering signals appear in opposite phase in the output circuits and cancel out, while the desired signals appear in like phase.

It is to be understood that, throughout this specification, a'connection to earth does not imply a direct connection to earth but may imply a connection to a point the potential with respect to earth of whichis substantially incapable of 40 variation at any frequency within the range of frequencies which the system is adapted to handle or is only capable of variations which have no adverse effect on the operation of the system.

The invention will be described by way of example with reference to the accompanying diagrammatic drawing in which Figs. 1 and 2 show two arrangements according to the present invention. 5

Referring to Fig. 1 a two wire line I comprising two conductors 2, 3 is connected at one end to a transmitter denoted ml and at the other end to a terminal station. Conductor 3 is earthed at a point adjacent the transmitter 4. In the terminal station the conductors 2, 3 are terminated by a terminating resistance 5 which may have a value equal to the characteristic impedance of the line I. Conductor 3 of the line I is connected to the cathode 6 of a thermionic valve 1 through a large condenser 8, for isolating steady potentials, and a grid bias battery 9. The other conductor 2 is connected to the control grid III of the valve 1 through another isolating condenser II. A high resistance grid leak I2 is provided to maintain the required steady potential on the grid ill from the battery 9. The cathode 6 of valve I is to a large extent isolated from earth by a large impedance I3, which may be a resistance of the order of 1000 times the terminating resistance 5, but which should be small compared with the anode slope resistance of the valve. For example, if the terminating resistance 5 is 100 ohms and the valve is a tetrode or pentode having an anode impedance of 10 ohms, the earthing resistance 13 may be 50,000 ohms.

The anode circuit may include an anode resistance l4 and a battery 15 with one end earthed, which supplies the anode voltage. If the valve 1 is a tetrode (as shown) or a pentode, a separate battery l6, insulated from earth, supplies the screen potential.

Differences of potential between the two conductors 2, 3 are amplified by the valve 1 and appear across the anode resistance l4, and may be taken off via a condenser I! to an output circuit denoted by IT.

Since conductor 3 is earthed at the transmitting end of the line, diflerences of earth potential between these two stations will cause current to flow through line conductor 3, which is connected to the cathode 6. Since the earthing resistance I3 is large, very little potential difference will be produced between the ends of line conductor 3, and still less potential diflerence will occur across the terminating resistance 5 due to current flowing through line conductor 2 and resistance 5. The potential developed across the earthing resistance l3 will cause very little variation of anode current, owing to the relatively high value of the anode slope resistance of the valve 1.

' Thus the interference is considerably less than that which would occur if the receiving end. of line conductor 3 where connected directly to earth, that is if the earthing resistance l3 were made zero.

In the arrangement of Fig. 2 the receiving ends of the line conductor 2, 3 are bridged by a terminating resistance 5, and directly connected respectively to the control grids l8, I3 of two input valves 20, 2| the cathodes 22, 23 of which are connected to earth through a biasing resistance 3|. Valve 2! is coupled by a resistance-capacity coupling comprising an anode resistance 241, a coupling condenser 25 and a grid leak 26 to a third valve 21 whose cathode 28 is connected to earth through a bias resistance 32 shunted by a decoupling condenser 33, and the output circuits of input valve 20 and of the third valve 21 are connected in parallel by means of a common anode resistance 29, which is preferably very small relative to the anode impedances of the valves 20, 21. The common output voltage of valves 20, 21 may be transferred by a coupling condenser 30 to an output circuit denoted by 30'.

The anode potentials for the three valves are furnished by a. battery l5 having its negative pole connected to earth.

The product or the voltage amplification of the input valve 2| and of the third valve 21 coupled thereto is arranged to be numerically equal to the amplification of the other input valve 20 alone, but is reversed in phase owing to the use of two valves as against one. Care must be taken to avoid introducing any phase differences other than the intentional 180.

Thus equal potential differences between earth and. the two ends of conductors 2, 3 of the transmission line I, due to interference, will produce at the anodes of input valve 20 and of the third valve 21 impulses which are in opposite phase and which therefore cancel out while differences of potential between the two line conductors 2, 3 will produce at these anodes signals which add in the same phase.

The transmission line I may be a concentric tube cable, or an open wire line,.as already suggested. In either case it is important that both conductors be insulated from earth. Alternatively, the cable may consist of two concentric tube cables lying side by side, with the sheaths connected together and earthed at both ends. The two inner conductors form the line conductors, and at the transmitter end either they may be fed in push-pull, or one only may be fed, and the other left as a dummy simply for providing equal interference to that picked up on the working cable, to be balanced against the latter at the receiving end.

To ensure sufliciently low losses at the modulation frequencies common in television, e. g., up to 3 x 10 cycles per sec., the dielectric of the concentric tube cable may consist of beads of glass or a suitable ceramic material, or may consist of hollow insulating shells of synthetic resin or similar material, providing supports for 'the central conductor at intervals of an inch or so.

A further type of cable which can be used in conjunction with the present invention comprises a conductor bent to a zig-zag shape and located within an insulating tube provided with conducting sheath.

I claim: I

1. A high frequency electrical transmission system comprising a transmission line having two conductors, a source of electrical signal variations connected between said conductors, said system being of the kind that spurious potential diflerences can be imposed between said conductors and ground by uncontrolled outside agencies, a

thermionic valve terminal circuit having two input terminals arranged at a point remote from said source, non-inductive coupling means arranged between said conductors and said input terminals, an output circuit having a point therein connected to ground, and means forming part of said terminal circuit for feeding to said output circuit signal variations derived from said source substantially free from said spurious potential differences.

2. A high frequency electrical transmission system comprising a transmission line having two conductors, a source of electrical signal variations connected between said conductors, said system being of the kind that spurious potential differences can be imposed between said conductors and ground. by. uncontrolled outside agencies, a thermionic valve terminal circuit arranged at a point remote from said source, and an output circuit having a point therein connected to ground, said terminal circuit comprising a terminating impedance element connected in shunt across said line, a thermionic valve having a cathode, a control grid and an anode, non-inductive coupling means between said conductors and said cathode and control grid respectively, a second impedance element connected between said cathode and ground, and coupling means arranged between said anode and said output circuit, the impedance of said second impedance element over the rangeof frequencies covered by said electrical variations being large compared with the resultant impedance due to the shunt impedance of said line and said terminating impedance element, so that the potential difference developed across said terminating impedance element due to said spurious potential differences is small compared put circuit.

3. A high frequency electrical transmission system according to claim 2, wherein over said range of frequencies theimpedance of said second impedance element is greater than ten times the impedance of said terminating impedance element.

4. A high frequency electrical transmission system according to claim 2, wherein over said range of frequencies theanode slope resistance of said valve is greater than ten times the impedance of said second impedance element.

5. A high frequency electrical transmission system comprising a transmission line having two conductors, a source of electrical signal variations connected 'between said conductors,

said system being of the kind that spurious po tential differences can be imposed between said conductors and ground by uncontrolled outside agencies, a thermionic valve terminal circuit arranged at a point remote from said source, and an output circuit having a point therein connected to ground, said terminal circuit comprising first and second thermionic valves, means comprising a non-inductive coupling for feeding variations from said line to said first and second valves in push-pull, phase-reversing means for reversing the phase of the output of said first valve, and means for feeding to said output circuit in parallel the output of said second valve and the output of said phase-reversing means. 7

6. A high frequency electrical transmission system according to claim 5, wherein said phasereversing means comprise a further thermionic valve having input and output circuits, said input circuit being coupled to the output circuit of said first valve and the output circuit of said phase-reversing means being connected in parallel with the output circuit of said second valve.

7. A high frequency electrical transmission system according to claim 5, wherein said phasereversing means comprise a further thermionic valve having input and output circuits, said input circuit being coupled to the output circuit of said first valve and the output circuit of said phase-reversing means being connected in parallel with the output circuit of said second valve,

the total amplification of said first valve and said further valve being substantially numerically equal to the amplification of said second valve.

ERIC LAWRENCE CASLING WHITE.

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