US 3697896 A
A signal transmission system comprising a means for eliminating or substantially reducing noise in a circuit including a transmission line for transmitting signals from a signal source for example a transducer to a utilization circuit, wherein the transducer has a predetermined ground and the transmission line comprises a pair of insulated signal conductors surrounded by a conductive shield with an insulated, shield-current relief conductor extending longitudinally along the signal conductors and parallel thereto outside of the shield and closely adjacent thereto. The shield relief conductor is grounded at one end to the transducer ground and means are provided at the other end for providing a connection to ground either through an intentional actual impedance or a stray impedance at the signal utilization circuit. This impedance may be of various types inherent in the utilization circuit or adapted to suit the utilization circuit.
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United States Patent Sarkozi et al.
 SIGNAL TRANSMISSION SYSTEM  Inventors: Miklos Sarkozi, Monroeville; An-
dras I. Szabo, Export; Perry J. Hite, Murrysville, all of Pa.
 Assignee: Westinghouse Electric Corporation,
 Filed: Feb. 11, 1971  Appl. No.: 114,621
 US. Cl. ..333/12, 333/24 R, 333/79, 333/96  Int. Cl. ..1104b 3/28  Field of Search ..333/1-6, 12, 84, 333/96, 24, 25, 26, 79; 174/102-109, 69, 32-36; 178/45, D16. 12; 179/78 R, 174, 170 E; 307/89-9l; 325/42, 52, 65, 357, 371
 References Cited UNITED STATES PATENTS 955,142 4/1910 Davis ..174/36X 2,376,101 5/1945 Tyzzer ..333/12 2,663,752 12/1953 Wier ..174/36 2,447,168 8/1948 Dean et al ..179/78 X 2,064,227 12/1936 Shumard ..333/9 2,280,950 4/1942 Harder ..333/12 X 3,518,577 6/1970 Baum ..333/12 3,534,146 10/1970 Fell ..174/35 MS STRAY MAGNETIC [451 Oct. 10, 1972 FOREIGN PATENTS OR APPLICATIONS 423,516 2/1935 Great Britain ..343/851 Primary Examiner-Herman Karl Saalbach Assistant Examiner-Marvin Nussbaum Att0rneyE. H. Henson, R. G. Brodahl and C. J. Paznokas  ABSTRACT A signal transmission system comprising a means for eliminating or substantially reducing noise in a circuit including a transmission line for transmitting signals from a signal source for example a transducer to a utilization circuit, wherein the transducer has a predetermined ground and the transmission line comprises a pair of insulated signal conductors surrounded by a conductive shield with an insulated, shield-current relief conductor extending longitudinally along the signal conductors and parallel thereto outside of the shield and closely adjacent thereto. The shield relief conductor is grounded at one end to the transducer ground and means are provided at the other end for providing a connection to ground either through an intentional actual impedance or a stray impedance at the signal utilization circuit. This impedance may be of various types inherent in the utilization circuit or adapted to suit the utilization circuit.
9 Claims, 5 Drawing Figures IO\ FLUX 3O '4 2| f T x \V i '7 32 If \L g: l 22 ll k J 1 r l J) M \J I I l I I L l I3 2'8 Z SIGNAL SYSTEM GROUND GROUND CROSS REFERENCE TO RELATED APPLICATION Patent Application Ser. No. 114,473 pending in 255, filed concurrently herewith, on Common Mode Noise Cancellation System, by Andras I. Szabo and Miklos Sarkozi, and assigned to the same assignee as the present application, is directed to other means for reducing noise in a signal transmission system and discloses means disclosed herein for use in the present system as one type of impedance connected between ground and the shield-current relief conductor at the utilization circuit end of the system.
BACKGROUND OF THE INVENTION l. Field of the Invention This invention pertains generally to a signal transmission system noise minimizing circuitry and more particularly to a system for transmitting relatively low input signals from a grounded transducer to a distant utilizing circuit, with means for minimizing the effect of electromagnetic and other electrical potential noise interferences with the signal transmission.
2. Prior Art In many instrumentation systems consisting of a signal source and a signal utilization circuit, undesirable electrical noise (stray currents and voltages) is introduced due to unavoidable electromagnetic coupling existing between the system and other devices such as electrical machinery operating in the proximity of the system. Often signals from sources at high off-ground voltages or from long distances are encountered. In neither case can a common ground be established. This complicates accurate information transmission, especially, as often is the case, where the information is in the form of low level electrical signals, such as those generated by remote sensing elements of types like strain gauges, thermocouples, and the like. Various common mode isolation and rejection techniques have been developed to separate useful signals from unwanted signals introduced into the system by various electromagnetic couplings along the signal path.
It is common practice to use a shielded, twisted instrumentation cable for the transmission of signals from distant transducers to signal utilization equipment in order to reduce the coupling of the signal wires to external noise sources, see for example US. Pat. Nos. 2,690,469-Perls; 3,032,604 and 3,274,329-Timmns; and 3,240,867-Maddox.
When the transducer is spaced a considerabledistance from the utilization circuit, a voltage, e.g., a differential ground potential, may exist between ground points of the transducer and at the utilization circuit. Connecting these ground points, as through ground connections of the shield at the two transmission line terminals, results in the flow of a differential ground loop current due to this differential ground potential. This can produce a differential mode noise in the information signal conductors, both by inductive and capacitive coupling therewith. Thus, although the prior practice of twisting signal cables longitudinally and encasing them in a conductive shield will reduce common mode noise induced by normal industrial and other environmental magnetic fields, it will not affect differential mode noise due to current in the shield. Balanced input circuits with high common mode rejection ratio eliminates much of the common mode noise. High input impedances and floating guarded input circuits also have been used to reduce noise loop currents. US. Pat. No. 3,204,201-Bahrs illustrates prior art circuitry and guards for reducing these noises.
SUMMARY OF THE INVENTION The present invention provides a signal transmission system, including a shielded transmission line having at least two signal conductors connecting a signal source for example a transducer with a predetermined ground to a utilization system, with a shield-current relief conductor extending along the signal conductors parallel thereto and grounded at one end to the transducer ground with means for providing a connection to ground either through an actual or stray impedance at the other end thereof at the signal utilization system. The signal conductors and the shield-current relief conductor are insulated conductors and the shield relief conductor is outside of the shield, parallel and closely adjacent thereto and minimizes and substantially inhibits the generation of differential mode noise in the shielded information signal conductors by providing a shield-current relief path which diverts the ground loop and other stray currents from the shield through a parallel conductive path outside of and insulated from the shield.
In some instances, as where the information signal conductors are first connected to a floating guarded analog-to-digital converter at the utilization receiver, the shield relief conductor is connected directly to the converter guard, which, by its distributed capacitance, is coupled to the utilization system ground. In other instances, the shield relief conductor may be connected to ground at the utilization system end thereof through an impedance network and a third winding of a three winding transformer, the other two windings of which are respectively connected one between each of the two signal conductors of the transmission line and the input of the utilization system. Each of the windings has the same number of turns and is wound and connected so that any common mode voltage induced in the signal conductor connected windings by current flow in the third winding will be opposed and equal to any common mode voltage induced in the signal conductors, thereby cancelling most of the common mode noise at the input to the utilization system. The ground connection of the transformer third winding may also be through a network which inhibits the flow of dc current but permits the flow of ac currents. This combination ensures normal operations in cases where a large dc potential difference can exist between the transducer ground and the utilization device ground by inhibiting the flow of a dc current which could magnetically saturate the transformer.
The foregoing and other advantages and features of novelty of this invention will be apparent from the following description referring to the accompanying drawing.
BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING In the drawing:
FIG. 1 is a schematic diagram illustrating the general circuitry of a signal transmission system incorporating this invention;
FIG. 2 is a schematic diagram illustrating the present invention in a system provided with a guarded analogto-digital converter, or similarly guarded device, at the input of the signal utilization circuit;
FIG. 3 is a schematic diagram illustrating the present invention in a system having a three winding commonmode noise cancelling transformer at the input to the signal utilization circuit;
FIG. 4 illustrates a cross section of one type transmission line for use in the present invention; and
FIG. 5 illustrates a cross section of another suitable type transmission line for use in the present invention.
DETAILED DESCRIPTION OF THE INVENTION Referring to the drawing, the same reference characters are used throughout the different figures to indicate the same features therein shown. FIG. 1 illustrates circuitry applicable in carrying out the present invention, wherein a transducer forms the information signal source for the transmission system and includes a suitable signal generating source 11 connected across a pair of impedances, such as resistors 12 and 13, and the transducer output terminals 14 and 15. The connecting junction point 16 between the resistors 12 and 13 has a predetermined connection 17 to ground for the transducer 10. As shown, the transducer also may be shielded by an enclosing conductive guard 18, which also may be connected to the transducer ground 17 by a suitable connection 19.
In order to transmit the information signal from the signal source (transducer 10) to a signal utilization system 30, which may be a computer, recorder, or the like, a transmission line 20 is provided having a pair of signal conductors 21 and 22, respectively connected to transducer terminals 14 and 15. Preferably, the input of the utilization system 30 is of the balanced or differential type, for example having in its input a differential amplifier 32. The system 30 includes a guard 33 for shielding against stray electromagnetic and electrostatic fields. These signal conductors are individually insulated, as shown at 23 and 24, respectively, in both illustrative cross-sectional views, FIGS. 4 and 5, and are enclosed in a suitable electrically conductive shield 25 which at its source end is electrically connected to the source ground 17. As used throughout herein, the term electrically connected refers to a connection that is capable of passing direct current. Preferably, the two conductors 21 and 22 are longitudinally twisted, as is conventional, in order to provide a substantially uniform exposure of these conductors to stray industrial and other environmental electromagnetic fields 26 along the transmission line. The shield 25 may be formed by a braided wire mesh or by a conductive foil spirally wrapped around the conductors 21 and 22. In the latter case the effectiveness of the shield can be enhanced by a third conductor 27 which is uninsulated and extends along the signal conductors parallel thereto and in good electrical contact with the shield 25. As shown in FIG. 4, the bare conductor 27 may be on the outside of the shield 25; or, as shown in FIG. 5, it may be inside of theshield 25. In most instances, it is preferable to ground the bare conductor 27 and thereby the shield 25 at one end thereof, such as shown at the transducer end (i.e., the source end) of the transmission line. This can conveniently be done by electrically connecting it to the transducer ground connection 17.
As has been pointed out, transducers often are spaced a considerable distance from the utilization circuit, and a consequent potential difference may exist between the system ground and the conductors of the cable at the utilization circuit. This voltage difference is normally referred to as the common-mode voltage. According to the present invention, such a common-mode potential is prevented from causing current flow in the transmission line shield which might induce noise voltages in the signal transmission conductors by the provi sion of a fourth transmission line conductor 28 which extends longitudinally along the signal conductors parallel thereto outside of the shield 25, closely adjacent to and parallel to the shield. This fourth conductor 28 is individually insulated, as shown at 29, FIGS. 4 and 5, so that it has no electrical contact with the shield 25 or any of the other three conductors along the transmission line. The conductor 28 is electrically connected to the transducer ground 17 at the transducer end of the line and is connected to ground through either an actual or a stray impedance Z at the utilization system. Impedance Z may for example be stray impedance inherent in system 30 with or without intentionally added impedance. The impedance 2 should impede dc and pass common mode frequencies. Thus, any tendency for current to flow through the shield 25 is relieved by the conductor 28 path which diverts such currents from the shield through this shield-current relief path. Both the bare conductor 27 and the insulated shield relief conductor 28 may be twisted around the other conductors. In the FIG. 4 arrangement, both could be twisted around the outside of the shield 25; and in the FIG. 5 arrangement, the three conductors 21, 22, and 27 could be twisted longitudinally around each other and enclosed by the shield 25, with the conductor 28 twisted longitudinally of the shielded conductor assembly. In both instances, a suitable insulating covering 31 encases all of the conductors and the shield.
FIG. 2 illustrates a specific application of the present invention, wherein the circuitry generally is the same as that previously described. It is here used in a signal transmission system in which the signals are supplied by the transmission line 20 to a floating guarded utilization system 30. In this figure, the signal conductors 21 and 22 are shown connected to the input of an analog-todigital converter 32 which forms the input to a utilization circuit. As shown, the converter 32 is shielded by a guard 33 having a distributed or stray capacitance coupling 34 to ground at the utilization system. In such a circuit, it has been found to provide excellent results to connect the shield relief conductor 28 at the utilization system 30 directly to the guard 33, as shown at 35. This eliminates the need of a high input impedance, high voltage driver circuit in the system.
FIG. 3 illustrates a signal transmission system utilizing the present invention generally as previously described but using a special three-winding common mode noise trap transformer 40, of the type disclosed in the previously mentioned concurrently filed application Ser. No. 1 14,473 pending in 255. This transformer does not per se form part of this invention. In this system, the three winding transformer 40 is provided with windings having equal numbers of turns, and with two of the windings 41 and 42 connected one between each of the signal conductors 21 and 22 respectively,
and the input to the utilization load, and with the third winding 43 connected between the utilization circuit end of the shield relief conductor 28 and ground through an impedance network 45 having high resistance to dc and low impedance to noise frequencies, preferably a resistance-capacitance network to ground at the utilization system. The transformer windings 41 and 42 are connected in the system such that any common mode voltages induced therein by current through the third winding 42 will be opposed to common mode voltages which may appear at the terminals of signal conductors 21 and 22. Thus, in this system, current in the transmission line shield 25 is avoided, and any shield diverted current through the shield relief conductor 28 is utilized by the transformer 40 to eliminate common mode noise voltages in the signal conductors, while the loading impedance Z minimizes dc current through the relief conductor 28 and the transformer winding 43.
While particular embodiments of this invention have been described, modifications thereof will occur to those skilled in the art. It is to be understood, therefore, that the invention is not to be limited to the exact details disclosed.
We claim as our invention:
1. In an electrical system including a signal source having a predetermined ground and a signal utilization system, the combination therewith of a transmission system comprising:
a. a pair of insulated signal conductors for connecting said source to said utilization system,
b. an insulation-covered electrically conductive shield surrounding said signal conductors, said shield at the source end thereof being electrically connected to said source ground, said shield being devoid of any electrical connection thereto at its other end,
c. an insulated shield current relief conductor located externally of, adjacent to, and extending along the length of said shield, said relief conductor at the source end thereof being electrically connected to said source ground, and
d. means connecting said relief conductor at the other end thereof to ground at said utilization system.
2. The combination as in claim 1 having a guard around said source, and means electrically connecting said source guard to said source ground.
3. The combination as in claim 1 wherein said shield relief conductor is connected to ground at said utilization system through effective impedance.
4. The combination as in claim 3 wherein said effective impedance comprises actual impedance made intentionally a part of the utilization system.
5. The combination as in claim 3 wherein said effective impedance comprises stray impedance of the utilization system.
6. The combination as in claim 1 wherein said transmission system is subject to common mode noise, and said shield relief conductor is connected to ground at said signal utilization system through a network which inhibits the flow of dc current but permits the flow of ac currents at the frequency of the common mode noise.
7. The combination as in claim 1 wherein said signal utiliz tion s stem includes elect onic circuitr with a guard therezi round that 15 free of electncal corinection to ground at the utilization system, and the utilization system end of said relief conductor is electrically connected to said guard.
8. The combination as in claim 1 having a transformer with first, second and third inductively coupled windings all having the same number of turns,
one of said windings being connected between an end of one of said signal conductors and an input terminal of said utilization system, a second of said windings being connected between an end of the other signal conductor and another inputter terminal of said utilization system, and the third of said windings being connected between the utilization system end of said shield relief conductor and ground at said utilization system.
9. The combination as in claim 8 wherein there is an impedance network through which the third winding is connected to ground, said impedance network offering high impedance to direct current and low impedance to common mode noise.