WO1999067880A1

WO1999067880A1 - Transmission of power and signals over coaxial cable, twisted pair cable, and other electric cables

Info

Publication number: WO1999067880A1
Application number: PCT/US1999/013890
Authority: WO
Inventors: Rafael Elul
Original assignee: Rafael Elul
Priority date: 1998-06-22
Filing date: 1999-06-20
Publication date: 1999-12-29
Also published as: AU4956799A

Abstract

This invention provides improvements in electrical transmission of digital and analog information and of electric power through cables of coaxial, twisted pair, and other types such as used for telephone local loop, local area networks (LAN), Cable Television, and other applications. The present invention describes methods for improving the electrical characteristics of such cables to allow reduced attenuation over longer distance and higher transmission rates. These improvements are achieved through neutralization of the longitudinal (series) inductance (110) of the cable through the addition of capacitors (300, 131) and inductors (400) selected to achieve resonance at the exact transmission frequency. Simultaneous transmission of signals at more than one resonant frequency is possible according to the present invention. Further improvement in transmission, which is applicable to the applications mentioned above as well as to transmission of electric power, involves neutralization of the leakage (shunt) capitance (130) through the addition of parallel inductor (400), or preferably a plurality of inductors, selected to produce resonance together with the intrinsic leakage or shunt capacitance of the cable.

Description

Transmission of Power and Signals over Coaxial Cable, Twisted Pair Cable, and other Electric Cables

FIELD OF THE INVENTION

This invention relates in general to conduction over coaxial, twisted pair, and other types of electric cable, such as used in telephone local loop, in local area networks (LAN), Cable Television (CATV), and other applications; in particular the present invention provides means for improving the conduction characteristics of such cables to make possible more efficient and more economical transmission of information and of electric power.

BACKGROUND OF THE INVENTION

Effective communication to the Internet requires passage of high data rates over the telephone local loop. Local Area Networks (LANs) achieve their connectivity by means of twisted pair wires or coaxial cables which pass data at high rate. Cable television requires passage of high data rates over coaxial cable for transmitting information from one node to the next. In all these systems attenuation of the signal is a major problem, limiting the maximal transmission distance, constricting the signal data rate, and requiring the use of repeaters whenever transmission over longer distances is required. Traditionally, signal attenuation has been managed by reducing either the frequency bandwidth, or the distance of transmission, or both parameters simultaneously.

It is useful to review how the reduction of transmission distance and of bandwidth, overcome signal attenuation. The fundamental fact of communication is that signal rate and bandwidth are directly related: given an efficient coding scheme, higher data rates are attainable only at higher signal frequencies. Conducting cables in general are incapable of accommodating this requirement. The impedance of the conducting medium, i.e the cable, has three distinct components: ohmic resistance, inductance in series with the resistance, and shunt capacitance either to ground or to the second lead. Resistance, the first of these three components, is not frequency-dependent. However, the impedance component contributed by the inductance increases with frequency, so that at higher frequencies the combined series impedance becomes so high as to make impossible the detection of the signal at the remote end. In a uniform cable, the total series impedance is proportionate to length. For this reason, reduction of the cable length is associated with lower signal attenuation.

In addition to this effect on the serial impedance, the shunt capacitance of the cable also contributes significantly to signal attenuation. Even well-msulated cable still exhibits signal attenuation sue to the capacitive shunt to ground and/or to the second conductor in a twisted pair cable. In this case, as well, lowering the top transmission frequency is effective in reducing attenuation: the ohmic resistance is not affected, but impedance presented by the shunt capacitance is increased , allowing only a smaller fraction of the signal to leak out

Nevertheless, economically it is advantageous to use higher data rates over longer distances, and much effort has therefore been spent in the past on producing cables with more attractive high-frequency electrical characteristics. The inductance of straight wire cannot be reduced, although it decreases at frequencies above 50 KHz because of the skin effect, typically cable inductance ranges around 1.0 mH (milliHenry) per mile for coaxial cable and 0.5 mH for each lead in a twisted pair cable. Capacitance, even with good materials technology, cannot be reduced beyond 20 nF (nannoFarad) to ground per mile of coaxial cable, or 60-80 nF per mile between the two leads of a twisted pair. Resistance is the only parameter which can be varied by the designer. Longitudinal resistance (i.e. the resistance between the two ends of the cable) is inversely proportional to conductor diameter, and varies from 80 Ohm per mile (19 gauge wire) to 425 Ohm (26 gauge) per mile. An accepted strategy has been to utilize heavier wire for applications requiring longer distances and higher data rates. Unfortunately, not only do the thicker, lower gauge, wires increase cost, but they also result in greater bulk and stiffness, causing handling difficulties and potential installation problems. It would therefore be desirable to find alternative means to decrease the impedance of cables used in the telephone local loop, in Cable Television and LAN communication. From a broader perspective, it would also be desirable to reduce signal leakage out of transmitting cables, whether for economical reasons, as in transmission of electric power, or to decrease EMF radiation. These and additional objectives are accomplished by the methods of the present invention.

PRINCIPLE AND METHODS OF THE PRESENT INVENTION

The overall objective of the present invention is to improve the transmission of high-frequency signals in coaxial and twin-lead cables. This objective is attained through use of L-C resonant properties to enhance longitudinal transmission while at the same time minimizing leakage to ground or to the second lead (in case of a twisted pair cable)

Transmission cables are characterized by their resistance, capacitance, and inductance However, what is often overlooked is that while the resistance and inductance are m series with one another along the cable, the capacitance is not a series capacitance, but rather reflects leakage from the conductor to ground (in a coaxial cable) , or to a second conductor (in a twisted pair cable) .

In a series circuit, i.e. one composed of a resistor, inductor, and a capacitor, all in series, at the specific frequency of resonance, the inductive and capacitive loads cancel one another and vanish, and the impedance of the cable reduces to the D-C resistance of the same cable. Therefore, and regardless of the transmission frequency, if the signal is transmitted at the exact frequency of resonance, voltage drop along the cable will decline to a small fraction of the attenuation at other frequencies, producing the most favorable conditions possible for signal transmission. In this way, resonance can provide a practical means to overcome the series impedance to high-frequency signals presented by electric cable

In a parallel LC circuit, the individual currents flowing through the capacitor and inductor are in opposite directions. At the frequency of resonance these currents cancel one another, so that the total impedance of the parallel resonant circuit is infinite. In practice, there is always in an LC circuit also a resistive component, representing the ohmic resistance of the inductor. This resistor (or any other resistor in the parallel LC circuit) limits the improvement produced by resonance. Nevertheless, the impedance of an LC circuit is much higher at the resonant frequency.

These two characteristics of resonant circuits provide effective means to: (I) cancel the longitudinal inductance of the cable, and

(II) minimize the leakage to ground or to the second lead.

These characteristics of resonant circuits are employed to achieve the goals of the present invention. Specifically, the present invention achieves enhanced transmission by following two methods:

1. Transmission of high frequencies along the cable is facilitated by adding a series capacitor, so as to achieve a series-resonant configuration between the intrinsic inductance of the cable, and the extrinsic, added capacitance.

2. At the same time, losses due to capacitive coupling of the cable are minimized by the addition of an inductor, or a plurality of inductors, between the cable and ground (for a coaxial cable) or between the two leads in a twisted pair cable, so as to achieve a parallel-resonant configuration with the leakage capacitor, where the leakage impedance is substantially increased at resonance.

Moreover, the added series capacitor and added parallel inductor can be chosen so that, together respectively with the series induction of the cable, and with the parallel leakage out of the cable, the series and parallel circuits will resonate at a single selected frequency which is identical for these two circuits. When signals are sent along the cable at that same resonant frequency, leakage losses are minimized, while series induction is "zeroed out".

The transmission improvements achieved by following these steps are quite significant. In a transmission cable 1 mile long (significantly longer than the typical LAN run, but shorter than most telephone local loops), the series inductance and the leak capacitance together constitute the preponderant high-frequency impedance of the cable. Cancellation of these factors is associated with a reduction in the overall impedance to the far lower level of D-C resistance. In that 1-mile long cable, Ohmic resistance is only about 2% of the total impedance at 1 MHz, 0.2% of the total impedance at 10 MHz and 0.02% of the total impedance at 100 MHz. In the same cable at resonance, leakage is decreased at least by a factor of 100 at 1 MHz, and by a factor of 10 at 10 MHz, and longitudinal impedance is decreased by a factor of 10 η

(or 10 in "loaded" telephone cables) .

Although these improvements are achieved at the circuit resonance frequency, and only at that frequency, the frequency of resonance of the circuit is not predetermined to a single, unique value. Rather, the resonant frequency is capable of adjustment to the frequency most favorable for transmission. This is because the frequency of resonance is dependent only on the magnitude of the capacitance and inductance. The D-C resistance of the cable has no effect on the frequency of resonance. Consequently, by adding capacitors of selected values to a given cable, practically any desired resonant frequency may be attained.

Additional advantages of the present invention are that the cable can sustain more than one resonant frequency at the same time. Duplex bi-directional communication is possible through the use of a number of transmitters connected to the cable through capacitors of different values, operating simultaneously at a number of different resonant frequencies.

Furthermore, communication at a narrow band around the resonant frequency facilitates signal detection. A detector with a resonant first stage, similar to a radio receiver, is capable of providing high level of selectivity and a far more favorable signal-to-noise ratio than any other existing technique. Resonant circuits are extremely attractive both for transmission and for reception of signals over the local loop.

Finally, reduction .of shunt leakage, by itself, can be of significant value in power lines and where reduction of EMF is required.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 presents a schematic of a typical electric cable, e.g. coaxial cable, showing the Ohmic resistance, self-inductance, and shunt capacitance.

Fig. 2 is a plot of resistance versus frequency response of the circuit of Fig. 1

Fig. 3 illustrates a "balanced" cable, e.g. a twisted pair cable such as used in many LAN applications and in the telephone subscriber loop.

Fig. 4 presents a non-balanced cable where self-inductance has been compensated by the addition of a capacitor of appropriate magnitude.

Fig. 5 illustrates for the circuit of Fig. 4 the relationship between impedance and frequency.

Fig. 6 presents a non-balanced cable where the leakage capacitance has been compensated by the addition of an appropriate inductor in parallel.

Fig. 7 shows the variation of the shunt impedance with frequency for the circuit of Fig. 6.

Fig. 8 illustrates the equivalent circuit of a non-balanced cable where both self-inductance and shunt capacitance have been compensated.

Fig. 9 presents the frequency response of the circuit of Fig. 8.

Fig. 10 shows a simplified presentation of the transmission cable circuit of FIG. 6, disregard- mg the series inductance of the cable, but including the correction for the leakage capacitance

(The shunt impedance of this circuit is shown m Fig. 7) .

Fig 11 is similar to Fig. 10, except that the correcting inductor is now placed at the other end of the cable.

Fig. 12 shows the change with frequency of the shunt impedance of the circuit of Fig. 11

Fig. 13 shows multiple transmitters and receivers operating simultaneously at different resonant frequencies, attached to the same cable.

DETAILED DESCRIPTION OF THE INVENTION

It is a common practice in analysis of transmission lines to represent by a diagram like the one m Fig. 1 , a coaxial cable, or any other cable with a single active lead. Return is provided in this case by ground, or alternatively by a grounded external sleeve. In the strict sense, a diagram such as Fig. 1 represents only a short segment of the cable, but as long as the load at both ends of that segment is equal to the termination impedance, no reflections will arise, and successive segments may be linked just as ratlcars in a tram are linked one to the next, and the entire cable will appear to both transmitter and receiver exactly the same as the short segment diagrammed in Fig. 1.

Referring to Fig. 1, a signal placed across input resistance (60) , one side of which is connected to ground at point ( 1) , travels to the other end of the cable terminated with load (50) . Typically, resistors (50) and (60) are identical, and equal to the nominal impedance of the cable. The cable possesses longitudinal resistance (100) , longitudinal inductance (110) , and leakage capacitance (130) . Although there may also be some leakage resistance, its value is so high in a new cable of good quality, that it need not be considered further. Inductive cross coupling of the conducting core of the cable with the sleeve also is negligible, primarily due to grounding of the sleeve (point 21 in Fig. 1) .

The situation of a twisted pair cable essentially is similar (Fig. 2) . A twisted-pair cable exhibits resistance on both leads, but if the load (60) is grounded so that the voltage measured between points (20) and (21) is exactly equal to the voltage measured between points (21) and (22) , the cable is balanced, and the circuit from point (21) to point (11) , from there to point (10) , thence to point (20) and back to (21) is identical with the circuit of Fig. 1. In the following discussion it will therefore be sufficient to consider the case of the unbalanced cable

The longitudinal inductance (1 10) and shunt capacitance (130) both degrade high fre- quency response, as shown in Fig. 3. The poor high-frequency response presents a major obstacle to Digital Subscriber Line (DSL) over telephone lines, and to LAN communication. Impaired high-frequency conduction also limits the programming capacity of Cable Television. Also, the leakage capacitance causes powei loss m cables employed for the transfer of electric power, especially at higher frequencies. It is therefore desirable to improve the high-frequency response of cables. The present invention provides methods whereby dramatic improvement in high-frequency transmission is achieved. This improvement is limited to a fairly narrow frequency band, but if that frequency band is at sufficiently high frequency, information transmission as well as electric power transfer are greatly enhanced.

The principle of the present invention is to first select the frequency band where the enhanced transmission is desired. For information transmission, typically this band will be at a frequency as high as feasible, since a higher "carrier" frequency makes possible encoding of information at a higher rate. The second step then is to add at some point along the cable a capacitor (300) which, together with the self-inductance of the cable (110), will produce resonance at the desired frequency. This circuit is displayed in Fig 4

Resonance in this system is independent of the longitudinal resistance (100) , and also is independent of the load resistors 50, 60. This independence is reflected in the equation which relates the resonant frequency f to the capacitance C and inductance L of the circuit. f = 1/ 6.28 (LC) ^1/2 ( 1)

It is generally most convenient to add capacitor 300 either at the input (close to point 20), or at the output (point 10) , but in fact the capacitor could be spliced any place along the cable with just as satisfactory outcome.

As noted in the "Principle and Methods of the Present Invention" above, at the frequency of resonance, the impedance of the inductor is exactly canceled by the impedance of the series capacitor. The decrease in total impedance is quite marked; assuming a cable 1 mile long, with 1 mH total inductance, Table 1 below shows the impedance without an added capacitor, and the decrease in impedance following the addition of a capacitor, assuming Ohmic resistance of 100 Ohm.

It is seen from Table 1 that at higher frequencies the impedance of the cable progressively increases. While the actual values of the cable parameters vary with the type of cable, length and installation, Table 1 shows that the impedance attributable to the inductance of the cable increases linearly with the increase in frequency At frequencies above 1 MHz, over 99% of the total impedance of the cable is due to the inductance. In the example in Table 1, the addition of a series capacitor of 0.002 pF will produce resonance at a frequency of 100 MHz, Table 1 Correction of Series Inductance of a Transmission Cable

(Cable Parameters Length 1 Mile, resistance 100 Ohm, Inductance 1 mH)

frequency Uncorrected Impedance Required Capacitor Corrected Impedance

0.1 Hz 700 Ohm 2,000 pF 100 Ohm

1.0 7,000 20 100

10 70,000 0.2 100

100 700,000 0.002 100

and the impedance at that frequency will drop from 0.7 MegOhm to 100 Ohm - a 7,000 fold decrease. At that exact frequency, a signal 7,000 times weaker than the signal required without the added capacitor (300) , when applied across input impedance (60) , will produce the same voltage across load (50) .

The situation with a twisted pair is essentially similar, except that the magnetic fields produced in the two twisted leads in a balanced circuit, are opposite in polarity and tend to cancel one another. Resonance can still be obtained, either by adding a single capacitor or, preferably, through use of two matched capacitors on the two leads The values of the capacitors needed to achieve resonance at a given frequency are higher, because the inductance per mile is likely to be lower. The frequency response of the circuit of Fig. 4 is shown in

Because of the series capacitor, impedance is extremely high at all frequencies except the frequency of resonance. The effect on transmission of the leakage capacitance (130) is summarized in Table 2. Leakage capacitance acts as a shunt resistance. The lower the value of the shunt resistance, the smaller the signal received at the output. As seen m Table 2, the leakage impedance drops with increased frequency. At 100 MHz, it is 1 ,000 times smaller than the shunt at 0.1 MHz.

Fig. 6 shows that the shunt due to capacitor (130) can be minimized through the addition of an inductor (400) m parallel. The value of the inductor is selected so that the parallel circuit (capacitor 130 and inductor 400) will resonate. At the frequency of resonance there is a substantial increase in shunt impedance. The magnitude of the increase depends inversely on the Ohmic resistance of the inductor (400) , and therefore is somewhat difficult to calculate. It is reasonable, however, to assume that the Ohmic resistance of the coil is linearly related Table 2 Correction of Shunt Capacitance of a Transmission Cable

(Cable Parameters Length 1 Mile, Resistance 100 Ohm, Shunt Capacitance 0 18 μF)

frequency Uncorrected impedance Required Inductor Corrected Shunt Impedance

0.1 MHz 0 Ohm 20 μH 1000 Ohm 1.0 1.0 0.2 1000 10 0.1 0.002 1000 100 0.001 0.00002 1000

to its inductance, since inductance depends on the number of turns of the wire on the coil core. Capacitance (130) is a structural feature of the cable and does not vary with frequency. Therefore, as shown in Table 2, the inductance required for resonance at different frequencies vanes linearly and inversely with frequency If the resistance of the inductor also varies linearly and inversely with the frequency, then in this situation the shunt impedance will be some 100 times higher than the impedance with the capacitor alone even at the relatively low frequency of 0.1 MHz, and will increase to be 100,000 times higher at 100 MHz.

The change in shunt impedance with frequency for the circuit of Fig. 6, is shown in Fig. 7 At the output (i.e. across load 50) , at low frequencies the parallel inductor shunts away most of the signal, and at high frequencies capacitor (130) shunts most of the signal, creating at the output the impression of extremely high circuit impedance. However, at the resonant frequency, signal amplitude at the output increases very markedly, as if the overall circuit impedance suddenly dropped.

It is possible to combine the correction for the series inductance (110) with the correction for the shunt capacitance (130) by selecting a series capacitor and a shunt inductor so that both series and parallel resonance will occur at the very same frequency, as shown in Fig. 8. An example of the required values of these matched added capacitor and inductor are given in Tables 1 and 2, respectively. In the example illustrated in Tables 1 ,2, each of the two correction improves the signal in/out ratio by a factor of 1,000. When both corrections are combined, improvement in the order of 1 ,000,000 is possible. This is illustrated in Fig. 9 (not to scale) .

In practice, a high level of shunt capacitance correction requires additional steps. Consider first the simplified shunt correction circuit of Fig. 10. For clarity, the series inductor and the associated correcting capacitor are not shown in this diagram. The correcting inductor in this diagram is placed in proximity of the shunt capacitor (130) . Concentrating on the parallel resonant circuit point (30) to (40) , to (41) through coil (400), thence to point (31 ) through the common ground, and back to point (30) through the shunt capacitor ( 130) , the only resistance m this circuit is the intrinsic resistance of the inductive coil As already noted, that resistance is linearly proportional to the length of the wire used in the coil. As seen in Table 2, higher resonant frequencies require coils of progressively lower inductance, so that the coil resistance also decreases linearly with frequency.

Now the impedance of the shunt circuit Xshunt at resonance, is approximately determined from the equation

Xshunt ⁼ (-^-capacitance ) / (■<-) where X_capaαtan_ce is the impedance of capacitor ( 130) at the resonant frequency (recall that Xcapaci_tance is equal in magnitude to the impedance of coil (400) at the same frequency, but opposite in sign) , and R is the overall Ohmic resistance of the shunt circuit (30) to (40) , to (41) , to (31), and back to (30) In this circuit, because R is very small, and is further decreased at higher frequencies, the impedance of the shunt circuit at resonance is very high, effectively eliminating the shunt current

Next consider the circuit shown in Fig 1 1 That circuit is identical with the circuit of Fig 10, except that the correction inductor (400) is now placed at the far end of the cable. The magnitude of shunt capacitor (130) and of correction coil (400) are the same as m Fig. 10 However, the overall resistance of the shunt circuit (30) to (40) , to (41 ) , to (31 ) , to (30) , now includes the longitudinal Ohmic resistance of the cable (100) .

Referring to equation (2) above, it is evident that the denominator R is now much larger, typically around 100 Ohm, whereas the Ohmic resistance of the coil is of the order of 1 Ohm or less. Consequently, Xshun_t in this circuit is much lower than in the circuit of Fig. 11. Using the figures of 100 Ohm and 1 Ohm respectively, it follows from equation 2 that Xshunt in the circuit of Fig. 11 will be only 1% of its value in the circuit of Fig. 10. Comparison of Fig. 12 with Fig. 7 shows that the correcting inductor still acts to increase the shunt impedance and thus decrease signal loss due to capacitance (130) , but the increase in impedance due to inductor (400) is now much smaller than in Figs. 6 and 10, even though the coil (400) is identical in both cases.

Both Fig. 10 and Fig. 11 represent abstractions of real cable circuits. In reality, capacitance (130) is the sum of smaller capacitances distributed evenly along the cable. To achieve optimal correction, inductor (400) would likewise need to be replaced by a multiplicity of larger inductors placed throughout the cable. From a practical perspective, clearly two shunt inductors placed at both ends of the cable will provide better shunt correction than a single inductor at one end only. In longer cable runs, inductors placed every 50 - 100 ft. (the Ohmic resistance of a typical cable is in the range of 0.01 Ohm/ft) should provide very significant reduction m signal loss due to capacitive leakage. Power line systems, where cable resistance typically is lower, probably will show marked reduction in leakage losses even with inductors spaced further apart.

In more general terms, correction of leakage requires a different approach from the correction of series inductance. To completely eliminate the effect of the series inductor, a single capacitor is sufficient, and the placement of that capacitor is immaterial, as long that it is in series with the cable conducting core. On the other hand, optimal control of signal leakage requires a multitude of shunt inductors placed as close as possible to one another, so that each short cable elementconsitutes a parallel LC circuit between the conducting core and ground This requirement can be best realised as part of the process of cable manufacture, and while it would require retooling, and possibly new materials technology, this approach will provide loss-less cables.

As noted earlier, in some situations, especially in communication over the telephone local loop, it is desirable to transmit a plurality of signals at the same time. Fig. 13 addresses the situation where a plurality of transmitters (600, 601, ...) are connected to the same cable, and the circuit is set up to resonate at different frequencies through the selection of different coupling capacitors (300, 301 , . .) Each transmitter is connected to the capacitor through a bandpass filter (700, 701, ...) set to the resonant frequency achieved by that same capacitor together with the inductance of the cable It can be seen that a signal at resonant frequency fl, injected by transmitter (600) , through bandpass filter (700) into capacitor (300) will flow into the cable, resonate with inductor (110) , and emerge across load (50) . That signal cannot be shunted into transmitter (601) because bandpass filter (701) , which is set to a different frequency f2 , presents to frequency fi an extremely high impedance. Insofar as relates to frequency fi, capacitor (301) is an open circuit. A similar situation in the reverse holds for signals sent by transmitter (601), through bandpass filter (701), into capacitor (301) and thence into the circuit to produce a voltage drop across load (50) . As long as fi and h are not harmonically related, the two transmitters can coexist on the same cable or telephone circuit, and the circuit will resonate concurrently at these two different frequencies fi and f_.

CONCLUSIONS, RAMIFICATIONS AND SCOPE

The present invention makes possible operation of existing cabling at data rates at least 10 times faster than currently possible, and most likely 100 times faster. Alternatively, consider- ably longer cable runs may be utilized with superior transmission quality, and without requiring repeaters.

The principal innovation in the present invention is the use of external capacitors and inductors to condition the cable so as to compensate for the intrinsic inductance and shunt capacitance of the cable. To be effective, this innovation requires that the circuit be operated at the resonant frequency.

In some applications, compensation of the series induction of the cable is the primary concern, as it is in "loaded" telephone local loops, where the present invention allows conditioning of the loop to high frequency signals without requiring removal of the loading coils, the location of which in many cases is not even known any longer. Moreover, the required conditioning of the local loop can be affected by a single series capacitor (or in certain "unloaded" local loops, by a single inductor placed in series with the existing local loop) . The required component can be placed either at the telephone company Central Office (CO) , or at the customer's premises, without need to physically manipulate the local loop cabling.

In other applications, as in power transmission, the compensation of the shunt leakage is the primary concern. In these situations, conditioning of the line requires a plurality of components which need to be placed at relatively close proximity, e g at each power pole

The present approach also entails one restriction, that a single frequency be employed, i.e the frequency of resonance. For use in LAN or in MAN (Metropolitan Area Networks) , this is not a major restriction, insofar that networks in any event operate at a set frequency Given a range of capacitors and inductors, it is possible to achieve resonance practically at any desired frequency, so that existing networks may be readily adapted to resonate at the set operating frequency without need for complete re-engineering of transceivers, repeaters, and other commercially available active components. In fact, the added capacitors and inductors can be placed in a simple adapter box interposed between the transceiver and the cable Such adapters may be manufactured at extremely low cost with different capacitor and inductor values, and include also trimmers and variable capacitors.

More elaborate designs, containing active components capable of simulating variable capacitance and inductance, have been known in the art for quite some time (e.g. Rafael Elul & Abba Tamaπ "An Amplifier with Constant Unity Gain for Microelectrode Studies" EEG Clin.Neurophysiol., 1963, 15: 1 18- 122) . With such designs a wider range of capacitances and inductances may be simulated. The mam drawback to the active component approach is the need for components with high slew rate to provide adequate feedback at the 100 MHz operating frequency of many current LAN, which is likely to increase cost. Nevertheless, in comparison to existing solutions to the problem of transmitting over long distance at high data rates, the present invention provides a more efficient and inherently less expensive alternative, both for retrofitting of existing networks and for new installations

Another application of the present invention is in Cable Television where the same benefits accrue, i.e. higher data rates and reduced need for repeaters

For telephone lines, the present invention offers a practical low cost solution to transmission over the many millions of telephone local loops which had been "loaded" with 60-100 mH coils in series with the telephone line. Using the methods of the present invention, the series inductive load can be completely "zeroed out" at the resonant frequency. Even m telephone local loops where extraneous inductive coils have not been added, the present invention allows to electronically eliminate the native inductance of the local loop (around 1 mH for a typical residential telephone installation) , and concurrently to decrease the shunt leakage. This approach makes it possible to provide DSL service or other high-frequency data services over the telephone line to residential and business locations which are inaccessible to current technologies.

A fourth potential application of the present invention is in transmission of electric power When the concern is only with RMS power, longitudinal self-inductance is of little significance, since current stored in the inductor is ultimately recovered In power transmission the mam concern is with leakage capacitance, which causes non-recoverable loss of energy The method described in the present invention to decrease shunt leakage is of greater interest in this particular context, in that it provides a means to decrease the shunt to ground. Again, the requirement to operate at a single set frequency is not a hindrance, since power transmission systems typically utilize only a single set frequency.

Unjustifiably, resonance is often associated with oscillations and with the uncontrolled broadcasting of signals to adjacent cables While it is true that broadcasting antennas generally utilize resonant circuits, it is important to keep in mind that any RLC circuit, on its own, is incapable of oscillations. Resonance is a condition of reduced energy loss and reduced damping, but not a condition of instability in the cable. Transceivers or repeaters which are inherently unstable and depend on the damping provided by the line to control their instability will of course become more prone to instability when the damping is removed. Such transceivers may need to be redesigned, or at least be fitted with substitute damping circuits in order to operate with lines conditioned according to the present invention. First and foremost, common sense needs to be applied; for example, in the situation illustrated in Tables 1 and 2 and in Fig. 8, where attenuation may be reduced by 60 dB, it is reasonable to decrease the input signal. Obviously, if the input signal also were decreased by 60 dB, the system will be no less stable than an unconditioned line. Thus, it is not resonance but overdriving which can produce oscillations and signal broadcasting. As long as the signals inserted into the cable in the present invention are kept withm appropriate limits, there is no risk of signal broadcasting In fact, the reduction of the effect of shunt capacitance can only contribute to reduce NEXT (Near End Cross Talk) and other forms of crosstalk.

One limitation of the present invention is that the bandwidth available is relatively narrow Some control over the bandwidth is available through use of capacitors with different Q (Quality factor) values. As is well known, the change in impedance at resonance is greater when the capacitor used has a high Q (i.e low loss), but on the other hand a high Q also implies a narrower bandwidth which will produce higher level of distortion in square waves The highest level of signal improvement probably is more compatible with coding schemes such as PSK (Phase Shift Key Coding) and related schemes, than with Manchester-type codes

At the same time, the narrow bandwidth is an advantage in that it provides the potential for more efficient utilization of the frequency spectrum As described m the preceding section, it is possible to transmit simultaneously a multiplicity of signals at different resonant frequencies Interference among these different frequencies is minimized in circuits possessing higher Q- factor.

When shunt capacitance was considered in the preceding section "Detailed Description of the Present Invention", the conclusion was reached that a larger number of parallel inductors distributed evenly throughout the length of the cable will minimize leakage more effectively than two inductors placed at both ends. In practice, such intermediate placements of inductor coils may be made at points where repeaters are present, but a more appropriate solution for some applications may be the placement of inductors every 50 or 100 ft during in the manufacture of the cable. In a coaxial cable these inductors may be placed between the core conductor and the sleeve, in a twisted pair the inductor will need to "short-circuit" between the two leads.

Claims

1 A method for decreasing longitudinal impedance of an electrically conducting cable, whether containing a single lead or a multiplicity of leads, whether surrounded by a conducting sleeve or not, by the addition to said cable of a capacitor or a plurality of capacitors connected in series with said cable, at any location between the source of signal or power, and the load at the other end of said cable, and adjusting the magnitude of these capacitors to obtain resonance at a desired frequency

2 A method for reducing shunt leakage from an electric cable to ground or to other nearby cables, by adding an inductor, or a multiplicity of inductors between the cable and ground or between the cable and an adjacent lead, and adjusting the magnitude of these inductors to obtain resonance at a desired frequency

3 A method for combining the methods of Claims 1 ,2, in the same cable, by adjusting the resonance frequencies described in Claims 1, and 2, respectively, until these two frequencies are identical

4. In a cable of a given longitudinal impedance and leakage, a method for producing a multiplicity of resonant frequencies by employing multiple capacitors and/or inductors of different magnitudes, to interact with the intrinsic inductance and capacitance of same cable so as to minimize the effects of said intrinsic inductance and capacitance.

5. Transmission of electric power over a cable configured according to Claims 1-4 above.

6. Transmission of information over a cable configured according to Claims 1 -4 above using encoding on the resonant frequency or on the multiple resonant frequencies, in either simplex or duplex mode.

7. Transmission of television programming and bi-directional video-on-demand over a cable configured according to Claims 1-4 above, using encoding at the resonant frequency or at multiple resonant frequencies.

8. Transmission of telephone signals, whether analog or digital, including both speech and data, either simplex or duplex, over subscriber (local) loops and trunk lines configured according to Claims 1-4 above using encoding on the resonant frequency or on multiple resonant frequencies.

9. In the systems described in Claims 1 -8, detection at one end of said cable, of a signal sent from the other end of the cable at a resonant frequency, through the use of a circuit tuned to the exact frequency employed for transmission of that same signal.

10. In the systems described in Claims 1-8, reducing cross-talk by tuning adjacent cables to transmit information at different "carrier" frequencies.

1 1. In the systems described in Claims 1-8, concurrent and/or simultaneous bi-directional communication at or close to the resonant frequency or resonant frequencies of the cable.

12. Coaxial cable, twisted-pair or multiple-lead cable with inductors inserted at intervals along the cable to connect one lead to the other, or the central conductor to the shield, in such manner so as to minimize signal leakage