WO2008084238A1

WO2008084238A1 - Filtering device

Info

Publication number: WO2008084238A1
Application number: PCT/GB2008/000083
Authority: WO
Inventors: Martinus Hermanus Maria Rijssemus
Original assignee: Technetix Limited
Priority date: 2007-01-12
Filing date: 2008-01-11
Publication date: 2008-07-17
Also published as: GB0700657D0; GB2446848A

Abstract

A filtering device (1) for providing a higher frequency information carrying current on the same conductor as a lower frequency power carrying current comprises a first conductor (5) suitable for carrying alternating current signals in a desired frequency range and a second conductor (2) connected to the first conductor and suitable for carrying direct current and alternating current below the desired frequency range. The second conductor comprises a first inductor (8) suitable for providing impedance to at least some electrical oscillations in the desired frequency range and a second inductor (9) connected in series with the first inductor (8), which is suitable for providing impedance to at least some electrical oscillations in the desired frequency range that are not sufficiently reduced by the first inductor.

Description

Filtering Device

Field of the Invention

This invention relates to a filtering device for providing a higher frequency information- carrying signal on the same conductor as a lower frequency power-carrying signal, in particular this invention relates to a device for providing power, cable television and Ethernet connectivity over coaxial cable.

Background to the Invention Ethernet over coax is a system that uses a single coaxial cable as a physical layer for

Ethernet rather than the standard Unshielded Twisted Pair (UTP) cables such as Cat 3 or Cat 5e. It is known to use special adapters to transform the standard current Ethernet physical format into an Ethernet over coax format. It is also known to provide power over a UTP cable in a system known as power over Ethernet.

Summary of the Invention

In its broadest aspect, this invention provides a filtering device for providing a higher frequency information-carrying signal on the same conductor as a lower frequency power- carrying signal. The device comprises a first conductor suitable for carrying alternating current signals in a desired frequency range and a second conductor electrically connected to the first conductor and suitable for carrying direct current or alternating current below the desired frequency range. The second conductor comprises a first inductor that provides an impedance to electrical oscillations having a first range of frequencies in the desired frequency range, and at least a second inductor connected in series with the first inductor. The second inductor provides an impedance to electrical oscillations having a second range of frequencies in the desired frequency range, the second range of frequencies being different to the first range of frequencies. Thus, the second conductor provides a high impedance path for signals in the desired frequency range to prevent substantial distortion of an information-carrying signal within the desired frequency range on the first conductor. The second conductor also provides a lower impedance path for a power-carrying signal having a frequency below the desired frequency range.

Thus, according to the invention, the second conductor includes at least two inductors each providing an impedance to different frequency ranges, such that the second conductor provides a high impedance path for signals in the desired frequency range to prevent substantial distortion of an information-carrying signal on the first conductor due to leakage onto the second conductor. The use of two or more inductors allows the frequency response of the second conductor to be "tuned" to ensure that the high impedance is maintained for all frequencies within the desired frequency range while still using practical components. Thus, typically, the first frequency range and the second frequency range may overlap or one may even be a sub-range of the other.

The filtering device may comprise at lease one additional inductor connected in series with the first and second inductors, wherein the additional inductor(s) provide(s) an impedance to electrical oscillations having a further range of frequencies in the desired frequency range, the further range of frequencies being different to the first range of frequencies and the second range of frequencies. Thus, further "tuning" of the frequency response of the second conductor may be achieved by the provision of additional inductors.

In some embodiments, the second conductor includes a connection to earth via an earth capacitor for grounding electrical oscillations in the desired frequency range. The inductors may be located between the earth connection and the electrical connection to the first conductor. The earth capacitor provides a low impedance path for signals within the desired frequency range that originate from the power supply. The first conductor may have an output for the combined higher frequency information- carrying signal and lower frequency power-carrying signal. The first conductor may include a first capacitor that provides an impedance to signals below the desired frequency range. The second conductor may be connected to the first conductor between the output and the first capacitor. In this way, the lower frequency power signal from the second conductor is directed in one direction along the first conductor and prevented from reaching the source of the higher frequency information-carrying signal.

The second conductor may provide an impedance to alternating currents in the desired frequency range that is at least five times greater than the impedance provided by the first conductor. The second conductor may provide an impedance to alternating currents in the desired frequency range that is at least 375 Ω.

In preferred embodiments, the alternating current signals (information-carrying signals) are Ethernet signals and the desired frequency range is the frequency range used by Ethernet, for example between 300 KHz and 125 MHz.

In preferred embodiments, the alternating current signals (information-carrying signals) are Ethernet and Cable Television signals and the desired frequency range is the frequency range used by Ethernet and Cable Television, for example between 300 KHz and 862 MHz.

The first inductor may have an inductance greater than 200 μH. All the inductors may have an inductance greater than 70 nH.

Another aspect of this invention relates to an apparatus for transmitting power and information carrying signals of a different frequency on the same conductor comprising a first filtering device and a second filtering device as described above, wherein a further conductor joins the first conductor of the first device to the first conductor of the second device. Brief Description of the Drawings

An embodiment of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 is a schematic representation of a filtering device according to one embodiment of the invention;

Figure 2 is a schematic representation of a filtering device according to a second embodiment of the invention; and

Figure 3 is a schematic representation of a system that incorporates devices according to the second embodiment of the invention.

Detailed Description of Exemplary Embodiments

Figure 1 shows schematically a filtering device 1 according to a first embodiment of the invention. Power is supplied to the device through a power port 2. This power can be AC or DC. An earth capacitor 3 is connected to earth so as to provide a low impedance path to ground for any high frequency leakage to the power port 2.

A high frequency information-carrying signal is provided to the filtering device 1 at an input port 4. hi the present embodiment, this signal is both an Ethernet over coax signal and a Cable Television signal. It is often necessary, particularly in the case of the Ethernet signal, that two signals can be carried along a cable, one a transmitting signal and the other a receiving signal. These signals can travel in opposite directions along the same cable.

The output of the filtering device 1 is provided at an output port 5. This output is to a coaxial cable and carries both the power supplied at power port 2 and the high frequency signal supplied at input port 4.

A protective capacitor 6 provides an impedance to direct currents and low frequency alternating currents, so that they will not be transmitted up the high frequency cable at input port 4, where they could cause damage to the equipment that is transmitting and receiving high frequency signals. Without some mechanism to prevent it, a significant portion of the high frequency information carrying signal would be lost as it passed through the filtering device 1. This would happen because the signal would be split between the output port 5 and the power path 7. That part of the signal that travelled down the power path 7 would escape through the power port 2, or to earth via the earth capacitor 3. This loss of signal strength would often result in the high frequency information carrying signal being rendered useless.

In order to preserve the characteristics of the high frequency signals therefore, it is desirable that the filtering device 1 should offer an impedance on the low frequency power path of at least five times that of the high frequency path to all oscillating electrical signals with frequencies in the high frequency range. In this embodiment, Ethernet over coax has a frequency range from 300 KHz to 125 MHz, although in access networks that include FM radio this range may be reduced to between 300 KHz and 65MHz. However, the requirement to provide transmission of Cable Television signals increases this range of frequencies. If the high frequency signals are both Ethernet and Cable Television signals then the low frequency path must provide high impedance to a range of frequencies from 300 KHz to 862 MHz, which is approximately 11.5 octaves.

The impedance of the high frequency path in this embodiment is 75 Ω, which is a typical value for this equipment. Therefore power path 7 must provide an impedance of at least:

5 x 75 = 375 Ω at a frequency of 300 KHz. The impedance of a perfect inductor, that is one that has no parasitic capacitances, is related to the frequency of the current passing through it by the equation: z = ωL

Where z is the impedance measured in Ohms, ω is the frequency measured in radians, and L is the inductance measured in Henrys. If the impedance is to be 375 Ω at a frequency of 300 KHz, then the inductance must be:

L ~ 200 μH In order to preserve the characteristics of the high frequency signals therefore, we need an inductor in the power path 7 with an inductance of at least 200 μH. Usually this would be an inductor of at least 220 μH, as such inductors are widely commercially available. With only a single inductor in the power path 7, the inductor must be able to maintain good working characteristics up to a frequency of 862 MHz. However, inductors are never perfect, and will always exhibit parasitic capacitances that cause their behaviour to break down at sufficiently high frequencies. Instead of the impedance they present to an alternating current increasing with ω, it will eventually begin to decrease. This phenomenon, known as self resonance, makes it extremely difficult to build an inductor that will have all the characteristics desired. Moreover, any non-superconducting inductor possesses an internal resistance that affects its quality factor (Q). As the current put through the inductor increases, the inductance it exhibits can decrease.

The present invention seeks to solve such problems by providing multiple inductors in the power path, three in the case of the current embodiment. There are two second inductors 9 and 10, which have smaller inductances, typically of the order of 10 μH, than the main inductor 8, which has a typical inductance of 220 μH. The second inductors are intended to provide significant impedance at higher frequencies, typically over 10MHz. Their inductances are chosen so that they will be sufficient to dampen the self resonance of the main inductor 8, but still be small enough that they will not have self resonance points in the frequency range 300 KHz to 862 MHz. In this way, the device 1 can provide the necessary impedance of at least 375 Ω across a frequency range of 300 KHz to 862 MHz.

In a simple embodiment such as this one, the smallest inductor 10 has an inductance of at least 70 nH, since a smaller inductor would not be able to provide a significant impedance of 375 Ω even at the highest desired frequency of 862 MHz. However, this may not always be the case since a plurality of inductors with low inductance could be used to provide a component that would behave very much like a single inductor with a higher inductance.

Although most AC power operates at frequencies far below the frequencies used by Ethernet and Cable Television, it is possible find some oscillations in the current of a power supply that are of a frequency whereby they can interfere with these signals. Even when using DC power, spikes in the power supply and residual oscillations from electrical components can cause similar problems. This is particularly true when the DC power is generated by converting AC power, as will be the case with any device that functions on mains power. The current embodiment also helps to prevent such oscillations from interfering with the high frequency information carrying signal. It does this by providing a route to ground through the earth capacitor 3 that has lower impedance for signals in the desired frequency range than the power path 7.

The current embodiment also provides a further capacitor 11 and a resistor 12 in parallel with the main inductor 8. These components are intended to help tune the response of the main inductor 8. The main inductor 8, further capacitor 11 and resistor 12 form a band- stop filter that has a high impedance around its resonant frequency. The values of the components can be chosen so that this resonant frequency lies in the lower part of the desired frequency range. In this embodiment, for example, the resistor has a value of 1,000 Ohms and the further capacitor has a value of 47 pF.

Figure 2 shows schematically another embodiment of the invention in a different context. In this embodiment, the filtering device 21 is provided with power from a power port 22. Two routes to earth are provided via two identical earth capacitors 23 and 24. Both earth capacitors 23 and 24 have a capacitance of 220 nF, and are intended to serve the same purpose as the earth capacitor 3 in the first embodiment.

The main inductor 25 provides an inductance of 330 μH. The main inductor is arranged in parallel with a resistor 26 which provides a resistance of IK Ohm. Three additional inductors are provided: the first additional inductor 27 typically provides an inductance of 15 μH, while the second and third additional inductors 28 and 29 are chosen so that their combined contribution is to provide an inductance similar to the main inductor 25; using two inductors in this way can be easier and cheaper than using a single, larger inductor. Between them, the inductors ensure that the device 21 provides a high enough impedance at all the required frequencies. The power signal is combined with the high frequency information carrying signal at a junction 30.

Ethernet signals for transmission are input through a modular connector 31. These signals are conveyed by an Unshielded Twisted Pair (UTP) cable to a balun 32. This provides a transition from UTP cable to a single line of transmission. The signal is then passed through band-pass filter 33 to help prevent interference before being passed through an amplifier 34. A splitter at 35 provides isolation between transmitted and received Ethernet signals. Received signals go through the same process as transmitted signals, but in reverse. They pass from the splitter 35 to an amplifier 36. They then travel through a band-pass filter 37 and a balun 38 onto a UTP cable and so on to the modular connector 31.

Cable Television signals are provided at an input 39. They are combined with the Ethernet signals by a splitter 40, which includes both inductors and capacitors arranged to act as filters and so prevent either signal being transmitted down the wrong wire.

In this way, an electric current that supplies power, Ethernet and Cable Television over a coaxial cable is provided at an output 41. This can be carried to the desired destination, where a substantially similar apparatus will serve to separate out the components again.

There are many conceivable uses of a system that provides these facilities. One example is illustrated in Figure 3, which includes two filter devices 51 and 52, both of which are in accordance with the embodiment of the invention illustrated in Figure 2.

The first device 51 is connected to a control system 53 via a modular connector through which an Ethernet connection is maintained. The first device 51 is supplied with power through an outlet 54 and is also connected to a television monitor 55.

The second device 52 is contained within a security camera 56, and connected to the first device 51 by a coaxial cable. The second device 52 is connected to a microprocessor 57 by a modular connector through which an Ethernet connection is maintained. This microprocessor controls a servo mechanism 58. The servo mechanism 58 can change the direction of the camera, the level of zoom, the depth of field, etc.

The second device 52 directs the power it receives along a connection 59 to both the servo mechanism 58 and to an image processing apparatus 60. The image processing apparatus 60 incorporates a charge coupled device and generates the cable television signal that is transmitted, via first and second filter devices 52 and 51, to the television monitor 55. In this way, a system is provided that allows an operator using the control system 53 and watching television monitor 55 to have control over the camera 56. There is no need to lay more than one cable between the control station and the camera 56, or to provide for an additional power supply at the camera's location.

In summary, a filtering device 1 for providing a higher frequency information carrying current on the same conductor as a lower frequency power carrying current comprises a first conductor 5 suitable for carrying alternating current signals in a desired frequency range and a second conductor 2 connected to the first conductor and suitable for carrying direct current and alternating current below the desired frequency range. The second conductor comprises a first inductor 8 suitable for providing impedance to at least some electrical oscillations in the desired frequency range and a second inductor 9 connected in series with the first inductor 8, which is suitable for providing impedance to at least some electrical oscillations in the desired frequency range that are not sufficiently reduced by the first inductor.

Claims

1. A filtering device for providing a higher frequency information-carrying signal on the same conductor as a lower frequency power-carrying signal, the device comprising: (i) a first conductor suitable for carrying alternating current signals in a desired frequency range; and

(ii) a second conductor electrically connected to the first conductor and suitable for carrying direct current or alternating current below the desired frequency range, wherein the second conductor comprises: a first inductor that provides an impedance to electrical oscillations having a first range of frequencies in the desired frequency range; and at least a second inductor connected in series with the first inductor, wherein the second inductor provides an impedance to electrical oscillations having a second range of frequencies in the desired frequency range, the second range of frequencies being different to the first range of frequencies, whereby the second conductor provides a high impedance path for signals in the desired frequency range to prevent substantial distortion of an information-carrying signal within the desired frequency range on the first conductor and the second conductor provides a lower impedance path for a power-carrying signal having a frequency below the desired frequency range.

2. A filtering device as claimed in claim 1, wherein the filtering device comprises at lease one additional inductor connected in series with the first and second inductors, wherein the additional inductor(s) provide(s) an impedance to electrical oscillations having a further range of frequencies in the desired frequency range, the further range of frequencies being different to the first range of frequencies and the second range of frequencies.

3. A filtering device as claimed in claim 1 or claim 2, wherein the second conductor includes a connection to earth via an earth capacitor for grounding electrical oscillations in the desired frequency range, and the inductors are located between the earth connection and the electrical connection to the first conductor.

4. A filtering device as claimed in any preceding claim, wherein the first conductor has an output for the combined higher frequency information-carrying signal and lower frequency power-carrying signal, the first conductor includes a first capacitor that provides an impedance to signals below the desired frequency range, and the second conductor is connected to the first conductor between the output and the first capacitor.

5. A filtering device as claimed in any preceding claim, wherein the second conductor provides an impedance to alternating currents in the desired frequency range that is at least five times greater than the impedance provided by the first conductor.

6. A filtering device as claimed in any preceding claim, wherein the second conductor provides an impedance to alternating currents in the desired frequency range that is at least 375 Ω.

7. A filtering device as claimed in any preceding claim, wherein the alternating current signals are Ethernet signals and the desired frequency range is the frequency range used by Ethernet.

8. A filtering device as claimed in claim 7, wherein the desired frequency range is between 300 KHz and 125 MHz.

9. A filtering device as claimed in any of claims 1 to 6, wherein the signals are Ethernet and Cable Television signals and the desired frequency range is the frequency range used by Ethernet and Cable Television.

10. A filtering device as claimed in claim 9, wherein the desired frequency range is between 300 KHz and 862 MHz.

11. A filtering device as claimed in any preceding claim, wherein the first inductor has an inductance greater than 200 μH.

12. A filtering device as claimed in any preceding claim, wherein all the inductors have an inductance greater than 70 nH

13. An apparatus for transmitting power and information carrying signals of a different frequency on the same conductor comprising a first filtering device as claimed in any preceding claim and a second filtering device as claimed in any preceding claim, wherein a further conductor joins the first conductor of the first device to the first conductor of the second device.