AU2019101212A4 - Galvanic Isolator with Improved High Frequency Screening for Coaxial Distribution Networks - Google Patents

Abstract

Abstract: A galvanic isolator with improved RF screening efficiency comprising a common mode filter and ferrite inductive elements. Cp l ci ---------- Figure 1. Figure 2.

Description

Galvanic Isolator with Improved High Frequency Screening for Coaxial Distribution Networks.

FIELD OF THE INVENTION [0001] The present invention relates generally to novel structures, component arrangements and assembly methods which improve the radio frequency screening performance of Galvanic Isolators.

[0002] The invention has been developed primarily for use as a galvanic isolator in coaxial communication distribution networks and will be described hereinafter with reference to this application. However, it will be appreciated that the invention is not limited to this particular field of use.

BACKGROUND OF THE INVENTION [0003] As the world’s demand for entertainment and information content increases, new means of distributing this content are being developed. Cable TV (CATV) networks have been deployed since the 1980’s and are an example of a telecommunication network that was built to offer subscribers a significantly increased range of content. Coaxial cable has traditionally been used for such distribution networks because it has relatively low cost and because it simplifies connection to network devices and customers premises. Network coaxial cables consist of outer plastic insulation, a conductive outer sheath, a low loss insulator and central conductor. Although original CATV networks were entirely made from coaxial cables, modern networks often employ a so called Hybrid Fibre Coax (HFC) structure where connectivity is provided using optical fibres for the core network and coaxial cables for connection to customer’s premises.

[0004] Although the content capacity of CATV networks has previously met subscriber’s requirements, there is a growing demand for subscriber customised content, for example in the form of streaming video on demand and other internet related sources of information or entertainment content. As a result network operators are under increased pressure to make use of the full

2019101212 04 Oct 2019 bandwidth capacities of their networks and/or to increase their network bandwidth capacities by upgrading network elements.

[0005] In typical installation scenarios, a Tap is installed on the network coaxial cable as it passes a user’s premises and a drop cable is run from the tap into the user’s building. This connection unusually terminates inside the building at a network element such as a set-top-box (STB) which decodes network signals and connects to user devices such as TVs or computer network devices.

[0006] When an electrically conductive cable enters a user’s premises there is an inherent risk that small voltage differences at each end of the cable can cause dangerous currents to flow along the cable. This may occur, for example, if the network coax cable in the street is connected to the neutral connection of the power grid and the power distribution system uses Main Earthed Neutral (MEN) connection schemes inside the premises. In this case, if a building has a high resistance power neutral connection, the neutral return current for the premises can potentially flow along the coax cable via set top box connection, thereby creating the risk of overload and fire. For this reason, network operators typically install galvanic isolators.

[0007] There is also a risk that cables entering a premises can convey dangerous voltages as a result of power grid faults or transient lightning surges. In this case, a galvanic isolator is designed to withstand dangerous voltages and limit the current flowing into the user’s premises.

[0008] A galvanic isolator is therefore a device which permits the passage of high frequency information-containing signals through the device and blocks the passage of low frequency current from mains frequency power systems and current surges such as produced from lightning strikes.

[0009] Figure 1 shows a circuit schematic of one type of conventional galvanic isolator. In this design, isolator functionality is split into two stages: a signal coupling stage and a filtering stage. Referring to Figure 1, components C1, C2 and T1 form the signal coupling stage. The values of the components are chosen to allow the passage of high frequency signals carrying data, for

2019101212 04 Oct 2019 example above 5MHz, and to block the flow of current from 50-60Hz power mains and surges, for example, surges induced by lightning strikes.

[0010] An unavoidable aspect of any isolator’s design is the need to break ground conductor continuity in order to introduce an isolating component between incoming and outgoing signal port grounds. This causes signals passing through an isolator to ‘leak’ out and appear as a differential signal between ground connections of the isolator ports, thereby potentially generating a common mode signal with respect to ground and producing interference in radio frequency bands. This same mechanism also allows signals to leak into the distribution network at each isolator, resulting in impaired network performance.

[0011] To lessen the effect of this problem, isolators employ a common mode filtering stage which attenuates signals leaving or entering the network at the isolator. Figure 1 shows an example of a conventional two stage filter. Components C3, F1 and C4 form a first ‘Pi-section’ filter and components, C4, F2, C5 and C6 form a second ‘Pi-section’ filter. Conventional isolators may use any number of common mode filtering stages.

[0012] Conventional isolators implement these common mode filtering stages as lengths of coaxial cable 103 which pass through ferrite beads, for example F1 and F2 in Figure 1. In this situation capacitors are connected to the coaxial cable outer screen on each side of the ferrite beads, for example C3, C4, C5 and C6 in Figure 1.

[0013] Conventional isolators use a low inductance capacitor as the final filter capacitor, shown as C6 in Figure 1. A low inductance capacitor is thought to be needed at this point to provide maximum attenuation of high frequency common mode signals. This capacitor is conventionally implemented as either a double-sided printed circuit board (PCB) with capacitor electrodes arranged

2019101212 04 Oct 2019 on opposing sides, or as a single sided PCB with an electrode on one side and where the opposing electrode is provided by one surface of the isolator’s shielding enclosure.

[0014] Figure 2 shows a front and back side view of a conventional double sided PCB 200 suitable for implementing capacitor C6 in Figure 1, where surface electrodes 201 and 202 are arranged on opposing surfaces.

[0015] Capacitor C5 in Figure 1 is optional and may not be fitted. If fitted, C5 will is conventionally a discrete capacitor, commonly a radial leaded ceramic capacitor.

[0016] Figures 3a and 3b show isometric views of an example of a conventional isolator comprising PCB capacitor 300 and associated components located inside isolator metallic shield 304. Conventional isolators comprise connectors 305 and 306 used to connect the isolator to network cables. One connector 306 is coupled directly to the coaxial cable 303 which forms the common mode filter described in Figure 1. Only one ferrite bead 307 is shown in Figure 3 for simplicity of illustration. Conventionally multiple ferrite beads are used.

[0017] Conventional isolators couple connector 306 directly to one electrode 301 of the PCB capacitor 300 by soldering the connector body directly to the PCB surface 308. This is conventionally regarded as the optimal connection strategy for connecting these components to minimise parasitic inductance and to achieve maximum screening efficiency, meaning that ingress and egress of signals to and from the isolator is minimised.

[0018] Despite this minimal inductance connection, conventional isolators are known to suffer from high degrees of signal ingress and egress, particularly at high frequency.

2019101212 04 Oct 2019 [0019] Accordingly, the inventor has realised that there is a need for a new isolator architecture which improves RF screening and minimises signal ingress and egress.

[0020] The discussion throughout this specification comes about due to the realisation of the inventor and/or the identification of certain prior art problems by the inventor and, moreover, any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material forms a part of the prior art base or the common general knowledge in the relevant art in Australia or elsewhere on or before the priority date of the disclosure and claims herein.

2019101212 04 Oct 2019

SUMMARY OF THE INVENTION [0021] It is an object of the present invention to provide a method and apparatus, which alleviates at least one disadvantage associated with related art arrangements as discussed herein.

[0022] According to a first aspect, the present invention provides a galvanic isolator with improved RF screening characteristics which results in reduced signal ingress and egress.

[0023] It is therefore an object of the preferred embodiments described herein to overcome or alleviate at least one of the above noted drawbacks of conventional systems or to at least provide a useful alternative to conventional systems.

[0024] These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. Accordingly, further scope of applicability of embodiments of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the disclosure herein will become apparent to those skilled in the art from this detailed description.

2019101212 04 Oct 2019

BRIEF DESCRIPTION OF THE DRAWINGS [0025] Further disclosure, objects, advantages and aspects of preferred and other embodiments ofthe present invention may be better understood by those skilled in the relevant art by reference to the following description of embodiments taken in conjunction with the accompanying drawings, which are given by way of illustration only, and thus are not limitative of the disclosure herein, and in which:

Figure 1 is a schematic diagram of a conventional galvanic isolator.

Figures 2a and 2b show a double sided PCB used to form a conventional filter capacitor.

Figures 3a and 3b are isometric views of a conventional isolator assembly.

Figures 4 is a schematic diagram of a conventional galvanic isolator showing parasitic ground impedance.

Figure 5 is a schematic showing a preferred embodiment ofthe present invention.

Figures 6a and 6b are isometric views of a preferred embodiment of the present invention.

2019101212 04 Oct 2019

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION [0026] Preferred embodiments of the present invention will now be described in relation to the drawings. Where possible, equivalent numbers have been used to identify the same element in each drawing or sub-drawing. Terms such as “upper” and “lower” or “top” and “bottom” are intended to aid description of the drawings as shown and are not meant to restrict the scope of the invention. In particular, a galvanic isolator is a bidirectional device, meaning that signals flow simultaneously in different directions through the isolator. Hence terms relating to “input” and “output” are used to describe certain features of the present invention and are not meant to restrict the scope of the invention.

[0027] The term “galvanic isolator” refers to a device, comprising at least two coaxial isolator ports, which permits the passage of high frequency information-containing signals through the device and blocks the passage of low frequency current from mains frequency power systems and transient voltage surges.

[0028] The term “isolator port” refers to the coaxial electrical interface point where a cable is coupled to a galvanic isolator.

[0029] The term “grounded port” refers to the isolator port where there is a low voltage DC resistance of less than 1 ohm between the outer conductor of the coaxial connector located at this port and the isolator’s metallic RF shield.

[0030] The term “isolated port” refers to the isolator port where there is a low voltage DC resistance of greater than 1 Mega-ohm between the outer conductor of the coaxial connector located at this port and the isolator’s metallic RF shield.

[0031] The term “signal path” refers to the passage of electromagnetic energy coupled from one isolator port, through isolator circuitry, to another isolator port in the form of differential voltages and corresponding currents which flow according to these voltages.

2019101212 04 Oct 2019 [0032] The terms “intrinsic capacitor” or “intrinsic capacitance” refers to capacitance unintentionally present in a circuit as a result of the proximity of electrically separate conductors.

[0033] The term “explicit capacitor” refers to a device or structure comprising two or more electrically conductive surfaces coupled to and separated by a dielectric material with relative permittivity greater than 1.

[0034] The term “intrinsic inductor” or “intrinsic inductance” refers to inductance unintentionally present in a circuit as a result of the electrical length of a conductor.

[0035] The term “explicit inductor” refers to a device or structure which deliberately provides more inductance at a point in a circuit than would be present at that point simply by the electrical length of a conductor.

[0036] The term “RF Screening” refers to the ability of an isolator to minimise the ingress or egress of signals to and from a coaxial distribution network, which occurs as a result of finite coupling impedance between the ground connections of the isolator’s signal ports.

[0037] The term “Bead” or “Ferrite Bead” refers to a piece offerrite material which has a hole through which a conductor or cable can be inserted.

[0038] According to a first aspect, the present invention provides a galvanic isolator comprising a common mode low pass filter formed from explicit inductive elements and explicit capacitive elements, and a coaxial connector wherein:

• said inductive elements of said filter comprise ferrite beads fitted to a length of coaxial cable • the outer conductor of said coaxial cable is electrically bonded to the outer surface of said connector, and

2019101212 04 Oct 2019 • at least one ferrite bead is fitted to said coaxial cable between said connector and any of said explicit capacitors of the filter.

[0039] Contrary to conventional belief, the inventor has realised that it is not advantageous to closely couple an explicit capacitor to the connector of the common mode filter. Instead, RF screening efficiency can be improved considerably by eliminating this capacitor and locating an explicit inductive element on the coaxial cable immediately next to this connector.

[0040] Referring to the generic, simplified circuit of a conventional isolator in Figure 1, PCB capacitor C6 is connected between the outer shield connection of coaxial connector J2 and the outer shield connection of coaxial connector J1. This capacitor is intended to provide a high frequency bypass current path between to ground connections of the isolator ports, thereby minimising high frequency signal ingress and egress.

[0041] The inventor has found that this technique is not only ineffective, it actually degrades high frequency screening of conventional isolators. Figure 4 shows a schematic of a conventional isolator including ground impedance Z1. This impedance is not strictly localised as implied by the Z1 circuit symbol but in reality is distributed along the common connection path of capacitors C1 to C6. This impedance is associated with the finite impedance of ground conductors inside the isolator as well as the isolator’s enclosing RF shield.

[0042] Referring to Figure 4, currents flowing in capacitors C3 and C4 from the first stage of the common mode filter (formed by these capacitors and inductive elements F1 and F2) cause a voltage to appear across the ground impedance Z1. If capacitors C5 or C6 are included in the circuit, these capacitors couple this voltage directly to connector J2, thereby compromising the performance of the filter and degrading screening efficiency.

2019101212 04 Oct 2019 [0043] Figure 5 shows one preferred embodiment of the present invention. Explicit capacitors associated with the isolator’s common mode filter, C3 and C4 in this example, do not have a common connection to coaxial connector J2. Instead, connector J2 is connected to the common mode filter stage through ferrite beads F2 and F3 which are fitted to the coaxial cable 503 passing between the filter and connector J2. Multiple ferrite beads are preferably used at this point of the circuit to improve screening efficiency and make up for reduction in filter efficiency as a result of the elimination of explicit capacitors C5 and/or C6.

[0044] Although eliminating capacitors C5 and C6 would decrease the effectiveness of an ideal common mode filter (with no ground impedance Z1), the inventor has found that eliminating these capacitors and instead introducing additional inductive decoupling circuit elements significantly improves isolator RF Screening effectiveness.

[0045] Figures 6a and 6b show isometric views of an example of one embodiment of the present invention. In this example, PCB 601 is retained although it no longer contains a PCB capacitor as shown in Figure 2. In this situation the PCB may allow filter capacitors 610 and C11 to me mounted and connected to coaxial cable 603 at selected points 612. Ferrite bead 609 is included between the nearest explicit filter capacitor 610 and coaxial connector 606 according to the present invention. Although a single ferrite bead 609 is shown in the illustration, one or more ferrite beads may be used in place of bead 609 within the scope of the invention.

[0046] It should be noted that the particular arrangement of components shown in figures 5 and 6a/6b are illustrative only and are not intended to limit the scope of the invention.

[0047] As noted above, while this invention has been described in connection with specific embodiments thereof, it will be understood that it is

2019101212 04 Oct 2019 capable of further modification(s). This application is intended to cover any variations, uses or adaptations of the invention following in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth.

[0048] “Comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.” Thus, unless the context clearly requires otherwise, throughout the description and the claims, the words ‘comprise’, ‘comprising’, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.

Claims (1)

1. THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

   1. A galvanic isolator comprising:

   • a common mode low pass filter formed from explicit inductive elements and explicit capacitive elements, • a coaxial connector, and • a coaxial cable wherein:

   • said inductive elements of said filter comprise ferrite beads fitted to said coaxial cable, • the outer conductor of said coaxial cable is electrically bonded to the outer surface of said coaxial connector, and • said coaxial cable passes through at least one ferrite bead before it is electrically connected to any of said explicit filter capacitors.