WO1998002022A1

WO1998002022A1 - Isolator for signal carrying lines

Info

Publication number: WO1998002022A1
Application number: PCT/AU1997/000422
Authority: WO
Inventors: Shaun Joseph Cunningham; Graham Ross Daniels; Michael Lawrence Brothers; Anthony Paul BIRCH
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 1996-07-03
Filing date: 1997-07-03
Publication date: 1998-01-15

Abstract

The invention relates to a method of providing a DC and low frequency currents isolation barrier, and coupling of high frequency signals between sections of transmission line (4, 5; 4a, 5a). Circuit boards (8, 9) including conductive elements, arranged to be connected to respective sections of a transmission line (4, 5; 4a, 5a), are spaced from each other and a capacitor (C1), leadless, is mounted between the circuit boards (8, 9) so that the first terminal of the capacitor (C1) is in intimate contact with the first circuit board (8) and the second terminal of the capacitor (C1) is in intimate contact with the second circuit board (9). The arrangement is such that the capacitor (C1) couples signals across the isolation barrier from the first circuit board (8) to the second circuit board (9).

Description

ISOLATOR FOR SIGNAL CARRYING LINES

The present invention relates to an apparatus for providing low frequency isolation between sections of a signaJ transmission line and, particularly, but not exclusively, to a device for providing isolation between sections of distribution line to prevent or minimise the circulation of common-mode currents in cables which span nodes of a transmission circuit that have different ground potentials. The invention also relates to a method of manufacturing the apparatus.

The use of devices which provide galvanic isolation (DC and mains frequency isolation) between sections of signal distribution line is well known (hereinafter these devices are referred to as "isolators"). Such devices are necessary to prevent common-mode currents circulating m cables which span nodes of a transmission circuit that have different ground potentials. For example, m cable connected television distribution systems (cable TV) it is sometimes necessary to isolate the distribution network ground connection from the subscribers premises ground connection to prevent fault currents from flowing m cables which may not be capable of carrying them safely. Such currents introduce the risk of fire. Isolators can also be employed to ensure electrical safety of people who inadvertently come into contact with signal distribution networks (eg at a distribution outlet) with elevated ground potentials. Isolators are also capable of providing network equipment with a degree of immunity to surges caused by lightning or other network-borne transients. The act of isolating cable-based transmission systems often necessitates breaking the ground connection of the transmission cable which allows signals to both enter the cable and degrade signal quality and also leave the cable and interfere with external services (eg radio) . Minimising this signal leakage (hereafter referred to as "signa_^ leakage") is an important requirement of isolator design ana requires different circuit techniques at the low end and high end of the signal frequency band.

It is also important for an isolator to minimise any effects which may impair the flow of signals of the desired frequency range through it. The parameters which are critical for optimum isolator performance are: insertion loss (i.e. the signal attenuation suffered passing through the isolator), and return loss (i.e. the degree of matching of the isolator to the nominal impedance of the connecting cables) . In practice, optimising return loss is much more difficult than optimising insertion loss.

Known isolators use capacitive and inductive components to couple high frequency signals across the isolating barrier while blocking DC and low frequency currents. In the prior art, it has proved particularly difficult to achieve satisfactory coupling of wide band signals (e.g. 5MHz to 1GHz) while providing high voltage (e.g. 3 - 6kV) isolation. This difficulty arises since conventional high frequency circuits require physically small components arranged in close proximity while high voltage circuits require large components with substantial separation between connections.

In an attempt to achieve satisfactory high frequency and high voltage performance known isolators either use intricate and expensive high voltage capacitive structures to couple the high frequency signals across the isolation barrier or elaborate circuits which use numerous "low- quality" parts in tandem to approximate the performance of the more costly equivalents. Isolators built along the latter principle often exhibit poor high frequency performance (le excessive signal leakage and/or poor return loss) and inferior reproducability . Nevertheless, cost constraints often dictate the use of these isolators.

Known isolators employ either one or two distinct stages. The first, essential stage is associated with coupling of signals across the isolation barrier, tne second, optional stage forms a filter which attenuates any portion of the coupled signal which is inadvertently introduced between the input and output ground connections.

Known isolators using only one stage typically employ non-standard capacitors which offer large capacitance and high operating voltage. Such non-standard capacitive structures are expensive and require complicated mounting arrangements .

Known isolators employing two stages use standard, low cost, leaded components in both the coupling and filtering stages. The applicants believe that the parasitic inductance associated with the mounting arrangement of these leaded components significantly degrades the high frequency performance of the isolator. This inductance can be lessened by using many components in parallel (e.g., capacitors) or can be cancelled over a narrow frequency range by introducing resonant capacitance. Leaded components used in coupling stages of known isolators need to be "tuned" to minimise the effect of the parasitic inductance by bending them towards ground planes or other components, thereby introducing additional capacitance. The applicants believe that this tuning process is expensive and may be incapable of fully cancelling parasitic impedances given the variability of leaded components and the imprecise nature of standard mounting arrangements. Although the inductance associated with leaded components m filtering stages of known isolators can also be reduced by using many components in parallel (e.g., capacitors), its effect cannot be adequately cancelled by introducing tuning capacitance. The parasitic inductance reduces the effectiveness of the filters and may necessitate the use of multiple filtering stages to counter this effect. This increases isolator cost.

It is also known in the prior art to employ standard surface mounted componentary m isolators. Such arrangements are limited to lower voltage applications, however. The use of surface mounted components for high voltage applications would require specially made components, and would therefore be very expensive.

From a first aspect, the present invention provides a method of manufacturing an isolator device for providing isolation of DC and low frequency AC currents and coupling of high frequency signal between sections of signal transmission line, the method comprising the steps of: l. forming an isolation barrier by arranging first and second printed circuit boards spaced from each other m a non-planar, three-dimensional manner, so that the isolation barrier exists between the spaced circuit boards; li. mounting a capacitor having a first terminal in intimate contact with the first circuit board and a second terminal in intimate contact with the second circuit board the capacitor being leadless, and the arrangement being such that the capacitor is arranged to couple signals across the isolation barrier from the first board to the second board; in. the respective circuit boards including conductive elements which are arranged to be connected to respective sections of the signal transmission line.

By "intimate contact" it is meant that the capacitor plates are connected, as far as is possible, directly to the conductive elements of the respective circuit boards. In one embodiment the plates may be soldered directly to the conductive elements of the circuit boards. In another embodiment, the printed circuit boards may be clamped to the capacitor physically, without any soldered connection. The elimination of capacitor leads and the three-dimensional topology of the device - printed circuit boards being spaced from each other by the capacitor - preferably minimises aspects of the isolator which may otherwise introduce unpredictable values of parasitic impedance. Preferably, the capacitor is absent any packaging which would be likely to introduce unpredictable values of parasitic impedance, and the capacitor and other isolator components are preferably positioned m precise predetermined locations and orientations to take into account stray capacitive and/or inductive effects, whereby to improve high frequency performance and reproducability of the isolator device.

The capacitor is preferably an industry standard, mass-produced, component, to which the packaging (which is normally present) has either not been added or has been removed.

By "high frequency" is meant anything substantially over mains frequency, preferably 100kHz and above, and by "low frequency" is preferably mains AC frequency and below. The isolator device of the present invention can preferably be arranged to pass wide bed signals of the range such as those discussed in the preamble above.

Preferably, the isolator may be arranged to provide high voltage isolation. By high voltage we mean mains voltage upward. An isolator may be economically designed according to the present invention to voltages much higher than isolators including leaded components e.g., 15kV.

7n isolator device must obviously be able to couple desired signals across the isolation barrier, while still maintaining the required barrier to circulation of common- mode currents and other low frequency signals. The device is thus generally made up of a number of discrete components which are mounted together between transmission line segments. For passage of desired signals it is obviously important that the isolator device, including all its components, matches the impedance of the transmission line. In other words, the components of the isolator device must not cause an impedance discontinuity at the interface between each transmission line segment otherwise the return loss characteristics of the isolator will oe degraded. The components of the isolator device will preferably include capacitive and inductive components forming coupling and filtering stages. Further, stray capacitance will exist between components mounted adjacent to each other, and stray capacitance will exist between the components and any housing of the isolator device. In the coupling stage of the isolator, these capacitive components are likely to be quite reJ evant at the high frequencies at which some isolation devices are required to operate. Further, industry standard relatively low cost components, of the type previously used m isolators having a coupling and filter stage, include parts which introduce unpredictable and counter-productive parasitic impedances. These include, for example, m standard capacitor components, the capacitor leads, which introduce parasitic inductance, and the capacitor encapsulation which affects stray capacitance. In the present invention, by "industry standard components" we mean generally available components, usually mass-produced, such as standard available capacitor components such as the type which have been conventionally used to provide isolation in the relatively low cost isolators and which are normally provided with component aspects such as leads and packaging.

In the present invention, the leads of capacitor components of the device are absent or eliminated to minimise the value of unpredictable and counter-productive parasitic inductance. The packaging of capacitors may also preferably be eliminated. Further, the positions of the isolation device coupling components are preferably determined to result m zero or minimal parasitic impedance. In other words, the capacitance between components and stray capacitance between components and the housing is preferably "balanced" to result m minimal parasitic impedance and a matching of the isolator device to the transmission line. Usually, some parasitic impedance will remain, even where capacitor leads and packaging has been eliminated and the positions of the components have been adjusted to the optimum. Preferably, m order to obviate the effect of this remaining parasitic impedance, predetermined values of impedance are added to adjust the characteristic impedance of the isolator device. The predetermined impedance may be added by adding controlled capacitive or inductive components m the form of extra circuit elements, such as circuit board patterns.

Preferably, control of the above parameters allows the isolator to be built using industry standard, relatively low cost, off the shelf isolator components, without the need for post-assembly tuning. Further, the spacial positioning of isolator elements witn respect to the circuit boards using standard assembly practices, is highly reproducible. The minimisation or elimination of unpredictable and counter-productive parasitic impedances, together with the adjustment of isolator impedance by determining positions of the isolator components and also perhaps by adding predetermined precise values of impedance in the form of controlled circuit elements, results, together with spaced printed circuit board structure, m being able to provide a highly reproducible isolator having a predetermined impedance (matching that of the transmission line) without the need for tuning. The positions of the components m the isolator can be predetermined because of the minimisation or elimination of unpredictable and counter-productive impedances. Such an isolator can be mass produced and the purchaser can be assured of satisfactory performance in every case.

The use of leadless capacitors with minimal parasitic impedance also optimises the effectiveness of the isolators filtering stage, where one is included. In fact, m a preferred embodiment, only a single filtering stage is required for adequate performance. This is as opposed to prior art isolators having a coupling stage and a filtering stage, where more than one filtering stage is normally required to provide reasonable performance.

Preferably, high voltage capacitors are useα to maximise the break down voltage of the isolator. The use of standard, mass-produced high voltage capacitors is advantageous m minimising isolator cost.

As discussed above, the method also preferably comprises the step of controlling the impedance of the isolator device by adding elements which provide known "controlled" predetermined values of capacitance and/or inductance. Known values of controlled inductance can be introduced by printing meandering tracks on circuit boards mounting the components. Similarly, controlled capacitance can be introduced by arranging circuit board features in close proximity. As discussed above these controlled impedances can be used to adjust the characteristic impedance of the device to match the transmission network. The capacitors used are preferably industry standard discoidal ceramic capacitors of a type which is preferably mass produced, normally with encapsulation and leads. The encapsulation and leads are eliminated from the standard "off the shelf" capacitor to minimise the value of "unknown" parasitic impedance. Preferably, the capacitors may be obtained from the manufacturer before the encapsulation and leads are added.

Any suitable standard capacitor may be used. What is important is that it is not necessary to specially manufacture a capacitor structure dedicated for this application, unlike the expensive prior art arrangements which utilise specially manufactured capacitive structures to provide isolation and coupling. The applicants have realised that parasitic impedance normally associated with industry standard components such as capacitors make it very difficult to design an effective, low cost and reproducible isolator. By eliminating the parasitic impedances associated with such industry standard components it is possible to realise a reproducible and relatively low cost isolator.

The isolator device preferably comprises transmission line elements which are provided by the circuit board mounted conductors. Transmission line elements on the circuit boards are preferably connected to transmission line connectors which are connected m the signal transmission line.

Preferably the transmission line elements on the PCB's are arranged such that at least where they contact the respective capacitor plates the conductors are parallel and adjacent to the plane of the respective plate of the capacitor to which they are intimately connected. This preferably results in minimal parasitic inductance.

Wherever an isolation barrier is provided m a shielded transmission line, it is necessary to break the transmission line shield which allows external signals to couple into the transmission line and, also signals to radiate out of the transmission line. Single leakage into and out of the transmission line can be minimised by using known circuit topologies together with bypass capacitors which have minimal parasitic inductance, m accordance with the present invention. These capacitors are preferably arranged with inductive elements to form a filter as with the coupling capacitors of the preferred embodiment of the invention. As discussed above, the capacitor of the filter stage is preferably an industry standard relatively low cost off the shelf capacitor the leads and encapsulation of which have been eliminated. The capacitor is preferably a ceramic discoidal capacitor. One capacitor plate is preferably connected directly to a conductor of a transmission line element, m a similar manner as discussed above, using a planar, low impedance connection. The other plate of tne capacitor is preferably connected directly to an earth connection, also preferably by a planar low impedance connectio .

In a preferred embodiment, the isolator components are mounted m a grounded conductive housing and are potted in an insulating compound, such as epoxy for example, having high dielectric strength and low dielectric constant. The dielectric constant of the epoxy is also taken into account when determining the mounting positions of components and the values of controlled impedances which are introduced, in order to provide the best match to the transmission lines.

Such an isolator may advantageously be used in isolation of cable TV signals, and general cable telecommunications .

The present invention, in at least preferred embodiments, is able to provide superior high frequency and high voltage performance at relatively low cost using highly reproducible techniques which eliminate the need for individual adjustment or tuning.

One application for preferred embodiments of the invention is in isolation of cable television networks at the point of connection to a customers premises. At this point the isolator is typically connected between sections of coaxial cable. Embodiments of the invention have been found to perform quite satisfactorily over the required frequency range (5MHz to 1GHz) , giving good low frequency isolation and minimal attenuation of high frequency signals. The invention is not limited to cable TV applications, and may be applied m other applications where isolation is required. Isolator signal transmission line elements may comprise any type of PCB mountable transmission line components such as, strip-line micro-strip etc. In its simplest form, the invention includes a pair of capacitors connected between two transmission line elements, one being connected between the active conductors and the other being connected between the shields or ground planes. In practical applications, for such isolators to meet low frequency signal leakage specifications, a large capacitance is required between the ground connections. This proves costly, particularly since high voltage capacitors are required. The high capacitance also increases the risk of electric shock for persons touching the isolated connection by allowing higher currents to flow at mams power frequencies. Therefore, although the invention does cover such an arrangement, it is not lιkelγ to be used m many practical situations. Instead, preferred embodiments, as discussed above, additional filter components are used to minimise signal leakage associated with the isolator.

Preferred embodiments of the invention are more complex arrangements, which, as well as incorporating DC and low frequency isolation using high voltage capacitors, also incorporate filter arrangements and like circuits for minimising signals which appear between the isolator input and output ground connections. This minimises signal leakage. In accordance with embodiments of the present invention, the performance of the filters s significantly enhanced by minimising any "unknown" parasitic inductance associated with the filter capacitors. In practice, the superior performance of each filter stage allows one stage to be used in the present invention whereas multiple filter stages are used in comparable known isolators. Preferably, standard unleaded ceramic capacitors are utilised to minimise cost.

In preferred embodiments, mounting topologies (relative positions of isolator components) are utilised which minimise parasitic inductance m places where it has a detrimental effect on circuit performance and introduce controlled amounts of inductance in places where it has a beneficial effect (e.g. where it is used to optimise high frequency return loss; . The use of leadless capacitors is crucial for both tasks. The use of leadless capacitors to minimise parasitic inductance has already been noted. Tne advantage of using leadless capacitors at points where inductance is needed is that this inductance can be predominantly provided by circuit board features (e.g. hair- pin tracks) thereby minimising the variation that would otherwise be experienced if equivalent leaded capacitors were used .

The present invention is particularly suited for galvanic isolation where the term implies isolation in an electric circuit which prevents the flow of direct current (DC) and minimises the flow of alternating current (AC) at the frequency of ma s power supplies (e.g. 50 to 60 hertz;. It may be suited to other applications, with different low frequency requirements. In general, the present invention finds application any situation where DC or low frequency AC (e.g. mams frequency AC) isolation is required, and m which signals (i.e. relatively high frequency signals compared to the frequency of the isolated signal) need to span the isolating barrier.

It is a surprising finding that the use of standard, relatively low cost, high voltage capacitors in a preferred embodiment of the present invention results in an isolator with superior high frequency performance, provided the capacitor plates are intimately connected to the transmission line elements on the PCB's. High voltage, leaded ceramic capacitors are generally considered by those skilled the art to perform poorly at high frequencies. For typical capacitance values required in an isolator (e.g., 2nF) , leaded ceramic capacitors begin to degrade (i.e., become inductive) at frequencies as low as 50MHz and continue to degrade proportionally with frequency above this point. This degradation has been found to be almost exclusively due to the inductance of the capacitor leads which can be minimised, if not eliminated, by using leadless capacitors. Those skilled in the art might also expect that bulk resonances of the ceramic material could also degrade capacitor performance. In practice, the applicants have found that this potential effect is insignificant in embodiments of the present invention.

The present invention further provides an isolator device for providing isolation of DC and low frequency AC currents and coupling of high frequency signals between sections of signal transmission line, the isolator device comprising: l. first and second printed circuit boards spaced from each other in a non-planar three-dimensional manner, so that an isolation barrier exists between the spaced circuit boards; li. a capacitor mounted between the first and second printed circuit boards, the capacitor having a first terminal in intimate contact with the first circuit board and a second terminal intimate contact with the second circuit board, tne capacitor being leadless and the arrangement being such that the capacitor s arranged to couple signals across the isolation barrier from the first board to the second board; in. conductive elements mounted on the respective circuit boards, which are arranged to be connected to respective sections of the signal transmission line . Preferably, the isolator device may include any or all of the features discussed above in relation to the previous aspect of the invention.

As discussed above, PCB mounted transmission line elements are used in the isolator. These transmission line elements need to be connected to the signal transmission line. A controlled impedance connection is desirable. Otherwise, the impedance of the connection can affect the impedance match to the transmission line.

In another aspect, the present invention relates to a novel transmission line connector for connecting a PCB mounted transmission line element to a transmission line cable .

From a further aspect, the present invention provides a transmission line connector for connecting a transmission line element formed on a printed circuit board directly to a transmission line cable, tne transmission line connector comprising means for receiving a portion of the printed circuit board, the printed circuit board having a conductor forming a central conductor of a transmission line, and a conductor forming a ground plane, a section of the printed circuit board mounting the central conductor including a slot for receiving a corresponding central conductor of the connector, and the connector also including means for shielding the connection.

The transmission line connector preferably has one portion which provides for a standard connection to a coaxial cable. The other portion is arranged to receive the printed circuit board and connect directly to the active conductor of the circuit board and the ground plane of the circuit board. This direct connection preferably provides an optimally shielded connection between the transmission line element on the circuit board and the transmission line which is important in achieving low signal leakage at high frequencies .

Since the purpose of an isolator is to provide high impedance to DC and low frequency AC currents, in certain situations it is possible for high voltages to appear across the isolator. These voltages can appear between the ground of the isolated connector and the grounded connector which is connected to the isolator housing (in the preferred embodiment) . These voltages can be extremely hazardous to personnel handling the isolator. It is therefore advantageous for isolators to incorporate a visual indicator which draws attention to the hazardous situation.

From yet a further aspect, the present invention provides a visual indicator for the purpose of indicating the presence of hazardous voltages between an isolator housing ground and an isolated ground connection. The indicator comprises a means of detecting a voltage difference between isolator ground connections, and a means for visually indicating that there is a voltage difference. The visual indication preferably provides an indication of the magnitude of the voltage difference. Preferably, the indicator further comprises a means for producing a modulated signal whose frequency is proportional to the detected voltage across the isolator, and this signal is preferably used to drive a visual indicator.

The voltage detector preferably comprises a plurality of rectifying devices such as diodes and a current limiting device such as a resistor or capacitor. The frequency modulator preferably comprises a capacitor and a gasseous discharge device such that the combination of these devices forms a relaxation oscillator when connected to the voltage detector components. The gasseous discharge device also provides a visible indication of the magnitude of detected voltage by flickering at a rate proportional to this voltage.

Features and advantages of the present invention will become apparent from the following description of embodiments thereof, by way of example only, with reference to the accompanying drawings, which: Figure 1 is a circuit diagram of an isolation device in accordance with a first, embodiment of the present invention;

Figure 2 is an exploded diagram showing components of an isolator embodiment implementing the circuit of Figure 1; Figure 3 is an exploded diagram showing components of an alternative embodiment implementing the circuit of Figure 1;

Figure 4 is a circuit diagram of a preferred embodiment of the present invention, for application m galvanic isolation of cable TV signals;

Figure 5 is an exploded diagram showing components of one preferred implementation of the circuit of Figure 4;

Figure 6 is a circuit diagram of an alternative preferred embodiment of the present invention, for application in galvanic isolation of cable TV signals;

Figure 7 is an exploded diagram showing components of one preferred implementation of the circuit of figure 6;

Figure 8 is a schematic circuit diagram of part of the circuit of figure 6, illustrating parasitic, stray and controlled elements of impedance; Figures 9, a, b and c illustrate a transmission line connector in accordance with an embodiment of the present invention;

Figures 10, a, b and c illustrate a further embodiment of a transmission line connector m accordance with the present invention;

Figures 11, a and b, illustrate a further embodiment of an isolator in accordance with the present invention, and

Figures 12 a, b, and c illustrate various embodiments of circuitry used to provide visual indication of hazardous isolator voltages in accordance with the present invention.

Figure 1 illustrates a simplistic circuit which provides high voltage, DC and low frequency isolation for a signal transmission line (transmission line connectors Fl and F2 are arranged to connect signal transmission line elements 3, in the signal transmission line (not shown) . The signal transmission line may comprise coaxial cables connecting to Fl and F2 respectively, and connecting to respective nodes in the signal transmission path (e.g. provider and user of cable TV signal) . The isolator incorporates a first signal transmission line segment 4,5 comprised of an active conductor 4 and a shield or ground plane 5. The isolator also incorporates a second signal transmission line segment 4a, 5a comprised of an active conductor 4a and a shield or ground plane 5a. The signal transmission line segments 4,5 and 4a, 5a may comprise any type of PCB mounted transmission line, such as , e.g. coaxial cable, strip-line, micro-strip, etc.

A first means, comprising capacitors Cl and C2, is mounted between the transmission line segments to provide high voltage, DC and low frequency isolation. Cl is mounted between the active conductors 4 and 4a and C2 between the shields or ground planes 5 and 5a. Cl and C2 are of the desired value and rating to perform the isolation function. For cable TV this may be 6kV and 2nF for each capacitor. These values are illustrative only and it will be appreciated that different capacitor values can be chosen for particular applications.

Figure 2 is an exploded view of one implementation of the circuit of Figure 1. Where convenient, the same reference numerals have been used as in Figure 1 for equivalent components.

The signal transmission line segments 4,5 and 4a, 5a of Figure 2 are implemented using a pair of printed circuit boards designated by reference numerals 6 and 7 respectively. Circuit boards are shown "exploded" and in practice each comprise an insulating substrate 8,9 and conductive tracks 10,11,12 13 on each respective side of substrates 8,9.

Tracks 11 and 12 are ground planes and are equivalent to elements 5 and 5a respectively of Figure 1. Tracks 10 and 13 form active conductors and are equivalent to elements 4 and 4a respectively of Figure 1. In operation, the tracks will be connected to connectors Fl and F2 (not shown) .

The arrangement is such that the printed circuit boards 6 and 7 are mounted above each other and are spaced by the capacitors Cl and C2. The capacitors Cl and C2 are preferably leadless ceramic disk capacitors whose plates are intimately connected to tracks 10,13 and ground planes 11,12 respectively. Note that the track 13 makes contact with the capacitor plate via piated-through hole 13a and mounting pad 13b. Track 10 makes contact with the other plate of the capacitor Cl via plated through hole 10a and pad 10b. Capacitor C2 is mounted on ground planes 11 and 12 via mounting pads 11A and 12A respectively.

The capacitors Cl and C2 are industry standard, relatively low cost off-the-shelf ceramic discoidal capacitors which are normally provided with encapsulation and leads. To minimise the incidence of "unknown" value of parasitic impedance, however, the leads and encapsulation of capacitors Cl and C2 are absent or eliminated. A planar low impedance connection is made between the respective capacitor plates and the conductors 10, 11, 12 and 13 of the transmission line segments. This also minimises "unknown" values of parasitic inductance.

The capacitors are preferably soldered to the respective mounting pads by means of a reflow process in an oven. It has been found that the reflow process facilitates automatic alignment of the capacitors on the circuit board. This is caused by the surface tension of the molten solder pulling the capacitors line with the features of exposed circuit board conductors which are in contact with the molten solder (not shown in the figure) . Alignment of the capacitors is important in achieving reproducible high frequency performance. The topology of the arrangement is such that the capacitors are mounted between the transmission line tracks 10,11 and 12,13 m a fashion which minimises parasitic inductance. This has been achieved by using a "vertical" topology, where the capacitors are intimately mounted between and against adjacent circuit boards. It will be appreciated that "vertical" is a relative term and the actual spatial orientation of the arrangement could be other than vertical. The term "vertical" is used here to denote this particular type of "top to bottom" topology.

The entire arrangement is mounted within a conductive housing (not shown) which is earthed, and is potted in a suitable insulating compound such as epoxy (not shown) . The epoxy has high dielectric strength and low dielectric constant and increases the break-down voltage of the isolator device, as well as increasing mechanical strength of the assembly. As discussed in the preamble of the specification, the impedance of the isolator device needs to be matched to the impedance of the adjoining signal distribution line. The factors affecting the characteristic impedance of the isolator include the relative positioning of the isolator components with respect to each other and with respect to the housing, and any parasitic impedances introduced by circuit elements.

Parasitic impedance of the capacitor components Cl and C2 is minimised by eliminating the encapsulation and leads and by intimately connecting the capacitor plates to the conductors of the transmission line segments. The isolator components are physically positioned in precise locations and orientations to take into account stray capacitive and/or inductive affects to provide the best matched impedance to the transmission line. Further, controlled amounts of impedance may be introduced by circuit board elements and by relative positions of the components, such as capacitors Cl and C2, to match the impedance to the transmission line. How this is done will be discussed more detail later m this description, with reference to figure 8.

The transmission line elements on circuit boards 6 and 7 are connected via transmission line connectors Fl and F2 to the transmission line. The transmission line connectors preferably connect directly to the transmission line elements 4a, 5a, 4, 5. This will be discussed more detail later in this description, with reference to figures 9 and lC.

Figure 3 shows an alternative implementation of the circuit of Figure 1. The same reference numerals as used in Figure 2 are used in Figure 3 for equivalent components, and no further description of the majority of components is required, apart from the capacitors Cl and C2. The only difference between the device of Figure 3 and the device of Figure 2 is that one of the capacitors, C2, is annular such that the other capacitor, Cl, fits within its central hole formed by the annular capacitor to form a coaxial structure. Mounting pads 11A, 12A are also suitably rearranged. It is believed that this topology may provide better high frequency performance than the topology of the device in Figure 2 because of its coaxial nature. In accordance with the invention, the plates of the capacitors Cl and C2 are directly connected to the conductive pads of the transmission line elements. The arrangement is potted m an insulating compound such as epoxy, mounted m a conductive housing (not shown) and connected to transmission line connectors, as with the embodiment of figure 2.

In the embodiment of figure 3, C2 is not an industry standard capacitor. Cl, however, is.

It will be appreciated that the transmission line of the arrangements of Figures 2 and 3 is essentially a micro- strip arrangement. It will be appreciated that a strip-line or other transmission line structure (e.g. co-planar waveguide etc) could also be used.

The embodiment illustrated in Figures 1 to 3, although useful for illustrating the operation and arrangement of the invention, will probably not be a practical arrangement for applications where the required signal bandwidth extends to relatively low frequencies (e.g. 5MHz) . As discussed m the preamble of the specification, because the high voltage isolation necessitates breaking the ground connection of the transmission line, as is clear from Figure 1, this allows signals to both enter the cable and degrade signal quality and also leave the cable and interfere with external services. It is important that suitable circuitry is included in a practical device which minimises signal leakage across the entire operating frequency band.

Figure 4 is a circuit diagram of a circuit which is adapted to provide high voltage, DC and low frequency isolation in a cable TV signal transmission system and includes circuitry to minimise signal leakage across a 200:1 signal frequency range.

In the circuit diagram, components having equivalent components in Figure 1 are given the same reference numerals. Connectors Fl and F2 are for connecting the device m the signal transmission path between a TV signal providers cable and a users cable. As before, high voltage, DC and low frequency isolation is provided by capacitors Cl and C2 connected in series with the active conductor 4,4a and shield or ground 5,5a of the signal transmission line 3. Note that the component values given m Figure 4 are suitable for standard cable TV operation, but it will be appreciated that they are illustrative only and may be varied.

In addition to these components, the arrangement comprises a transformer Tl connected in the transmission line and a capacitor C3 connected across the transformer Tl. The transformer and capacitor C3 are used in a known manner to reduce signal leakage at the bottom end of the signal frequency band by specifically forming a capacitive divider which reduces signals which appear between the isolator ground connections 5 and 5a. A gasseous discharge tube N, such as a standard low cost indicator light like those commonly used in electric appliances, is provided to protect the transformer Tl from high voltages which may appear from one winding to the other. A resistor Rl is provided in parallel with the discharge device N. Gasseous discharge tubes have been used because they offer low capacitance, tolerance to relatively high energy surges and low cost. Each of these factors are important the present invention. The applicants believe that the use of one or more neons or similar discharge devices, to protect low voltage components of isolators is novel .

The circuit further comprises an inductive component 20 the transmission line 3 and a further capacitor C4 connected between the shield or ground plane 5a and earth E to which ground plane 5 and conductive housing (not shown, of isolator are connected. Inductor 20 and capacitor C4 form a filter which attenuates any high frequency components remaining between the ground connection E and 5a. This reduces signal leakage at high frequencies, a known manner . Figure 5 illustrates an exploded view of an implementation of the circuit of Figure 4 accordance with the principles of the present invention.

The same reference numerals are used in Figure 5 for equivalent components in Figure 4. Circuit boards 30 and 31 are utilised to mount the components and also form signal transmission line segments. Circuit board 31 is a strip-line arrangement having a central track 32, forming the active conductor 4a of figure 4, and an upper ground plane 33 and a lower ground plane 33a (which cannot be seen but which is on the underside of the board 31) form the shield segment 5a. In practice, the circuit board 31 includes copper plating on each edge (not shown) which connects the ground planes on each surface together to improve shielding and minimise high frequency signal leakage. The transmission line segment 4a, 5a m Figure 4 is therefore totally shielded the implementation of Figure 5, which is critical to achieve low signal leakage at high frequencies.

The upper circuit board 30 has one track 35 which forms the active conductor segment 4 in Figure 4 and a lower portion 39 (which cannot be seen, but which is a ground plane) forming ground plane 5 segment. Note that the lower portion of board 30 is connected to an earthed conductive housing E m which the device is mounted. Capacitors Cl and C2 are connected to the upper circuit board via plateα- through holes 36 and 38 which connect to the respective terminals of the transformer Tl in accordance with Figure 4. Capacitor C3 is connected to the ground plane on the upper circuit board 30 and the ground plane 33 of the lower circuit board.

The inductance 20 is provided by surrounding tne transmission line elements 4a, 5a with ferrite material such as a ferrite bead.

All capacitor plates are preferably intimately soldered or glued with conductive adhesive to either the transmission line conductors or the various circuit components or ground, m accordance with the present invention. Again the topology used is "vertical" in order to minimise parasitic inductance. The capacitors used are preferably relatively low cost ceramic dιsκ capacitors. The encapsulation and leads of the capacitors Cl through C4 is eliminated (these are standard ceramic capacitors which would normally be provided with encapsulation and leads) . This enables a planar, low impedance connection with the transmission line elements of the circuit boards 30 and 31. As discussed above, the positioning of the isolator components is important in matching to the impedance of the transmission line. The arrangement is such, however, that once the optimal positions of the components have been determined the precise positions can be reproduced many times. Controlled impedances may also be introduced in the form of circuit elements, such as conductive tracks on the PCB, as well as controlling impedance by varying the distance between capacitors. How positions of components and controlled impedance are determined will be discussed relation to figure 8.

The entire arrangement is encapsulated in an insulating compound such as epoxy and mounted m a conductive housθ g (not shown) . The transmission line segments on PCB' s 31 are connected to a transmission line cable via transmission line connectors, such as will be discussed later m relation to figures 10 and 11. Note that transmission line segment 4a, 5a will be insulated from the conductive housing.

The ferrite bead 20 is placed over the PCB 31 before the connector F2 is connected to the PCB. Ferrite bead 20 can therefore be an integral cylindrical bead instead of a split bead which would need to be clipped over the PCB assembly.

This allows for the implementation of at least partial coaxial filter structure. Further, these types of ferrite beads are low cost.

Figure 6 is a circuit diagram of an alternative circuit to figure 4, which is also adapted to provide high voltage, DC and low frequency isolation in a cable TV signal transmission system and includes circuitry to minimise signal leakage. The same reference numerals are used figure 6 as figure 4, for equivalent components. The only difference between the arrangement of figure 6 and the arrangement of figure 4 is that, in the arrangement of figure 6, the capacitors Cl and C2 are connected on the Fl connector side of the transformer Tl . This allows for a different isolator topography as is clear from figure 7.

Figure 7 is an exploded view of an implementation of the circuit of figure 6 in accordance with the principles of the present invention. The same reference numerals are used m figure 7 for equivalent components of figure 6 and also equivalent components of figure 5. The topology of the figure 7 arrangement is different from that figure 5. Essentially, the position of the circuit boards carrying the transmission line elements 4a, 5a and 4, 5 respectively, is reversed. In the figure 7 topology, board 30 is the lower board and board 31 is the upper board. Tne ground plane 39 is also on the top side of board 30, being on the same side as active conductor 35. This topology has been found to be an improvement on the topology of figure 5. Again, the entire arrangement is potted in an insulating compound such as epoxy and mounted m a conductive housing. PCB' s 30 and 33 are connected to the transmission line via connectors Fl and F2 to be described later.

Figure 8 is a schematic circuit diagram of a portion of the circuit of figure 6, to illustrate the effects of parasitic impedance, stray impedance and controlled impedance. The same reference numerals as for figure 6 are used for equivalent components. It can be seen that figure 8 shows only the components on the Fl side of the transformer Tl of the circuit of figure 7. Nor does it show the connection to the gasseous discharge device Nl .

In a real life situation, as well as the component impedances, a number of other impedance effects need to be taken into account when matching total impedance to the transmission line impedance. Each capacitor Cl and C2 will have associated with it a parasitic inductance, Lp. This parasitic inductance is generally an "unknown" factor. In the present invention therefore, this inductance is minimised as much as possible by eliminating capacitor Cl and C? leads. There is therefore a minimum of "unknown" and uncontrollable parasitic impedance.

There is also a value of capacitance between Cl and C2, being Cc, the characteristic capacitance of parallel transmission line formed by Cl and C2. This is inversely proportional to the spacing of the Cl and C2 capacitors. The capacitors Cl and C2 can therefore be spaced to introduce a controlled capacitance value, which can oe varied by varying the spacing.

Furthermore, Cs represents the stray capacitance between the capacitors and the housing of the isolator. Again, this can be varied and controlled by varying tne position of the capacitors.

When the arrangement of the components of the isolator device is being designed, (this goes for all previous embodiments) account must be taken of Cs and Cc to ensure that the characteristic impedance of the isolator device matches the impedance of the transmission line. The positioning of the components of the isolator device are determined by trial error and by some mathematical modelling, as is known to a skilled person. Once the ideal positions have been established, however, the precise positions and locations of components can be reproduced to enable mass production of isolator devices with wel] controlled impedance values. No "tuning" is required, as with prior art isolators. This results m a lowering of manufacturing costs and an improvement in reproducability of the isolator device.

In most practical cases, it will not be possible to obtain an ideal match to the transmission line merely by appropriate positioning of the isolator components, taking into account Cc and Cs . It will usually be necessary to incorporate a controlled amount of inductance Lt to offset excessive Cc or Cs capacitance. This controlled amount of inductance Lt can be provided by circuit elements, such as PCB tracks or lengths of wire. Again, the amount of Lt required can be determined empirically, but once established can be reproduced accurately any number of times. No Lt is illustrated n the described embodiments, but techniques for introducing Lt are as simple as, for example, replacing a portion of track 35 of figure 7 with a thin meandering track to provide added inductance. It is merely a matter of adding some conductor pattern on the board which adds some inductance .

By eliminating capacitor leads, thereby minimising parasitic inductance, and by controlling other impedances , such as Cs, Cc and Lt, component placement and circuit board features, an isolator device can be manufactured with parameters which are eminently reproducible and which are relatively low cost to mass produce (no "fiddly" tuning is required during manufacture) . Preferred embodiments of the present invention utilise printed circuit board mounting transmission line elements to mount other components of the isolator, such as capacitors, transformers and inductors. To minimise high frequency signal leakages, a connection between the transmission line elements on the PCB' s and the 'isolated' signal transmission line cable is desirable which maintains the shielding characteristic of a 'coaxial' structure. The present applicants have designed a novel transmission line connector which at least minimises signal leakage associated with this connection. Referring to figures 9a, b and c, a transmission line connector 100 m accordance with an embodiment of the present invention, is illustrated.

The transmission line cable connector 100 comprises a first end 101 which is a conventional coaxial connector, connectable to coaxial transmission line. End 102, however, is adapted for connection directly to a PCB 110 carrying a transmission line element. The portion 102 has a slot 103 therein which exposes the central conductor 104 of the transmission line connector. PCB 110 which has a ground plane 105 and a central conductor 106 is sized to fit into the slot 103 in a direction indicated by arrow 107. Central conductor 106 on PCB 110 is connected to a slot 108 the side of PCB 110, which is a plated through slot arrangement. When the PCB 110 is inserted into the slot 103, the slot in the PCB 108 receives the central conductor pin 104, so that the central conductor pin 104 is directly connected to the active conductor 106 of the PCB. As PCB 110 is inserted, the arrangement is such that the pin 104 is bent up slightly and snaps into the slot 106 to hold the PCB 110 place. This serves to align the upper PCB in a fixed orientation prior to it being soldered in place. The PCB 110 slides in under the pin 104 and rests on the sides of the connector slot 103. Because the PCB slot 108 is slightly narrower than the active conductor 104 the pin is bent up slightly as the PCB is inserted and then snaps back into the slot to make a good connection with the active conductor 106 and also assists in holding the PCB place.

The pin 104 may be soldered to the plated through hole 106. Once this has been done, a metal shim cap 111 is placed over the slot m the connector case to complete the shielding. Note that the ground plane 105 may also be soldered to the connector 100. The shield 111 snaps over the connector case like a "cir-clip". The shield is soldered to the connector case and PCB at the point where the PCB enters the connector. The portion 102 has flat sides 112, 113. This assists to prevent the connector from turning in the isolator mount. An alternative embodiment of the connector as shown m figure 10. Figure 10b is a section on line X-X of figure 10a. Figure 10c is a section on line Y-Y, m plane view. A conventional coaxial connector portion 120 connects to a coaxial cable transmission line (not shown) . A central conductor 121 of the transmission line terminates in a pin 122. The pin 122 extends into a chamber 123 formed by a "block" housing 124 for passages 125, 126 are provided in the walls of the housing for receiving a PCB 127, so that the PCB can extend into the chamber 123. The PCB 127 includes a ground plane conductor 128 and a plated through slot 129 to an active conductor. The PCB fits into the connector m a similar manner to the embodiment of figure 10, the plated through slot connecting to the pin 122 and the ground plane 128 connecting to the conductive walls of the housing 124.

The totally screened connection between the transmission line elements on a PCB and a transmission line, provided by the connector of figures 9 and 10, assists in providing the improved features of the isolator of the present invention, but the connector is novel in its own right and could be used with other devices.

In the isolator of the present invention, the PCB' s will be configured appropriately so that they can fit to the connectors of the present invention.

Figure 11a is a plan view of a further embodiment of an isolator in accordance with the present invention. Figure lib is a plan view of a circuit board 100 forming part of the isolator. The circuit board 100 is shown in outline 100a on figure 11a. Circuit board 100 sits below circuit board 101 and is spaced therefrom by the width of capacitors .

The circuit implemented by the embodiment of figures 11a and lib is essentially the same as that of figure 7. The only difference is that capacitor C3 of figure 7 is actually formed by two capacitors in the arrangement of figure 12a, capacitors C3 and C3A. Otherwise, equivalent reference numerals are used in figures 11a and lib, the circuit boards 100 and 101 are also shown connected to connectors Jl and J2, the connectors being in accordance with the embodiments of the present invention.

The entire arrangement is mounted within a conductive housing 102 (note that connector J2 is insulated from the conductive housing 102 by insulators 103, 104) and potted in epoxy. Note that the dimensions given in figure 11 are exemplary only and are not limiting.

Figure 12 shows several embodiments of circuitry used to implement hazardous voltage indicators for isolators. In each case, the circuits will be connected between the isolator's ground connections le: between the ground of the connector connected to the isolator housing (eg Fl in Figure 4) and the ground of the isolated connector (eg F2 m Figure 4) . Also m each case, the magnitude of the detected voltage is indicated by the flickering rate of the gasseous discharge devices N10, N20 and N40 which protrude through an aperture m the isolator housing. Neon tubes are suitable for this purpose. Capacitors connected parallel with these devices, CIO, C20 and C40 integrate charging current caused by voltage across the isolator. When the capacitor is charged to the striking voltage of the discharge tube, the tube fires discharging the capacitor and causing a visible flash. By integrating the charging current for some time, eg: 1 second, sufficient energy accumulates the capacitor to give a bright flash which is easily visible m typical levels of ambient light. Also, because the rate at wnicn voltage accumulates on the capacitors is proportional to tne voltage across the isolator, the time taken to reach the striking voltage decreases with increasing voltage le: the discharge tubes flicker at a rate which is proportional to the voltage across the isolator. Component values are chosen such that minimal voltages (eg 32 volts) cause the discharge tubes to flicker at, for example, 0.5 Hertz (one flash every 2 seconds) . Higher voltages would increase this flicker rate proportionally (eg 5 Hertz or 5 flashes per second for 320 volts) . In this manner, the indicator allows personnel to anticipate the risk involved in handling operational isolators .

Figure 12a shows a circuit which detects either DC or AC voltages using a high-voltage-rated current limiting resistor R10 and bridge rectifier formed by D10 ..D13. Figure 12b is similar to Figure 13a except an additional capacitor C21 is provided to act as an AC voltage doubler, thereby increasing the sensitivity of the indicator for low AC voltages. C21 is also a high-voltage-rated component . Figure 12c is a simplified circuit and detects peak-to- peak AC voltages only. The advantage of such a circuit lies in its lower component count and lower cost.

It will be appreciated by persons skilled m the art that numerous variations and/or modifications may be made tc the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

THE CLAIMS REGARDING THE INVENTION ARE AS FOLLOWS:

1. A method of manufacturing an isolator device for providing isolation of DC and low frequency AC currents and coupling of high frequency signals between sections of signal transmission line, the method comprising the steps of:

1. forming an isolation barrier by arranging a first and second printed circuit boards spaced from each other a non-planar, three-dimensional manner, so that the isolation barrier exists between tne spaced circuit ooards; li mounting a capacitor having a first terminal in intimate contact with the first circuit board and a second terminal in intimate contact with the second circuit board the capacitor being leadless, and the arrangement being such that the capacitor is arranged to couple signals across the isolation barrier from the first board to the second board; in . the respective circuit boards including conductive elements which are arranged to be connected to respective sections of the signal transmission

2. A method accordance with claim 1, comprising the step of employing a relatively low cost, industry standard ceramic capacitor.

3. A method accordance with claim 2, wherein the capacitor is a disc capacitor..

4. A method in accordance with claim 3, wherein the capacitor has a voltage rating which is at least 150v.

5. A method m accordance with any preceding claim, comprising the further step of physically positioning isolator components precise predetermined locations and orientations to take into account stray capacitive and/or inductive effects, whereby to improve high frequency performance and reproducab±lity of the isolator device.

6. A method accordance w th claim 5, comprising the further step of adding predetermined, controlled capacitance and/or inductance whereby to adjust tne characteristic impedance of the isolator device.

7. A method m accordance with claim 6, wherein the step of adding predetermined controlled capacitance and/or inductance comprises incorporating the predetermined capacitance and/or inductance in a circuit board pattern or extra circuit elements.

8. A method in accordance with any preceding claim, wherein the isolator components include transmission line elements formed on the printed circuit board, and wherein the transmission line elements are connected to a transmission line connector which is connectable to the signal transmission line, the isolator also including a choke the form of a ferrite bead, the method of manufacture also comprising the step of placing the ferrite bead over an appropriate portion of the printed circuit board, so that the ferrite bead surrounds a section of the transmission line element, before connecting the transmission line connector.

9. A method in accordance with any preceding claim, comprising the step of mounting isolator components in a material having a relatively low dielectric constant and high dielectric strength, thereby increasing the breakdown voltage of the isolator device.

10. A method m accordance with any preceding claim, comprising the further step of providing a filter stage for attenuating signals which are inadvertently introduced between the isolator grounds, the filter including a leadless capacitor having one terminal intimately mounted to a transmission line element and the other terminal connected to ground and the ferrite bead.

11. A method in accordance with any preceding claim, the arrangement being such as to provide high voltage isolation between the sections of signal transmission line.

12. An isolator device for providing isolation of DC and low frequency AC currents and coupling of high frequency signals between sections of signal transmission line, the isolator device comprising: l. first and second printed circuit boards spaced from each other in a non-planar three-dimensional manner, so that an isolation barrier exists between the spaced circuit boards; li. a capacitor mounted between the first and second printed circuit boards, the capacitor having a first terminal m intimate contact with the first circuit board and a second terminal in intimate contact with the second circuit board, the capacitor being leadless and the arrangement being such that the capacitor is arranged to couple signals across the isolation barrier from the first board to the second board; in. conductive elements mounted on the respective circuit boards, which are arranged to be connected to respective sections of the signal transmission line.

13. .An isolator device in accordance with claim 12, comprising a relatively low cost industry standard ceramic capacitor .

14. An isolator device in accordance with claim 19, wherein the capacitor is a disc capacitor.

15. An isolator device in accordance with any one of claims 12, 13 or 14, wherein the capacitor has a voltage rating of at least 150v.

16. A device accordance with any one of claims 12 to 15, wherein isolator components are physically positioned in predetermined precise locations and orientations to take into account stray capacitive and/or inductive affects, whereby to improve high frequency performance and reproducability of the isolator device.

17. .An isolator device in accordance with any one of claims 12 to 16, further comprising additional controlled capacitive and/or inductive elements having predetermined values, whereby to adjust characteristic impedance of the isolator.

18. A device in accordance with claim 17, controlled capacitance and/or inductive elements comprising circuit board patterns or extra circuit elements.

19. An isolator device in accordance with any one of claims 12 to 18, further comprising a filter for attenuating signals which are inadvertently introduced across the isolator device, the filter comprising a capacitor having one plate connected intimately to a conductor of one of the printed circuit boards and the other terminal connected to ground and the ferrite bead.

20. A device in accordance with any one of claims 12 to 19, isolator components being mounted in a material having a relatively low dielectric constant and high dielectric strength, whereby to increase the breakdown voltage of the isolator device.

21. A device in accordance with any one of claims 12 to 20, further comprising transmission line elements formed on a printed circuit boards, and a choke in the form of a ferrite bead, the cylindrical ferrite bead surrounding an appropriate portion of the printed circuit board, to proviαe common mode filtering.

22. A device accordance with any one of claims 12 to 21, comprising transmission line elements formed on printed circuit boards, the transmission line elements being connected to the sections of the transmission line by a connector arrangement which connects directly to ground plane and central conductor arrangements on the pπnteα circuit boards.

23. A device in accordance with claim 22, the printing circuit board including a conductor forming a central conductor of the transmission line and a conductor forming a ground plane, a section of the printed circuit boarc mounting the central conductor including a slot for receiving a corresponding central conductor of a connector.

24. A transmission line connector for connecting a transmission line element formed on a printed circuit board directly to a transmission line, the transmission line connector comprising means for receiving a portion of the printed circuit board, the printed circuit board having a conductor forming a centra] conductor of a transmission line, and a conductor forming a ground plane, a section of the printed circuit board mounting the central conductor including a slot for receiving a corresponding central conductor of a connector, and the connector also including means for shielding the connection.

25. A transmission line connector in accordance with claim 23, wherein the means for receiving a portion of the printed circuit board includes a slot within the connector which is dimensioned to receive the portion of the printed circuit board.

26. A transmission line connector accordance claims 24 or 25, the pin being of slightly greater width than the slot in the printed circuit board so as to form an interference fit with the slot.

27. A method of manufacturing an isolator device m accordance with any one of claims 1 through 17, comprising providing a transformer to assist in reduction of signal leakage and providing a gasseous discharge element to protect the transformer from high voltage surges.

28. A visual indicator for indicating the presence of hazardous voltages between a voltage isolator housing ground and an isolator ground connection, the indicator comprising a means of detecting a voltage difference between isolator ground connections, and means for visually indicating that there is a voltage difference.

29. A visual indicator in accordance with claim 28, further comprising a means for producing a modulated signal whose frequency is proportional to the detected voltage across the isolator, and a means for changing the visual

SUBSTITUTESHEET (RULE28) indication in accordance with the modulated signal.

30. An indicator arrangement m accordance with claim

29, the arrangement being such as to provide a visible indication of the magnitude of the detected voltage by

J causing the visual indicator to flicker at a rate which is proportional to the voltage.