Methods and devices for protecting CATV circuits from combination and ring waves

In a grounded electrically conductive housing for an electrical device, common electrically conductive connection points for one or more components connected thereto to be protected from harmful voltage surges, are provided with a spark gap formed between the connection points and ground, via either electrically conductive surge rods, electrode chips, or eyelets for conducting the voltage surges across the spark gap to ground thereby protecting the associated components from damage due to a voltage surge.

FIELD OF THE INVENTION

The present invention relates generally to methods and devices for protecting electrical circuitry from surge voltages and currents, and more particularly to protecting the circuitry associated with CATV devices, apparatus, and systems.

BACKGROUND OF THE INVENTION

A Cable TV system is exposed to environmental forces since it consists of metal clad coaxial cables and signal conditioning equipment that is physically spread throughout a geographical service area either aerially on utility poles or buried in the ground. One such environmental force is the discharge of atmospheric electrical charge known as lightening. It is well known that such charge currents can travel along the outer conductor of the coaxial cable, and that periodic connection of the cable to the power utility ground wire and also to ground rods buried in the earth are required to keep the power utility ground and the Cable System ground at the same voltage potential to insure public safety and protect subscriber equipment. However, currents that travel along the outer conductor of a coaxial cable will induce currents on the center conductor of the cable. These electrical disturbances on the center conductor of the coaxial cable, which may be induced by lightening discharge or by switching transients on the power grid, can damage the electrical components connected to the Cable TV system. Therefore, various methods of surge protection have evolved to keep power and communication utilities operating during atmospheric disturbances that may impact either network's distribution system at any point.

A standard developed by the Institute of Electrical and Electronic Engineers, based on empirical studies of typical voltage disturbances, defines standard voltage and current waveforms that any device in the Cable TV system or the electrical power grid may experience. The IEEE C62.41 1999 standard specifies service categories relative to the network architecture and defines the maximum voltage and current expected in these service categories.

Typically, devices connected to the CATV outside plant are classified as category B3 where the voltage may reach 6 kilovolts in an open circuit and the current into a short circuit may reach 3 kiloamps. The waveform is unipolar and is known as a “combination” waveform.

Devices connected inside the customer premises are usually classified as category A3 where the voltage may reach 6 kilovolts, but the waveform is a damped sinusoid known as a “ring” wave. Currents typically are limited by the impedance of the path to ground and typically may reach 500 amperes. These surge waveform standards make it possible to standardize and test the ability of a device in the network to survive the conditions expected for its position in the CATV network.

The Combination Wave is specified in paragraph 9.4.2 of IEEE C62.41. The 1.2/50-8/20 μs (microsecond) combination wave is defined by both an open circuit voltage waveform and a short circuit current waveform. The open circuit voltage waveform has a front time of 1.2 μs and duration of 50 μs (seeFIG. 1). The short circuit current waveform has a front time of 8 μs and duration of 20 μs (seeFIG. 2).

The Ring Wave is specified in paragraph 9.4.1 of IEEE C62.41. The 0.5 μs-100 kHz (kilohertz) ring wave has an initial rise time of 0.5 μs and an oscillating frequency of 100 kHz, where the frequency is calculated from the first and third zero crossing after the initial peak (seeFIG. 3). The current associated with this voltage is determined by the source impedance, which is 12 ohms for a category A3 ring wave, and 2 ohms for a category B3 ring wave.

The methods used to protect devices connected to the CATV system depend upon several factors. First, the impedance of the protected circuit and the frequency at which it functions differentiate the types of surge protection devices that can be used effectively. Internal Power Regulators found in active circuits that require power such as amplifiers usually are protected by devices that function like zener diodes. These devices have a high impedance below a threshold voltage. This enables power to flow to the circuit without being affected by the protection device. Above this threshold voltage, current flows through the protection device to ground thus limiting further voltage rise that could be detrimental to the regulator circuit. Other threshold devices known as crowbar circuits become short circuits themselves when the threshold voltage is exceeded. The gas tube diode and triac semiconductor devices are in this group.

If the protected circuit is a path for high frequency signals, it is not possible to use these types of protection devices. With the exception of the gas tube, they all have a low impedance at radio frequencies that would shunt the communication signals to ground making the device ineffective under any condition. When powering an active device through a port that also carries radio frequency signals, an inductor must be in series with the power regulator input to mask its low impedance from the signal path with a high impedance at radio frequencies.

One common means of protecting a high frequency circuit from low frequency surges is to shunt the path with an inductor to ground that has a high impedance in the operating frequency range of the device but a low impedance below that frequency range. In a CATV system, the lowest frequency is 5 megahertz. A coupling capacitor typically follows the inductor in series with the signal flow into the device. The combination of the capacitor and inductor creates a simple highpass filter. Inductor values in this case are typically 6-8 microhenries and the capacitor value is typically about 1,000 picofarads.

But there are cases, especially in passive splitting devices, where the use of an inductor to protect the circuit is not desirable either because it introduces group delay at the lower band edge, or because it introduces extra cost in a competitive commodity market. In this case, the coupling capacitor takes the full impact of the surge. This places high demands upon the voltage breakdown rating of the dielectric in the capacitor. A high voltage capacitor with the temperature stability required to insure consistent signal parameters at low temperatures is typically larger than the economically sized housing of the device will allow.

SUMMARY OF THE INVENTION

In various embodiments of the invention, a surge rod or post of electrically conductive material is formed to be secured to and extend upward from an interior portion of a grounded electrically conductive housing of a device, with the free end of the rod being spaced from and opposing either a connection point on a PCB or the center conductor of an electrical contact protruding into the housing from an electrical port thereof, for example. The rod is electrically connected to the housing. A spark gap is provided within the space or separation between the free end of the rod and the connection point or center conductor of the electrical contact, whereby voltage surges that would otherwise damage circuitry connected to the connection point or the center conductor are protected by the discharge of the voltage surge therefrom across the spark gap, and through the surge rod to ground via the housing. Similarly, any other connection point on a PCB can be protected from voltage surges through use of a surge rod extending from the housing to a spark gap opposing the connection point.

In a second embodiment of the invention, the surge rod has one end electrically secured to a connection point or an electrical contact of a printed circuit board (PCB), with the electrical contact also being electrically connected to electrical components to be protected from voltage surges, such as a capacitor included in a high pass filter in a passive splitter device. The free end of the surge rod is spaced from an interior wall of the housing to form a spark gap therebetween for protecting the electrical circuitry on the PCB, and/or a coupling capacitor as previously described. Similarly, any other electrical contact of a PCB can be protected from voltage surges to prevent damage to circuit components associated with the electrical contact or connection point.

Other embodiments of the invention include an electrode secured to an edge of a PCB, electrically connected to a connection point on the PCB associated with components to be protected, whereby the electrode is spaced from an opposing wall of the housing to provide a spark gap therebetween. Yet another embodiment includes two spaced apart electrically conductive eyelets secured to a PCB, with one eyelet being grounded, and the other eyelet being electrically connected to a connection point on the PCB connected to components to be protected. The eyelets are spaced apart to form a spark gap therebetween.

DETAILED DESCRIPTION OF THE INVENTION

A typical 2-way splitter circuit8is shown inFIG. 4. The various embodiments of the present invention can be employed to particularly protect capacitors C1, C3, and C4from destruction by the magnitude of the surge voltages that may occur at their associated Input port10, output port12, and output port14, respectively, through use of surge rods16,18, and20, respectively. Similarly, any other port or circuit connection point of any other electrical device or apparatus can be protected against voltage surges. Through use of the various embodiments of the present invention in a 2-way splitter8ofFIG. 4, capacitors C1, C3, and C4can be employed each with a lower voltage rating and smaller size by using lightning or surge rods16,18, and20, respectively, to arrest excess voltage before passing through the associated capacitors. Tests conducted by the inventors have shown that without the use of the surge rod16, once the dielectric of capacitor C1breaks down, current passes directly to ground through the transformer T1comprising the signal processing circuit and the capacitor may explode due to the sudden heating of its ceramic body.

The diagram of a typical 2-way splitter8ofFIG. 4used extensively in many CATV systems shows the relationship of the coupling capacitors C1, C3, C4to the core of the circuit that splits an input signal into two output signals. The connection of T1directly to the grounded housing15provides a current path through the transformer windings from any port10,12,14, if the coupling capacitor C1breaks down. This current is also damaging to the ferrite cores that the transformers T1and T2are built around. Magnetization of the associated core produces non-linear signal distortion that introduces a phantom signal at twice the frequency of the fundamental. This is known as second harmonic distortion. By shunting the surge to ground through use of surge rods16,18, and20, before the surge voltage can discharge through the transformer windings, the degree of magnetization and thus the distortion is substantially reduced.

There are a number of inventive embodiments to accomplish an effective surge rod. The first method or embodiment inferred inFIG. 4, shows surge rods16,18, and20protruding upward from the housing15to form a spark gap22,24,26, respectively, with the tail or tab of an associated connector that connects the coaxial cables (not shown) with the associated ports10,12,14, respectively, for example. Surge voltages are thereby discharged via surge rods16,18,20to ground, thereby preventing the voltage ratings of the associated capacitors C1, C3, and C4from being exceeded. More specifically, the first embodiment of the invention is shown inFIG. 5, which is a partial cutaway view of the typical splitter8ofFIG. 4. As shown, looking into the housing15portion where a center conductor28of an input connector port10, in this example, protrudes into the interior of the housing15for electrical connection to a printed circuit board PCB (not shown). In this example of use of the first embodiment of the invention in a splitter8, a surge rod16is electrically connected and secured at one end to a grounded metallic and electrically conductive housing15, as shown. In this example the surge rod16is made from tin plated brass, and is formed from a solid rod, having an outside diameter ranging from 1 mm (millimeter) to 2 mm. The free end17of the surge rod16is spaced apart from a position to directly oppose the center conductor28, as shown, for forming a spark gap22. The housing15is connected to ground, whereby if a voltage surge of greater than a predetermined value occurs on center conductor28, the surge arcs over from the center conductor28, across the spark gap22, and through the surge rod16to ground via grounded housing15. Note that this first and other embodiments of the invention is not meant to be limited to use in CATV splitter devices8, and can otherwise be employed to protect circuit connection points of other devices or apparatus from voltage surges. Such protection can be provided at any circuit connection point, and is not limited to protecting signal input connections.

In another embodiment of the invention as previously described, and more specifically shown inFIGS. 6A,6B and6C, each representing a partial cutaway view of a splitter8, in this example. A surge rod16has one end secured to and electrically connected to a circuit connection point3to be protected on an associated PCB19, with the free end of the surge rod16extending toward and spaced apart from the housing15to create a spark gap21therebetween. Surge voltages occurring at connection point3are discharged through surge rod16, across the spark gap21to ground via housing15. In this embodiment, the copper trace conducting the surge current must be of sufficient width W to conduct the current without itself self-destructing.FIG. 6Cis a partial cutaway view looking toward the bottom of the PCB19, showing that a copper pad23is relatively wide compared to typical conductive circuit tracings on a PCB19. The copper pad23is electrically and mechanically secured to one end of surge rod16, and by via25(a plated through hole) to circuit connection point on the top of PCB19(typically by soldering). The width of the copper pad23, in this example, was 2 mm, and the thickness of the copper cladding on the circuit board is typically 0.1 mm thick. As indicated, tests made by the inventors have shown that the width of copper traces need to be short and wide to be able to survive the high current of the category B3 surges. The inventors found that by insuring that the copper pad23on the PCB was at least 2 mm wide, for the splitter8ofFIG. 4, and soldering the associated surge rod16to the pad23, with the PCB19circuit connection point3also electrically connected to the associated capacitor C1in this example, no damage occurred to PCB pad23during the occurrence of combination or ring surges. This alternative embodiment has the advantage of lower cost and reduced labor impact, when compared to the previously described embodiment, but it is more difficult to control the spark gap tolerance due to the presence of soldering and the orientation of the flat part of the surge rod16. Although the surge rod16can be placed at any port to protect the associated coupling capacitor, in a splitter such as but not limited to splitter8, it should be noted that the input port10usually is more susceptible to a high energy B3 combination surge since it may be connected to the outside CATV main feed cable.

In a third embodiment of the invention, as shown inFIG. 7, a surge rod30is secured at one end to the grounded housing15, while insuring electrical connection therebetween. The free end of the surge rod30is spaced from a connection point3, in this example, to form a spark gap32therebetween, as shown. Otherwise, the surge protection provided by this embodiment of the invention is substantially the same as that described above for other embodiments of the invention. As shown in this example, as inFIG. 5,FIG. 7shows a center conductor or input electrode28from port10electrically connected to the connection point3, which is also connected to one end of the capacitor C1, in this example, on the PCB19. In addition to copper pad23on the top of PCB19, another thick copper pad27is secured to the bottom of PCB19as shown, for enhanced reliability.

A fourth embodiment is shown inFIGS. 8A through 8D. As shown, the fourth embodiment of the invention includes a tin-plated brass U-shaped electrode34that is secured to the PCB19, as shown. Outer edge35of the U-shaped electrode34is parallel to an opposing sidewall of housing15, in this example, forming a spark gap38therebetween. A centrally located hole36on a top portion of the electrode clip34provides for soldering to connection point3, and also attaching an end of capacitor C1thereto, and a conductor28from port10thereto, as shown. Clip electrode34is U-shaped, and has a top portion40and bottom portion42spaced apart from one another for forming a slotway44as shown. The clip electrode34is pushed onto to the PCB19as shown inFIG. 8B, with the hole36of clip electrode34positioned directly over the connection point3of PCB19. The connection point3is provided by via25(seeFIG. 6C) on top and bottom portions of the PCB19, whereby clip34also includes other centrally located throughhole portion36, as shown inFIG. 8C.

A fifth embodiment of the invention is shown inFIG. 9A, includes an eyelet46mounted upon PCB19electrically connected to connection point3. Eyelet46is positioned opposing a wall51of housing15for forming a spark gap48therebetween. As shown, a capacitor C1is connected between connection point3and another connection point50on PCB19, with connection point3also being electrically connected to center conductor28of port10. The eyelet46is formed from tin-plated brass material in this example. As shown inFIGS. 9B and 9C, the eyelet46has a centrally located hole52. In this example, centrally located hole52has a diameter A of 0.8 millimeter (mm), and an outside diameter B of 2.0 mm. Eyelet46is T-shaped, as shown inFIG. 9C, includes a topmost portion54, and a lowermost portion56that is narrower than the topmost portion54. The topmost portion54has a thickness C of 0.3 mm, whereas the narrowed circular portion56has a length D of 1.7 mm, and an outside diameter E of 1.4 mm. Note that the dimensions given for the eyelet46are not meant to be limiting, and are provided from dimensioning used in a prototype of the invention by the inventors to test the same for the fifth embodiment illustrated.FIG. 9Dis an exploded assembly view showing that the hub portion56of eyelet46is secured in a via-hole25of PCB19, and electrically connected thereto. The eyelet46is typically inserted into the PCB19by a machine and the leading end of the eyelet46is rolled to create a flange that fastens it securely to the PCB19prior to soldering.

A sixth embodiment of the invention is shown inFIG. 10. This embodiment includes eyelet46, and an identical second eyelet60secured to the PCB19in a spaced apart relationship. The spacing between eyelets46and60provides a spark gap58, as shown. Eyelet46is connected to connection point3, with one end of capacitor C1, the other end of the latter being connected to connection point50on PCB19. Eyelet60, in this example, has the identical design to that previously described for eyelet46. Eyelet60is electrically connected to ground, either via a ground plane (not shown) associated with PCB19, or via a direct electrical connection provided by an electrical lead62or PCB19mounting boss shown in phantom between eyelet60and an electrical connection point64on housing15.

The preferred gap dimension for the six embodiments of the invention was determined by testing a series of gap distances to quantify the spark-over or surge voltage with respect to gap distance. The surge voltage was applied to the port and was increased incrementally by 500 volts to a maximum of 6 kV. The voltage where spark-over occurred was noted as shown in the curve ofFIG. 11. As shown, the voltages at which the spark arcs over are recorded with respect to the gap distance22between the surge rod16and the center conductor28of port10, for example. The results are shown for the polarity of the respective voltage when the spark over arcs occurred. Carbon residue formed each time there was an arc. Note that a gap of 1 mm appears to arc at a lower voltage than a gap of 0.5 mm. This is due to the build-up of carbon on the circuit board in the gap. This effect was repeatable. When a given gap was tested by incrementally increasing the voltage until it fired, the initial spark over voltage was higher than the spark over voltage after numerous discharges to test the durability of the spark gap. This indicates that the spark gap surge protection means in all of the embodiments of the invention actually improves with repeated arcing. From previous experiments, the ring wave did not cause damage to the device under test, even with a 50V spark gap type capacitor at the associated input port10. The combination waveform can deliver surge energy that damages the coupling capacitor, in this example. In other words, the use of a surge rod, of the present invention, is more effective in protecting against the effects of the more damaging type of discharge. A 500 volt capacitor used in these surge tests survived surges up to 6 kV @ 3 kA.

A side by side RF performance data comparison shows no significant change in insertion loss. Changes observed in return loss in high frequency voltage surge tests (see below) might come from the anticipated repetitive soldering, which inevitably affects RF performance in higher frequencies. Further tests of multiple prototypes have verified this explanation. For CATV splitter8devices, the inventors determined that a 0.8 mm spark gap is preferred for various embodiments of the invention. The inventors also determined that a standard capacitor (non spark gap type) with at least a 500 volt dielectric rating can be used in conjunction with a surge rod for reliable 6 kV/3 kA combo surge survival. It is expected that the same result will be obtained for the other embodiments of the invention.

FIGS. 12,13, and14, show curves of insertion loss, return loss, and isolation, respectively, for tests made before applying a surge voltage at the input port10using the first embodiment of the invention in a typical CATV splitter8circuit as shown inFIG. 4.FIGS. 15,16, and17are tables showing test results before surge for insertion loss, return loss, and isolation, respectively.FIGS. 18,19, and20, show curves of insertion loss, return loss, and isolation, obtained from applying 6 kV (kilovolts) combination voltage surges to the input port10.FIGS. 21,22, and23are tables showing test results after applying surge voltages for insertion loss, return loss, and isolation, respectively. It was observed that damage from surge voltage to capacitor C1, for example, occurred mainly in the low frequency range 5 Mhz to 30 MHz.

The spark gap embodiments of the present invention are intended to prevent damage to an associated capacitor connected to an associated port. The fact that the inventors observed minimal change to the low frequencies indicates that the capacitor is being protected by the spark gap, which limits the peak voltage appearing across the plates of the capacitor. Slight changes in the return loss are due to the carbon residue formed between the surge electrodes during the arc over owing to the conductivity of carbon. These changes in the RF performance of the circuit are not detrimental to the operation of the circuit. However, without the surge protection of the spark gap, capacitors exposed to high voltage surges are likely to explode or crack, or otherwise be damaged. This interrupts the operation of the associated circuit severely. Spark gap surge protection allows the use of capacitors with lower working voltage5(500 volts, for example) to be used. Such capacitors are smaller in size than the typical capacitors of 1 kilovolt, which is the minimum voltage without spark gap. The ability provided by use of the present embodiments is an important factor in determining the minimum cavity size of a splitter housing, for example.

The parameter that is most important to the functionality of the circuit from a user point of view is the insertion loss. This is a measurement of the amount of signal that is lost as the signal travels through the circuit. The return loss is a measurement of the ratio of the incident wave to its reflection. As the impedance of the circuit approaches the characteristic impedance of the medium (75 ohms, for example), the return loss decreases toward an infinite value at exactly 75 ohms. This indicates that none of the incident signal is lost to reflection. This can occur at any frequency. This parameter is sensitive to subtle changes in the nature of the transmission path which could arise from carbon deposits caused by a spark or by the erosion of the transmission path. The isolation is a measure of the separation of the outputs. Signals injected into one output are attenuated from another output by phase cancellation. The measurement of this attenuation reveals the isolation parameter with respect to frequency. Changes in the impedance of the circuit8from input10to outputs12and14affect the isolation signature over the operating frequency range of the circuit. Surge effects are often seen in the low frequency isolation performance due to damage to the associated coupling capacitor (C1, in this example).

The tables ofFIGS. 15 to 17, and21to23are associated with the graphs showing the RF (radio frequency) parameters associated with the input port10and output ports12and14at various frequencies. Insertion loss is the signal loss from the input port10to ports12and port14. The return loss is the logarithm of the ratio of the reflected signal to the incident signal at each port. The isolation is the attenuation of the signal between ports1and2. All signal losses are expressed in decibels. The graphs give a pictorial representation of the tables with respect to the signal frequency in Megahertz.

For all embodiments of the invention, it was observed from prototype tests that a spark gap range from 0.5 mm to 1.0 mm was operable to protect coupling capacitors at an input or output port from surge voltage. The preferred gap was determined to be 0.8 mm. For each embodiment, it was also determined that the thickness of the surge rods17,30, clip electrodes34, and eyelets46,60must be sufficient to insure long-term reliability. Also, although tin-plated brass material was used for each embodiment, other suitable material can be used. Further note that the material thickness must be controlled to insure long term reliable surge protection. In the prototypes the diameter or thickness of surge rods16,17, and30was 1.2 mm, and of clip electrode34was 0.5 mm. The dimensioning of eyelets46and60is given above.

Although various embodiments of the present invention have been shown and described, they are not meant to be limiting. Those of skill in the art may recognize certain modifications to these embodiments, which modifications are meant to be covered by the spirit and scope of the following claims. The present embodiments of the invention have been described hereon in association with a cable television splitter circuit, as an example, but the present invention is not to be so limited, and is applicable for use with any electrical or electronic devices that require surge voltage protection. Such other devices, for example, may include amplifiers, filters, directional couplers, and so forth.