Wave transmission lines and networks – Coupling networks – Frequency domain filters utilizing only lumped parameters
Reexamination Certificate
2002-12-03
2004-09-21
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Frequency domain filters utilizing only lumped parameters
C333S176000
Reexamination Certificate
active
06794957
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to electronic filter assemblies used in the cable television (CATV) industry, and more specifically to structural features of such filter assemblies directed to providing improved electrical grounding of a filter circuit carried on a printed circuit board.
In typical CATV applications a filter circuit or network is provided to pass signals having frequencies within one or more specified band widths, sometimes with a desired amount of signal attenuation, while blocking signals of other frequencies. It has been found convenient to mount the electrical components, e.g., capacitors, inductors and resistors, on one or more printed circuit boards in essentially conventional fashion. That is, for such CATV filters as highpass and lowpass filters, diplex filters, windowed highpass filters, and step attenuator (or return path) filters, tunable filter circuits and shielding between filter components and circuits are not normally required. The circuit board(s) carrying the filter circuit components is/are mounted within a suitable, protective housing. Prime requirements for the circuitry and housing means are preventing ingress of moisture and foreign matter through the filter connectors (such as shown in U.S. Pat. No. 5,278,525 to Palinkas) and the provision of a solid and reliable electrical ground between the filter circuit(s) and the housing (such as disclosed in U.S. Pat. No. 6,323,743 to Zelenz et al). The latter requirement is particularly critical when the filter is intended to handle signals of high frequency. In fact, the grounding arrangements of the prior art, including that of the Zelenz et al patent, are not fully reliable for frequencies above about 300 Mhz.
Filter circuits of this type are typically designed with a plurality of interconnected “sections,” sometimes referred to as “poles,” “stages,” or by other terms. As generally understood in the art, and as used herein, the term “section,” as applied to filter circuits, denotes a discrete portion of the circuit including a group of two or more components which establish a resonance (e.g., a capacitor and an inductor). In general, the performance of highpass, lowpass, and other types of CATV filters is enhanced by increasing the number of sections. That is, other factors being equal, a circuit having four sections is superior to a circuit having two sections.
Oftentimes, the greatest obstacle to achieving optimum circuit performance is proper grounding. Each section of the circuit requires a connection to ground. Traditionally, filter circuits carried in housings less than 2.1 cm in diameter make the ground contact between the housing and circuit sections at the male connector side of the housing. This is the case, for example, in the previously referenced Zelenz, et al patent where grounding pads are provided at each of the two corners of the board at the male connector end and on both surfaces of the circuit board. When the ground contact is placed at one end of the board, the electrical path from the circuit sections to the ground contact lengthens as the section components are placed farther away from the housing ground contact. Inductance and resistance parasitics are introduced into the electrical path in proportion to its length, i.e., the longer the path between the circuit section and the ground contact, the greater the level of undesirable inductance and resistance which are introduced. The additional inductance limits the circuit operating frequency range and the resistance severely degrades the “Q” (quality factor) of the circuit. The present invention is based upon recognition of these effects and minimizing their impact on circuit performance.
The principal object of the present invention is to provide a filter assembly for CATV applications with improved electrical grounding of the filter circuit.
A further object is to provide a CATV filter assembly having a filter circuit with a plurality of sections each of which is connected to ground potential through an electrical path of minimal length.
Another object is to provide a CATV filter assembly including a circuit board carrying a filter circuit and mounted in a housing at ground potential wherein the board is mechanically connected and the circuit is electrically connected to the housing without a soldering operation.
A still further object is to provide a CATV filter assembly including a filter circuit carried on a printed circuit board mounted within a housing having two mating portions wherein the filter circuit is placed in electrical (grounding) communication with the housing by mutual assembly of the two portions.
Other objects will in part be obvious and will in part appear hereinafter.
SUMMARY OF THE INVENTION
The invention will be shown and described in three physical embodiments, although it will be understood that the electrical and structural components and their arrangement are subject to many modifications within the scope and spirit of the invention. In each disclosed embodiment the circuit components are mounted upon a single circuit board using surface mount components, although the invention may be implemented with filter circuits utilizing through-hole components, or a plurality of circuit boards. Likewise, the invention may be practiced with a wide variety of filter circuit designs although it is directed to use with circuits having a plurality of sections, each of which has a direct path to ground. That is, at least one component of each section of the circuit is connected by a path on the circuit board to a member which is at ground potential. The components are connected to one another and to the grounding member by electrically conducting paths on the board which are sometimes referred to as traces or runs, but in the present description and claims are uniformly called “paths.”
The housing or casing wherein the circuit board is mounted is of electrically conducting material at ground potential, as in many conventional filter assemblies of this type, and provides the ultimate ground for the circuit. The paths from the respective sections may contact portions of the housing directly, or (as in the preferred embodiment) may extend from each section to a grounding bus on one or both surfaces of the board. The grounding bus is in direct electrical communication with the housing and thus provides a connection to ground for each section of the circuit components. The grounding bus may be (again, as in the preferred embodiment) in the form of an elongated strip, in which case it is important the strip be in direct contact with the housing over substantially all of its length so that an ungrounded portion of appreciable length will not act as a lengthy electrical path between the ground connection and one or more circuit sections, thereby contributing to the problem of inductance and resistance parasitics. In any case, the length of the electrical path from each section of the circuit to a member at ground potential, i.e., not simply to the grounding bus but to a point on the grounding bus which is in direct, grounding contact with the housing, is kept to a minimum which may be as small as one millimeter or less, but in no event is more than 5 millimeters.
In the first disclosed and preferred embodiment, the housing includes inner and outer, hollow, cylindrical portions. The inner portion includes a pair of open slots on opposite sides of the cylinder extending from one end for part of the axial length thereof. The printed circuit board carrying the filter circuit components and connecting paths has parallel, linear side edges with strips of conducting metal extending along marginal portions of the board adjacent each side edge on both the top and bottom surfaces of the board. The widths of the slots in the inner housing portion are greater than the thicknesses of the respective edge portions of the circuit board, and the outside diameter of the inner housing portion is slightly greater than the distance between the outer edges of the board.
In addition to the circuit board and compo
Purdy Eric J.
Shafer Steven
John Mezzalingua Associates Inc.
Muldoon James R.
Pascal Robert
Takaoka Dean
Wall Marjama & Bilinski
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