Wave transmission lines and networks – Plural channel systems – Having branched circuits
Reexamination Certificate
1999-05-12
2001-03-27
Bettendorf, Justin P. (Department: 2817)
Wave transmission lines and networks
Plural channel systems
Having branched circuits
C333S012000
Reexamination Certificate
active
06208219
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to radio frequency (RF) circuits and particularly broadband RF circuits such as those used for instance in signal splitters, routers and switching matrices in multichannel systems.
2. Background Information
Typically, multiple RF signals such as television signals offered by cable television (CATV) systems have been distributed using routers made up of many branching elements such as splitters, combiners and mechanical or solid state switches interconnected by a large number of cables. Building large, multi-port routing switchers and matrix switching systems capable of operating in the upper VHF and low microwave frequencies (L-band to 2500 Mhz and beyond) requires a high concentration of splitting, switching and combining elements in limited rack space. To maintain acceptable return loss, insertion loss and isolation performance for each individual path in a “traditional” VHF or microwave switch, each circuit element, switch module and interconnecting coaxial cable has to be individually optimized for all possible (n*M) switching combinations, which is a very time consuming and expensive process, requiring technical expertise, and the outcomes are not predictable due to widely varying physical layouts and cost limitations. This process inevitably results in phasing and gainslope problems, with some paths being deficient at some frequencies over the very broad frequency bands (multi-octaves) of the intended applications.
U.S. Pat. No. 5,481,073 describes a modular broadband switching system suitable for such applications which eliminates most of the cabling by using lumped elements (5-1000 MHz) or distributed elements (above 1000 MHz) and using microstrips to interconnect the multiple branching elements mounted on printed circuit boards (PCB). The routers or matrices are assembled from transversely oriented stacks of parallel sets of such boards (stack & tier configuration). The circuit on an individual board is laid out in a candelabra pattern so that multiple branches between inputs and outputs all contain the same number of branching elements and are all of substantially the same length so that signal loss and phase of each signal in all branches are substantially the same. Initially, the switches used in these circuits were mechanical switches which provide very good isolation, e.g., 90 dB. As the performance requirements placed upon these systems increase, such as by adding data, voice and two-way internet access capability to cable services, the frequency band must be expanded up into the gigahertz range. In addition, efforts have been directed toward increasing the number and switching speed of the channels. This has lead to the use of solid state switches, such as (1*4) element GaAs switches which simultaneously reduce the total count of elements on the circuit boards, while increasing the number of circuit branches and the speed of switching. Unfortunately, these solid state switches do not provide the isolation available from mechanical switches, e.g., typically only 40 dB. This isolation can be improved by adding switches in series with each branch output to increase isolation by as much as about 20 dB at the highest frequency of operation and considerably more at the lower frequencies.
Even with such tandem switching, isolation between branches remains a problem. At least 60 dB of isolation is required between any circuit paths carrying analog TV signals. I have realized that poor isolation is due in part to the virtually universal practice of laying out microstrips on circuit boards in straight lines or arcs of constant radii with adjacent lines or arcs often parallel to each other. While this practice simplifies circuit board design and renders the software used in automated design (CAD) simpler, it leads to coupling between closely spaced lines. In addition, when a signal is split between adjacent parallel lines which radiate the signal in phase, an antenna array may be created. Similarly, an antenna array can be created by two identical lines which diverge about a common axis. Where a common signal is distributed through stacks of identical parallel circuit boards the problem is compounded.
The problem is not limited to broadband multi-channel splitters, couplers, routers and matrices. As the frequency of digital processors increases into the high megahertz and gigahertz ranges the typical linear, parallel patterns of the microstrip lines on mother boards as used on computers and other advanced PCBs also leads to signal isolation difficulties.
There is a need therefore for RF circuits with improved isolation between circuit elements and reduced radiation of signals.
SUMMARY OF THE INVENTION
This need and others are satisfied by the invention which is directed to a broadband RF circuit which includes a circuit board, a number of circuit elements on the circuit board, and a plurality of microstrips on the circuit board interconnecting the circuit elements. The term microstrip as used herein refers to conductors printed, etched or machined into rigid or flexible dielectric material and an associated ground plane and includes striplines suspended between two ground planes. The microstrips are laid out in randomly meandering paths such that no two paths are parallel. By randomly meandering, it is meant that the paths change direction in an irregular and non-symmetrical fashion and have a random relationship to the wavelengths of RF signals carried by the microstrip. Each of the randomly meandering paths is non-parallel to adjacent randomly meandering paths and no two adjacent paths are symmetrical about a common axis. Such non-parallel randomly meandering paths reduce the coupling between microstrips. They also reduce the tendency of adjacent microstrips carrying the same signal to act as a multi-element antenna which would radiate the signal.
The invention has particular applicability to broadband RF circuits such as those used in multi-media routers and switching matrices. Such circuits tend to have multiple branches made up of branching elements and interconnecting microstrips which form a symmetrical pattern such as a candelabra pattern. The symmetrical patterns promote the coupling between microstrips and the creation of multiple element antennas arrays, especially where a signal is split into a number of parallel signal paths.
The invention also has particular applicability to modular broadband RF circuits where a number of the circuit boards are mounted in spaced apart parallel planes. The isolation between multiple paths and reduction of radiation of a common signal can be further enhanced in such modular circuits by having different randomly meandering paths on corresponding microstrips on adjacent boards. Where such boards are symmetrical about a central axis, the random meandering of corresponding paths on the two sides of the axis can be different so that adjacent boards can be reversed about the common axis to maintain similar circuit configurations but without having microstrips with the same random meandering of adjacent microstrips. This arrangement permits improved isolation to be achieved with a single, standardized circuit board.
In accordance with another aspect of the invention, the circuit board can have electrically isolated surfaces with a first randomly meandering microstrip on one surface and a second microstrip on the second surface which is opposed to the first microstrip and follows a randomly meandering path which is a mirror image of the randomly meandering path of the first microstrip. This same arrangement of using mirror image randomly meandering paths can be applied to corresponding microstrips on adjacent circuit boards in the modular configuration.
REFERENCES:
patent: 3475700 (1969-10-01), Ertel
patent: 3736534 (1973-05-01), Chaffee
patent: 4293858 (1981-10-01), Hockham
patent: 4525689 (1985-06-01), Wagner et al.
patent: 4785135 (1988-11-01), Ecker et al.
patent: 5285175 (1994-02-01), Edwards
patent: 5357227 (1994-10-01), Tonegawa et al.
patent: 54143
Bettendorf Justin P.
Buchanan Ingersoll P.C.
LandOfFree
Broadband RF circuits with microstrips laid out in randomly... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Broadband RF circuits with microstrips laid out in randomly..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Broadband RF circuits with microstrips laid out in randomly... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2513708