Wave transmission lines and networks – Long lines – Strip type
Reexamination Certificate
2002-04-05
2004-01-13
Wells, Kenneth B. (Department: 2816)
Wave transmission lines and networks
Long lines
Strip type
C333S157000, C333S239000, C333S248000
Reexamination Certificate
active
06677838
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention is based on a priority application 101 19 717.9 which is hereby incorporated by reference.
The invention relates to a coplanar waveguide, which has parallel conductor strips that are arranged on a substrate and have an electrically insulating intermediate space between one another.
In coplanar waveguides which are formed by striplines, the characteristic impedance depends on the distance between the conductor strips. In one case, however, for instance the case below, it is not possible to achieve the small distance required for a desired low characteristic impedance. On silicon submounts of laser modules for a transmission rate of 10 Gbit/second, it is necessary to have a circuit to compensate for the stray capacitance of the laser, in order to increase the bandwidth. In the case of coplanar thin-film technology, this compensation circuit requires a short transmission line with a low (characteristic) impedance of e.g. 15 ohms. A low impedance of this type cannot be achieved in the form of a coplanar waveguide: because of the small intermediate spaces which would be needed between the conductor strips, it would be necessary to go below the smallest technically feasible distance (critical distance).
It is an object of the invention, for a device of the type described in the introduction, to offer a way of providing a line section with a low impedance by using simple means.
SUMMARY OF THE INVENTION
This object is achieved, according to the invention, by arranging a material with a high dielectric constant on the conductor strips, on their side facing away from the substrate.
Preferably, the material with a high dielectric constant is held in close contact with the conductor strips, so that any air gaps can be kept very small and the arrangement is well defined.
The material with a high dielectric constant, which may be any suitable material in any suitable form, and may preferably be a piece of ceramic and, in particular, may be in the form of a plate, has a dielectric constant which is high enough to obtain a capacitance that is sufficiently high for a low characteristic impedance. In one example, the relative permittivity =65 in order to create the desired relatively low characteristic impedance in the vicinity of the plate, or the ceramic piece, in the coplanar line. This low characteristic impedance owes its existence to the fact that the ceramic piece increases the effective capacitance between the conductors of the coplanar line, compared with the situation in which the ceramic piece is not present. The ceramic piece is fastened onto the top of the conductors of the coplanar line, that is to say the conductors of the coplanar line are situated between the substrate of this line, which e.g. consists of (high-resistivity) silicon, and the ceramic piece. In other applications, it may be possible to use a material with a relative permittivity of only about 10 in order to increase the capacitance.
In one embodiment of the invention, the side of the material with a high dielectric constant facing away from the conductor strips is designed to be electrically conductive, and is preferably provided with a metallisation. The advantage of this is that the metallisation enhances the capacitance-increasing effect of the ceramic piece, specifically because the field strength in the ceramic piece is increased by shortening the field lines.
In one embodiment of the invention, the length over which the material with a high dielectric constant is applied, or the length of the material with a high dielectric constant, is shorter than the 4. Waveguide according to one of the preceding claims, wherein the length over which the material with a high dielectric constant is applied, or the length of the material with a high dielectric constant, is shorter than the stripline. Here, a lower characteristic impedance is hence present over a limited part of the length of the stripline.
REFERENCES:
patent: 4216450 (1980-08-01), Linke et al.
Avanex Corporation
Suzue Kenta
Wells Kenneth B.
Wilson Sonsini Goodrich & Rosati
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