System and method for improving performance of coplanar waveguide bends at mm-wave frequencies

1. Field

The invention relates to systems and methods for improving the performance of 90 degree coplanar waveguide (CPW) bends at mm-wave frequencies. More particularly, the CPW bends may be chamfered on the signal conductor and the ground plane and additional vias may be placed near the CPW bends.

2. Background

Microwave and mm-wave RF circuits may be integrated on a dielectric substrate with transmission lines (e.g., CPW) that feed the RF signals between the circuits. Such transmission lines often include bends that turn the direction of energy propagation (i.e., change the direction of field orientation) from one direction to another. A right angle transmission line bend, for example, turns the direction of energy propagation around 90 degrees. One drawback is that transmission line bends introduce losses.

One type of loss, called a return loss, relates to the energy that is reflected back from the transmission line bend. Return losses can be created due to capacitance and inductance being formed around the transmission line bends. For example, capacitance may arise through charge accumulation at the right angle transmission line bend, particularly, around the outer point of the transmission line bend where the electric fields concentrate. Inductance may arise due to current flow constriction. In addition, the change of field orientation at the right angle transmission line bend is influenced by mode conversions. These influences significantly increase the return loss.

Focusing on the return loss, several techniques have been implemented in the past to compensate for the transmission line bends in order to reduce the effect of the capacitance and inductance. For example, the transmission line bends may be mitered and rounded where the miter technique removes metal where there is no current flow, and that reduces the capacitance and inductance. Doing so improves the voltage standing wave ratio (VSWR) and reduces the return loss.

A coplanar waveguide (CPW) is an attractive choice for the development of monolithic microwave integrated circuits (MMICs). A CPW is formed from a conductor separated from a pair of ground planes, all on the same plane, atop a dielectric medium. Several advantages of CPWs include ease of shunt and series connections, low radiation, low dispersion, and avoidance of the need for thin fragile substrates. One drawback of a prior art CPW bend is that the two slots and the two ground planes on each side of the center conductor have different lengths. The different lengths cause unwanted slot-line and parallel plate modes which tend to radiate and reduce the overall performance of the transmission line.

FIG. 1A is a schematic view of a prior art CPW bend 104 that utilizes air-bridges 102 for performance improvements. FIG. 1B is a schematic view of a prior art chamfered CPW bend 106 that utilizes air-bridges 102 for performance improvements. The prior art CPW may include a center signal plane and a pair of ground planes. The center signal plane may have a width W, and it may be spaced between the pair of ground planes with a slot width S. Referring to FIGS. 1A and 1B , the placement of air-bridges 102 near the CPW bends 104 and 106 has been used to eliminate unwanted slot-line and parallel plate modes. However, the inclusion of air-bridges 102 may add unwanted capacitance on the transmission lines which can further degrade the CPW performance. CPW performance is especially important at mm-wave frequencies.

FIG. 2 is a schematic view of a prior art CPW bend that utilizes high-impedance transmission line sections 204 under the air-bridges 202 for performance improvements. The high-impedance transmission line sections 204 under the air-bridges 202 are narrower and therefore add less parasitic capacitance on the transmission line. However, the high-impedance transmission line sections 204 require the addition of short matching networks.

Although the foregoing techniques are helpful in reducing the return loss for the transmission line bends, additional improvements can be made to improve the VSWR and reduce the return loss. Moreover, they require the fabrication of air-bridges which is complex. Therefore, a need exists in the art for systems and methods for improving the performance of CPW bends at mm-wave frequencies without the need for air-bridges.

An apparatus including a liquid crystal polymer substrate having a top surface and a bottom surface, a coplanar waveguide formed on the top surface of the liquid crystal polymer substrate, the coplanar waveguide having a 90 degree bend with a mitered edge, an inner via positioned adjacent to an inner corner of the 90 degree bend, and an outer via positioned adjacent to the mitered edge of the 90 degree bend, the inner and outer vias positioned along a first plane that is perpendicular to a second plane defined by the mitered edge.

The features, objects, and advantages of the invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, wherein:

FIG. 1A is a schematic view of a prior art CPW bend that utilizes air-bridges for performance improvements;

FIG. 1B is a schematic view of a prior art chamfered CPW bend that utilizes air-bridges for performance improvements;

FIG. 2 is a schematic view of a prior art CPW bend that utilizes high-impedance transmission line sections under the air-bridges for performance improvements;

FIG. 3 is a schematic top view of a three-dimensional automotive radar RF front-end according to an embodiment of the invention;

FIG. 4 is a schematic bottom view of the three-dimensional automotive radar RF front-end according to an embodiment of the invention;

FIG. 5 is a schematic diagram showing a back-to-back 90 degree CPW bends on a liquid crystal polymer (LCP) according to an embodiment of the invention;

FIG. 6 is a graph of return loss (dB) versus frequency (GHz) for the apparatus shown in FIG. 5 according to an embodiment of the invention; and

FIG. 7 is a graph of insertion loss (dB) versus frequency (GHz) for the apparatus shown in FIG. 5 according to an embodiment of the invention.

Apparatus, systems and methods that implement the embodiments of the various features of the invention will now be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate some embodiments of the invention and not to limit the scope of the invention. Throughout the drawings, reference numbers are re-used to indicate correspondence between referenced elements and may not be described in detail for all drawing figures in which they appear.

FIG. 3 is a schematic top view of a three-dimensional automotive radar RF front-end 300 having a plurality of CPW bends 305 according to an embodiment of the invention. FIG. 4 is a schematic bottom view of the three-dimensional automotive radar RF front-end 300 having a plurality of CPW bends 305 according to an embodiment of the invention. The plurality of CPW bends 305 achieve optimum performance by exploiting the capabilities provided by the use of a liquid crystal polymer (LCP) substrate. The plurality of CPW bends 305 are wideband (e.g., 60-90 GHz) to increase fabrication and assembly tolerances and have low-insertion loss (e.g., less than −1 dB) to reduce the loss between the antenna and the T/R module. The loss generally dominates the overall noise figure of the radar and eventually limits its sensitivity and read range. The low return loss, small size to allow for the co-location of multiple transitions in close proximity to the chip, low cost, and minimum number of vias are compatible with LCP design rules.

The automotive radar RF front-end 300 achieves very good RF performance (i.e. low insertion and return loss and wide bandwidth) by utilizing a chamfered or mitered bend on the signal conductor and the ground plane and by strategically placing vias adjacent to the CPW bends 305 . The automotive radar RF front-end 300 may be implemented using hardware, software, firmware, middleware, microcode, or any combination thereof. One or more elements can be rearranged and/or combined, and other radars can be used in place of the radar RF front-end 300 while still maintaining the spirit and scope of the invention. Elements may be added to the radar RF front-end 300 and removed from the radar RF front-end 300 while still maintaining the spirit and scope of the invention.

FIG. 5 is a schematic diagram showing a back-to-back 90 degree CPW bends 505 on a LCP 510 according to an embodiment of the invention. The CPW bends 505 may be used with automotive radar and passive mm-wave imager applications. Each CPW bend 505 has a mitered corner 506 . In one embodiment, signal plane 507 and ground planes 508 are chamfered. The CPW bends 505 are formed on a LCP 510 or any other low-cost organic substrate. The CPW bends 505 are made of a metallic material and are used to propagate signals.

An inner via 515 and an outer via 520 are positioned adjacent to each CPW bend 505 . The inner via 515 and the outer via 520 are preferably metalized vias. The inner via 515 and the outer via 520 lie along a first plane 521 that is perpendicular to a second plane 522 defined by the CPW bend 505 . The inner via 515 and the outer via 520 enhance and optimize the performance of the CPW bend 505 . The enhancement and optimization of performance is shown in FIGS. 6 and 7 . That is, the return and insertion losses are reduced from 605 , 705 (no chamfering and no vias) to 610 , 710 (chamfering and no vias) to 615 , 715 (chamfering and vias). In one embodiment, as depicted in FIG. 5 , the inner via 515 is about 0.15 millimeters in diameter, about 0.1 millimeters in depth, and positioned about 0.15 millimeters away from the CPW bend 505 . Similarly, the outer via 520 is about 0.15 millimeters in diameter, about 0.1 millimeters in depth, and positioned about 0.15 millimeters away from the CPW bend 505 . The inner via 515 and the outer via 520 are used to suppress the parasitic parallel plate and slot-line modes. The inner and outer vias 515 and 520 are created with either laser or mechanical drilling and are filled with metal.

FIG. 6 is a graph of return loss (dB) versus frequency (GHz) for the apparatus shown in FIG. 5 according to an embodiment of the invention. Curve 605 shows the return loss of a bend with a sharp corner, curve 610 shows the return loss of a bend with a chamfered end, and curve 615 shows the return loss of a bend with a chamfered end and inner and outer vias 515 and 520 of FIG. 5 . The bend with a sharp corner (curve 605 ) can be used for frequencies up to 60 GHz, the bend with a chamfered end (curve 610 ) can be used for frequencies up to 75 GHz, and the bend with a chamfered end and inner and outer vias 515 and 520 of FIG. 5 (curve 615 ) can be used for frequencies up to 90 GHz and even higher. Hence, the performance (i.e., return loss) of the bend with a chamfered end and inner and outer vias 515 and 520 of FIG. 5 is better than other bends without the inner and outer vias 515 and 520 of FIG. 5 .

FIG. 7 is a graph of insertion loss (dB) versus frequency (GHz) for the apparatus shown in FIG. 5 according to an embodiment of the invention. Curve 705 shows the insertion loss of a bend with a sharp corner, curve 710 shows the insertion loss of a bend with a chamfered end, and curve 715 shows the insertion loss of a bend with a chamfered end and inner and outer vias 515 and 520 of FIG. 5 . The bend with a sharp corner (curve 705 ) can be used for frequencies up to 60 GHz, the bend with a chamfered end (curve 710 ) can be used for frequencies up to 75 GHz, and the bend with a chamfered end and inner and outer vias 515 and 520 of FIG. 5 (curve 715 ) can be used for frequencies up to 90 GHz and even higher. Hence, the performance (i.e., insertion loss) of the bend with a chamfered end and inner and outer vias 515 and 520 of FIG. 5 is better than other bends without the inner and outer vias 515 and 520 of FIG. 5 .

Those of ordinary skill would appreciate that the various illustrative logical blocks, modules, and algorithm steps described in connection with the examples disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the disclosed apparatus and methods.

The previous description of the disclosed examples is provided to enable any person of ordinary skill in the art to make or use the disclosed methods and apparatus. Various modifications to these examples will be readily apparent to those skilled in the art, and the principles defined herein may be applied to other examples without departing from the spirit or scope of the disclosed method and apparatus. The described embodiments are to be considered in all respects only as illustrative and not restrictive and the scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.