Patent Grant
6952143
This invention relates generally to a millimeter-wave signal transition, and, more specifically, to a signal transition for transiting a mm-wave signal between two different geometric planes.
Automated cruise control (ACC) for automobiles is gaining popularity in recent years. ACC allows a user to set the desired speed and minimum following distance of his/her vehicle. The system then controls the speed of the user's vehicle to ensure that the minimum following distance is maintained. Critical to such systems is the effective implementation of a radar system, typically those operating in the 77 GHz range. Such systems must be capable of transmitting, receiving and manipulating millimeter-wave (mm-wave) signals. As with most electronics, there is continuous pressure to miniaturize such systems to reduce their space and material requirements. Consequently, the circuitry of these systems is becoming more compact and sophisticated, employing such techniques as stack circuit technology to reduce size. With stacked circuits, there is often a need to transmit a signal between circuit substrates while operating in the mm-wave domain. For example, in ACC system applications, transreceiver and antenna are placed on either sides of a thick support plate. This makes it necessary to transmit the mm-wave signal between two microstrips on either side of the relatively thick metal support plate. This transmission is performed by a “signal transition” or “transition” as used herein. Design of this transition is critical to the overall system performance.
The purpose of a signal transition in an electrical circuit is to transfer the radio frequency (RF) energy from one point to another point with minimum interference and loss. The key requirements of a good signal transition are high return loss and low insertion loss. Note that, in general, these two specifications are independent from each other, but must be satisfied simultaneously. In other words, one may achieve a relatively good return loss using a particular signal transition, however, without having a low insertion loss, mm-wave energy is absorbed in the transition, thereby diminishing the total performance of the system. Having a low insertion loss is especially important in high frequencies due to increased conductor and radiation losses.
Transitions designed to transfer electrical signals from transverse plane of microstrip lines to another plane, which is parallel to the first one, with a vertical connection are now going to be explained in more detail because the invention is related with such structures. Via holes employed in standard multi-layer printed circuit board (PCB) technology are very good examples of such transitions. The critical issue here is the electrical length of the vertical connection. As the length of vertical connection increases, design of the transition becomes more challenging because of the increased parasitic inductance. There are a number of reported developments for transferring a signal from one transverse plane to another one. For example, the microstrip-to-slot transition along with its variants which use a vertical waveguide section is one of the more commonly used techniques for this purpose. This approach, however, has a number of disadvantages. First, this transition relies on the resonance phenomenon to achieve a good match. Therefore it is particularly susceptible to geometry variations in the transition. Additionally, since the transition has no back short, it suffers from relatively high insertion loss due to radiation. This is especially important because the spurious radiations that may occur in such a transition may increase the cross talk or affect the antenna pattern in a mm-wave system. Alternatively, a transition can be used which exploits an E-plane probe with a back short to transfer the energy through a waveguide section. Although this approached is well established in the literature, it has a significant disadvantage in mm-wave frequencies. Specifically, at these frequencies, one must position a back short over a microstrip probe within a tolerance in the order of sub-millimeters in a 77 GHz application. This is clearly an expensive procedure for a high volume manufacturing.
Therefore, there is a need for a mm-wave transition to overcome the aforementioned difficulties. The present invention fulfills this need among others.
The present invention provides a mm-wave signal transition which overcomes the problems of the prior art. Specifically, the transition of the present invention uses a transducer to convert signals between transverse electromagnetic (TEM) and waveguide modes, rather than relying on the precise positioning of a transmission line relative to a waveguide to launch a signal down the waveguide. By using a transducer, the sensitive signal conversion between TEM mode and waveguide mode is performed in a single, modular unit, which lends itself to mass manufacturing using well-known techniques. Once the delicate operation of converting a signal between TEM and waveguide modes is performed, the converted signal can be transmitted to an orthogonally positioned transmission line or waveguide with relative ease. If desired, the signal can then be converted back to either a TEM mode or waveguide mode signal for transmission down a different orthogonally positioned transmission line or waveguide. This allows the signal to be transmitted over various types of transmission lines over relatively large distances between circuits with efficiency.
This approach offers a number of advantages over prior art approaches with respect to both manufacturing and performance. As mentioned above, since the TEM/waveguide mode conversion is performed in a transducer, which can be manufactured discretely using well-known techniques, the need for close tolerance positioning between the other components of the transition is alleviated, thereby facilitating large-scale manufacturing techniques and modularization. For example, the waveguide need not be precisely aligned with the transition line, but may instead be based on a relatively loosely toleranced borehole through a support plate. This borehole may be adapted to receive a separately manufactured, modular waveguide filler to aid in the propagation of the waveguide mode signal. Additionally, by converting the TEM/waveguide mode in a modular transducer, there is no need to interconnect probes or the like through soldering or other welding techniques which are time-consuming and prone to failure or performance variations. The transducer not only simplifies the assembly of the transition, but also, in its preferred embodiment, it is planar and eliminates the need for back short, thereby simplifying its own manufacture. Therefore, the present invention's exploitation of a transducer in a transition offers significant manufacturing benefits over the prior art.
In addition to the manufacturing benefits of the present invention, it also offers important performance advantages over the prior art. Specifically, by converting between TEM and waveguide modes in a relatively simple, modular unit, a complex assembly of components is eliminated along with its attendant inefficiencies and variances. This results in a transition that provides consistent performance with both low insert loss and low reflective loss. Additionally, since the signal transition between orthogonal transmission lines is performed by converting the mode of the signal, the distance over which signals may be communicatively connected to parallel transmission lines is limited by the loss of the vertical hollow-waveguide which can be relatively low. This is in stark contrast to many prior art devices which experience difficulty in transmitting mm-wave signals between parallel transmissions that are further than 10% of the operating signal's wavelength. Finally, since the transition does not use probes or similar antennas like devices to launch the signal into the waveguide, radiation losses are very low and there is no need for a back short.
Accordingly, one aspect of the present invention is a transition for transmitting a mm-wave from one plane to another plane using a transducer. In a preferred embodiment, the transition comprises: (a) first and second transmission lines on parallel planes; (b) a third transmission line orthogonal to the first and second transmission lines, wherein either the first and second transmission lines are suitable for transmitting a TEM mode signal and the third transmission line is suitable for transmitting a hollow waveguide mode signal, or the third transmission line is suitable for transmitting a TEM mode signal and the first and second transmission lines are suitable for transmitting a waveguide mode signal; and (c) first and second transducers, the first transducer coupled between the first and third transmission lines, the second transducer coupled between the second and third transmission lines, each of the transducers being suitable for converting a signal between TEM and hollow waveguide modes.
Another aspect of the present invention is a method for transmitting a mm-wave signal from a first plane to a second plane using a transition comprising a transducer. In a preferred embodiment, the method comprises: (a) transmitting a mm-wave signal along a first transmission line in a first plane; (b) converting the signal from one mode of either a TEM mode or a waveguide mode to the other mode of either the TEM mode or the waveguide mode using a transducer; (c) transmitting the signal along a third transmission line orthogonal to the first transmission line in the other mode to a second plane parallel to the first plane; (d) converting the signal back to the one mode; and (e) transmitting the signal in the one mode along a second transmission line in the second plane.
Another aspect of the present invention is a method of manufacturing a transition which lends itself to large-scale manufacturing. In a preferred embodiment, the method comprises: (a) providing a support plate; (b) boring a hole in the support plate to form the waveguide; (c) inserting a waveguide filler in the hole; (d) providing first and second mm-wave boards, each board comprising an integrated transmission line and a transducer having a waveguide portion; (e) affixing the first and second mm-wave boards to each side of the support plate such that the transition lines are orthogonal to the waveguide and that the waveguide is axially aligned with the waveguide portion of each transducer.
Yet another aspect of the invention is a system incorporating the transition of the present invention. In a preferred embodiment, the system comprises an ACC system with the transition described above.
a and 4b show performance data for the transition of FIG. 1.
Referring to
Transition 1 comprises first and second parallel transmission lines 2a, 2b, and a third transmission line 4 orthogonal to the first and second transmission lines 2a, 2b. In this particular embodiment, the first and second transmission lines are incorporated into first and second mm-wave boards 6, 7, which are on different transverse planes. The first and second transmission lines 2a, 2b are suitable for transmitting a signal having a TEM mode, while the third transmission line 4 is a waveguide 4a disposed in a support plate 5 and is suitable for transmitting a signal in a waveguide mode. The transition 1 also comprises first and second transducers 3a, 3b on the first and second mm-wave boards 6,7, respectively. The first transducer 3a is coupled between the first and third transmission lines 2a, 4, while the second transducer 3b is coupled between the second and third transmission lines 2b, 4. Each of the transducers converts a signal between a TEM mode and a waveguide mode. These components are considered below in greater detail.
In the embodiment of
Transmission lines for transmitting TEM and waveguide mode signals are well known. Examples of transmission lines for transmitting TEM signals include coaxial lines, striplines, microstrip lines, coplanar waveguides (CPW), and fin strips. Preferably, at least one of the transmission lines suitable for transmitting TEM signals is a coplanar transmission line, specifically, a microstrip. More preferably, both the first and second transmission lines are microstrips.
Referring to
The microstrip may comprise any known conductor such as copper, gold, silver or aluminum. The dimensions of the microstrip can vary depending upon the application and the material used. The width of the microstrip line depends on the characteristic impedance required. For example, on a 5 mils thick Duroid 5880 material, which has the dielectric constant of 2.2, the 50-Ohm microstrip transmission line is 15 mils wide.
The substrate 26 may be any structure that provides a platform for supporting the conductive path 21. Preferably, the substrate is also suitable for supporting other electrical and optical components such as the transducer. The conductive path 21 and other components may be mounted in or on the substrate or may be integrally formed or integrated with the substrate. As a matter of convention, when referring to a component's position with respect to a substrate, the terms “on,” “in,” “incorporated into,” and “integrally-formed” are used interchangeably throughout this disclosure. Preferably, the substrate 26 is rigid to provide a stable platform for the electrical components affixed thereto, although flexible substrates are contemplated herein as well. Additionally, the substrate is preferably, although not necessarily, planar.
Aside from its physical configuration, the substrate is often an integral component of a transmission line or transducer, and, thus, its electrical properties may be critical. Suitable materials for the substrate include dielectrics having a dielectric constant between about 2 and 10. Examples of suitable materials include ceramics such as Alumina, single crystal semiconductors such as Gallium Arsenide and Silicon, single crystal sapphire, glass, quartz, and plastics such as Teflon®. Satisfactory results have been obtained with a substrate of Duroid® 5880 (a Teflon based material, commercially-available through Rogers Corporation) which has an effective dielectric constant of 2.2.
The substrate should be adequately dimensioned to provide a sufficient base for the first conductive path 21, and, preferably, the first transducer 3a, although it should be understood that the transducer and transmission lines may be supported by discrete substrates and coupled via an additional transition suitable for coupling TEM mode signals between different transmission lines on the same plane (well known). One of ordinary skill in the art can determine the appropriate thickness for a particular substrate material.
In the embodiment shown in
Referring to
In the embodiment shown in
After determining the thickness of the dielectric and the backside metallization of the filling material through the design process, they are cut in the shape of rectangular prisms to form the completed dielectric substrate filling 31 and dropped into the rectangular opening previously prepared in the metal plate 5a. This way, a rectangular dielectric-filled waveguide 4 is formed in the metal plate 5a, which is used to transfer the mm-wave energy from one side of the metal plate 5a to the other side.
The length of waveguide 4 may be as thick as the support plate 5 or the vertical distance between the first and second transmission lines 2a, 2b. This means that the waveguide may have a length which is greater than 10% of the wavelength of the mm-wave signal. For example, if the wavelength is 2.8 mm (77 GHz), the length may be greater than 0.28 mm. Such lengths have proven problematic in the prior art, however, since the present invention employs a filled waveguide section to transfer the mm-wave energy, it is possible to transfer the energy through thicker support plates with relatively low loss. In a preferred embodiment, the length of waveguide section is at least 0.25 mm, more preferably, at least 1 mm, and, even more preferably, at least 1.5 mm.
The first and second transducers 3a, 3b serves to convert the signal between the TEM mode and waveguide mode. The concept of using a transducer is discussed generally in U.S. Pat. No. 6,087,907 which is hereby incorporated by reference. Referring to
For illustrative purposes, the first transducer 3a may be separated into three different portions: the transmission portion 23, the conversion portion 24 and the waveguide portion 25. The transmission portion 23 of the transducer 3a is electrically coupled to the conductive path 21 of the first transmission line 2a. It should be understood that the transducer and transmission line may be printed on the same substrate as the transmission line and consequently a clear line of demarcation between the two may not exist. Nevertheless, for purposes of discussion herein suffice it to say that, at some point 22 (perhaps hypothetical), the conductive path 21 is no longer part of the transmission line 2a but rather part of the transmission portion 23 of the transducer 3a.
The transmission portion 23 is connected to the conversion portion 24. The conversion portion 24 comprises a plurality of conductive converting fins 28 printed onto the first substrate 26. The use of fins minimizes the reflective loss of the transducer. Each fin 28 is disposed in perpendicular relation to the direction of TEM mode propagation. In the embodiment shown in
In operation, it can be thought that the fins 28 electrically behave as transmission lines. At the operating frequency, the appropriate length of the transmission line electrically creates what appears to be an open circuit near, but away from the center of the TEM axis by virtue of the approximately one-quarter wavelength dimension. The transmission line, however, may also be emulated using a lumped element equivalent circuit instead of the fin 28, for example a parallel inductor and capacitor combination having appropriate values at the operating frequency. In alternate embodiments, it is not necessary that the fins 28 in each pair be co-linear with each other or that there be an equal number of fins 28 on either side of the conversion trace 27. Modifying these characteristics, however, will vary performance characteristics. These characteristics, therefore, may be used to optimize performance of the transformer for specific applications.
The conversion portion is adjacent the waveguide portion 25 of the transducer 3a. The waveguide portion 25 comprises the first substrate 26 and a U-shaped conductive barrier 29 defining a portion of the first waveguide's perimeter. The barrier 29 may be formed in known ways including etching or machining a trench or series of recessions in the substrate and filling or lining the trench or recessions with a conductive material such as, for example, gold, silver, copper, or aluminum. Rather than forming a continuous trench in the substrate, it may be preferable to use closely spaced circular vias to approximate a trench wall. Such an approach may be preferred for a printed circuit board. However, a continuous trench would improve the isolation between the neighbor transitions significantly.
A waveguide mode signal is launched into the waveguide portion by the conversion portion. Specifically, since adjacent fins 28 are electrically close together, the currents flowing through the fins are approximately in phase. The currents through the fins induce magnetic and electric fields that interfere destructively in air, but interfere constructively in the dielectric. Most of the energy, therefore, is transferred into the first substrate 26 of the waveguide portion 25.
The specific configuration of the transducer and the waveguide may be determined using commercially available full-wave electromagnetic simulators. For example, the design process may employ a simulation and optimization of appropriately portioned structures using a full-wave 3D electromagnetic simulator, available though, for example, Ansoft HFSS. The optimization feature of the simulator allows one to vary the dimensions of the transition for different material properties, sizes, and operating frequencies.
Referring to
It should be understood that although the function of the transducer was described above with respect to the transducer converting a TEM mode signal inputted into its transmission portion to a waveguide mode signal which is outputted through its waveguide portion, the transducer may work in reverse as well. Specifically, in the preferred embodiment, the same transducer can be used to convert a waveguide mode signal inputted into its waveguide portion to a TEM mode signal which is outputted through its transition portion.
As mentioned above, the configuration of the transition of the present invention provides for improved manufacturability. Specifically, the design avoids the close tolerances required in prior art transitions such as, for example, microstrip-to-slot and E-plane probe transitions. By relying on a transducer to convert the signal between TEM and waveguide modes, the conversion is effected in a modular component and complex alignment between components and waveguides can be avoided. Consequently, production methods can be used which lend themselves to volume and automated assembly. In particular, since the transmission line to waveguide position is not critical, the waveguide can be made separately from the transition—that is, it does not need to be formed integrally with the transition. This allows it to be manufactured using high-volume manufacturing techniques. For example, in the embodiment shown in
The transition of the present invention not only lends itself to high-volume manufacturing techniques, but also offers improved performance. For example, referring to
The transition of the present invention may be utilized in any assembly in which a mm-wave signal is transferred from one plane to another plane. Examples of such assemblies include ACC systems, LMDS systems and HRR systems.
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