This disclosure relates generally to microwave transmission lines and more particular to coplanar waveguide (CPW) microwave transmission lines to provide a different attenuation to an unwanted mode of propagation from that provided to a desired mode of propagation.
As is known in the art, a coplanar waveguides (CPW) structure includes: a center conductor disposed over a surface of a substrate; and a pair of ground plane conductors disposed over the surface of the substrate, the center conductor being disposed between the pair of ground plane conductors. Microwave energy fed to an input of the CPW propagates to an output in a differential transmission mode relative to the pair of ground plane conductor with the electromagnetic field being near the surface substrate. CPW has been and continue to be used in wide variety of integrated circuit and circuit board applications. However, being a three conductor system, CPW structures are vulnerable to propagation of unwanted common mode(s). For example, in many applications the integrated circuit having active elements interconnected on a top, or upper, surface of a common substrate and a conductor is disposed on the bottom surface of the substrate for mounting to a heat sink or to a system ground conductor, for example. In this example, a parallel plate region is formed between the conductors on the upper surface, particularly, when larger ground plane conductors are used for the CPW transmission line, and the conductor on the bottom surface.
More particularly, a microwave parallel plate region includes a pair of conductors disposed over opposite surfaces of a substrate. When such parallel plate region is used as a portion of a microwave transmission line, unwanted, parasitic, parallel plate modes may be generated (moding), supported between the pair of conductors, and then transmitted through the parallel plate region. In one application, a substrate may be used to realize a Monolithic Microwave Integrated Circuit (MMIC) chip having an amplifier with a conductor on the bottom of the substrate, for providing a system ground or for soldering to a printed circuit board or heat sink, for example, and conductors on the top of the substrate. In such a chip, transmission lines are used to interconnect elements of the amplifier. As a result of the top and bottom conductors, parallel plate moding may be generated. If the generated moding has frequencies within the bandwidth of the amplifier with magnitudes equal to, or greater than, the forward gain of the amplifier, a portion of the output energy produced by the amplifier may be coupled back to the input of the amplifier providing positive feedback thereby generating unwanted oscillations.
Common mode generation may also result from interference from other sources, such as, for example, coupling of external signals generated by other sources, unbalanced excitation or unbalanced ground paths.
Thus, while CPW transmission uses a differential mode transmission, these other sources can generate common modes that can propagate through the CPW transmission lines as unwanted signals and become a source of parasitic unwanted common mode signals that propagate through the one or more of the center conductors and pair of ground plane conductors and adversely affect the performance and operation of the MMIC.
In accordance with the present disclosure, a transmission line structure is provided having: a substrate; and a coplanar waveguide transmission line disposed over a surface of the substrate. The coplanar waveguide transmission line includes: a center conductor disposed over a surface of the substrate; and a pair of ground plane conductors disposed over the surface of the substrate, the center conductor being disposed between the pair of ground plane conductors. The coplanar waveguide structure is configured to provide a different attenuation to an unwanted mode of propagation from that provided to a desired mode to propagate.
In one embodiment, the unwanted mode is a common mode of propagation and the desired mode is a differential mode of propagation.
In one embodiment, at least one of the center conductor and the pair of ground plane conductors is configured as an inductor reactive element.
In one embodiment, the pair of ground plane conductors and the center conductor is each spiral shaped.
In one embodiment, the pair of ground plane conductors and the center conductor is each a meander line.
In one embodiment, each one of the center conductor and pair of ground plane conductors provides an inductor to suppress parasitic common mode signal propagation in the center conductor or in either one, or both, of the pair of ground plane conductors.
In one embodiment, a microwave structure includes: an input section for receiving both a common mode signal and a CPW differential mode signal; an output section; and a CPW transmission line, having a center conductor disposed between a pair of coplanar ground plane conductors, connected between the input section and the output section. The conductors of the CPW transmission line are configured to provide the common mode signal a different attenuation in passing to the output section than the CPW transmission line provides to the differential mode signal passing between the input section and the output section.
In one embodiment, the center conductor and the pair of ground plane conductors are each configured as an inductor.
In one embodiment, a capacitor is connected in parallel with the inductor.
With such an arrangement, the structure presents different impedances to the desired differential mode and the unwanted common mode. The structure provides attenuation of the unwanted common mode while allowing the desired differential mode to propagate. Thus, the structure appears as a spiral inductor to the common mode while appears as a matched transmission line to the differential mode. The structure can be used alone or part of a resonant circuit to block the common mode leaving the differential mode transparent to the resonant circuit.
The structure serves as a choke to common mode microwave signals and a CPW transmission line for differential mode microwave signals.
In one embodiment, a resistor is connected in parallel with the inductor.
The resistor is used for dissipating the energy of the unwanted mode signal.
The details of one or more embodiments of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.
Referring now to
More particularly, the input to the coplanar waveguide transmission line 14 includes a center conductor input pad 30 (
As noted above, at least one of the center conductor 18 and the pair of ground plane conductors 20, 22; here all three conductors 18, 20 and 22 are shaped as a spiral inductor. The spiral inductors are to provide an impedance to the common mode signals to suppress such common mode signals in attempting to pass between the input pad 30 and the output pad 32; however, the three conductors 18, 20 and 22 forming a CPW transmission line, allow differential mode signals at the input pad 30 to pass to the output pad 32 substantially unattenuated. Thus, the structure resembles a spiral inductor, however unlike the common spiral inductor where the signal line only wraps around, in structure two ground conductor strips 20, 22 also follow the signal line 18 and wraps around as well.
It is noted that the ground plane conductors 34a, 34b are separated from ground plane conductors 36a, 36b by a portion of the surface of the substrate 12. The ground plane conductors 34a, 34b is electrically connected to ground plane conductors 36a, 36b through a resistor R and a capacitor C (
Referring now to
A number of embodiments of the disclosure have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other embodiments are within the scope of the following claims.
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Number | Date | Country | |
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20150255847 A1 | Sep 2015 | US |