Patent Description:
A typical implementation of an FSL includes a stripline transmission line structure using two layers of dielectric material disposed about the stripline, with the stripline having a fixed length and a fixed width along the length of the FSL. Such structures are relatively simple to fabricate and provide adequate magnetic fields to realize a critical power level of approximately <NUM> dBm when using a single crystal material. One method of reducing the threshold power level is to use a lower-impedance stripline at the cost of degraded return loss. An external matching structure can be used to improve the impedance match, but this technique reduces the bandwidth and increases the insertion loss of the FSL. Permanent biasing magnets can be mounted to, or near, the FSL structure to produce a bias field. The strength of the magnetic field within the structure establishes the operating bandwidth of the limiter.

For further background, <CIT> describes a frequency selective limiter (FSL) having a transmission line structure with a tapered width. The FSL includes a substrate having a magnetic material, a signal conductor disposed on the substrate and first and second ground plane conductors disposed on the substrate. The signal conductor has a first end with a first width and a second end with a second different width such that the signal conductor is provided having a taper between the first and second ends of the signal conductor. First and second ground plane conductors are spaced apart from first and second edges of signal conductor, respectively, by a distance that changes from the first end of signal conductor to the second end of signal conductor such that signal conductor, and first and second ground plane conductors form a coplanar waveguide transmission line.

<CIT> describes multilayer ceramic devices, and more particularly to multilayer tapered transmission line devices.

According to one aspect the present disclosure provides a frequency selective limiter, FSL, having an input port and an output port, the FSL comprising: a plurality of vertically stacked transmission line structures, wherein each one of the plurality of vertically stacked transmission line structures is electrically coupled to a transmission line structure disposed directly above it and with a first one of the plurality of vertically stacked transmission line structures having one end corresponding to the FSL input port and a second one of the plurality of vertically stacked transmission line structures having one end corresponding to the FSL output port; and wherein each of the plurality of vertically stacked transmission line structures comprises: a magnetic material having first and second opposing surfaces; and one or more conductors disposed on at least one of the surfaces of the magnetic material.

In some embodiments, the FSL can include a substrate disposed between each of the plurality of vertically stacked transmission line structures.

In certain embodiments, the FSL can include a first bias magnet disposed along a first length of the plurality of vertically stacked transmission line structures and a second bias magnet disposed along a second length of the plurality of vertically stacked transmission line structures. In some embodiments, the first and second bias magnets can be disposed such that they establish a DC magnetic field having a direction which is substantially parallel to a direction of an RF magnetic field. In particular embodiments, the first and second bias magnets can be disposed such that they establish a DC magnetic field having a direction which is substantially perpendicular to a direction of an RF magnetic field.

In some embodiments, each subject transmission line structure can be shorter than a transmission line structure disposed directly below the subject transmission line structure. In particular embodiments, the FSL can include an input connector coupled to a bottom-most transmission line structure of the plurality of vertically stacked transmission line structures, and an output connector coupled to a top-most transmission line structure of the plurality of vertically stacked transmission line structures.

In certain embodiments, the FSL can include an input connector coupled to a top-most transmission line structure of the plurality of vertically stacked transmission line structures, and an output connector coupled to a bottom-most transmission line structure of the plurality of vertically stacked transmission line structures.

In some embodiments, the magnetic material of any of the plurality of vertically stacked transmission line structures can include a ferrite material. The ferrite material can include one or more of: a Yttrium iron garnet (YIG), a single crystal (SC) YIG, polycrystalline (PC) YIG, hexagonal ferrite, or a variety of doped YIG materials, as well as calcium vanadium garnet (CVG), Lithium Ferrite, or Nickel Zinc Ferrite. In certain embodiments, two or more of the plurality of vertically stacked transmission line structures may include different ferrite material from one another.

In particular embodiments, the FSL may include a fixture configured to house the plurality of vertically stacked transmission line structures. In some embodiments, the FSL can include wire bonds configured to electrically couple each of the subject transmission line structures is to the transmission line structure disposed directly above the subject transmission line structure.

In certain embodiments, one or more conductors are disposed on the first surface of the magnetic material to form a coplanar waveguide (CPW) transmission line. A first one of the one or more conductors may correspond to a first signal conductor having a width that decreases from a first end of the CPW transmission line to a second end of the CPW transmission line.

In some embodiments, one or more conductors disposed on a first surface of the magnetic material may correspond to a signal conductor having a width that decreases from a first end to a second end. In certain embodiments, the FSL may include two ground conductors disposed on a second surface of the magnetic material and defining a gap therebetween. A width of the gap may decrease from the first end to the second end. In particular embodiments, the FSL may include two first ground conductors disposed on a second surface of the magnetic material and defining a gap therebetween. As the width of the first signal conductor decreases, a spacing between the two first ground conductors may taper so as to maintain a transmission-line impedance the same as at first and second ends of the first signal conductor.

The foregoing and other objects, features and advantages will be apparent from the following more particular description of the embodiments, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the embodiments.

The dynamic range of frequency selective limiters (FSLs) is related to the length of the transmission line used. In general, to increase FSL dynamic range, the length of the FSL transmission line must be increased. To minimize the packaging footprint and reduce cost, the additional length can be achieved using a so-called "in-plane meandering" configuration whereby the microstrip does not extend along a straight line, but instead traverses side to side along the length of the substrate. Various FSLs have been proposed that use meandered variants of either microstrip or coplanar waveguide (CPW) transmission lines.

It is recognized herein that in-plane meandering can lead to a restriction in the useful bandwidth of the FSL device, typically resulting in a useful fractional bandwidth of only <NUM>% for state-of-the-art devices. For example, conventional FSLs may use meandering in the plane of the bias magnetic field, ensuring that a substantial portion of the RF magnetic field (or "H-field") is perpendicular to the bias magnetic field, thereby restricting available bandwidth. This can cause two problems that restrict bandwidth. First, magnetostatic surface waves (MSW) generated in the lower frequency regions may block off a wide range of available frequencies on the low end. Second, the perpendicular bias has a power threshold with a strong frequency dispersion, which can limit the useful frequency range above the MSW band to only <NUM>-<NUM>% bandwidth.

Prior approaches to solve these problems involve in-plane tapering using a variety of coplanar and microstrip transmission lines with both parallel and perpendicular biasing. However, prior approaches exhibit restricted bandwidth performance. Lumped element approaches have also been attempted to reduce component size, however these components also restrict bandwidth and may have reduced limiting capability.

The present disclosure overcomes these and other limitations found in the prior art by enabling co-packaging of multiple FSLs (e.g., three or more FSLs) within the same magnetic bias fixture while using so-called "vertical meandering" to ensure that no perpendicular field components are present in the ferrite material. Embodiments of the present disclosure can perform over multi-octave bandwidth while achieving a significant reduction in component length.

<FIG> shows an example of a frequency selective limiter (FSL), according to some embodiments of the present disclosure. The illustrative FSL <NUM> includes a plurality of transmission line structures provided in a vertically stacked arrangement (also referred to herein as "vertically meandered"). In this example, the FSL includes three transmission line structures 115a, 115b, 115c (<NUM> generally). A skilled artisan will understand that an FSL according to the present disclosure can include other numbers of transmission line structures <NUM>, such as two, four, five, or more than five transmission line structures <NUM>. In some embodiments, an FSL can have at least three transmission line structures <NUM>.

Each of the plurality of vertically stacked transmission line structures <NUM> can include a substrate comprised of magnetic material and one or more conductors disposed on one or more surfaces of the substrate. In some embodiments, the magnetic material may comprise a ferrite material, such as yttrium iron garnet (YIG), single crystal yttrium iron garnet (SC-YIG), polycrystalline yttrium iron garnet (PC-YIG), hexagonal ferrite, calcium vanadium garnet (CVG), lithium ferrite, or nickel zinc ferrite. In certain embodiments, two or more substrates within the same FSL <NUM> may be comprised of different ferrite materials from one another. For example, one substrate may comprise PC-YIG and another substrate may comprise SG-YIC.

The one or more conductors disposed over the magnetic substrate may form a coplanar waveguide (CPW) transmission line, as illustrated in <FIG>. In particular, each transmission line structure <NUM> can include a signal conductor together with a pair of ground conductors, one to either side of the signal conductor and separated therefrom by gaps. In the example of <FIG>, a first transmission line structure 115a includes signal conductor 130a separated from ground conductors 120a and 122a by gaps 124a and 126a, respectively; a second transmission line structure 115b includes signal conductor 130b separated from ground conductors 120b and 122b by gaps 124b and 126b, respectively; and a third transmission line structure 115c includes signal conductor 130c separated from ground conductors 120c and 122c by gaps 124c and 126c, respectively.

Each of the CPW transmission line structures 115a, 115b, 115c (<NUM> generally) may have a so-called "tapered" design. In particular, the width of the signal conductor <NUM> may decrease along the length of corresponding transmission line structure <NUM>. As the width of the signal conductor <NUM> decreases, a spacing between the two corresponding ground conductors <NUM>, <NUM> may taper to maintain a particular transmission-line impedance along the length of the signal conductor <NUM>.

In some embodiments, the direction of the tapering may alternate between vertically adjacent pairs of transmission line structures <NUM>. For example, as shown in <FIG>, the width of conductor 130a on the first transmission line structure 115a can decrease from left to right (relative to the drawing page), the width of conductor 130b on the second transmission line structure 115b can decrease from right to left, and the width of conductor 130c on the third transmission line structure 115c can decrease from left to right.

In some embodiments, at least one transmission line structure <NUM> may be provided having a signal conductor <NUM> disposed on a first surface of the substrate (e.g., a YIG material) and having ground conductors <NUM>, <NUM> disposed on a second surface of the substrate.

Each of the plurality of vertically stacked transmission line structures <NUM> can be electrically coupled to the transmission line structure <NUM> disposed directly above it. In certain embodiments, vertically adjacent pairs of transmission line structures <NUM> may be electrically coupled to each other using a wire bonding technique. For example, as shown in <FIG>, first transmission line structure 115a may be electrically coupled to second transmission line structure 115b via wire bonds 140a, and second transmission line structure 115b may be electrically coupled to third transmission line structure 115c via wire bonds 140b. Each set of wire bonds 140a, 140b (<NUM> generally) may include three wires to connect the three conductors of one transmission line structure <NUM> (i.e., the signal conductor <NUM> and the two ground conductors <NUM>, <NUM>) to the corresponding three conductors on the adjacent transmission line structure <NUM>, as illustrated in <FIG>.

While embodiments of the present disclosure are shown and described using tapered CWL elements, a skilled artisan will understand that the concepts and structures sought to be protected herein are compatible with various types of FSL transmission line and lumped element topologies. For example, the vertical stacking approach disclosed herein can be used with many different classes of parallel-biased transmission lines.

The FSL <NUM> can also include an input port (or "connector") <NUM> and an output port <NUM>, as shown in <FIG>. Input port <NUM> may be coupled to a transmission line structure located at the bottom of the vertical stack (e.g., structure 115a) and output port <NUM> may be coupled to a transmission line structure located at the top of the stack (e.g., structure 115c). In other embodiments, input port <NUM> may be coupled to the top-most transmission line structure and output port <NUM> may be coupled to the bottom-most structure. In some embodiments, ports <NUM>, <NUM> may be provided as SMA (SubMiniature version A) connectors.

In some embodiments, the vertically stacked transmission line structures <NUM> may be spaced apart from each other using a laminate material, as shown in <FIG> and discussed below in conjunction therewith. In particular embodiments, the FSL <NUM> may include a fixture (not shown) configured to house the plurality of vertically stacked transmission line structures <NUM>.

Referring to <FIG>, the FSL <NUM> can include bias magnets disposed along the lengths of the vertically stacked transmission line structures <NUM>, according to some embodiments of the present disclosure. In the example of <FIG>, the FSL <NUM> includes a first bias magnet <NUM> disposed along one side of the transmission line structures <NUM> and a second bias magnet <NUM> disposed along an opposite side of the transmission line structures <NUM>. The bias magnets <NUM>, <NUM> may be disposed such that they establish a DC magnetic field (or "bias field") having a direction which is substantially parallel to a direction of an RF magnetic field generated by the transmission line structures <NUM> during operation. In other embodiments, the bias magnets <NUM>, <NUM> can be disposed such that they establish a DC magnetic field having a direction which is substantially perpendicular to a direction of the RF magnetic field. It is appreciated herein that maintaining a shared DC magnetic field across the plurality of vertically stacked transmission line structures can achieve wider bandwidth than is possible using prior art techniques. In addition, sharing bias magnets across multiple stacked transmission line structures can significantly reduce the packaged component size compared to prior art techniques.

As shown in the embodiment of <FIG>, each transmission line structure <NUM> may be shorter than the transmission line structure disposed directly below it in the vertical stack. For example, transmission line structure 115b may be shorter than transmission line structure 115a and transmission line structure 115c may be shorter than transmission line structure 115b. This arrangement can provide clearance for the wire bonds <NUM> (<FIG>) that interconnect those structures while allowing for the use of wire bonds that are shorter than would otherwise be possible, thereby reducing parasitics.

<FIG> shows an example of a stacked biplanar FSL <NUM>, according to some embodiments. The illustrative FSL <NUM> includes a plurality of transmission line structures 315a, 315b, 315c (<NUM> generally) arranged in a vertical stack. Each transmission line structure 315a, 315b, 315c can include a respective first substrate 320a, 320b, 320c (<NUM> generally) and one or more conductors disposed thereon (such as the conductors illustrated in <FIG> and described in detail above). A first substrate <NUM> may be comprised of a ferrite material, such as PC-YIG or SC-YIG. In some embodiments, a first substrate <NUM> may have a thickness of about <NUM> mil. Because the power threshold of the FSL depends on the smallest spacing between the signal and ground conductors, thinner substrates will translate into lower power thresholds. Thus, a wide range of substrate thicknesses can be used, for example but not limited to <NUM> mil thick substrates to <NUM> mil thick substrates. Substrates with tapered thickness across the length of the FSL, or in parallel to the length of the FSL, or in other directions can also be used.

The transmission line structures <NUM> may be electrically coupled to each other using wire bonds. For example, as shown in <FIG>, a first transmission line structure 115a may be electrically coupled to a second transmission line structure 115b via first wire bonds 340a, and the second transmission line structure 115b may be electrically coupled to a third transmission line structure 115c via second wire bonds 340b.

Each transmission line structure 315a, 315b, 315c can include a respective second substrate 350a, 350b, 350c (<NUM> generally) disposed below the first substrate <NUM>. The second substrates <NUM> may serve to maintain a suitable spacing between the vertically stacked transmission line structures <NUM>. In some embodiments, a second substrate <NUM> may have a thickness of about <NUM> mil. Because the power threshold can be lower with thinner substrates, one embodiment may have a thicker <NUM>-<NUM> mil YIG substrate on the input portion of the FSL, while additional YIG substrates that are closer to the output side of the FSL can have thinner substrates such as <NUM>-<NUM> mil. A second substrate <NUM> may be comprised of a laminate material, such as a laminate material manufactured by the ROGERS CORPORTATION (sometimes referred to as a "Rogers material").

<FIG> shows an example of a magnetically biased FSL <NUM>, according to some embodiments of the present disclosure. The FSL <NUM> includes includes a plurality of transmission line structures (e.g., three or more structures) arranged in a vertical stack <NUM> and bias magnets <NUM>, <NUM> disposed along opposing lengths of the transmission line structures <NUM>. In this example, the FSL <NUM> includes three transmission line structures <NUM> having lengths L1, L2, and L3, respectively, and each having a width W1. In some embodiments L1 may be in the range of <NUM> to <NUM> mils, L2 may be in the range of <NUM> to <NUM> mils, L3 may be in the range of <NUM> to <NUM> mils, and W1 may be in the range of <NUM> to <NUM> mils. Bias magnets <NUM>, <NUM> may each have a length L4 and a width W2. In some embodiments L4 may be in the range of <NUM> to <NUM> mils and W2 may be in the range of <NUM> to <NUM> mils.

It is appreciated herein that vertically stacked FSL design disclosed herein can achieve performance comparable to the prior art but with significant (e.g. ><NUM>%) reduction in component length.

Claim 1:
A frequency selective limiter, FSL, (<NUM>) having an input port (<NUM>) and an output port (<NUM>), the FSL (<NUM>) comprising:
a plurality of vertically stacked transmission line structures (<NUM>), wherein each one of the plurality of vertically stacked transmission line structures (<NUM>) is electrically coupled to a transmission line structure (<NUM>) disposed directly above it and with a first one of the plurality of vertically stacked transmission line structures (<NUM>) having one end corresponding to the FSL input port (<NUM>) and a second one of the plurality of vertically stacked transmission line structures (<NUM>) having one end corresponding to the FSL output port (<NUM>); and
wherein each of the plurality of vertically stacked transmission line structures (<NUM>) comprises:
a magnetic material having first and second opposing surfaces; and
one or more conductors (<NUM>) disposed on at least one of the surfaces of the magnetic material.