TECHNICAL FIELD
[0001] This disclosure relates generally to frequency selective limiter.
BACKGROUND
[0002] As is known in the art, a Frequency Selective Limiter (FSL) is a nonlinear passive
device that attenuates signals above a predetermined threshold power level while passing
signals below the threshold power level. A key feature of the FSL is the frequency
selective nature of the high-power limiting: low power signals close in frequency
to the limited signals are unaffected. In this sense, the FSL acts as a high-Q (>1000
demonstrated) notch filter that automatically tunes to attenuate high power signals
within a narrow frequency band as illustrated in FIGS. 1A, 1B and 1C which illustrate
the frequency selectivity of a typical YIG FSL; the frequency response of: an input
to the FSL being illustrate in FIG. 1A, the transmission loss through the FSL being
illustrated in FIG. 1B, it being noted that there is significant attenuation to the
frequency components in the input signals having power levels above the predetermined
power threshold level, P
TH (FIG. 1A) while the frequency components in the input signals having power levels
below the predetermined power threshold level, P
TH pass through the FSL unattenuated (except for by the small signal losses (resistive
losses, impedance mismatch, etc.) and output power spectra being illustrated in FIG.
1C, for multiple weak and strong signals. With FSL, the power threshold level is set
primarily by the structure of a ferrite material. For example, single-crystal YIG
material is a ferrite material that provides a lower power threshold than polycrystalline
YIG, which is then lower than hexaferrite materials. The difference in power threshold
between these materials is on the order of 10-20 dB, with single-crystal YIG providing
the lowest of around 0 to +10 dBm. As is also known in the art, ferrite FSLs rely
on the non-linear response of a magnetized ferrite material. Above a critical RF magnetic
field level the spin precession angle saturates in the ferrite and coupling to higher
order spin-waves starts to occur. RF energy fed to the FSL is coupled efficiently
to spin-waves at approximately one-half the signal frequency and then converted to
heat.
[0003] The threshold power levels for the onset of limiting range from <-30 dBm for magnetostatic
wave FSLs to >40 dBm for polycrystalline ferrite in subsidiary resonance FSLs. The
critical RF magnetic field is directly proportional to the spin-wave linewidth of
the ferrite material. Liquid Phase Epitaxy (LPE) Yttrium-Iron-Garnet (YIG) is typically
used because it has the narrowest spin-wave linewidth of all measured materials, on
the order of 0.2-0.5 Oersted (Oe). This single crystal YIG approach provides the lowest
insertion loss for weak signals, the highest-Q filtering response, and provides a
power threshold on the order of 0 dBm - collectively making the material the most
attractive for a wide variety of applications. A typical implementation of an FSL
includes a strip conductor disposed between a pair of ground plane conductors in a
stripline microwave transmission structure using two YIG slabs or films for the dielectric,
as shown in FIG. 2, to couple the magnetic energy of the interfering signal into the
magnetic material. Permanent biasing magnets are mounted to the sides, as shown, or
may be mounted to the top and bottom of the structure. The strength of the magnetic
field within the structure establishes the operating bandwidth of the limiter. An
electro-magnet may be used in which case a wire, not shown, is wrapped around the
entire structure to provide windings in a direction perpendicular to the stripline.
DC current flows through the windings to provide a bias magnetic field. The bias is
selected to establish the operating bandwidth of the limiter. The slab thickness is
generally 100 um or less because of the difficulty in growing thick YIG films, requiring
stripline widths on the order of 20 um to achieve an input impedance Z
0 matched closely to 50 ohms. This approach is simple to fabricate and provides adequate
magnetic fields to realize a critical power level of approximately 0 dBm when using
single crystal YIG material. One method of reducing the power level threshold of the
FSL is to use a lower input impedance stripline (i.e., less than 50 ohms); however,
at the cost of degraded return loss. Thus, when using a lower input impedance structure,
an impedance matching structure is sometimes used to improve the impedance match;
however, this technique reduces the bandwidth and increases the insertion loss of
the FSL; the approach reduces the resistive losses associated with the transmission
structure for weak signals, and slightly increases the magnetic coupling of the signals
with the ferrite material.
[0004] US 2015/130564 A1 relates to a selective frequency limiter having a magnetic material and a slow wave
structure disposed to magnetically couple a magnetic field, produced by electromagnetic
energy propagating through the slow wave structure, into the magnetic material.
[0005] US 4980657 A relates to a frequency selective limiting device incorporating a planar ferrite member
and at least one signal-carrying conductor positioned thereon. A conductor is located
on the ferrite member to confine a portion of an RF magnetic field produced by the
microwave signals within the ferrite member.
[0006] US 2005/093737 A1 relates to a device for phase shifting on at least one single-layer or multilayer
substrate, in particular a substrate also having at least one metallic layer, to which
is applied at least one planar line, in particular in the form of a strip line or
in the form of a symmetrical or asymmetrical coplanar line or in the form of a microstrip
line or in the form of a slot line or in the form of a coplanar dual-strip line so
that the advantages of a slow-wave structure may also be used in mechanically controllable
phase shifters.
SUMMARY
[0007] The present disclosure is directed to a frequency selective limiter having a combination
of magnetic material and dielectric material. The dielectric material has a lower
relative permittivity or relative dielectric constant, εr, than the magnetic material,
which results in an enhanced microwave transmission line. In an embodiment, this design
improves an overall frequency selective limiter (FSL) performance by increasing the
local magnetic interaction of the signal with the magnetic material, thereby achieving
a lower threshold for the onset of the desired nonlinear behavior. The FSL may be
implemented in any strip conductor configuration including but not limited to a microstrip
configuration, a stripline configuration or a co-planar configuration.
[0008] With a lower power threshold, the present disclosure also enables the use of lower-cost
materials (e.g. polycrystalline instead of single-crystal YIG), with significantly
reduced complexity associated with manufacturing. Further, the insertion loss remains
low with the proposed structure and the FSL performance parameters can be tuned via
design changes in the transmission line structure rather than modifying material properties
of the dielectric material. By using a pair of low dielectric substrates in addition
to the pair of magnetic substrates, a slow wave FSL structure can be fabricated using
common manufacturing techniques without requiring micromachining or etching of the
magnetic materials, thereby resulting in a low cost solution.
[0009] An embodiment of the invention is defined in claim 1.
[0010] The inventors have recognized that while slow wave structures (SWS) have been used
to produce larger time delays for the same physical length, they exploit the property
of the SWS in producing locally-strong magnetic fields. The structure creates locally-strong
magnetic coupling, thereby decreasing the effective power threshold via electrical
design rather than modification to the material properties. Further, using periodic
segments of very low characteristic impedances, the inventors increase the magnetic
interaction of the microwave signals with the magnetic, e.g., YIG substrate, thereby
reducing the effective power threshold of when nonlinearity occur and thereby achieves
a lower threshold for the onset of the desired nonlinear behavior. This enables the
use of lower-cost polycrystalline YIG material with similar threshold and loss performance
to single-crystal YIG substrates, or when used with single-crystal material enables
lower threshold power for improved compatibility with sensitive receiver architectures.
Additionally, the ability to design for localized strengths of magnetic field enable
engineering of the FSL transfer characteristics of its limiting region of operation
without changes to the material itself. Further, when high and low impedance segments
of equal length are used and the product of their native characteristic impedances
is equal to Z
02 and a 50Ω characteristic impedance is maintained for the composite transmission line.
[0011] It is noted that with a slow wave structure, repeating pair of high and low impedance
segments is used where each segment is much less than a wavelength (λ, where λ is
the nominal operating wavelength of the slow wave structure) (in practice, <( λ)/10,
but the smaller the better). Because the segments are electrically small, the effective
impedance of the entire transmission line structure is the square root of the product
of the two impedances. This is why it is desired the product be Zo
2. For example, a structure could have 100 ohm and 25 ohm impedance segments; however,
10 ohms and 250 ohms, or even 5 ohms and 500 ohms, may be preferred. The difficulty
here is achieving the >100 ohm line; however, with this last embodiment using the
vertical vias for the low impedance sections makes this easier to achieve as the ground
plane is moved away from the strip conductor sections to achieve the high impedance
rather than making the center conductor extremely small.
[0012] Further, the FSL performance parameters can be tuned via design changes in the transmission
line structure rather than optimize material properties of the dielectric. Here, the
power threshold is now a function of both the material properties and of the transmission
line structure. Because the slow wave structure features stronger magnetic coupling
into the magnetic material, the effective threshold of power is lower because less
RF power is needed to achieve the same magnetic field strength. An additional benefit
is the ability to design for a specific threshold power. It is much easier to design
a slow wave structure to provide a specific magnetic field strength (hence threshold
power level, P
TH) than it is to tune the material properties of the magnetic material.
[0013] 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.
DESCRIPTION OF DRAWINGS
[0014]
FIGS. 1A, 1B and 1C illustrate the frequency response of an Frequency Selective Limiter
(FSL) according to the PRIOR ART; FIG. 1A showing the frequency spectrum of an input
signal to the FSL; FIG 1B showing the transmission loss through the FSL, it being
noted that there is significant attenuation to the frequency components in the input
signals having power levels above the predetermined power threshold level, PTH (FIG. 1A) while the frequency components in the input signals having power levels
below the predetermined power threshold level, PTH pass through the FSL unattenuated (except for by the small signal losses (resistive
losses, impedance mismatch, etc.); and FIG. 1C showing the output power spectra of
the FSL for multiple weak and strong signals;
FIG 2 shows an FSL according to the PRIOR ART;
FIG. 3 is an exploded, isometric view of an FSL according to the prior art;
FIGS. 4 and 4A are diagrammatical isometric and cross sectional views, respectively,
of an FSL according to the prior art;
FIGS. 5A-5E, are different views of an FSL according to the prior art; FIG. 5A being
a cross sectional view of a FSL having a helical slow wave structure formed on a magnetic
substrate, the substrate having a helical coil conductor disposed around it, the substrate
being bonded to a dielectric slab, the dielectric slab having a metal trace to provide
a ground conductor for the FSL structure; FIG. 5B being a plan view of a top of the
magnetic substrate; FIG. 5C being a plan view of a bottom plan of the magnetic substrate;
FIG. 5D being a plan view of bottom of the lower dielectric slab; and FIG. 5E being
a diagrammatical isometric of the FSL having the helical slow wave structure of FIGS.
5A-5D; and wherein the cross section of FIG. 5A is taken along line 5A-5A n FIG. 5D,
the top view of FIG. 5B being designated by the line 5B-5B in FIG. 5A, the bottom
view of FIG. 5C being indicated by the line 5C-5C in FIG. 5A, and the bottom view
of FIG. 5D being indicated by the line 5D-5D in FIG. 5A;
FIG. 6 is a cross-sectional view of an FSL having a microstrip transmission line according
to an embodiment of the disclosure;
FIG. 7 is an end view of an FSL having a stripline transmission line according to
another embodiment of the disclosure; and
FIG. 7A is a cross-sectional view of an FSL taken across lines 7A-7A in Fig. 7.
[0015] Like reference symbols in the various drawings indicate like elements.
DETAILED DESCRIPTION
[0016] Referring now to FIG. 3, a frequency selective limiter (FSL) 10 is shown. The limiter
10 is a slow wave structure comprising a stripline microwave transmission line having
a series of different impedances Z
HIGH and Z
LOW from an INPUT of the limiter 10 to an OUTPUT of the limiter 10. More particularly,
the limiter 10 includes a pair magnetic members, slabs 12, 14, here, for example,
ferrimagnetic slabs, such as, for example, YIG slabs, 12, 14, having a strip conductor
16 sandwiched between the slab and ground plane conductors 18, 20 on the outer surface
of the magnetic slabs 12, 14, as shown. The strip conductor 16 varies in width between
a narrow width sections 16N and wider width sections 16W, as shown. Here, the slow
wave structure 10 has in input impedance Z
0 of 50 ohms; the narrow section 16N providing impedances of here for example, 250
ohms and the wider sections 16W providing here for example, 10 ohms. The length of
each section is less than the nominal operating wavelength of the electromagnetic
energy pass into the FSL. The impedance of each section is established by the width
of the strip conductor of such section. The size and spacing of the wide and narrow
section 16N and 16W provide the slow wave structure with the input impedance Z
0 of 50 ohms. Thus, the impedances of the narrow sections and wider sections 16N and
16W here periodically change from an impedance greater than Z
0 to an impedance less than Z
0 as the electromagnetic energy propagates through the slow wave structure 10. It is
noted that a conventional pair of bias magnets, 11 here permanent magnets, for example,
are mounted to the sides of the structure. The permanent biasing magnets 11 may be
mounted to the top and bottom of the structure. The strength of the magnetic field
within the structure establishes the operating bandwidth of the limiter. An electro-magnet
may be used in which case a wire, not shown, is wrapped around the entire structure
to provide windings in a direction perpendicular to the stripline. DC current flows
through the windings to provide a bias magnetic field. The bias is selected to establish
the operating bandwidth of the limiter.
[0017] The slow wave structure 10 couples the magnetic energy of the input interfering signal
that has higher power level (a power level above the predetermined FSL power threshold
P
TH) of the slow wave structure 10 into the magnetic material of the magnetic slabs 12,
14. In other words, the slow wave structure 10 is used to magnetically couple a magnetic
field, produced by electromagnetic energy propagating through the slow wave structure,
into the magnetic slabs 12, 14.
[0018] Referring now to FIGS 4 and 4A, a slow wave structure FSL 10' is shown. The limiter
10' is a slow wave structure comprising a stripline microwave transmission line having
a series of different impedances Z
HIGH and Z
LOW from an INPUT of the limiter 10' to an OUTPUT of the limiter 10'. More particularly,
the limiter 10' includes a two pairs magnetic slabs 12a, 12b, and 14a, 14b, having
a strip conductor 16 sandwiched between the slabs and ground plane conductors 18,
20 on the outer surface of the ferrimagnetic slabs 12a and 14a, as shown.
[0019] More particularly, a magnetic material, here for example, a ferrimagnetic slab 12a,
has a ground plane conductor 18 on its outer surface and a series of conductive pads
21 laterally spaced by regions 27a on its inner surface, as shown. The conductive
pads 12 are connected to the ground plane conductor 18 by conductive vias 22 passing
through the slab 12a between the conductive pads 21 and the ground plane conductor
18, as shown.
[0020] Disposed between the upper surface of the strip conductor 16 and the conductive pads
21 is the ferromagnetic slab 12b, as shown.
[0021] Similarly, magnetic slab 14a, here, also, for example, a ferrimagnetic slab, has
a ground plane conductor 20 on its outer surface and a series of conductive pads 23
laterally spaced by regions 27b on its inner surface, as shown. The conductive pads
23 are connected to the ground plane conductor 20 by conductive vias 25 passing through
the slab 14a between the conductive pads 23 and the ground plane conductor 20, as
shown.
[0022] Disposed between the bottom surface of the strip conductor 16 and the conductive
pads 23 is the ferrimagnetic slab 14b, as shown.
[0023] It is noted that the distance D1 between the conductive pads 21, 23, (and hence,
in effect, the electrically connected ground plane conductors 18, 20) respectively,
and the strip conductor 16 is lower that the distance D2 between the strip conductor
16 and the ground plane conductors 18, 20 in the regions 27a, 27b. Thus, the impedance
in the regions 27a, 27b Z
HIGH is greater than the impedance Z
LOW in the regions having the conductive pads 21, 23. Hence, here again the slow wave
structure 10' has in input impedance Z
0 of 50 ohms; the regions 27a, 27b providing impedances of here for example, 250 ohms
and the regions through the conductive pads 21, 23 providing here for example, 10
ohms. The size and distance D1, D2, provide the slow wave structure with the input
impedance Z
0 of 50 ohms. Thus, the impedances of again periodically change from an impedance greater
than Z
0 to an impedance less than Z
0 as the electromagnetic energy propagates through the slow wave structure 10'. The
impedance of each section is established by the distance D1 and D2.
[0024] In this example, width of the strip conductor 16 is set to a constant that minimizes
small-signal insertion loss, and the impedance is set by varying the vertical distance
of the ground planes 18, 20 using vias 22. While the complete FSL component is matched
to 50Ω, the numerous low-impedance sections of the slow wave structure couple significantly
higher magnetic energy into the ferrimagnetic slabs, locally reducing the P
TH power threshold. This reduces the total effective power threshold, without also degrading
the return loss or instantaneous bandwidth of the device. Referring now to FIGS. 5A-5E,
another example of an FSL is shown. Here, the FSL is a helical slow wave structure
10' having a magnetic body 30 made of a magnetic, here ferrimagnetic (e.g., YIG) substrate
30, as shown). The substrate 30 provides a magnetic core, for a helical conductor
or coil 32. The helical conductor 32 is used to create a strong magnetic field within
the ferrimagnetic material center, or core 30 due to reinforcement from adjacent turns
in the coil 32. The coil 32 is implemented with conductive vias 34 to connect the
top side of the coil 32 to the bottom side of the coil 32. Since the magnetic field
outside of the coil is relatively small, it may not be beneficial to have additional
magnetic, for example, YIG substrates (not shown), outside of the coil structure 32.
In one application, the ground reference for the coil includes a metal trace 36 defined
on the bottom side of a supporting dielectric slab 38. The dielectric slab 38 is bonded
to the bottom of the magnetic body 30, whereby the supporting dielectric is attached
to the ferrimagnetic core (or substrate) containing the coil 32. In this application,
the dielectric material of dielectric slab 38 is a non-magnetic material such as FR-4
or a Rogers Corporation, Rogers, CT laminate material. In one application, the lowest
critical fields are achieved when the static and RF induced magnetic fields are parallel.
[0025] It is noted that a pair of bias magnets 11, here permanent magnets, are included.
The strength of the magnetic field within the structure establishes the operating
bandwidth of the limiter. The coil structure is oriented perpendicular to the axial
direction of the magnetic field produced by the magnets 11. For the case of biasing,
it is noted that the permanent magnets 11 are disposed on either end of the coil rather
than along the sides or the top and bottom.
[0026] Now referring to FIG. 6, a frequency selective limiter 40 includes a magnetic material
42 disposed over a dielectric material 44 which in turn is disposed over a ground
plane 50. Magnetic material 42 has first and second opposing surfaces 42a, 42b and
dielectric material 44 also has first and second opposing surfaces 44a, 44b. In the
illustrative embodiment of FIG 6, the second surface 42b of magnetic material 42 disposed
over the first surface 44a of dielectric material 44. A strip conductor 46 is disposed
over the first surface 42a of magnetic material 42 such that ground plane 50, dielectric
material 44 and magnetic material 42 form a microstrip transmission line structure.
[0027] In an embodiment, dielectric material 44 has a lower relative permittivity or relative
dielectric constant, ε
r, than magnetic material 42. In some embodiments, magnetic material 42 may be provided
as a ferromagnetic material, such as Yttrium iron garnet (YIG), and dielectric material
44 may be provided as a non-magnetic material, such as FR-4 laminate material or a
Rogers Corporation, Rogers, CT laminate material (e.g., RO 4003 laminates). Other
materials having similar mechanical and electrical properties, may of course, be used.
For example and without limitation, magnetic material 42 may be provided as single
crystal YIG, polycrystalline YIG, hexaferrite YIG or a variety of doped YIG materials.
Further and without limitation, dielectric material 44 may include any material having
a low relative permittivity (i.e., a relative dielectric constant of less than 4).
In some embodiments, dielectric material 44 may be provided as alumina or low-temperature
co-fired ceramics (LTCC).
[0028] Conductive vias 54a-54x may be disposed through dielectric material 42 and at least
electrically couple ground plane 50 to a first set of conductive pads 52 disposed
between second surface 42a of magnetic material 42 and first surface 44a of dielectric
material 44. Conductive vias 54a-54x may be spaced a predetermined distance from a
neighboring or adjacent conductive via 54. In an embodiment, each conductive via 54a-54x
is aligned with at least one conductive pad 52. In embodiments, conductive vias 54a-54x
may be formed such that they are perpendicular to a plane in which lie ground plane
50 and strip conductor 46.
[0029] In an embodiment, a region 56 is formed between each conductive pad 52. Region 56
may include portions of dielectric material 44 that have reflowed into the gaps (i.e.,
regions 56) formed between each conductive pad 52 during fabrication. In some embodiments,
region 56 includes an adhesive material that bonds dielectric material 44 to magnetic
material 42. For example, the adhesive material may be provided as a lower melting
temperature version of the same material provided in dielectric material 44. In other
embodiments, region 56 may be provided as a different dielectric medium than the material
provided in dielectric material 44.
[0030] In some embodiments, each of the conductive pads 52 may include an adhesive material
disposed over at least one surface to adhere each conductive pad 52 to magnetic material
42. The adhesive material may be formed in a very thin layer over conductive pad 52,
(e.g., thickness in the range of about 0.5 mil to about 2 mil). It should be appreciated
that one of ordinary skill in the art will understand how to adhere dielectric layer
44 to the magnetic material layer, once a particular set of materials is selected.
[0031] Conductive vias 54a-54x may operate as a ground plane for low impedance portions
within frequency selective limiter 40. For example and in the illustrative embodiment
of FIG. 6, conductive vias 54a-54x form alternating sections of low impedance and
high impedance microstrip transmission lines within frequency selective limiter 40.
In an embodiment, the number of low impedance sections in frequency selective limiter
40 is equal to the number of high impedance sections.
[0032] In an embodiment, the characteristic impedance of a particular system establishes
an impedance threshold between a low impedance section and a high impedance section.
For example, a section having an impedance less than the characteristic impedance
of the system can be a low impedance section and a section having an impedance greater
than characteristic impedance of the system can be a high impedance section. In one
embodiment, with a system having a characteristic impedance of 50 ohms, a low impedance
section refers to a section having an impedance less than 50 ohms. In said embodiment,
a high impedance section refers to a section having an impedance greater than 50 ohms.
Of course other systems may have a characteristic impedance greater than or less than
50 ohms (e.g., a characteristic impedance of 40 ohms or 60 ohms may be desired). In
one example embodiment, a low impedance section has an impedance less than 30 ohms
and a high impedance section has an impedance greater than 75 ohms.
[0033] Thus, in an embodiment, frequency selective limiter 40 is a slow wave structure having
a microstrip microwave transmission line and having a series of different impedances
Z
HIGH and Z
LOW from an INPUT of frequency selective limiter 40 to an OUTPUT of frequency selective
limiter 40.
[0034] In some embodiments, a pair of neighboring or adjacent sections (i.e., one low impedance
section and one high impedance section) form a unit cell. The spacing between each
unit cell may be the same or substantially similar. For example, each unit cell may
be of equal length and width. The lengths and widths of the unit cells may be selected
based upon a particular operating frequency or range of operating frequencies of frequency
selective limiter 40. For example, in one embodiment, each unit cell may have a length
of about 40 mil, which provides useful operation up to a frequency of about 5 GHz.
In other embodiments, each unit cell may have a length of about 20 mil, which provides
useful operation up to a frequency of about 10 GHz.
[0035] In some embodiments, a length (i.e., a dimension parallel to a length of strip conductor
46) of each conductive pad 52 may be equal to or about half the length of its corresponding
unit cell. For example, in an embodiment with a unit cell having a length of about
20 mil, the respective conductive pad 54 would have a length of about 10 mil.
[0036] Each conductive pad 52 may be provided having a width (i.e., a dimension perpendicular
to a length of strip conductor 46) that is wide enough to support a microstrip (or
stripline) transmission line mode. For example, in some embodiments, each conductive
pad 52 may be provided having a width that is at least three times a distance between
the respective conductive pad 52 and strip conductor 46.
[0037] In some embodiments, a width (e.g., a dimension along a plane parallel to the plane
in which first set of conductive pads 52 is disposed between second surface 42a of
magnetic material 42 and first surface 44a of dielectric material 44) of each of the
conductive vias 54a-54x may be provided such that it is less than a smallest dimension
of the corresponding conductive pad 52 (i.e., length or width).
[0038] In one embodiment, each of the conductive pads 52 have the same or substantially
similar dimensions and each of the conductive vias 54a-54x have the same or substantially
similar dimensions, thus frequency selective limiter 40 may be provided as a generally
symmetric structure.
[0039] In an embodiment, the impedance within frequency selective limiter 40 may be set
or controlled by varying a vertical distance between a ground plane and strip conductor
46. For example, a distance, D1, between conductive pad 52 (i.e., acting as a ground
plane to which conductive pad 52 is coupled to) to strip conductor 46 is less than
a distance, D2, between ground plane 50 and strip conductor 46 in regions 56 where
no conductive via 54 is disposed. Thus, an impedance in regions 56, Z
HIGH, is greater than the impedance, Z
LOW, in regions having conductive pads 52.
[0040] The alternating sections of low impedance microstrip lines and high impedance microstrip
lines couple magnetic energy propagating through the slow wave structure and into
magnetic material 42. In an embodiment, magnetic energy having a power level above
or equal to a predetermined power level threshold of frequency selective limiter 40
is coupled into magnetic material 42. A combination of magnetic material 42 and dielectric
material 44 in frequency selective limiter 40 increases the magnetic coupling of magnetic
energy into magnetic material 42. For example, multiple low-impedance microstrip transmission
lines couple significantly higher magnetic energy into magnetic material 42, thus
reducing a total effective power threshold.
[0041] Now referring to FIGs. 7 and 7A in which like elements are provided having like reference
designations, a frequency selective limiter 60 includes a pair of magnetic materials
62, 63 disposed about a strip conductor 66 and a pair of dielectric materials 64,
65 with a first one of the dielectric materials 64, 65 disposed over a first one of
the magnetic materials 62, 63 and a second one of the dielectric materials 64, 65
disposed over a second one of the magnetic materials 62, 63. In an embodiment, frequency
selective limiter 60 is provided as a multi-layer frequency selective limiter structure
having a stripline transmission line structure. For example, strip conductor 66 is
disposed between surface 62b of the first magnetic material 62 and surface 63 a of
the second magnetic material 63. A second surface 64b of first dielectric material
64 is disposed over a first surface 62a of first magnetic material 62. A first ground
plane 70a is disposed over a first surface 64a of second dielectric material 64. Further,
a second surface 63b of second magnetic material 63 is disposed over a first surface
65a of second dielectric material 65. A second surface 65b of dielectric material
65 is disposed over a second ground plane 70b.
[0042] In an embodiment, frequency selective limiter 60 includes two sets of conducting
pads 72, 73. Each set disposed may be disposed between magnetic material 62, 63 and
dielectric material 64, 65. For example, and as illustrated in FIG. 7, a first set
of conductive pads 72 are disposed between second surface 64b of dielectric material
64 and first surface 62a of magnetic material 62. Further, a second set of conductive
pads 73 are disposed between second surface 63b of magnetic material 63 and first
surface 65a of dielectric material 65.
[0043] As may be most clearly seen in FIG. 7A, two sets of conductive vias 74a-74d, 75a-75d
are disposed through respective ones of dielectric material layers 64, 65. Respective
ones of conductive vias 74a-74d, 75a-75d electrically couple respective ones of pads
72a-72d and 73a-73d to respective ones of ground planes70a, 70b. To vary an impedance
presented to an RF signal propagating along the stripline transmission line formed
by strip conductor 66 and the ground planes, through frequency selective limiter 60,a
vertical distance between ground planes 70a, 70b and strip conductor 66 may be controlled.
[0044] In the illustrative embodiment of FIG. 7A, conductive vias 74a-74d, 74a-d disposed
through respective ones of the dielectric materials 64, 65 electrically couple respective
ones of ground planes 70a, 70b to respective ones of conductive pads 72a-72d, 73a-73d
to thereby form alternating sections of low impedance stripline sections 76 and high
impedance stripline sections 78 within frequency selective limiter 60. Thus, in an
embodiment, frequency selective limiter 60 is a slow wave structure having a stripline
microwave transmission line having a series of different impedances Z
HIGH 78 and Z
LOW 76 from an input of frequency selective limiter 60 to an OUTPUT of frequency selective
limiter 60.
[0045] The alternating sections of low impedance striplines 76 and high impedance striplines
78 couple magnetic energy propagating through the slow wave structure and into the
pair of magnetic materials 62, 63.
[0046] In an embodiment, using alternating (i.e., periodic) segments having very low characteristic
impedance (e.g., low impedance striplines 76 having an impedance less than a system
characteristic impedance), a magnetic interaction of signals with magnetic materials
62, 63 is increased. The combination of magnetic material 62, 63 and dielectric material
64, 65 may couple higher magnetic field into magnetic material 62, 63 in low impedance
stripline sections 76. Thus, an effective power threshold of when nonlinearity occurs
for frequency selective limiter 60 is reduced. In an embodiment, by lowering the power
level required to cause nonlinear behavior, frequency selective limiter 60 provides
protection for even lower levels of input power. For example, in one embodiment with
a power threshold of about 10 dBm, an interfering signal of about 5 dBm may still
cause problems. However, frequency selective limiter 60 with a reduced power threshold
level of about 0 dBm would provide protection against the same 5 dBm interfering signal.
[0047] In an embodiment, a width of the strip conductor 66 is set to a constant that reduces
(and ideally minimizes) small-signal insertion loss, and the impedance is set by varying
the vertical distance of the ground planes 70a, 70b and hence the length of the conductive
vias 74a-74d, 75a-75d. For example, in low impedance striplines 76, first and second
ground planes 70a, 70b are closer to strip conductor 66 (providing higher capacitance
thus lower impedance) and in high impedance striplines 78, first and second ground
planes 70a, 70b are farther away from center strip conductor 66 and have an effective
dielectric constant (a function of the combination of magnetic material 62, 63 and
dielectric material 64, 65) that is lower thus providing a higher impedance.
[0048] Impedances at input and output ports of the frequency selective limiter 60 may be
matched to a desired characteristic impedance (e.g. a characteristic impedance of
a system in which the FSL is included such as a 50Ω characteristic impedance). At
the same time, however, the numerous low-impedance sections of the slow wave structure
couple significantly higher magnetic energy into magnetic material 62, 63, locally
reducing the power threshold (PTH). For example, when a section of frequency selective
limiter 60 has a low impedance, a magnetic field of a radio frequency (RF) signal
is higher than a section of frequency selective limiter 60 having a high impedance.
Thus, the FSL structures described herein are capable of both reducing the total effective
power threshold, without also degrading the return loss or instantaneous bandwidth
of the device.
[0049] In one example embodiment, frequency selective limiter 60 is formed having two layers
of 100 µm thick polycrystalline YIG as magnetic material 62, 63 and two layers of
60 mil thick Rogers 4003 as dielectric material 64, 65. First ground plane 70a is
disposed over first surface 64a of first dielectric material 64. Second surface 64b
of first dielectric material 64 is disposed over first surface 62a of first magnetic
material 62. Strip conductor 66 is disposed between second surface 62a and first surface
63a of second magnetic material 63. Second surface 63b of second magnetic material
63 is disposed over first surface 65a of second dielectric material 65. Second dielectric
material 65 is disposed over second ground plane 70b.
[0050] In such an embodiment, a twenty (20) ohm section of transmission line is provided
from a strip conductor having a width of about 175 µm (i.e., Z
LOW 76) when the YIG ground planes (i.e., conducting pads 72, 73) are used, while a 50
µm wide stripline conductor (i.e., Z
HIGH 78) achieves a 120 ohm impedance when the ground planes 70a, 70b on the outside portions
of dielectric materials 64, 65 (e.g., Rogers material) is used.
[0051] In an embodiment, stripline segment lengths 76, 78 are formed to be electrically
small, such as less than a wavelength (λ, where λ is the nominal operating wavelength
of frequency selective limiter 60). For example, in one embodiment, stripline segment
lengths 76, 78 are formed to be less than 1/10 of a wavelength (< (1/10)(λ) at a maximum
frequency of operation), which results in a 49 ohm characteristic impedance and a
slow wave factor of 1.43. Thus, an increased magnetic field intensity produced by
the low impedance segments 76 decreases the frequency selective limiter's 60 power
threshold by activating spin waves in dielectric material 64, 65 (i.e., YIG material)
at an earlier onset that if a 50 ohm line had been used.
[0052] In some embodiments, conductive vias 74, 75 and ground planes 70a, 70b may be formed
by fabricating on or within dielectric material 64, 65, thus no micromachining or
etching of dielectric material 64, 65 is required.
[0053] 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 scope
of the disclosure as defined in the appended claims. For example, the high and low
impedance lines may be by varied using both the ground plane height and the width
of the center conductor line.
1. Frequenzselektiver Begrenzer (40; 60), der Folgendes umfasst:
eine Verzögerungswellenstruktur;
eine erste Schicht von Magnetmaterial (42; 62), um ein Magnetfeld, erzeugt durch elektromagnetische
Energie, die sich durch die Verzögerungswellenstruktur fortpflanzt, magnetisch in
das Magnetmaterial (42; 62) zu koppeln;
eine erste Schicht von dielektrischem Material (44; 64), die auf der ersten Schicht
von Magnetmaterial (42; 62) angeordnet ist,
wobei die Dielektrikumschicht (44; 64) eine niedrigere relative Permittivität als
das Magnetmaterial (42; 62) hat;
eine erste Grundebene (50; 70a), die auf einer ersten Oberfläche (44b; 64a) der ersten
Schicht von dielektrischem Material (44; 64) angeordnet ist;
einen ersten Satz von Leiterplatten (52; 72), die zwischen der ersten Schicht von
dielektrischem Material (44; 64) und der ersten Schicht von Magnetmaterial (42; 62)
angeordnet sind;
einen ersten Satz von Vias (54; 74), die innerhalb der ersten Schicht von dielektrischem
Material (44; 64) angeordnet sind,
wobei der erste Satz von Vias (54; 74) die Grundebene (50; 70a) mit dem ersten Satz
von Leiterplatten (52; 72) koppelt, um abwechselnde Abschnitte von Microstrip-Abschnitten
mit niedriger Impedanz und Microstrip-Abschnitten mit hoher Impedanz innerhalb der
Verzögerungswellenstruktur zu bilden; und
wobei die Verzögerungswellenstruktur eine Eingangsimpedanz Z0 hat, und wobei die Impedanzen periodisch von einer Impedanz größer als Z0 zu einer Impedanz kleiner als Z0 wechseln, während sich die elektromagnetische Energie durch die Verzögerungswellenstruktur
fortpflanzt.
2. Frequenzselektiver Begrenzer (60) nach Anspruch 1,
wobei eine zweite Oberfläche (64b) der ersten Schicht von dielektrischem Material
(64), die der ersten Oberfläche (64a) der ersten Schicht von dielektrischem Material
(64) gegenüberliegt, auf einer ersten Oberfläche (62a) der ersten Schicht von Magnetmaterial
(62), die der zweiten Oberfläche (62b) der ersten Schicht von Magnetmaterial (62)
gegenüberliegt, angeordnet ist;
der frequenzselektive Begrenzer ferner Folgendes umfasst:
eine zweite Schicht von dielektrischem Material (65), die eine erste und eine zweite
gegenüberliegende Oberfläche (65a, 65b) hat;
eine zweite Schicht von Magnetmaterial (63), die eine erste und eine zweite gegenüberliegende
Oberfläche (63a, 63b) hat,
wobei die erste Oberfläche (65a) der zweiten Schicht von dielektrischem Material (65)
über der zweiten Oberfläche (63b) des zweiten Magnetmaterials (63) angeordnet ist;
und
eine Leiterbahn (66) zwischen der ersten und zweiten Schicht von Magnetmaterial (62,
63) angeordnet ist;
eine zweite Grundebene (70b) auf der zweiten Oberfläche (65b) der zweiten Schicht
von dielektrischem Material (65) angeordnet ist;
ein zweiter Satz von Leiterplatten (73) zwischen der zweiten Schicht von dielektrischem
Material (65) und der zweiten Schicht von Magnetmaterial (63) angeordnet ist;
ein zweiter Satz von Vias (75) innerhalb der zweiten Schicht von dielektrischem Material
(65) angeordnet ist;
wobei die Kombination der ersten und zweiten Schicht von dielektrischem Material (64,
65) und der ersten und zweiten Schicht von Magnetmaterial (62, 63) die Verzögerungswellenstruktur
umfasst, die eine Eingangsimpedanz Z0 hat, und
wobei die Impedanzen periodisch von einer Impedanz größer als Z0 zu einer Impedanz kleiner als Z0 wechseln, während sich eine elektromagnetische Energie durch die Verzögerungswellenstruktur
fortpflanzt.
3. Frequenzselektiver Begrenzer nach Anspruch 2,
wobei der erste Satz von Vias die erste Grundebene mit dem ersten Satz von Leiterplatten
koppelt, und der zweite Satz von Vias die zweite Grundebene mit dem zweiten Satz von
Leiterplatten koppelt, um abwechselnde Abschnitte von Streifenleitungsabschnitten
mit niedriger Impedanz und Streifenleitungsabschnitten mit hoher Impedanz innerhalb
der Verzögerungswellenstruktur zu bilden.
4. Frequenzselektiver Begrenzer nach Anspruch 3,
wobei die abwechselnden Abschnitte von Streifenleitungsabschnitten mit niedriger Impedanz
und Streifenleitungsabschnitten mit hoher Impedanz die magnetische Energie, die sich
durch die Verzögerungswellenstruktur fortpflanzt, in die der ersten und zweiten Magnetschichten
koppeln,
wobei die magnetische Energie einen Leistungspegel über einer vorbestimmten Leistungsschwelle
hat.
5. Frequenzselektiver Begrenzer nach Anspruch 2,
wobei der frequenzselektive Begrenzer eine Übertragungsleitung mit einer Eingangsimpedanz
Z0 ist, und
wobei die Übertragungsleitung einen ersten Übertragungsleitungsabschnitt aufweist,
der zwischen einem Paar von zweiten Übertragungsleitungsabschnitten angeordnet ist,
wobei der erste Übertragungsleitungsabschnitt eine Impedanz ZH höher als Z0, und das Paar von zweiten Übertragungsleitungsabschnitten eine Impedanz ZL niedriger als Z0 hat.
6. Frequenzselektiver Begrenzer nach Anspruch 5,
wobei der erste Übertragungsleitungsabschnitt und das Paar von zweiten Übertragungsleitungsabschnitten
jeweils eine Länge haben, die kürzer als eine Nennbetriebswellenlänge der elektromagnetischen
Energie ist, die sich durch die Verzögerungswellenstruktur fortpflanzt.