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BACKGROUND OF THE INVENTION
1. Field of the Invention:
This invention relates to attenuating devices, and more particularly, to
devices which utilize a YIG material to provide frequency selective
attenuation of microwave signals above a preselected threshold power
level.
2. Description of Related Art:
Frequency selective limiting (FSL) or attenuating devices which utilize a
yttrium-iron-garnet (YIG) material have the property of being able to
attenuate higher power level signals while simultaneously allowing lower
power level signals, separated by only a small frequency offset from the
higher level signals, to pass with relatively low loss. YIG-based FSLs are
capable of limiting or attenuating across more than an octave bandwidth in
the 2-8 GHz range. Higher power level (above-threshold) signals within a
selectivity bandwidth will be attenuated without requiring tuning of the
FSL. Lower power level (below-threshold) signals, separated from the
higher power level signals by more than a few spinwave linewidths, will
pass through the FSL without experiencing any greater loss than if the
higher power level signals were not present. For an attenuating device
based on YIG, this selectivity bandwidth is on the order of between 20-50
MHz.
YIG-based FSLs have many applications in microwave signal systems. One such
application is illustrated in FIG. 1. The microwave signal system 10
includes an antenna 12 for collecting and passing microwave RF signals, a
YIG-based FSL device 16 and a broadband receiver 14 (hereafter sometimes
referred to as receiver 14). Microwave signal processing equipment 18 is
responsive to the output of the receiver 14. The microwave signal
processing equipment 18 is of a type presently known in the art and will
not be further described.
FSL 16 is utilized to increase the dynamic range over which microwave
signals collected by the antenna 12 can be accepted by the receiver 14.
Because known receivers such as broadband receiver 14 generally have a
dynamic range of approximately 35 dB, and signals of interest arriving at
antenna 12 may have a dynamic range of, for example, 85 dB, it can be
readily appreciated that a power mismatch is created within system 10. The
mismatch is corrected by utilizing the FSL device 16 which may be designed
to provide a dynamic range of about 50 dB, approximately mid-point between
the signal level at the antenna 12 and the dynamic range of the receiver
14.
FSL 16 is designed to provide that the ratio of power out to power in
(P.sub.out /P.sub.in), below a predetermined threshold value of TP.sub.in,
is substantially linear. As the value of input power P.sub.in seen by FSL
16 increases above the predetermined threshold value of TP.sub.in, the
ratio of P.sub.out /P.sub.in becomes smaller. Stated in another manner,
FSL device 16 operates to attenuate an above threshold, high power input
microwave signal having a large dynamic range to provide an output signal
having a smaller dynamic range.
A YIG-based frequency selective limiting (FSL) device 20 discussed in U.S.
Pat. No. 4,845,439; entitled "Frequency Selective Limiting Device," in the
name of Steven N. Stitzer et al. and assigned to Westinghouse Electric
Company the assignee of the present invention is illustrated in FIGS. 2A
and 2B. Attenuation in the FSL 20 is proportional to the volume of YIG
material in layers 22 and 24 which is coupled to the RF magnetic-field 26
generated by the signal-carrying conductor 28. While the configuration and
positioning of the YIG layers 22 and 24 relative to the signal-carrying
conductor 28 results in satisfactory coupling of the RF magnetic field 26
with YIG material in layers 22 and 24, the configuration of the FSL 20 is
difficult and expensive to fabricate.
According to current manufacturing procedures a narrow signal-carrying
conductor 28 is sandwiched between two thin layers 22 and 24 of single
crystal yttrium-iron-garnet (YIG). The YIG layers are typically about
0.002 to 0.005 inch thick. Effective limiting requires a strong coupling
of RF magnetic-field 26 with the YIG for a given RF power level.
Accordingly, in order to confine the magnetic-field 26 within the YIG
layers 22 and 24, the sandwich is surrounded by a ground plane 29. The
arrangement of FIGS. 2A and 2B ensures that substantially all the RF field
lines 26 pass through the YIG layers 22 and 24.
The device 20 is currently made by epitaxially forming separatly, each YIG
layer as a thin layer of single crystal YIG on a gadolinium-gallium-garnet
(GGG) substrate (not shown). Each layer of GGG is then removed by a
grinding step. It is also necessary to use a separate metalized GGG
substrate 30 as a device support. The metalized surface 32 separates the
GGG substrate 30 from the YIG material of layer 24 and completing the
group plan. The current process is expensive and time consuming.
In order to achieve a greater level of attenuation, a plurality of
individually formed separate devices 20 may be connected in series as
shown in FIG. 3. The FSL devices 20 are positioned in parallel with a
plurality of D.C. biasing magnets 34 located therebetween which provide a
transverse biasing field 36. Each FSL 20 is separately manufactured.
SUMMARY OF THE INVENTION
The present invention simplifies the configuration and fabrication process
of FSL devices for attenuating microwave signals. According to the present
invention, a plurality of FSLs are formed on a common substrate with a
common biasing magnet. According to one embodiment of the present
invention, a plurality of signal-carrying conductors for carrying
microwave signals are positioned on a planar ferrite member.
In another embodiment of the invention a second generally planar ferrite
member is positioned on the plurality of signal-carrying conductors in
confronting relationship with the first ferrite member to further enhance
the attenuation of the microwave signals. First and second ferrite members
are posted between a pair of planar magnets. In a preferred embodiment,
the signal-carrying conductors are separated from each other by a distance
sufficient to ensure negligible magnetic field coupling between adjacent
conductors.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic block diagram of a microwave circuit in which the
frequency selective limiting device of the present invention may be
utilized;
FIG. 2A is a fragmentary prespective view of a frequency selective limiting
device disclosed in the above-identified copending U.S. Pat. application
assigned to the assignee of the present application;
FIG. 2B is a side sectional view of the FSL device of FIG. 2A;
FIG. 3 is a top view of eight frequency selective limiting devices, as
shown in FIG. 2A and 2B, connected in series;
FIG. 4 is a fragmentary perspective view of a frequency selective limiting
device according to the present invention;
FIG. 5 illustrates graphically the measured limiting of one of the strips
in an FSL device according to the embodiment shown in FIG. 4; and
FIGS. 6A through 6E illustrate in a series of side sectional views the
sequence of steps for assembling the FSL device of FIG. 4.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The theory of operation and the construction of frequency selective
limiting (FSL) devices which utilize a yttrium-iron-garnet (YIG) material
are described in the following articles, which are incorporated by
reference herein: "Frequency Selective Microwave Power Limiting in Thin
YIG Films," IEEE Transactions on Magnetics, Vol. MAG-19, No. 5, September
1983, Steven N. Stitzer; "A Multi-Octave Frequency Selective Limiter,"
1983 IEEE MTT-S Digest, page 326, Steven N. Stitzer and Harry Goldie;
"Non-Linear Microwave Signal-Processing Devices Using Thin Ferromagnetic
Films," Circuits Systems Signal Process, Vol. 4, No. 1-2, 1985, page 227,
Steven N. Stitzer and Peter R. Emtage.
FIG. 4 illustrates an FSL device 50 (hereinafter sometimes referred to
generally as FSL 50) in accordance with the preferred embodiment of the
present invention. The FSL 50 includes a plurality of signal-carrying
conductors 52 of a predetermined length positioned between a pair of
confronting ferrite members 54 and 56. The ferrite members 54 and 56 are
yttrium-iron-garnet (YIG) based materials having a generally planar
configuration respectively grown on nonmagnetic gadolinium-gallium-garnet
(GGG) substrates 58 and 60. To enhance the operation of the device, a
ground plane 55 may be positioned adjacent to YIG layer 54. A pair of
planar D.C. biasing magnets 62 and 64 are positioned on the GGG substrates
58 and 60. The magnets 62 and 64 produce a D.C. biasing field and also
provide RF shielding for the signal-carrying conductors 52. In the present
invention, GGG substrates 58 and 60 are also utilized to provide
mechanical support for the YIG ferrite members 54 and 56. The second YIG
layer 56 provides additional attenuation of the microwave signals carried
by the signal-carrying conductors 52, but is not essential to the
operation of the FSL 50.
The present invention, as illustrated in FIG. 4, provides for the
fabrication of a plurality of signal carrying conductors 52 on a single
YIG substrate. This arrangement, incorporating only one YIG substrate,
significantly reduces the volume occupied by the YIG layers 54 and 56 and
biasing magnets 62 and -64 within the FSL device 50.
In the compact FSL configuration of the present invention when a ground
plane 55 is employed, the signal-carrying conductors 52 are separated by
about 10 times the distance from the ground plane 55 to the conductors 52.
The distance from the ground plane 55 to the conductors 52 be the
thickness of the YIG layer 54 plus the thickness of the GGG substrate 58.
However, when the GGG layer 58 has been removed, the distance from the
ground plane 55 to the conductors 52 is equal to the thickness of YIG
layer 54. The YIG layer thickness is approximately 100 .mu.m. The amount
of separation is sufficient to ensure negligible coupling between adjacent
conductors. When a plane 55 is not present, the separation between the
conductors 52 must be increased to achieve negligible coupling between
adjacent conductors. Any known fabrication process used to manufacture
single strip YIG-based FSL devices may be used to manufacture the compact
FSL configuration of the present invention.
The performance of the compact limiter (FIG. 4) of the present invention is
generally comparable to that obtained from single conductor FSL devices
(FIG. 3). One of the differences, however, is that the compact FSL 50
allows a lower biasing field than that required for single conductor FSL
devices. For example, a low biasing field of about 35 Oe (Oersted or
ampere per meter) is required for the compact FSL 50, whereas
approximately 120 Oe is required in the conventional single conductor
devices. The difference in the required bias field arises from the
reduction in the demagnetizing field 96 inherently present within the YIG
layers 54 and 56. The demagnetizing field 96 is dependent, in an inverse
relationship, upon the aspect ratio (width (w)/thickness (t)) of the YIG
layer 54 and 56 (see FIG. 3). In other words, a higher aspect ratio allows
a lower bias field. In the single conductor configuration, as shown in
FIG. 3, the aspect ratio is calculated to be about 12.5, as shown below.
##EQU1##
In the compact multiple conductor configuration, as shown in FIG. 4, the
aspect ratio is calculated to be about 63.5, as shown below.
##EQU2##
FIG. 5 illustrates graphically the limiting characteristics of each of the
strips in the FSL device 50 of FIG. 4. FIG. 5 plots the attenuation of the
fourth strip of an 8 strip compact FSL 50 as a function of the frequency
(2.4-5.4 GHz) versus different input power levels (0-19 dBm). The higher
aspect ratio of the compact multiple conductor configuration (FIG. 4)
allows for a more consistent limiting characteristic (see FIG. 5) than is
provided by the single conductor configuration (FIG. 3) which has a lower
aspect ratio. The compact FSL 50 provides a more consistent limiting
characteristic curve over a wide range of frequencies by reducing the
degree to which the level of limiting is frequency dependent. The flatter,
or more uniform, limiting characteristic curve is a great advantage for
the compact multiple conductor configuration since the attenuation loss at
low frequencies is negligible compared to the attenuation loss at low
frequencies with the conventional single conductor configuration.
Therefore, the higher aspect ratio of the compact configuration allows for
a more predictable level of attenuation across a range of frequencies than
provided by the single conductor devices.
Although GGG substrate layers 58 and 60 are illustrated and described
herein, other suitable materials may be utilized in forming the substrate
layers. The material from which substrate layers 58 and 60 are formed
should be selected to have a thermal expansion coefficient (TEC) which
approximates that of YIG layers 54 and 56. For example, a high nickel
alloy (70% Ni, 17% Mo, 7% Cr, 6% Fe), which has substantially the same TEC
of YIG (.DELTA.L/L-10.4.times.10.sup.-6 /.degree.C.) may be utilized if
desired.
FIGS. 6A through 6E illustrate in stepwise fashion a process of forming the
FSL 50 shown in FIG. 4. FIG. 6A illustrates the GGG substrate layer 58
having a metalized surface 59. GGG substrate 61 has the first YIG layer 54
epitaxially grown thereon. A metalized surface 63 is formed over YIG layer
54. The metalized surfaces 59 and 63 are bonded together in confronting
relationship as shown by a conductive epoxy 65 and form part of the ground
plane 55 referred to in FIG. 4.
In FIG. 6B the GGG substrate 61 has been removed by grinding, leaving the
upper surface 67 of the first YIG layer 54 exposed to receive a
metalization layer of gold (not shown) which is thereafter etched by a
photolithographic technique to form the plurality of signal carrying
conductors 52 (FIG. 6C). The etching process is such that the resulting
signal-carrying conductors 52 have a width W.sub.s of about 25 .mu.m and
are separated from each other by nonconductive gaps having a width W.sub.g
about 1 mm. Therefore, one embodiment of the present invention, as shown
in FIG. 6C, has a single ferrite layer 54 and a planar ground plane 55
formed of the metalization layers 59 and 63 and the conductive epoxy 65.
Alternately the device may be formed by stretching gold wire or ribbon
across the YIG layer 54 if desired.
In the preferred embodiment, the second YIG layer 56 is epitaxially grown
on a GGG substrate 68. Thereafter a thin layer of non-conductive epoxy
paste 65, preferably of a thickness equal to the thickness of the
conductors 52, is utilized to attach the second YIG layer 56 to the first
YIG layer 54 in confronting relationship as shown in FIG. 6D. The GGG
substrate 68 may thereafter be removed by grinding and polishing. A
metalization 70 may surround the two YIG layers 54 and 56 and be in
contact with the planar ground plane 55 to form a surrounding ground plane
(FIG. 6E).
The principle of combining multiple FSL devices on a single YIG substrate
with the magnetic bias field 69 supplied by a common magnet 62, is shown
in FIG. 4. As stated earlier, the configuration of the present invention
may also be applicable to different FSL device structures such as
microstrip, coplanar waveguide, or slot lines as well as to different
fabrication procedures such as a monolithic approach.
Although present invention has been described in terms of what are at
present believed to be the preferred embodiments, it will be apparent to
those skilled in the art that various changes may be made without
departing from the scope of the invention. It is therefore intended that
appended claims cover such changes.
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