Noise-limiting transformer apparatus and method for making

Embodiments of the invention include a transformer device having a saturation region for limiting ingress noise and other noise. The transformer comprises a magnetic core, an input coil and an output coil arranged so that the output signal caused by the magnetic linkage between the input and output coils through the magnetic core is based on the magnitude of the input signal. According to an embodiment of the invention, the magnetic core includes a saturation region that limits the output signal regardless of the magnitude of the input signal once the saturation region reaches its saturation magnetization state. The saturation region comprises a reduced saturation magnetization level caused by a geometrically constricted region of the magnetic core or, alternatively, by a modified, magnetic-equivalent region having properties similar to a geometrically constricted region.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to transformers. More particularly, the invention 
relates to noise-limiting transformers suitable for use in, e.g., 
bidirectional communications schemes using local access coaxial cables. 
2. Description of the Related Art 
Most homes in the United States have coaxial cable lines installed to their 
homes, primarily for entertainment purpose. The use of the coaxial cable 
lines for two-way communications, either entirely or locally as a part of 
a Hybrid-Fiber-Coax (HFC) communications system arrangement with the 
coaxial cable portion connecting the distribution fiber node to each home, 
is an economically viable alternative to the use of existing telephone 
lines (i.e., conventional twisted pair copper wires), or yet-to-be 
realized fiber-to-home or wireless local access configurations. 
Currently, the HFC architecture is used mostly for one-way (downstream) 
transport of multiplexed signals to subscribers, e.g., with a downstream 
bandwidth of about 50-750 Megahertz (MHz). However, the HFC architecture 
is being considered as a promising, bidirectional, broadband 
communications infrastructure for the multibillion dollar communications 
market in part because of the low deployment cost expected. Conventional 
system architecture uses an upstream bandwidth of about 5-40 MHz, but the 
possibility of higher frequency (e.g., 750-1000 MHz) regimes also exist as 
information transmission increases in the future with the addition of more 
video, Internet and other applications. Accordingly, there exists a need 
for improved devices and equipment to support and maintain reliable, 
bidirectional HFC communications systems using this high rate of 
information transmission. 
However, one of the main barriers to the reliable operation of 
bidirectional HFC communications systems is the noise problem, commonly 
know as the "ingress". Because the HFC architecture typically consists of 
conventional tree-and-branch arrangements, the upstream transmission from 
various subscribers to the headend (central) office is shared. Thus, 
ingress noise, e.g., from individual subscriber homes and the overall 
cable structure, are added onto the main upstream transmission signals, 
inadvertently affecting the transmission of other subscribers. Such 
behavior typically is referred to as noise funneling. See, e.g., C. A. 
Eldering, e.g., "CATV Return Path Characterization for Reliable 
Communications", IEEE Communications Magazine, August 1995, p. 62. 
Accordingly, a need exists for noise-limiting devices to reduce the problem 
of ingress noise and thereby improve efficiency among two-way 
communications using, e.g., HFC architecture. 
SUMMARY OF THE INVENTION 
The invention is embodied in a transformer device having a saturation 
region for limiting ingress noise and other noise within communications 
and other systems. The transformer comprises a magnetic core, an input 
coil and an output coil arranged so that the output signal caused by the 
magnetic linkage between the input and output coils through the magnetic 
core is responsive to the magnitude of the input signal. According to an 
embodiment of the invention, the magnetic core includes a saturation 
region having a reduced saturation magnetization level that limits the 
output signal to be regardless of the magnitude of the input signal once 
the saturation region reaches its saturation magnetization level. The 
reduced saturation magnetization level of the saturation region is caused 
by geometric constriction of a portion of the magnetic core, which reduces 
the potential maximum magnetic flux flow through the region. 
Alternatively, the saturation region is a geometric 
constriction-equivalent region with a "magnetic-equivalent" region 
modified to exhibit material properties similar to a geometrically 
constricted region. The, transformer is suitable for high frequency (e.g., 
.gtoreq.0.1 MHz) use in communications and other systems for limiting 
noise such as ingress noise often inherent in such systems.

DETAILED DESCRIPTION 
In the following description, similar components are referred to by the 
same reference numeral in order to simplify the sequential aspect of the 
drawings. 
Referring to FIG. 1, a conventional Hybrid Fiber-Coax (HFC) architecture is 
shown. The conventional architecture includes at least one headend or 
central office 12 operably connected, e.g., via one or more single mode 
fibers 14, to at least one node 16 for downstream transmission of signals 
thereto. Typically, each node 16 serves approximately 500 to 2000 homes or 
other subscriber or customer premises (shown generally as 18) through a 
plurality of actives 22, taps 24 and coaxial fiber lines 26, as shown 
generally. Node 16 converts optical signals received from central office 
12 to corresponding electrical signals as well as provides amplification 
for transmission of these signals to the coaxial cable portion 26 of the 
network that connects to homes 18. 
As previously discussed, upstream transmission (i.e., transmission from 
communications peripherals in homes 18 to headend office 12) suffers from 
ingress noise appearing on the cable return path. Such peripherals 
include, e.g., computers, telephones and televisions. Ingress noise 
includes, e.g., narrowband short-wave signals, impulse noise (e.g., noise 
generated by electrical motors, engine ignitions, power switching, 
computers, digital equipment, lightning, electrostatic discharge), common 
mode distortion (e.g., distortion caused by nonlinearities in the cable 
equipment such as oxidized connectors and fittings), and 
location-specific, subscriber-induced interference (e.g., interference 
caused by the operation of amateur radios or other high-power, 
high-frequency devices). 
A noise-limiting transformer 10 according to an embodiment of the invention 
is shown in FIG. 2a. Transformer 10 comprises a magnetic core material 28, 
an input coil 32 and an output coil 34. Magnetic core 28 is made of one or 
more thin or thick film soft magnetic materials such as iron-based 
nanocrystalline amorphous materials, permalloys and ferrites. As shown, 
magnetic core 28 has one or more input core regions 36 for the input 
signal and at least one output region 38 for the output signal. 
Input coil 32, which typically carries the upstream signal (i.e., the 
signal from homes 18 to headend office 12), is wound around input core 
region 36 of magnetic core 28 and magnetizes the core material in 
proportion to the magnitude of the input signal current. As shown, output 
coil 34 is wound around output core region 38 of magnetic core 28, whose 
output region generates a current within output coil 34 whose magnitude 
typically is based on the magnitude of the input current. Alternatively, 
as shown in FIG. 2b, input coil 32 is wound on only one side of output 
coil 34 if satisfactory output signal intensity and quality are 
obtainable. 
According to embodiments of the invention, as shown in FIGS. 2a-b, output 
region 38 of magnetic core 28 includes at least one saturation region 42. 
As will be discussed in greater detail below, for purposes of discussion 
in this description, the term "saturation region" is understood to be any 
region that alters the conventional input/output relationship of an 
electromagnetic device such as a transformer. Furthermore, it is 
understood that the term "saturation region" includes any region within an 
electromagnetic device that reaches a characteristic saturation 
magnetization in such a way as to effectively limit the maximum magnetic 
flux density therein. 
For example, in FIGS. 2a-b, saturation region 42 is a geometrically 
constricted region around which output coil 34 is wound. An output region 
38 having a geometrically constricted saturation region 42 typically is 
characterized by a smaller cross-sectional area and/or volume than the 
corresponding input region(s) 36 around which input coil 32 is wound. Such 
geometrically constricted regions are prepared, e.g., by patterned 
deposition or patterned removal of selected portions of magnetic core 28. 
In typical operation, the larger volume (and hence the larger magnetic 
flux) of input core region 36 serves to amplify the output signal by 
magnetizing the smaller volume of output core region 38 to a state of 
higher magnetic flux density approaching its saturation value. When the 
input signal reaches a certain level (e.g., by ingress noise), the 
saturation region portion of output region 38 reaches its saturation 
magnetization, at which point the output signal is limited by the 
saturation value, even if the level of the input signal (noise) keeps 
increasing. 
In this manner, the presence of geometrically constricted saturation region 
42 places an upper limit on the maximum magnetic flux density in the 
portion of magnetic core 28 around which output coil 34 is wound, and 
hence limits the magnitude of the output signal generated within output 
coil 34, regardless of, e.g., an excessive input signal at input coil 32. 
Desirably, magnetic core 28 is made of a material that exhibits some 
combination of a strong anisotropy and a square magnetic hysteresis (M-H) 
loop along the easy axis of magnetization. For example, in FIG. 3, a 
representative loop is shown for a core material having a thickness of 
approximately 1000 .ANG. and made of, e.g., Fe-4.6% Cr-0.2% Ta-7.4% N 
atomic % alloy, that was triode sputter deposited near room temperature on 
a quartz substrate containing an approximately 100 .ANG. thick chromium 
(Cr) top layer. Advantageously, the addition of the Cr top layer improves 
the squareness of the M-H loop. 
For the device of FIGS. 2a-b, the material of magnetic core 28 is 
magnetically biased so that the easy axis of magnetic core 28 is in the 
direction indicated by arrow 44. Easy axis biasing is achieved, e.g., by 
applying an external field (such as by placing a permanent magnet nearby) 
or by adding an exchange interaction bias layer (such as by depositing a 
thin NiO or Fe--Mn film above or below a soft magnetic permalloy 
80%Ni--20%Fe! film). 
The easy axis is approximately orthogonal to the direction of the magnetic 
field (indicated by arrow 46) applied by input coil 32 and sensed by 
output coil 34. The plot of the M-H loop for the hard and easy axes is 
shown in FIG. 3. 
In typical operation of transformer 10, alternating current (AC) signals 
are applied in the direction of the hard axis loop, i.e., in the direction 
indicated by arrow 46. The material of magnetic core 28 is pre-saturated 
by a direct current (DC) field along the easy axis (arrow 44) so that the 
magnetic domain walls are essentially removed. The easy axis saturation is 
maintained by either bias field, exchange coupling, or high coercivity of 
the material itself in magnetic core 28. With high-frequency alternating 
current (AC) field operation along the hard axis loop, the pre-saturation 
along the easy axis allows magnetization change to occur predominantly by 
spin rotation without domain wall motion. The elimination of domain wall 
motion during the hard axis operation of transformer 10 helps the M-H loop 
to be tightly closed, e.g., as shown in FIG. 3, and reduces energy loss 
such as hysteresis loss. 
The desired magnetic properties of the material of magnetic core 28 for 
embodiments of the invention are as follows. The easy axis loop should be 
square, with a squareness ratio (i.e., remanence to saturation ratio, 
M.sub.r /M.sub.s) greater than approximately 0.85 or even greater than 
approximately 0.95. The easy axis coercivity, H.sub.c, should be high 
enough for the sake of stability but should still be low enough to 
maintain soft magnetic properties. For example, it is desirable to have 
H.sub.c in the range of approximately 1-1000 oersted (Oe), or even within 
the range from approximately 5 or 20 Oe to approximately 100 Oe. Also, it 
is desirable to have a saturation magnetization (4.pi.M.sub.s) greater 
than approximately 3 kilogauss (kG), or even a saturation magnetization of 
greater than approximately 8 kG or approximately 16 kG. 
In terms of hard axis loop characteristics, a closed and linear loop such 
as shown in FIG. 3 is desirable. The anisotropy field, H.sub.a, defined as 
the field strength required to overcome the anisotropy and achieve 
saturation in the hard axis direction, should be at least approximately 2 
Oe. However, having H.sub.a greater than approximately 10 Oe or even 30 Oe 
also is desirable. 
Having the magnetic properties of the material of magnetic core 28 
characterized by such a value of H.sub.a along with such a saturation 
magnetization value is desirable because it is possible for magnetic 
fields associated therewith to push the onset of undesirable ferromagnetic 
resonance and accompanying energy absorption and deterioration of magnetic 
permeability to higher frequencies well beyond the operating frequency 
range of the transformer, e.g., operating frequencies greater than 
approximately 1 MHz, 10 MHz or even 100 MHz. 
FIG. 4 shows the permeability spectrum as a function of frequency for a 
magnetic film having the characteristics shown in FIG. 3. The AC 
permeability is measured along the hard axis using a magnetic field of 
approximately 10 mOe) after saturation along the easy axis. The AC 
permeability is maintained and the onset of substantial loss does not 
occur within an operating frequency of up to at least 2 GHz, as the 
ferromagnetic resonance (FMR) frequency is pushed beyond the 2 GHz level 
in the film characterized by FIG. 3. As is known to those skilled in the 
art, ferromagnetic resonance is a resonating phenomenon occurring between 
the applied AC field and the resonating frequency of the magnetic core. 
Suitable magnetic materials processed to exhibit the above mentioned 
properties include thin or thick films and ribbons of soft magnetic 
materials such as Fe-based, Co-based or Ni-based alloys, ferrites, 
amorphous materials. Furthermore, suitable materials include permalloy 
(80%Ni-20%Fe), Fe-refractory metal films such as Fe--Ta, Fe--Zr, Fe--Hf, 
Fe--Nb, Fe--Ti with the refractory metal content typically in the range of 
approximately 0.5-10 wt %, Co--Fe based soft magnetic films with the Fe 
content typically in the range of approximately 10-80 wt %, and soft 
ferrites such as Ni--Zn ferrite or Mn--Zn ferrite. 
In general, thin film magnetic materials are more desirable than thick film 
or ribbon materials for minimizing eddy current loss in high frequency 
operations, e.g., frequencies above approximately 10 Mhz. For example, the 
overall thickness of magnetic films according to embodiments of the 
invention often are within the range from approximately 0.01 to 100 .mu.m, 
with the thickness of individual layers often within the range from 
approximately 0.05 to 5 .mu.m. Also, according to embodiments of the 
invention, it is possible to use a multilayer configuration with 
insulating intermediate layers to further reduce the eddy current effect 
in a larger volume core materials. 
The thin films are deposited by any one of a number of known processing 
methods, such as physical vapor deposition (using sputtering), 
evaporation, ion beam deposition, laser ablation, electrochemical means 
(using electroplating or electroless deposition) and chemical vapor 
deposition. Often, as-deposited films are used without post heat treatment 
at high temperatures to minimize the danger of degrading other components 
or materials in the inventive devices. Characteristics of the magnetic 
properties of such as-deposited films are shown, e.g., in FIG. 3 and 4, 
discussed previously. Also, such thin-film deposition methods are 
disclosed in detail, e.g., in co-pending application Ser. No. 08/595,543, 
filed Feb. 02, 1996 and assigned to the assignee of this application. 
It is possible to use films having a thickness greater than approximately 
20 .mu.m) if the material of the magnetic core has relatively high 
electrical resistivity (e.g., greater than approximately 500 
.mu..OMEGA./cm) and the operating frequency is relatively low (e.g., less 
than approximately 100 MHz). Such films are deposited and patterned using 
known techniques such as spray coating, screen printing, ink jet printing, 
doctor-blade coating using a powder-slurry approach, or plasma spray, 
electroplating, and sol-gel coating. Films processed by using a 
powder-containing precursor generally require post heat treatment for the 
purpose of sintering or stress relief annealing. 
Substrates for deposition of films according to embodiments of the 
invention include insulators such as glass, quartz, Al.sub.2 O.sub.3, 
Y.sub.2 O.sub.3, Y-stabilized zirconia, LaAlO.sub.3, LaGaO.sub.3, 
SrTiO.sub.3, polyimide, or semiconductors such as Si or GaAs. 
Alternatively, a combination substrate having a thin interlayer coating of 
an insulator, semiconductor or metal formed on the surface of the base 
substrate is used for the purpose of enhancing texture formation, 
crystallization or adhesion. For example, such combination substrate 
includes a thin Cr coating (20-500 .ANG. thick) formed on Si, glass or 
quartz. 
Referring to FIG. 5, an alternative embodiment of the invention is shown. 
Instead of having a geometrically constricted region as described above, 
the noise-limiting transformer has a "magnetic-equivalent" constriction 
region 48. For purposes of discussion in this description, a 
"magnetic-equivalent" constriction region is a region whose material 
properties have been modified in such a way that the region demonstrates 
the saturation magnetic induction characteristics of a geometrically 
constricted region as described herein. Typically, the region is modified 
to intentionally deteriorate the local magnetic properties of the region 
to cause a lower saturation moment. Specifically, a "magnetic-equivalent" 
constriction region is created, e.g., by locally modifying the chemical 
composition, crystal structure or internal stress state of the material of 
magnetic core 28. 
Methods used to alter the chemical composition and hence reduce the 
saturation magnetization of the region of interest include, e.g., local 
ion implantation of alloying elements (e.g., C, N, O, B, C), local 
carburizing, nitriding, and oxidizing heat treatment. Methods such as 
local laser beam heating are used for the carburizing, nitriding or 
oxidizing heat treatment. Also, local laser beam heating is used to modify 
the crystal structure by inducing phase transformation through rapid 
cooling to a metastable, lower saturation or non-magnetic phase, e.g., 
body-centered cubing (bcc) martensite to face-centered cubing (fcc) 
austenite phase in iron-rich alloys or from a crystalline to an amorphous 
structure. 
Alternatively, internal stress is created in the local region of interest 
by rapid heating and cooling thereof (e.g., by laser) to bring about 
intentional, magnetostriction-induced deterioration therein. Also, 
internal stress is created by depositing a foreign material (e.g., a 
narrow strip of thin films with a different volume expansion coefficient) 
locally over the soft magnetic film so as to cause deposition-induced or 
transformation-induced stress. 
The windings for input and output coils 32, 34 in transformer 10 are wire 
windings or, alternatively, as shown in FIGS. 6-8, are thin film 
metallization layers of Cu, Al or other suitable materials deposited and 
patterned, e.g., as shown. Such metallization layers are prepared in a 
conventional manner, e.g., as described in C. R. Sullivan and S. R. 
Sanders, Proceedings, 24th Annual Power Electronics Specialists Conf., p. 
33-40, Jun., 1993. 
FIG. 6 illustrates a noise-limiting transformer 60 according to an 
alternative embodiment of the invention. As shown, transformer 60 
comprises a thin, square-shaped magnetic core 62 having leg regions 64a-d, 
an input coil thin film conductor 66 and an output coil thin film 
conductor 68. Magnetic core 62 and conductors 66, 68 are made, e.g., by 
deposition in a conventional manner using a known multi-step deposition 
and patterning procedure. 
Magnetic core 62 is deposited in such a way that its hard axis of 
magnetization is in the direction indicated by arrow 72, i.e., parallel to 
leg regions 64a and 64c. Also, the magnetic bias of transformer 60 is such 
that the easy axis orientation is in the direction indicated by arrow 74. 
A saturation region, e.g., a geometric constriction region 76, is formed 
within magnetic core 62, e.g., along leg region 64b. Constriction region 
76 of transformer 60 has reduced physical dimensions as shown. In this 
configuration, the cross-sectional area and thus the overall volume along 
constriction region 76 is reduced, resulting in less magnetic flux flow 
within the region compared to other areas along magnetic core 62. 
Although not shown, it is possible to form a "magnetic-equivalent" 
constriction region within magnetic core 62 in the manner discussed 
previously. In using such a region, reduced magnetic flux is achieved by 
modifying the properties of the region as discussed previously herein. 
Typically, it is necessary for one or more saturation features to be 
included in the soft magnetic material of magnetic core 62. For example, 
constricted region 76 as shown in FIG. 6 is prepared, e.g., by partial 
masking during film deposition, or by removal of materials through partial 
etching, laser ablation or other suitable techniques. 
FIG. 7 illustrates a noise-limiting transformer 70 according to yet another 
embodiment of the invention. Specifically, transformer 70 has an E-core 
type configuration, as shown. In this embodiment, the magnetic core is 
made up of a first, top layer 84 formed on a second, bottom layer 86, as 
shown. An input coil thin film 92 and an output coil thin film 94 are 
formed to pass through first and second layers 84, 86 as shown. Also, 
transformer 70 is magnetically biased in such a way that the easy axis 
orientation, which is indicated by arrow 96, is perpendicular to the long 
axis of the magnetic core, which is indicated by arrow 98. 
In the embodiment shown in FIG. 7, the easy axis biasing is advantageously 
convenient because the easy axis direction is the same for both the top 
and bottom legs of the core material. Thus, only one biasing step is 
needed. As discussed previously, easy axis biasing is achieved by applying 
an external field or by adding an exchange interaction bias layer. Such 
easy axis biasing is compared, e.g., with that of transformer 60 shown in 
FIG. 6, in which the easy axis biasing directions of horizontal legs 64a, 
64c are different than those of vertical legs 64b, 64d. In those 
arrangements, separate biasing steps are needed. 
FIG. 8 illustrates a transformer 100 according to yet another embodiment of 
the invention. In this embodiment, magnetic core 62 is not in a 
closed-loop configuration. An input coil thin film conductor 104 and an 
output coil thin film conductor 106 are formed around magnetic core 62 as 
shown. Also, a saturation region 108 is formed within magnetic core 62 and 
between input and output film layers 104, 106. As discussed previously, 
saturation region 108 is, e.g., a geometrically constricted region (as 
shown in FIG. 8) or a "magnetic-equivalent" constriction region (not 
shown). 
In the configuration shown in FIG. 8, some loss in magnetic permeability is 
possible due to the presence of demagnetizing fields. However, such loss 
typically is not problematic because of a high aspect ratio (i.e., a 
length to thickness ratio of the core material) in the film length 
direction (indicated by arrow 112), especially when transformer 100 is 
used in the higher-frequency, hard axis operation (for which the 
permeability is somewhat limited) as compared to the lower-frequency, easy 
axis operation. 
In operating the noise-limiting transformer according to embodiments of the 
invention disclosed herein, the magnetic core first is easy axis saturated 
(e.g., by applying a DC field) to obtain a single-domain magnetic state. 
Then, the magnetic core is operated in the hard axis direction by applying 
an AC field. In this manner, any hard axis magnetization changes occur by 
a spin rotation mechanism rather than the loss-laden domain wall motion 
mechanism. Also, in such operation, the stability of the single-domain 
magnetic state needs to be secured against exposure to stray fields in 
excess of the coercivity of the core material, which can partially 
demagnetize the magnetic core and introduce domain walls. It is not 
uncommon for stray magnetic fields of a few to several oersted to be 
present. However, in an effort to mitigate the presence of such magnetic 
fields, it is possible to operate the inventive transformer in various 
modified manners as shown in FIGS. 9a-c and described below. 
As shown in FIG. 9(a), an external bias field is applied in the easy axis 
direction (indicated by arrow 116) by placing at least one permanent 
magnet 118 on the sides of transformer 10. The strength of the external 
bias field is at least approximately 2 Oe, but typically is greater than 
approximately 20 Oe. In this manner, the applied DC field essentially will 
restore and maintain the single domain state in the easy axis direction, 
even after exposure to a temporary stray magnetic field. Input coil 32 and 
output coil 34 shown in FIG. 9(a) are wires similar to those shown in 
FIGS. 2a-b and described previously. Alternatively, coils 32, 34 are thin 
films similar to those shown in FIGS. 5-8 and described previously. 
Magnets 118 are made of, e.g., known materials such as ferrites, alnico, 
Fe--Cr--Co, rare earth cobalt, or Nd--Fe--B. 
As shown in FIG. 9(b), a bias field along the easy axis is provided by 
using a surface film layer 122 of antiferromagnetic or ferrimagnetic 
material such as Fe-50% Mn or NiO on magnetic core 28. Typically, the 
thickness of exchange bias film layer 122 is, e.g., within the range from 
approximately 20 .ANG. to approximately 1000 .ANG.. 
When the material in layer 122 is magnetized along the easy axis (indicated 
by arrow 124), a magnetic exchange interaction at the interface of layer 
122 and magnetic core 28 causes the M-H loop of magnetic core 28 to be 
shifted along the field axis. With a sufficient shift of the M-H loop 
(e.g., in excess of the coercivity value), the material of magnetic core 
28 is maintained at the single-domain, saturated state along the easy axis 
even after exposure to stray fields. 
Typically, the width of the M-H loop to be within the bias field range. 
Layer 122 is applied on either side or, alternatively, on both sides of 
magnetic core 28. Input coil 32 and output coil 34 are similar to those 
windings discussed previously herein. 
Yet another way of providing the stability of the single-domain state as 
discussed above within a transformer, e.g., transformer 10 shown in FIG. 
9(c) is to provide a high coercivity, H.sub.c, to the material of magnetic 
core 28 while still maintaining the square M-H loop characteristics 
thereof. In this manner, neither a bias magnet nor an exchange bias film 
layer is used. 
For example, a coercivity H.sub.c, is established within the range from 
approximately 10 Oe to 50 Oe. It is possible to establish H.sub.c greater 
than approximately 50 Oe, but H.sub.c should not exceed approximately 200 
Oe for the sake of a reasonably high permeability in the hard axis 
operation. It is possible to process the material of magnetic core 28 to 
exhibit a high H.sub.c and a square M-H loop by introducing second phase 
precipitate particles during, e.g., film deposition processes or by post 
heat treatment. The presence of the particles impedes the domain wall 
motion. Also, the application of uniaxial stress in the easy axis 
direction (indicated by arrow 126) creates a high H.sub.c and a square M-H 
loop. 
FIG. 10 illustrates a typical application of the inventive transformer 
shown, e.g., in FIGS. 2-9 and described herein. Transformer 10 is useful 
in limiting ingress noise generated from, e.g., a customer premise 18. 
Typically, transformer 10 is operably connected in a conventional manner 
in the upstream portion of a communications network beyond the point where 
the frequency is separated, e.g., by a frequency separator 128, into a 
downstream portion 132 and an upstream portion 134. As shown, transformer 
10 is placed in upstream signal portion 134, e.g., in series between a 
computer 136 and frequency separator 128. Downstream signal portion 132 
connects, e.g., to a television 138 or other suitable equipment. 
Alternatively, transformer 10 is positioned, e.g., within the 
communication network at each subscriber premise before the point where 
the frequency is separated into downstream and upstream portions (not 
shown), e.g., between node 16 and frequency separator 128. 
It will be apparent to those skilled in the art that many changes and 
substitutions can be made to the embodiments of the noise-limiting 
transformer and the bidirectional communications system herein described 
without departing from the spirit and scope of the invention as defined by 
the appended claims and their full scope of equivalents.