U.S. patent number 3,743,978 [Application Number 05/088,042] was granted by the patent office on 1973-07-03 for coated ferrite rf filters.
Invention is credited to William Baird Fritz.
United States Patent |
3,743,978 |
Fritz |
July 3, 1973 |
**Please see images for:
( Certificate of Correction ) ** |
COATED FERRITE RF FILTERS
Abstract
In a low pass RF filter, a coating of barium titanate is applied
to a ferrite substrate. In one embodiment, the RF filter is an
extruded tube of ferrite coated with barium titanate. The tube is
used as an RF filter for a connector pin. In another embodiment, a
thin strip of ferrite is coated with barium titanate. This forms a
filter strip for use on circuit boards or for use as a high
capacity lossy power bus.
Inventors: |
Fritz; William Baird (Hershey,
PA) |
Family
ID: |
26778097 |
Appl.
No.: |
05/088,042 |
Filed: |
November 9, 1970 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
883501 |
Dec 9, 1969 |
|
|
|
|
Current U.S.
Class: |
333/182 |
Current CPC
Class: |
H01R
13/719 (20130101); H01P 1/215 (20130101); H03H
1/0007 (20130101); H01R 13/7197 (20130101); C04B
35/4682 (20130101) |
Current International
Class: |
C04B
35/462 (20060101); H03H 1/00 (20060101); C04B
35/468 (20060101); H01P 1/215 (20060101); H01P
1/20 (20060101); H01R 13/719 (20060101); H03h
007/04 () |
Field of
Search: |
;333/70,79,24.1,31,73
;317/242 ;334/4 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Saalbach; Herman Karl
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of my prior copending
application Ser. No. 883,501, filed Dec. 9, 1969 now abandoned, and
to which priority is asserted as to subject matter common
therewith.
Claims
What is claimed is:
1. A unitary low pass filter element for mounting on a conductor of
a low frequency transmission line to attenuate high frequencies
thereon comprising,
a conductive tubular member for receiving a conductor therein,
a semiconductive substrate in the form of a sleeve secured to the
outer surface of the tubular member, in intimate engagement
therewith,
a layer of dielectric material covering the outer surface of the
sleeve in direct intimate contact therewith,
and an outer conductive layer disposed about and secured to the
dielectric material substantially the length thereof for connecting
the unitary filter element to ground.
2. The filter recited in claim 1 wherein said substrate is a
semi-conducting ceramic.
3. The filter recited in claim 2 wherein the semi-conducting
ceramic is doped barium titanate.
4. The filter recited in claim 1 wherein said dielectric is undoped
barium titanate.
5. The filter recited in claim 1 wherein the layer of dielectric
material is coated thereon.
6. The filter recited in claim 1 wherein the tubular member is a
metallic plating over the inner surface of the sleeve.
7. The filter recited in claim 5 wherein the outer conductive layer
is metallic plating over said layer of dielectric material.
8. A unitary low pass filter strip for mounting on a circuit board
provided with a ground plane conductor comprising,
a substrate in the form of a flat strip of semiconductive
material,
a conductive metal plating on one surface of the substrate,
a coating of dielectric material on the opposite surface of the
substrate, and a conductive metal plating on the outer surface of
the dielectric coating,
one of said platings being connected to one terminal of a low
frequency source and load, and the other plating being in
conductive engagement with the ground plane conductor.
9. The filter of claim 8 in which the substrate is a semiconductive
ceramic.
10. The filter of claim 8 in which the substrate is doped barium
titanate.
11. The filter of claim 8 in which the dielectric is undoped barium
titanate.
12. A unitary low pass filter element for mounting on a conductor
of a low frequency transmission line to attenuate high frequencies
thereon comprising,
a conductive tubular member for receiving a conductor therein,
a substrate of ferrite in the form of a sleeve secured to the outer
surface of the tubular member, in intimate engagement
therewith,
a layer of dilectric material covering the outer surface of the
sleeve in direct intimate contact therewith,
and an outer conductive layer disposed about and secured to the
dielectric material substantially the length thereof for connecting
the unitary filter element to ground.
13. The filter recited in claim 12 wherein said dielectric is
undoped barium titanate.
Description
BACKGROUND OF THE INVENTION
This invention relates to low pass RF filters and more particularly
to a layer of dielectric material deposited on a ferrite substrate
to form a filter.
Low pass RF filters are used extensively in electrical circuits to
suppress stray radio frequency noise. Lumped impedance filters
perform well at the lower frequencies but resonances limit their
utility as the frequency is increased. Also, these type filters are
large in size compared to the circuits with which they are used. To
overcome this, RF filters of the type disclosed in U.S. Pat. No.
3,275,953 -- Coda et al. were developed and used as feed through
filters or on connector pins. These filters are small and have good
insertion loss characteristics at high frequencies. However, there
are several problems associated with filters of the type shown in
the Coda et al. patent. First, they include an inner sleeve of
ferrite coated with a metal layer and an outer metallized ceramic
sleeve, usually barium titanate. Therefore, they require several
fabrication steps. Also, the capacity is limited by the thickness
to which the outer sleeve can be made, usually 8 to 10 mils
minimum.
Finally, even though resonances are minimized at high frequencies,
the filter, because of the type of construction, is still lumpted
at the lower frequencies of interest, 1-50 megahertz. Accordingly,
resonances can result at these frequencies. It is desirable then
that filters of this type have a completely distributed impedance,
that they be easier to fabricate and that they not be limited in
capacity by the titanate sleeve thickness.
SUMMARY OF THE INVENTION
This invention concerns an RF filter in which a thin coating of
dielectric material is laid down on a ferrite substrate. In one
specific embodiment a layer approximately 2 mils thick of barium
titanate is laid down on the ferrite substrate to produce an
electrical filter. The filter produced in this manner has low cost
because fewer fabrication steps are involved. Also, the electrical
properties are better than prior art filters. The impedance is
completely distributed and the filter has a high capacity.
In one form of the invention, the filter is an extruded tube of
ferrite upon which a layer of barium titanate has been deposited.
These filters are used for connector pins.
In another form of the invention the filter is a thin strip of
ferrite upon which a barium titanate layer has been deposited.
These are used as filter strips, or filtered buses, for circuit
boards.
The foregoing and other objects, features and advantages of the
invention will be better understood from the following more
detailed description, the drawings, and the claims.
DESCRIPTION OF THE DRAWINGS
FIG. 1 depicts a prior art type of filter for a connector pin;
FIG. 2 depicts a filter for a connector pin constructed in
accordance with the present invention;
FIG. 3a shows the insertion loss versus frequency for the prior art
filter and for the filter of this invention;
FIG. 3b shows the attenuation versus frequency for the prior art
filter and for the filter of this invention;
FIG. 4 shows the equivalent circuit of a prior art type filter;
FIG. 5 shows the filter of the present invention in place on a
connector pin;
FIG. 6 shows the invention embodied in a filter strip for a circuit
board; and
FIGS. 7-9 show modifications of the filter strip.
DESCRIPTION OF A PARTICULAR EMBODIMENT
Referring to FIG. 1, most prior art connector pin filters are
constructed of two concentric sleeves. The inner sleeve includes an
extruded ferrite tube 1 with metal plating 2. The outer sleeve
includes barium titanate 3 with the metal plate 4. The two sleeves
are joined together by conductive epoxy or by soldering. It will be
appreciated that the fabrication process includes extruding two
sleeves, two steps of plating, one for each sleeve, and the step of
joining the two sleeves together.
Contrast this with a filter constructed in accordance with the
present invention as depicted in FIG. 2. The extruded ferrite tube
5 is coated with the barium titanate layer 6. Barium titanate may
be laid down on ferrite with several known techniques.
Electrophoretic deposition is a particularly good technique for
coating barium titanate on a ferrite tube. The electrophoretic
deposition described in Senderoff et al. U.S. Pat. No. 2,843,541
may be used to lay down the barium titanate layer. After the barium
titanate has been deposited on the ferrite, the device is metal
plated, the metal plating being indicated at 7. Gaps 8 and 9 are
left in the metal plating to isolate the ground and center pin
electrodes.
Note that in FIG. 1 the same provision must be made for gaps 10 and
11 in the metal plating. Additionally, a gap 12 must be provided on
the inside of sleeve 3. When the inner sleeve 1 is electroplated, a
gap 12a must be provided so that the ferrite is not shielded out of
the circuit. It can be seen that the fabrication of prior art
devices include additional difficult fabrication steps not required
in constructing the filter of this invention.
The filter of this invention also has improved electrical
characteristics over prior art filters. This can best be shown by
an example. Two filters were constructed, one in accordance with
the prior art and one in accordance with this invention. The
filters were 0.1 inches in diameter by 0.465 inches long. The
capacity of the prior art filter was 6,000 .mu..mu.F. The capacity
of the filter of this invention was 5,000 .mu..mu.F. The insertion
loss versus frequency of the filters as measured in a 50 ohm system
is shown in FIG. 3a. This response for both filters is good. Note,
however, that the attenuation of the two filters, shown in FIG. 36,
is quite different and that the prior art filter actually shows an
undesirable gain between 5 and 10 megahertz. This gain is the
result of the shunt capacity resonating with the filter series
inductance. These circuit elements are shown in the equivalent
circuit of FIG. 4. Because the impedance of the circuit in which
these filter devices are used is not always known or easily
established, the prior art filter in actual use may show less loss
or even a gain from that determined by measurement made in a
circuit with predetermined source and load impedance, such as mil
standard 220. The filter of this invention, because of its
distributed construction and inherent low Q, does not show gain in
the attenuation curve of FIG. 3b, regardless of the circuit
impedances.
As previously pointed out, another advantage of this invention is
that it is possible to get an extremely thin film of barium
titanate, about 2 to 4 mils being common. This thin film gives a
much higher capacity per unit length of filter and for a given
dielectric constant there is more attenuation per unit length than
in a conventional filter.
FIG. 5 shows one of the filters constructed in accordance with the
invention in place on a connector pin. The filter 13 is positioned
over the connector pin 14. A ground plane 15 is snapped onto the
filter to provide the ground connection. For a connector pin filter
having a length of 1 centimeter, the noise attenuation is
approximately 60 db at 100 megahertz. That is, the noise power is
reduced by a factor of 10.sup.6.
FIG. 6 shows an embodiment of the invention in which a coated strip
of ferrite is used as a filter strip for a circuit board. The strip
of ferrite 16 has a layer of barium titanate 17 deposited thereon.
The strip has a metal plating 18 on one side and a metal plating 19
on the other side. The metal plating 19 is soldered to the ground
plane 20 of the circuit board. The circuit components 21 and 22 are
connected to the RF filter strip by the leads 23 and 24
respectively. These leads are soldered to the metal plating 18.
In this form, the invention provides good filtering for components
connected to the metal plating 18 which may be a DC bus.
Particularly in digital circuits, when one of the circuits such as
21 or 22 is triggered, high frequencies are normally imposed on the
DC bus. This high frequency noise may interfere with the other
circuits on the board. However, the use of the lossy bus according
to the present invention will isolate the components. Also, it will
prevent noise from other sources from entering the power bus and
possibly causing a malfunction.
In one actual application of the invention a 1/2 inch wide bus bar
was constructed of the form shown in FIG. 6. The attenuation was 90
db per centimeter at 100 megahertz. Stated another way, the power
of the noise was attenuated by a factor of 10.sup.9.
Many modifications of the invention will be apparent. While barium
titanate has been described as a particularly good dielectric
material, other dielectric materials with lower dielectric
constants may be deposited on the ferrite as a means of controlling
the cut off frequency of the filter. For example, a filter with a 2
mil epoxy coating resulted in a filter with a cut off frequency
(frequency at which insertion loss is 3db) of 50 megahertz, whereas
the equivalent filter with barium titanate had a cut off at 2
megahertz.
Various modifications of the filter strip may be made. For example,
in FIG. 7 the ferrite 25 has coatings of barium titanate 26 and 27
on both sides. Conductive metal platings 28 and 29 are applied over
the barium titanate. Such a filter strip has a higher breakdown
voltage. However, it would also have lower attenuation for a given
thickness of barium titanate.
In FIG. 8 the layers 30, 31, and 32 are conductive metal coatings.
Layers 33 and 34 are ferrite and layers 35 and 36 are barium
titanate. The metal 31 may be the conductor and the metal coatings
30 and 32 the ground. This embodiment has a higher capacity and
loss per unit length.
In FIG. 9 the filter strip includes a single ground conductor 36, a
ferrite 37 and a barium titanate layer 38. Laid down on this are a
plurality of metal conduction strips 39-42. This embodiment can be
used where multiple circuits are required.
While the invention is particularly suitable for use with a ferrite
substrate as previously described, the substrate may, in accordance
with a further aspect of the invention, be constructed of other
materials. One practical alternative in the use of a doped
semiconducting ceramic material for the substrate. It is well known
that the normally high resistivity of barium titanates can be
greatly reduced by the introduction of proper additives. The
resulting semiconductive barium titanates, produced by known
methods of treatment referred to in U.S. Pat. No. 3,268,783 to
Osamu Saburi, are termed "controlled valency semiconductive barium
titanates." As is pointed out in the Saburi patent, semiconductive
ceramic material can be produced via valency control and can be
carried out upon members of the family of materials generically
designated by E.sup.2.sup.+ M.sup.4.sup.+ O.sub.3.sup.2.sup.-,
wherein E is an alkaline earth element material selected from the
group consisting of barium, magnesium, calcium, strontium, lead and
mixtures thereof, M is a metal chosen from the group consisting of
titanium, tin, and zirconium, and O is of course oxygen. Barium
titanate is one member of the aforesaid family of materials. As
further pointed out in the Saburi patent, the additives used for
valence control may comprise a material A selected from the group
consisting of yttrium, actinium, thorium, antimony, bismuth, the
members of the rare earth elements, and mixtures thereof, or a
material B taken from the group consisting of vanadium, niobium,
tantalum, selenium, tellurium, tungsten, and mixtures thereof. The
total amount of additive should be between 0.01 atomic percent to
0.50 atomic percent of the host material, the alkaline earth
material E being the host with additive A, and the metal M being
the host in the case of additive B. In FIGS. 1 and 2 of the Saburi
patent, the semiconductive plate 3 is an illustration of a
semiconductive barium titanate substrate in a capacitor device.
In accordance with this further aspect of the invention, the
substrate may consist of a semiconducting ceramic, such as the
aforementioned semiconductive barium titanate, which is then coated
with a suitable dielectric material to produce a filter, the
coating being deposited in the same manner as in the case of the
ferrite substrate above. For example, a semiconducting barium
titanate sleeve coated with a low conductivity titanate forms a
large lossy capacitor. Such a device does not have the loss
characteristics associated with the magnetic ferrite and it is not
as effective as the ferrite device at high frequencies. However,
for some applications the filter constructed with a semiconducting
ceramic substrate is quite satisfactory and can be inexpensively
manufactured.
* * * * *