U.S. patent number 3,789,263 [Application Number 05/223,624] was granted by the patent office on 1974-01-29 for rf filters with glass on a substrate.
This patent grant is currently assigned to AMP Incorporated. Invention is credited to William Baird Fritz, Neil Harrison Sanders, John Wayne Tiley.
United States Patent |
3,789,263 |
Fritz , et al. |
January 29, 1974 |
RF FILTERS WITH GLASS ON A SUBSTRATE
Abstract
A low pass RF filter for high voltage feed-through to the
filament of a magnetron. The filter comprises a ferrite substrate
in the form of a sleeve with the outer surface of the sleeve
carrying a dielectric layer comprising a lead oxide glass. The
dielectric layer also comprises aluminum oxide to provide a rough
surface for metal plating without increasing the coefficient of
thermal expansion of the dielectric layer relative to the ferrite
substrate.
Inventors: |
Fritz; William Baird (Hershey,
PA), Tiley; John Wayne (Hatboro, PA), Sanders; Neil
Harrison (Carlisle, PA) |
Assignee: |
AMP Incorporated (Harrisburg,
PA)
|
Family
ID: |
22837318 |
Appl.
No.: |
05/223,624 |
Filed: |
February 4, 1972 |
Current U.S.
Class: |
315/39.51;
315/39.53; 315/85; 333/182; 336/233 |
Current CPC
Class: |
C04B
35/4682 (20130101); H03H 1/0007 (20130101); H01R
13/719 (20130101); H01R 13/7197 (20130101); H01P
1/215 (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); H01j
023/20 () |
Field of
Search: |
;333/79
;315/39.51,39.53,85 ;117/124C ;336/233 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Rolinec; Rudolph V.
Assistant Examiner: Chatmon, Jr.; Saxfield
Claims
What is claimed is:
1. An RF low pass filter for use in a high temperature environment
comprising:
a substrate, and
a dielectric layer comprising glass characterized by low electrical
leakage at ambient temperatures of 200.degree.C to 300.degree.C,
wherein said substrate is in the form of a tube and said dielectric
layer is carried directly by the exterior surface of said tube,
said dielectric layer being characterized by a coefficient of
thermal expansion which is less than the coefficient of thermal
expansion for said substrate,
and electrically conductive means on the outer surface of the glass
and inner surface of the substrate for connection in a circuit.
2. The filter of claim 1 wherein said dielectric layer is
characterized by a rough surface, and said conduction means
comprising metal plating adhering to said rough surface.
3. The filter of claim 1 wherein said dielectric layer is uniformly
applied to said substrate.
4. The filter of claim 1 wherein said glass comprises lead
oxide.
5. The filter of claim 1 wherein said dielectric layer further
comprises aluminum oxide.
6. The lossy filter of claim 1 wherein said substrate comprises
ferrite.
7. The filter of claim 1 wherein said glass is a low sodium glass
characterized by low sodium ion migration at high temperatures.
8. The filler of claim 7 wherein the resistance of said glass is no
less than one megaohm per cube.
9. A microwave system for use in an ambient of high temperature
comprising:
a magnetron having a filament;
a filament transformer supplying a high voltage to said filament;
and
a distributed low pass RF filter means for feeding said high
voltage through to said filament while preventing microwave energy
of said magnetronx from being propogated back to the input of said
filament transformer, said filter means comprising:
a filter housing having a ground plane;
a filter comprising a substrate and a dielectric layer carried by
said substrate, said dielectric layer comprising glass
characterized by low electrical leakage at ambient temperatures of
200.degree.C to 300.degree.C, wherein said substrate is in the form
of a tube and said dielectric layer is carried directly by the
exterior surface of said tube, said dielectric layer being
characterized by a coefficient of thermal expansion which is less
than the coefficient of thermal expansion for said substrate, the
outer surface of said glass and the axial opening of said substrate
being metal plated, the metal plating of said glass making
electrical contact with said ground plane, and
a pin connected between said filament and said filament transformer
inserted within the metal plated axial opening of said substrate in
electrical coupling relation therewith.
Description
BACKGROUND OF THE INVENTION
This invention relates to low pass RF filters and more particularly
to low pass RF filters having a substrate carrying a dielectric
material.
This invention also relates to low pass filters for high voltage
feed-through applications.
One very important high voltage feed-through application for such a
filter is the microwave oven. The filament of the microwave oven
magnetron is supplied with a high voltage from the filament
transformer. Unless a low pass RF filter is utilized, it is
possible for microwave energy to be propagated back to and along
the power supply line creating a risk of radiation as warned by the
Department of Health, Education and Welfare.
A relatively low cost RF filter charcterized by excellent
electrical properties, a distributed impedance and a high capacity
is described in copending application of William B. Fritz, "Coated
Ferrite RF Filters," Ser. No. 88,042, filed Nov. 9, 1970, which is
assigned to the assignee of this invention. In one embodiment of
the filter disclosed in the aforesaid application, a dielectric
layer comprising barium titanate is carried by a ferrite substrate
in the form of a tube or sleeve. The barium titanate is
particularly desirable since it is relatively low cost and
corrosion resistant. In addition, it may be electrophoretically
deposited on the substrate to assure a uniform self-leveling
dielectric layer.
However, the dielectric strength of barium titanate makes it
ill-suited for many high voltage feed-through applications, e.g., a
microwaVe filter for use with the high voltage filament of a
magnetron in a microwave oven. Tests on barium titanate indicate
that it will undergo a breakdown with sustained use at a voltage of
600 to 700 volts which is the operating range of the magnetron
filament on a microwave oven. Furthermore, many materials which
could conceivably be substituted for the barium titanate exhibit
leakage at the high ambient temperatures generated by the microwave
oven. Other materials do not provide the desired electrical
characteristics and other properties which are exhibited by the
filter disclosed in the aforesaid Fritz application.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a low pass RF filter
of the type comprising a substrate carrying a dielectric layer
which exhibits the desired electrical characteristics at high
operating voltages.
It is another object of this invention to provide such a filter
wherein the dielectric layer is not characterized by leakage at
high ambient temperatures and high operating voltages.
It is another object of this invention to provide such a filter
wherein the dielectric layer is corrosion resistant.
It is a further object of this invention to provide such a filter
wherein the dielectric layer is relatively inexpensive.
It is a still further object of this invention to provide such a
filter wherein the dielectric layer is uniformly applied to the
substrate.
In accordance with these and other objects of the invention, a low
pass RF filter of one specific embodiment comprises a ferrite
substrate carrying a dielectric layer comprising glass. The glass
is characterized by low leakage at ambient temperatures in excess
of 200.degree.C. A lead oxide glass is particularly well suited for
this purpose since it is characterized by low leakage due to a low
migration of sodium ions at these ambient temperatures.
In accordance with one important aspect of the invention, the
ferrite substrate is in the form of a tube and the dielectric layer
including the glass is carried on the outer surface of the tube. By
utilizing a glass with a lesser coefficient of expansion than the
ferrite substrate, a compressive force is developed between the
glass and the substrate to increase the mechanical strength of the
glass.
In accordance with another important aspect of the invention, the
dielectric layer further comprises aluminum oxide (Al.sub.2
O.sub.3) so as to provide a dielectric layer with a rough surface
permitting metal plating thereon. Since aluminum oxide has a
coefficient of expansion less than the ferrite substrate, the
addition of aluminum oxide does not adversely affect the
compressive force between the glass and the ferrite substrate.
In accordance with another important aspect of the invention, the
dielectric layer is uniformly applied to the substrate without pin
holes through the dielectric layer. This is accomplished by the
electrophoretic deposition of the dielectric layer.
In accordance with still another aspect of the invention, the
central opening in the tube of the filter receives a connector pin
associated with the high voltage filament of a magnetron which may
be used as a source of microwave energy in a microwave oven. The
filter is effective to prevent the microwave energy from the
magnetron from propagating back to and along the power supply line
for the oven at the high ambient temperatures and operating
voltages encountered in this particular application.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an axially sectioned view of a filter constructed in
accordance with the present invention;
FIG. 2 is an axially sectioned view of a connector assembly
utilizing the filter of FIG. 1; and
FIG. 3 is a schematic diagram of a magnetron system wherein the
filter of FIGS. 1 and 2 is utilized in conjunction with the
magnetron filament.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
A filter constructed in accordance with this invention will now be
described with reference to FIGS. 1 and 2. A substrate in the form
of an extruded ferrite 10 carries a dielectric layer 12 comprising
a glass characterized by low electrical leakage at ambient
temperatures of 200.degree. to 300.degree.C and operating voltages
of 600 to 700 volts. Metal plating 14 is applied to the glass 12
and the ferrite 10 along its axial opening 6. Gaps 18 and 20 are
left in the metal plating 14 to isolate the pin 22, which is
received in the axial opening 6, from the ground plane 24.
It has been found that a lead oxide glass may be utilized in the
dielectric layer 12 to obtain the desired low leakage at high
ambient temperatures. For example, a soft, G12 glass such as the
KG12 manufactured and sold by Owens-Illinois and 0120 glass
manufactured and sold by Corning Glass Works may be utilized. These
glasses are particularly advantageous since they are corrosive
resistant at ambient temperatures of 200.degree.C to 300.degree.C
and are also commercially available at a relatively reasonable
cost. A lead oxide glass such as G12 which is a low sodium glass
displays the necessary low leakage characteristics at ambient
temperatures of 200.degree.C - 300.degree.C since there is little
sodium ion migration through the glass.
It will of course be understood that the desired low leakage at
these temperatures might also be achieved with other low sodium
glasses wherein the sodium ions are replaced by other alkali metal
ions such as barium or potasium. In general, it is preferable to
utilize a glass which is characterized by a resistance greater than
1 megaohm per cube.
In accordance with one aspect of the invention, the glass of the
dielectric layer 12 is characterized by a coefficient of thermal
expansion which is only slightly less than the coefficient of
thermal expansion for the ferrite substrate 10,
90.times.10.sup.-.sup.7 per .degree.C as compared with
100.times.10.sup.-.sup.7 to 110.times.10.sup.-.sup.7 per .degree.C
at room temperature. These relative coefficients of thermal
expansion establish a compressive force between the dielectric
layer 12 and the ferrite substrate 10 to improve the mechanical
strength of the glass.
In accordance with another aspect of the invention, the dielectric
layer 12 includes aluminum oxide (Al.sub.2 O.sub.3) which provides
a rough outer surface to permit adhesion of the metal plating 14,
e.g., silver, to the dielectric layer 12. Aluminum oxide is
particularly desirable since it too has a coefficient of thermal
expansion which is less than the ferrite substrate so as not to
adversely affect the compressive force between the dielectric layer
12 and the ferrite substrate 10.
In accordance with another aspect of the invention, the dielectric
layer 12 is uniformly applied to the substrate 10 without any pin
holes through the layer 12. This is accomplished by ball milling
the glass for some 16 hours and then passing the glass through a
200 mesh screen. This is followed by the electrophoretic deposition
of the glass and aluminum oxide using a slurry made with an organic
solvent which will not adversely affect the glass components of the
slurry to any appreciable degree. Ethyl acetate has been found to
be particularly well suited for use as the organic solvent when
utilized in accordance with the electrophoretic deposition process
described in the Senderoff et al. U.S. Pat. No. 2,843,541. The
filter is then fired in a suitable atmosphere such as nitrogen,
argon or air and at a temperature sufficient to fire the glass
particles into a smooth dense layer. However, the firing
temperature must not be too high to avoid damaging the ferrite.
Although a ferrite substrate has been described, it will be
understood that other high magnetic permeability, resistive
substrates may be utilized. For example, a doped semiconductive
ceramic material might be utilized as disclosed in the aforesaid
application Ser. No. 88,042 which is incorporated herein by
reference.
The electrical characteristics of a filter constructed in
accordance with this invention compare quite favorably with the
excellent electrical characteristics of the filter described in the
aforesaid Fritz application Ser. No. 88,042. For example, the
filter of this invention exhibits an attenuation of 20dB at 915
MH.sub.z which is more than adequate to prevent any microwave
radiation hazard due to the propogation of microwave energy from
the magnetron filament back to the power supply line.
The filter of FIGS. 1 and 2 will now be described as part of a
source of microwave energy which is particularly well suited for
use in a microwave oven. As shown in FIG. 3, a system comprises a
magnetron 26 with a high voltage input circuit 28 and a filament
30. The filament 30 is supplied by a filament transformer 32
through a low pass RF filter means 34 constructed in accordance
with this invention. The filter means 34 which is a distributed
impedance filter is so illustrated.
It will be understood that each line 36 of the filament 30 employs
a filter assembly such as that shown in FIG. 2. In other words, a
pin 22 of the assembly shown in FIG. 2 is connected between each
line 36 and one terminal of the filament transformer 32. It will
also be understood that ground as shown in FIG. 3 is established by
the ground plane 24 as shown in FIG. 2.
Although the filter described above is particularly useful for high
voltage feed-through to the magnetron filament in a microwave oven,
other high voltage feed through applications will occur to those of
ordinary skill in the art.
It will be understood that, even though a particular embodiment of
the invention has been shown and described, various modifications
may be made without departing from the spirit and scope of the
invention as set forth in the appended claims.
* * * * *