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.

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.

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.