Miniature broad band low pass filters with multiturn tape wound inductors

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present application relates generally to the field of miniature filters for broadband high frequency low pass applications.

permeability and Q is the quality factor. The value defined previously drops very little even at higher frequencies, up to about 50 MHz., after which it begins to decrease more rapidly. Such performance was unobtainable with previously used ⁵ laminated metal or ferrite cores. It should be noted that if a high permeability is needed only at lower frequencies, the thickness of the tape can be as great as 0.001 inch or more. On the other hand, if the high permeability must be maintained at j θ higher frequencies, the thinnest tape obtainable today which is about 0.00004 inch thick may be used. Various configurations of invented filters (e.g. L-type, pi-type, T-type, double L-type, double Pi-type, double T-type etc.) can be made utilizing separate ceramic capacitors and the multiwound tape wound 15 ^core inductors taught by the inventive concept disclosed herein.

DESCRIPTION OF THE DRAWINGS

II. Description of the Prior Art

In the past high frequency low pass filters have been constructed utilizing inductors having laminated or pulverized ferromagnetic metal cores (dust cores) or ferrite cores. Many of the filter structures also used ceramic capacitors in combination with the core structures previously mentioned. However, 25 the core materials and core structures had certain performance deficiencies and disadvantages which greatly limited their use in high frequency low pass filter applications.

For example, laminated core structures made of obtainable lamina thicknesses exhibit poor frequency response charac- 30 teristics and their permeability generally starts to diminish quite drastically at frequencies of about 0.1 MHz. which is in frequency range where high inductances are normally required for high frequency low pass filter applications.

If it were possible to make laminated cores of metal laminae 35 of the same thickness as the thickness of the tape, the frequency dependence of the permeability would be substantially the same. However, no practical method for making laminated ring stack cores is presently known. For example, to obtain a 0.3-inch long core would require approximately 2,400 rings or each 0.000125 inch in thickness. The presently known techniques would make fabrication of such a core awkward and completely uneconomical. As another example, dust cores either exhibit a permeability of about 500 in the low kilohertz ranges, but the permeability decreases sharply at about 50 kHz. or if operation is required in megahertz frequency range the permeability of the core is limited to below about 100.

While ferrite cores have reasonably high permeabilities (in the range of about 3,000 and above) up to the few hundred ⁵⁰kilohertz ranges, their saturation induction is quite low (e.g. 3,000 to 5,000 Gauss). Therefore, the permeability of a miniature multitum ferrite may drop to about 10 at a field strength of about 3 Oersted which may correspond to a current as low as 0.01 amperes, for example. Generally, broadband low pass filters are used in circuit applications where certain amounts of DC or low frequency current must pass. The effectiveness of the prior art filters is usually completely diminished by current bias when in order to obtain miniaturized filters the prior art type of inductor cores (e.g. ferrites) of higher permeabilities are employed.

SUMMARY OF THE INVENTION

The present invention relates to the use of multiturn tape 65 wound inductors for use in low pass filter applications. It has been found that the attenuation characteristics of filters using multitum tape wound inductors are such that there is no appreciable change even when fairly substantial amounts of DC or low frequency current are applied to the filter. It has been 70 found that the permeability and especially the value μ^/l-|—L which is generally considered to be the most important inductor characteristic regarding filters is very stable up to the megahertz ranges if thin tapes, (e.g. 0.000125 inch thick) are 75 wound and used. In the above recited formula, μ stands for the

FIG. 1 is a cross-sectional end view of a tape wound core having a plurality of turns wound therethrough;

FIG. 2 is a cross-sectional, front elevational view which illustrates an L-type filter utilizing a multiturn tape wound core inductor;

FIG. 2α is a schematic diagram of the filter shown in FIG. 2;

FIG. 3 is an enlarged cross-sectional view of one form of a multilayer ceramic capacitor which may be utilized in the invention and is shown also in FIG. 2;

FIG. 4 is a cross-sectional front elevation view which illustrates another embodiment of the invented filter in which multi¹¹¹¹·¹¹ tape wound core inductors are used in a pi-type filter;

FIG. 4α is a schematic diagram of the filter shown in FIG. 4; and

FIG. 5 is a cross-sectional front elevation view illustrating an L-section feedthrough filter having a multiturn tape wound core and a single ceramic capacitor envelope.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It has been found that for DC or low frequency current (e.g. 60 to 400 c.p.s.) carrying miniature low pass filters, multiturn tape wound cores yield superior performance compared with any of the filters using inductors made according to presently known techniques.

Tape wound core inductors for use in filters were disclosed in U.S. Pat. Ser. No. 3,456,215. In this patent single turn tape wound cores were employed in pi filters, inside of a ceramic double-capacitor. Another copending application of mine (Ser. No. 831,142, filed June 6, 1969) entitled “High Frequency Low Pass Filter with Embedded Electrode Structure, discloses the use of Multiturn tape wound core inductors inside of the double-capacitor of a pi filter.

The present invention utilizes filters with individual ceramic capacitors and hollow multitum tape wound core inductors which make possible the building of broad band high frequency low pass filters of various types, for example, L-type, pitype, T-type, double L-type, double pi-type, double T-tyne etc. ’

In theory, the tape material for the core may be any ferromagnetic metal or alloy. However, in practice, those magnetic materials are chosen which have a permeability suitable to the required characteristics of the filter, and exhibit as high a saturation induction as possible. The thickness of the material will generally be determined by the desired permeability versus frequency performance. While a laminated core using 0.004 inch thick laminates (about the lowest practical limit) may start to drop its permeability rapidly from 20 kHz. upwards, a tape wound core e.g. with a tape thickness 0.000125 inch would roughly maintain the original permeability up to about 2 MHz. Iron, cobalt, and iron-cobalt alloys are especially favored materials because their saturation induction is about 21,000 Gauss or higher. However, for special purposes, other known alloys, like e.g., iron-nickel, iron-cobalt-nickel, iron-nickel-molybdenum, etc. would also be employed. If desired nonferromagnetic metal may be alloyed with fer3,603,902 romagnetic materials of the general type previously mentioned. The addition of the nonferromagnetic material may allow improvements of certain inductor properties. For example, the permeability may be increased and/or the Q factor increased or diminished depending on alloys used and the composition required to obtain the desired results. FIG. 1 shows a hollow tap wound core 10 formed from a ferromagnetic tape 11 (e.g. the materials description above) with an insulation layer 12 (e.g. a suitable binder of epoxy resin or a very thin, e.g. 0.000002 inch thick, oxide layer) between the layers of tape 11. A single length of wire W is wound through the core and around the outer surface thereof to provide a multiturn structure as shown in FIG. 1. As will be discussed below the type of tape material and the number of wire turns can be selected to suit the various application requirements for inductors in broadband low pass filters.

Ceramic capacitors which may be utilized in these filters are known per se in the art and are not a part of this invention. While in principle the capacitors can be a single ceramic tubular or discoidal capacitor generally a plurality of such tubes or discs are connected in parallel to obtain the needed capacitance values. For higher voltages, often tube clusters or disc stacks may be built up from single self-sustaining capacitors. In practice, generally multilayer ceramic capacitors are used which may be in the form of ceramic rolled capacitors, multidisc capacitors or multitubular capacitors as described in my copending application entitled “High Frequency Low Pass Filter with Embedded Electrode Structure,” filed June 6, 1969 (our File PD-1072-387 CIP(A)).

In order to illustrate the superior performance characteristics obtained by using the inventive concept the following structural examples are presented.

EXAMPLE I

With reference to FIG. 2, an L-type filter 13 is shown having a standard enclosure 14, a tape wound core inductor 10 (as described previously) and a ceramic multilayer capacitor 15. Typically, the enclosure 14 and parts of the enclosure may be made from metal such as steel or copper ad has openings 16 and 17 therein. The tape wound core inductor 10 is positioned so that the ends W, and W₂ of the wire W which is wound around and through the inductor 10 pass trough openings 16 and 17 insulated from parts of the enclosure and form external terminals for circuit connections. The wire end W₂ is also connected at point 18 to the capacitor 15 at the electrode connecting washer 19. The capacitor 15 is typically formed by a number of parallel embedded electrodes 20 and 21 (e.g. palladium or other noble metals) which are insulated from each other by ceramic layers C. One set of parallel electrodes 20 are the plate forming electrodes of the capacitor and are connected together by outer conductive layers 20α and 20ft to the electrode connecting washer 19 for connection to the wire end W₂ of the inductor. (See FIG. 3 The outer set of parallel electrodes 21 are the ground forming electrodes which are connected together by outer conductive layers 21α and 21ft to the electrode connecting washer 22 for connection to the enclosure 14. The enclosure 14 is generally at some base potential such as ground potential. The washer 22 is generally joined to or in contact with the casing at point 23. The electrodes 20 and 21 are physically and electrically insulated from each other as are their respective connecting conductive layers 20α, 20ft, and 21α and 21ft, respectively, as shown in FIG. 3. The wire end W₁ passes through an opening 22α in the connecting washer 22 without making any physical or electrical contact with washer 22.

Normally, the filter 13 is hermetically sealed at points 224 and 25 using known techniques and the internal spaces between the components of the filter in the interior of the enclosure 14 are encapsulated with a suitable potting material 26 such as an epoxy material or silicone rubber. The hermetic sealing and potting of the filter give it better mechanical and vibration resistant characteristics. A schematic diagram of the filter structure of FIG. 2 is shown in FIG. 2α, using the structure shown in FIG. 2.

By way of example, a miniature 50 VDC L-type filter having the structure shown in FIG. 2 can be built to obtain the improved operating characteristics previously described. The capacitor 15 would have a capacitance of 1 mfd. and the tape wound core 10 may be made of “Armco iron,” a product of Armco Steel Co. The tape 11 of which the core 10 is formed should be about 0.00025 inch thick. The initial permeability of this core material is approximately 1,000 up to 1 MHz. and the saturation induction is about 21,500 Gauss. The core 10 is wound with 25 turns of wire W and its inductance should be about 500 microhenries. The attenuation of the filter 13 should be 25 db. at 30 KHz., 45 db. at 100 kHz. and 85 db. at 1 MHz. Utilizing the multiturn tape wound core described hereinabove, in the filter structure shown in FIG. 2, the attenuation figures previously recited should not drop appreciably even with DC or low frequency (60 to 400 c.p.s.) tape 11. loads of up to W amps. A 50 VDC L-type filter 13 as described could have a cylindrical enclosure 14 which would measure only 0.3 inch in diameter and 0.5inch in length.

EXAMPLE II

With reference to FIG. 4, a pi-type filter 30 is shown having a standard enclosure 31, a tape wound core inductor 10' (of similar structure to that previously described) and two multilayer ceramic capacitors 15' and 15. The enclosure 31 can be of a similar type to that described for enclosure 14. The tape wound core 10' is positioned so that the ends W/ and W₂' of the wire W' which is wound around and through the inductor 10' pass through the openings 32 and 33 and form external terminals for circuit connections. The wire ends W/ and W₂' are also connected at points 34 and 35 to the capacitors 15' and 15, respectively, at the electrode connecting washers 36 and 37. The capacitors 15' and 15 may be formed by a number of parallel electrodes 20' and 21' in capacitor 15' and 20 and 21 in capacitor 15. The various set of capacitor electrodes are insulated from each other by layers of ceramic material C' and C, respectively. One set of parallel electrodes 20' and 20 are plate forming electrodes of the capacitors 15' and 15, respectively, and are connected by outer conductive layers 20α' and 20α to electrode connecting washers 34 and 35 for connection to the wire ends W and W₂'. The other set of parallel electrodes 21' and 21 are ground forming electrodes of the capacitors 15' and 15, respectively, which are connected together by outer conductive layers 21α' and 21α to electrode connecting washer 38 and 39 for connection to the enclosure 31. The enclosure 31 is generally at some base potential such as ground potential. The washers 38 and 39 are typically joined to or in contact with the casing at points 40 and 41. The capacitors shown in FIG. 4 are slightly different in configuration from those shown in FIGS. 2 and 3 (e.g. the electrode orientation) but operate in substantially the same manner. Either configuration may be used to obtain excellent results in the various filters described.

The wire ends W,' and W₂' pass through the openings 38α and 39α in the connecting washers 38 and 39, respectively, without making any physical or electrical contact with the washers 38 and 39.

Normally, the filter 30 is hermetically sealed at points 42 and 43 using known techniques and the internal spaces between the components of the filter in the interior of the enclosure 31 are encapsulated with a suitable potting material 44 such as an epoxy material or silicone rubber. A schematic diagram of the filter structure of FIG. 4 is shown in FIG. 4α .

Using the structure shown in FIG. 4, a 100 v. DC miniature pi-type filter can be built as described below. The multitubular capacitors 15' and 15 each would have capacitance of 1 mfd. The tape wound core 10' may be made of a 50 percent iron and 50 percent cobalt containing alloy. The thickness of the tape used should decrease about 0.001 inch. The initial permeability of this core 10' is approximately 800 up to 4

3,603,902 □

MHz., the saturation induction is about 24,500 Gauss. The core 10' is then wound with 15 turns of wire W'. The inductance should be about 160 microhenries. The insertion loss of the filter 30, measured according to MIL-STD-220 A., should be 28 db. at 100 kHz. and 100 db. about 500 kHz. The insertion loss will not decrease with a through flowing DC or low frequency current (60 to 400 c.p.s.) of 1.5 amps. The miniature pi-type filter 30 described may be mounted in a cylindrical enclosure 31 as small as 0.32 inches in diameter and 0.65 inches in length. As a comparison, if a filter of the same performance and size were built with a ferrite core, a DC or low frequency current higher than 0.2 amps would start to appreciably decrease its insertion loss.

The structure shown in FIGS. 2 and 4 are illustrative of two types of filters which may be formed using the same basic type of multitum tape wound inductor in combination with at least one ceramic capacitor. More than one inductor may be used for example in a T-type filter. Also, a plurality of inductors and capacitors may be used for example in a double-L-type filter. The various structures for these and other low pass filters in accordance with the teachings described and taught for the filters in FIGS. 2 and 4 would be apparent to those skilled in the art. The following configurations of filters using the inventive concept may also be built to give superior results when compared with any of the know filters of similar size using the prior art inductor cores previously described.

EXAMPLE III

A miniature T-type filter may be made with a 50 v. DC multilayer capacitor of 1 mfd. and two identical multitum tape wound core inductors. Each inductor could have a tape wound core fabricated of a 65 percent iron and 35percent cobalt from a tape 0.00025 inch thick. The initial permeability should be about 300 and the saturation induction about 23,500 Gauss. The winding of the core would have 30 turns and an inductance of about 240 microhenries. The insertion loss of the filter should be 24 db. at 30 kHz. 55 db. at 100 kHz. and more than 90 db. above 500 kHz. when measured according to MIL-STD-220A . The insertion loss will not decrease when a 2amp. DC or low frequency current (60 to 400 c.p.s.) flows through the filter. The size of the enclosure for this filter would be as small as 0.3 inch in diameter and 0.7 inch in length.

EXAMPLE IV

A miniature 50 V. DC double-L-type filter may be constructed with two ceramic multilayer capacitors, each 1 mfd., and two identical multiturn tape wound core inductors. The tape may be 0.00025 inch thick and its material may be an alloy composed of 45 percent nickel and 55 percent iron. The initial permeability of this alloy is approximately 3,500, but its saturation induction is only 16,000, which is still much much higher than that of ferrites. The core is wound with 20 turns and the induction should be approximately 1,200 microhenries. The attenuation of this filter should be 65 db. at 30 kHz. and more than 100 db. above 100 kHz. The size of the enclosure for such a filter can be as small as 0.3 inch in diameter and 1 inch in length. The maximum DC or low frequency current (60 to 400 c.p.s.) which would not lower the attenuation is only 0.1 amp. in this embodiment. As a comparison, however, if a filter of the same performance and same size would have been built with a ferrite core, the maximum DC or low frequency current not lowering the attenuation would be only approximately 0.025 amp. Further, if a filter of the same attenuation performance had been built with a known ferrite inductor core in order to carry currents up to 0.1 amp. without appreciably decreasing the attenuation, the volume of the necessary filter enclosure would be approximately five times larger than the enclosure described hereinabove.

EXAMPLE V

An L-section feedthrough configuration filter as shown in FIG. 5 has been found to give similar performance to filters which were the smallest available prior to the filter embodiments taught by the present invention. However, the smallest prior art filters required volumes approximately twenty times greater than the volume of the filter shown in FIG. 5 and described hereinbelow.

The envelope of the filter 50 is a single multitubular capacitor 51 (ceramic) which can be soldered to a planar ground member 52 as a feedthrough unit. The capacitor 51 is cylindrical and is made of a plurality of electrodes embedded in a ceramic insulating material C'. The electrodes 53 may be ground forming electrodes and are interconnected by a conductive (e.g., metal) layer 54 which is placed in contact with the member 52. The member 52 may be made from any suitable conductive material, typically a metal such as copper or steel. The plate electrodes alternate with the ground electrodes 53 and 55 and are insulated from the latter electrodes by layers of the ceramic material C' located between the various plate and ground electrodes. The plate electrodes 55 are connected to each other by a conductive (e.g., metal) layer 56.

A hollow ferromagnetic tape wound core inductor 10'” of the structure shown in FIG. 1 and described above, is wound with turns of wire W. The wire W” is wound around and through the inductor 10”' and one end W, of the wire W” passes through an opening 52α in the member 52 to form an external connection point for the filter. The end of the wire W, is in electrical and physical contact with the member 52 at point 52b and also in contact with the ground electrodes 54. The end W₂ of the wire W is not in electrical contact with the capacitor 51 and forms a second external connection point. This inductor-capacitor structure forms an L-Section feedthrough filter since the inductor is in contact with only one side of the single capacitor as described above. A suitable potting material 57 may be used to encapsulate the components to keep them in position and to give good mechanical properties to the filter.

Using the structure shown in FIG. 5 a miniature L-section feedthrough filter may be made for example, as follows:

The inductor 10'” is a tape wound core made of a composition of 17 percent iron, 79 percent nickel and 4 percent molybdenum. Thickness of the tape is 0.000125”. The number of the turns of wire W is 4. The permeability of this material is about 15,000 at 150 kHz., which is still much higher than any other type of magnetic material, e.g. ferrite. However, the saturation induction is only 8,000 Gauss, hence the DC or low frequency current which will not influence the attenuation, is not as high as in the examples previously described above.

The dimensions of the filter 50 were only 0.120 inch in diameter and 0.4 inch in length. The ceramic capacitor 51 had a capacitance of 0.2 μ/F. and a working voltage of 50 v. DC. The inductance was about 120 microhenries. The attenuation at 0.15 MHz. was 25 db. and at 1 MHz. 50 db. Such filters yield a very dense multipackage for filtering a number of through going connections between miniature integrated subsystems, and yet give very good attenuation performance at the lower end of the frequency spectrum too.

Other miniature low pass filter configurations may be made within the scope and spirit of the invention by those skilled in the art. The specific configurations described in this specification are merely exemplary of the inventive concept and technique and are not to be considered as limitations of the invention.