Background of the Invention
1. Field of the Invention
[0001] This invention relates to a filter connector for reducing electromagnetic interference
in electrical devices. More particularly, it refers to a filter connector having a
series of thick film capacitors with holes within the various elements of the capacitors,
each accommodating an electrically conductive pin and attenuating various frequencies
applied to the pin.
2. Background of the Invention
[0002] Filter connectors for attenuating high frequency interference from electrical devices
are well known from several patents; e.g., U.S. Patent 3,538,464, U.S. 4,126,840,
U.S. 4,144,509 and U.S. 4,187,481. In each of these patents, a capacitor employed
with the filter is a series of ceramic layers forming a monolithic structure. Thick
film capacitors are also well known from U.S. Patent 4,274,124. Although monolithic
capacitors are currently used in filter connectors, it has not been practical heretofore
to substitute thick film capacitors such as shown in U.S. 4,274,124 for these monolithic
capacitors. Problems have occurred in designing a thick film capacitor for a filter
connector which has a low enough inductance to attenuate high frequencies.
[0003] In recent years, the common usage of computers and particularly home computers has
resulted in the generation of significant additional amounts of high frequency electromagnetic
signals interfering with other electrical devices. For the purpose of reducing the
output of such signals, the United States Federal Communications Commission (
FCC) has promulgated regulations requiring attenuation at their source. See 47 CFR
15, Subpart J.
[0004] Available monolithic capacitor structures used in filters are not cost effective
for use in low-cost electronic equipment such as the personal computer. Since the
cost of producing a filter connector can be greatly diminished by using thick - film
capacitors, a filter connector employing such a thick film capacitor with a low inductance
is needed. A useful commercial filter attenuates the electromagnetic signal at least
30 decibels (dB) at a 1000 megahertz (MHz) frequency.
Sumnary of the Invention
[0005] This invention is a cost effective electrical filter connector for filtering a wide
band -of frequencies up to 1000 MHz using a particular design of thick film capacitor
in repeating sequence to form the filter element. The filter element comprises a multiplicity
of closely spaced thick film capacitors, each one having a conductive pin mounted
in a hole through a capacitor. The capacitor has multiple layers of screen printed
materials over an alumina substrate having two horizontal surfaces and which is generally
rectangular in shape. One layer is a metallization forming a ground electrode. This
electrode is grounded to the connector housing. It substantially covers an entire
horizontal surface of the alumina substrate and has holes sufficient in size to accommodate
the conductive pins but without touching any of the pins.
[0006] Another layer is a metallization forming a pin electrode, but its area is limited
to a portion around a given hole in the substrate. This layer is in electrical contact
with the pin through a solder joint. In between the two electrodes is a layer, dielectric
in nature, applied directly over one of the electrodes. This layer substantially overlaps
a horizontal surface of the ground electrode when it is the first layer but allows
the two longest edges on each side of the ground electrode to remain exposed. This
layer also has holes barely sufficient to allow conductive pins to pass through without
touching the dielectric material. The dielectric material also covers the vertical
surface of the ground electrode which is nearest each hole.
[0007] A fourth and last layer is a nonconductive encapsulant for excluding moisture covering
all layers except electrical contacting or soldering areas. This filter connector
maintains a substantial attenuation in the ultra high frequency range up to at least
1000 MHz.
Brief Description of the Drawings
[0008] The present invention may be best understood by those having ordinary skill in the
art by reference to the following detailed description when considered in conjunction
with the accompanying drawings in which:
FIG. 1 is an isometric view of an assembly, partially sectioned, of the filter connector;
FIG. 2 is a partial elevational view of the filter connector in section;
FIG. 3 is a transverse sectional view along lines 3-3 of the filter connector of FIG.
1;
FIG. 4 is a section through a single capacitor unit of a filter element assembled
to a pin;
FIG. 5 is an exploded view of a filter element containing multiple capacitor units
shown in FIG. 4;
FIG. 6 is a perspective view of the filter element member shown in FIG. 5;
FIG. 7 is a magnified view in cross section along lines 7-7 of FIG. 6;
FIG. 8 is a'partial sectional view of the filter connector having a ferrite sleeve
around each pin; and
FIG. 9 is a graph showing an attentuation curve for a filter connector where the ground
electrode does not cover the substrate compared with one shown in FIGS. 1-7.
DESCRIPTION OF THE INVENTION
[0009] Referring first to FIG. 1, filter connector 8 comprises a housing 10 having a top
shell 12 and a bottom shell 14. Housing 10 encloses two rows of pins 18 mounted on
a filter member 16. The interior of connector 8 is protected by a top insulator 20
and a bottom insulator 38. Pins 18 are individually mounted on filter element 16 by
solder joints 22.
[0010] Threaded insert 28 can be included in the connector optionally to provide a mounting
fixture to a cabinet. Ground contacts 32 are made available on the top shell 12 to
provide a ground contact for a female plug (not shown) inserted over the pins 18.
The two shells 12 and 14 are crimped together by a tab 40. Pins 18 can be either straight
or right-angled 34 as shown in FIGS. 1-3. FIGS. 2-4 show the solder joints 22 where
the pin 18 is attached to the filter element 16. Holes 31 in the bottom insulator
38 provide bottom exit for pins 18. Hole 30 in the filter member 16 provides the means
for passage of pins 18 through the filter member and the location of solder joint
22.
[0011] The structure of filter element 16 is seen by reference to FIGS. 4 and 5. FIG. 4
shows only one capacitor unit within the filter element 16 for illustration purposes.
The filter element comprises an alumina substrate 42 which has screen printed on one
horizontal surface-a metallization 44. This metallization forms a ground electrode
that is subsequently soldered 36 to the shell 14. The ground electrode covers substantially
the entire surface of the alumina substrate 42. It has holes 24, seen in FIG. 5, which
are large enough to accommodate the pins 18 without touching the pins.
[0012] The ground electrode 44 is partially covered by a screen printed layer of dielectric
46. For purposes of this specification, a single layer of dielectric is mentioned
although in practice two layers of dielectric 46 and 48 are screen printed over the
ground electrode to provide more than adequate protection against shorting between
electrodes. As seen in FIG. 5, the dielectric layer 46/48 also has holes 26 which
are slightly larger than the diameter of the pins 18. The dielectric 46/48 covers
the horizontal surface of the electrode 44 except for the edges 43 and 45 which are
soldering areas used for the ground to the shell 14. The dielectric 46/48 also is.
applied on the vertical edge of the ground electrode 44 which is contiguous with the
holes 24 as seen in FIG. 4.
[0013] A second metallization layer 50.is screen printed intermittently in a regular pattern
usually arrowhead shaped over, the dielectric layer. This forms a series of pin electrodes
50, each of which is in electrical contact with a pin 18 through solder joint 22.
This electrode is screen printed in such a manner as to form a series of discrete
spaced apart arrowhead-shaped layers distributed over the surface of dielectric 46/48
as seen in FIGS. 5 and 6. There is one electrode 50 contiguous with each hole 26 and
also annularly surrounding the holes 41. The last layer, glass encapsulant 52/54,
covers both the electrodes 50 and dielectric 46/48. Although only one layer is shown
in FIG. 5, in practice two layers of encapsulant are usually screen printed over the
electrode 50 for added safety. For purposes of this specification, when talking about
a layer of encapsulant, one or more layers of encapsulant is meant. The arrowhead
design of the electrode 50 provides a means for closely spacing the capacitors used
in the filter connector and, hence, increasing the area of the capacitor and therefore
its capacitance value. Of course, other designs could be used which satisfy the purpose
of producing capacitors of the type employed in this invention.
[0014] It is preferred that the metallizations used in layers one and three be a noble metal
or an alloy of a noble metal. However, copper metallization compositions could be
employed. Particularly preferred is a palladium/silver alloy metallization. Each layer
is applied using conventional screen printing methods. The dielectric employed can
be any type commonly used in capacitors. However, barium titanate is preferred.
[0015] The glass encapsulant can be any one of the types used in capacitors having a coefficient
of" expansion compatible with the other components employed.
[0016] A ferrite sleeve 19 also can be attached to the pin 18, as seen in FIG. 8._ Such
sleeves are well known as seen in U.S. Patent 4,144,509. The use of the particular
filter member of this invention will increase the filtering action of filter connectors
employing ferrites.
[0017] Metallizations used in this.invention are made from compositions containing a finely
divided metal powder of either a noble metal or copper, a binder for the metal and
a vehicle to disperse the powders evenly. The composition is applied by screen printing
methods and the vehicle is removed from the applied composition by firing the screened
on layer by conventional techniques.
[0018] Although the drawings FIGS. 4-5 depict the ground electrode 44 as being.applied as
the first metallization layer and the pin electrode 50 as the third layer, this can
be reversed. Therefore,.pin electrode 50 can be screen printed directly to the alumina
42 around each hole 41. The layers 46 and 48 are then applied to overlap the layer
50 except for the solder area 22. The ground electrode 44 would then be screen printed
over the layers 46 and 48 and all exposed horizontal surfaces of the alumina substrate
42. The encapsulant 52/54 is applied in the same manner as in FIG. 4. The encapsulant
covers all exposed surfaces except for edges 43 and 45 which are solder areas.
[0019] The low inductance at high frequencies achieved by this invention is a direct result
of the geometry of the ground electrode as related to the pin electrode. If the ground
electrode and dielectric are placed only to one side of the pin, the attenuation curve
(a) of FIG. 9 results. This curve shows a reduced attenuation and hence reduced filtering
action in the ultra high frequency range, particularly above 200 MHz and more particularly
above 700 MHz. The reason for this reduced attenuation is that the capacitor has a
series resonance around 200 MHz (shown by the sharp peak in curve (a)) caused by the
inductance of the electrodes of the capacitor.
[0020] When the ground electrode extends substantially over the'entire substrate and the
dielectric surrounds the hole, the current flow from the pin can divide into two components,
each flowing toward a ground connection on each side of the filter element 16. This
results in a decreased effective electrode inductance by providing two parallel current
paths. The decreased inductance results in an increased series resonant frequency
and an increased attenuation such as is shown in curve (b) of FIG. 9.
1. In an electrical filter connector for attenuating electromagnetic interference
having a housing, a filter element enclosed within the housing . and electrically
conductive pins mounted within the filter element, the improvement whereby the filter
element comprises a multiplicity of closely spaced thick film capacitors formed by
screen printing multiple layers over an alumina substrate having two flat horizontal
surfaces containing holes and electrically conductive pins mounted thereon, one layer
being a thick film metallization forming a ground electrode in electrical contact
with the connector housing and substantially covering one horizontal surface of the
substrate and having holes within the ground electrode sufficient in diameter to allow
the conductive pins to pass without touching the electrode.
2. A filter connector according to claim 1 wherein the ground electrode layer is the
first layer applied to the substrate, a second layer being an insulating dielectric
material applied over the ground electrode at least in the area surrounding each substrate
hole but exclusive of an electrical contacting area and a third layer being a thick
film metallization forming a discrete pin electrode applied over the second layer
in the area surrounding each substrate hole thereby being in electrical contact with
a pin and insulated from the ground electrode.
3. A filter connector according to claim 2 wherein a fourth layer is a nonconducting
encapsulant having a compatible coefficient of expansion covering all exposed layers
exclusive of electrical contacting areas.
4. A filter connector according to claim 3 wherein the first layer of the filter element
is a. noble metal metallization.
5. A filter connector according to claim 3 . wherein the first layer of the filter
element is a palladium/silver alloy metallization.
6. A filter connector according to claim 3 wherein the third layer of the filter element
is a noble metal metallization.
7. A filter connector according to claim 3 wherein the third layer of the filter element
is a palladium/silver alloy metallization.
8. A filter connector according to claim 3 wherein the first layer of the filter element
is a copper metallization.
9. A filter connector according to claim 3 wherein the third layer of the filter element
is a copper metallization.
10. A filter connector according to claim 2 . wherein the third layer metallization
is in the shape of an arrowhead.
11. A filter connector according to claim 1 wherein a ferrite sleeve encloses each
conductive pin.
12. In an electrical filter connector for attenuating electromagnetic interference
having a housing, a filter element enclosed within the housing and electrically conductive
pins mounted within the filter element, the improvement whereby the filter element
comprises a multiplicity.of closely spaced thick film capacitors, each one accommodating
a pin within holes in an alumina substrate having two flat horizontal surfaces and
having multiple layers screen printed over the substrate, a first layer being a noble
metal metallization forming an electrode grounded to the connectot housing and substantially
covering one horizontal surface of the substrate, the first layer having holes therein
sufficient in diameter to allow the conductive pins to pass without touching the first
layer, a second layer being a dielectric insulat-ing material, the second layer substantially
covering the first layer except for exterior electrical contacting areas and annularly
overlapping the first layer around each hole, a third layer being a metallization
forming a discrete pin electrode surrounding each substrate hole and applied to annularly
overlap the second layer, and a fourth layer being a nonconducting encapsulant having
a coefficient of expansion compatible with the other layers together with substrate
and covering all exposed layers exclusive of electrical contacting areas.
13. A filter connector according to claim 12 wherein the pin electrode metallization
extends within and adheres to each substrate hole.
14. A filter connector according to claim 1 wherein the ground electrode is a third
layer applied over a second layer and substantially all the horizontal surface of
the substrate, a first layer being a metallization forming a discrete pin electrode
applied_over the substrate to surround each substrate hole and the second layer being
a dielectric material applied over each pin electrode.
15. A filter connector according to claim 14 wherein a fourth layer is a nonconducting
encapsulant having a coefficient of expansion compatible with the other layers together
with substrate and covering all exposed layers exclusive of electrical contacting
areas.
16. A filter connector according to claim 15 wherein the first layer of the filter
element is a noble metal metallization.
17. A filter connector according to claim 15 wherein the first layer of the filter
element is a palladium/silver alloy metallization.
18. A filter connector according to claim 15 wherein the third layer of the filter
element is a noble metal metallization.
19. A filter connector according to claim 15 wherein the third layer of the filter
element is a palladium/silver alloy metallization.
20. A filter connector according to claim 15 wherein the first layer of the filter
element is a copper metallization.
21. A filter connector according to claim 15 wherein the third layer of the filter
element is a copper metallization.
22. A filter connector according to claim 14 wherein the first layer metallization
is in the shape of an arrowhead.
23. In an electrical filter connector for attenuating electromagnetic interference
having a housing, a filter element enclosed within the housing and electrically conductive
pins mounted within the filter element, the improvement whereby the filter element
comprises a multiplicity of closely spaced ' thick film capacitors, each capacitor
accommodating a pin within holes in an alumina substrate having two flat horizontal
surfaces and having multiple layers screen printed over the substrate, a first layer
being a noble metal metallization forming a discrete pin electrode applied over the
substrate around and within each hole, the first layer being in electrical contact
with a conductive pin passing through the substrate hole, a second layer being a dielectric
insulating material overlapping the first layer except for electrical contacting areas,
a third layer being a noble metal metallization forming a ground electrode and overlapping
the second layer and substantially all of one horizontal surface of the substrate,
and a fourth layer being a nonconducting encapsulant having a coefficient of expansion
compatible with the other layers-together with substrate and covering all exposed
layers exclusive of electrical contacting areas.
24. A filter connector according to claim 23 wherein the pin electrode metallization
extends within and adheres to each substrate hole.
25. A filter connector according to claim 1 wherein an electrical connection is made
between the ground electrode and the conductive housing on one horizontal surface
of the substrate.