Information
-
Patent Grant
-
6236287
-
Patent Number
6,236,287
-
Date Filed
Wednesday, May 12, 199925 years ago
-
Date Issued
Tuesday, May 22, 200123 years ago
-
Inventors
-
Original Assignees
-
Examiners
Agents
- Alkov; Leonard A.
- Lenzen, Jr.; Glenn H.
-
CPC
-
US Classifications
Field of Search
-
International Classifications
-
Abstract
A coaxial-to-microstrip vertical transition includes a dielectric substrate having formed on a first surface thereof a primary microstrip conductor trace, and on a second surface a secondary microstrip conductor trace. A first conductive via extends through the dielectric substrate and electrically connects the primary conductor trace to the secondary conductor trace. A second conductive via is spaced from the first conductive via and extends through the dielectric substrate to electrically connect the secondary conductor trace to the coaxial center conductor. A bottom microstrip ground plane layer is defined on the second substrate surface. A conductive base plate structure has a cavity formed therein, the substrate positioned such that the base plate structure is in contact with the bottom ground plane layer, and the secondary conductor trace is positioned over the cavity. The substrate is positioned between a cover structure and the base plate structure, the cover structure disposed in spaced relation with respect to the first surface of the substrate. A coaxial transmission line structure includes an outer shield and a coaxial center conductor structure disposed within the outer conductor and transverse to the substrate, the center conductor passed through an opening in the cover structure to contact the second via. A conductive plate structure is positioned between the plane of the cover structure and the substrate, providing shielding surrounding the center conductor between the cover and the substrate.
Description
TECHNICAL FIELD OF THE INVENTION
This invention relates to RF devices, and more particularly to a shielded coaxial to microstrip orthogonal launcher with multiple matching junctions for wideband microwave frequency operation with improved shielding and flexible routing of RF signals along the transmission line.
BACKGROUND OF THE INVENTION
There is a need in many RF systems to provide an orthogonal transition from a microstrip transmission line to a coaxial transmission line. A known technique of accomplishing this is to end launch a right angle coax connector onto microstrip along the substrate edge. Disadvantages of this approach include the relatively large space and volume requirements, and the requirement that the transition be made at the edge of the substrate.
It would therefore be an advantage to provide a transition technique which required less space, and offered the flexibility to vertically launch anywhere along the microstrip circuit board.
Another problem not addressed by known transition techniques between coaxial and microstrip transmission lines involves the issue of complete ground shielding the coaxial launcher as it contacts the microstrip center conductor vertically from an air dielectric side. In known techniques, the coaxial outer ground shield is partially removed to prevent that shield from short circuiting the microstrip center conductor. Exposing the coaxial section to air will result in RF leakage and the generation of the higher order waveguide modes, and thus degrades the RF performance when used at higher frequencies. Commercially available coaxial launchers are thus limited at the high frequency end to about 7 Ghz. Launchers for channelized microstrip transmission line described in U.S. Pat. No. 5,416,453 are limited at the high frequency end to about 14 Ghz.
SUMMARY OF THE INVENTION
A coaxial-to-microstrip vertical transition in accordance with this invention can operate at higher frequencies with better RF performance than what has been accomplished in the past. A coaxial-to-microstrip transition in accordance with an aspect of this invention is completely shielded with little possibility of leakage or generation of higher order waveguide modes at higher frequency. The transition incorporates matching junctions for improved performance, and a compressible center conductor to allow for blind mate connections.
In an exemplary embodiment, the coaxial-to-microstrip vertical transition includes a dielectric substrate having formed on a first surface thereof a primary microstrip conductor trace, and on a second surface a secondary microstrip conductor trace. A first conductive via extends through the dielectric substrate and electrically connects the primary conductor trace to the secondary conductor trace. A second conductive via is spaced from the first conductive via and extends through the dielectric substrate to electrically connect the secondary conductor trace to the coaxial center conductor. A bottom microstrip ground plane layer is defined on the second substrate surface. A conductive base plate structure has a cavity formed therein, the substrate positioned relative to the base plate structure such that the base plate structure is in contact with the bottom ground plane layer, and the secondary conductor trace is positioned over the cavity so that the secondary conductor is not in electrical contact with the base plate structure. A conductive cover structure is disposed such that the substrate is positioned between the cover structure and the base plate structure, the cover structure disposed in spaced relation with respect to the first surface of the substrate. The transition further includes a coaxial transmission line structure having an outer shield, a coaxial center conductor structure disposed within the outer conductor and transverse to the substrate, the center conductor passed through an opening in the cover structure to contact the second via. A conductive plate has an opening formed therein, and is positioned between the cover structure and the substrate, the plate providing shielding surrounding the center conductor in a space between the cover and the substrate.
BRIEF DESCRIPTION OF THE DRAWING
These and other features and advantages of the present invention will become more apparent from the following detailed description of an exemplary embodiment thereof, as illustrated in the accompanying drawings, in which:
FIG. 1
is a cross-sectional diagram illustrating a coaxial-to-microstrip transmission line transition in accordance with the invention.
FIG. 2A
is a top view of the top microstrip conductive layer pattern of the microstrip circuit board of the transition of FIG.
1
.
FIG. 2B
is a bottom view of the bottom microstrip conductive layer pattern of the circuit board.
FIG. 3
is an end cross-section view showing how the microstrip transmission line is “quasi-channeled” to control RF leakage and prevent the generation of higher order modes.
FIG. 4A
is a top view of a portion of an alternate embodiment of a transition substrate embodying the invention;
FIG. 4B
is a bottom view of this substrate.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A coaxial-to-microstrip vertical transition
50
is illustrated in cross-section in FIG.
1
. The transition is capable of operation from DC to 18 Ghz with low loss and excellent match. The microstrip transmission line
60
is provided by a dielectric substrate
62
having on an upper surface thereof a conductive layer pattern
64
defining a primary microstrip conductor
64
A, and on a lower surface thereof a bottom microstrip conductive layer pattern
66
. The relative thicknesses of the conductive layer patterns
64
and
66
in relation to the substrate thickness are exaggerated in the figures for illustrative purposes. In a typical implementation, the conductive layer patterns will be relatively thin, the patterns being defined by photolithographic techniques as is well known in the art.
The coaxial transmission line
80
in this exemplary embodiment includes a solid metal center conductor
82
disposed within an outer cylindrical conductor shield
84
. The outer periphery of the shield
84
is threaded in this embodiment to form a coaxial connector, to which a matching coaxial connector can be connected to make connection to a coaxial line.
In this exemplary embodiment, the microstrip circuit board comprising the substrate
62
, primary conductor
64
and ground plane
66
is installed in a metal housing
90
comprising a base plate
92
and a metal cover
94
. The base plate
92
has an air dielectric cavity
92
A formed therein in a region underlying the transition. The cover has a circular bore
94
A formed therein to receive the coaxial transition elements. A second air dielectric cavity
92
B is formed in the region between the substrate and the cover. A metal plate portion
94
B is disposed between the plate
92
and cover
94
. The metal plate portion
94
B has formed therein the cavity or bore
94
A, to form a shielded coaxial outer conductor in the region between the circuit board substrate and the plane of the cover plate. In this exemplary embodiment, the metal plate portion
94
B is formed as an integral structure with the cover
94
, although in another embodiment, the metal plate portion can be fabricated as a separate structure from the cover.
The top microstrip conductive layer pattern
64
is illustrated in
FIG. 2A
, and the bottom layer pattern
66
is illustrated in FIG.
2
B. The top layer pattern
64
defines the primary microstrip conductor line
64
A, which ends in a widened region
64
A
1
and a microstrip pad
64
A
2
for impedance matching. Region
64
A
3
is a clearout area in which the conductive layer is removed from the top surface of the substrate. The top layer pattern
64
further defines a top ground plane
64
B which extends about the primary conductor line
64
A and the clearout region
64
A
3
. Another clearout region
64
A
4
surrounds a connecting via
64
A
5
. Ground vias
68
extend through the substrate between the top layer pattern and bottom layer pattern.
As shown in
FIG. 2B
, the bottom layer pattern
66
defines the microstrip ground plane, provides a termination for the ground vias
68
, and defines a secondary microstrip transmission line
66
A
1
. A clearout region
66
A
2
surrounds the secondary line
66
A
1
. The secondary line
66
A
1
is widened at region
66
A
3
where the top and bottom cavities
92
A and
92
B (
FIG. 1
) overlap, to provide impedance matching. A connecting via
70
runs through the substrate between pad
66
A
4
and pad
66
A
2
(FIG.
2
A), to electrically connect the primary and secondary microstrip conductors. A conductive via
72
runs through the substrate, from pad
66
A
5
to pad
64
A
5
(
FIG. 2A
) in the top conductive layer pattern
64
, for connection between the secondary microstrip conductor and the interconnect structure
86
(
FIG. 1
) for the coaxial center conductor.
The space between the center conductor
82
and the outer shield
84
is filled by a dielectric spacer
88
A, which is a material such as TEFLON(™), for example.
The diameter of the center conductor
82
is stepped down from its diameter in the coaxial shield
84
to a smaller diameter at the plane of the top surface of the cover
94
, to improve the impedance match to the pad. The smaller diameter portion
82
A has a length equal to the thickness of the cover
94
in this exemplary embodiment. A compressible conductive interconnect structure
86
extends between the tip
82
B of the conductor
82
and the conductive pad
64
A
5
formed by the top conductor layer pattern
64
. The compressible interconnect structure
86
in this exemplary embodiment is a bundle of thin, densely-packed gold-plated wire. Other interconnect structures can alternatively be employed, e.g. a conductive bellows structure which is compressible, or a solid conductor which has a spring-loaded telescoping conductor pin extending from one end. Alternatively the coaxial center conductor tip can simply extend to the substrate pad, instead of using a compressible interconnect. This will result in more risk in creating a gap between the tip and the pad, due to manufacturing tolerances.
A dielectric spacer structure
88
B fills the cavity space between tip region
82
/interconnect
86
and the walls
94
A
1
of the plate
94
. This structure can also be fabricated of TEFLON(™) or other suitable dielectric material. The structure
88
B has formed therein a central bore which receives the tip
82
A and the interconnect structure
86
.
The transition structure
50
operates in the following manner. The coaxial center conductor
82
through the interconnect structure
86
contacts the pad
64
A
5
(
FIG. 2A
) on the circuit board which is connected to the secondary or “transition” microstrip center conductor
66
A
1
with a plated through hole formed in the substrate, defining the connecting via
72
. This transition or secondary microstrip line
66
A
1
is located on the one (bottom) side of the circuit board while the main or primary microstrip line comprising primary conductor
64
A is located on the opposite (top) side. The secondary microstrip line is then connected to the main microstrip by another plated through hole extending through the substrate defining the connecting via
70
. The corresponding groundplane for the primary and secondary microstrip lines are also connected by a series of plated through holes (ground vias
68
).
At the connecting vias
70
,
72
, the diameter of the microstrip pads
64
A
2
,
64
A
5
is designed to cancel out parasitic inductance contributed by the respective plated through vias. Also, the traces of both the primary and secondary microstrip conductor lines are intentionally widened at respective regions
64
A
1
,
66
A
3
to assure continuous
50
ohm characteristic impedance when the two lines enter the region where their respective air cavities
92
B,
92
A overlap.
The microstrip transmission line used in this invention is “quasi-channeled” as illustrated in the end cross-section view of
FIG. 3
to control RF leakage and prevent the generation of higher order modes. Thus, the cover
94
has a channel
94
C which defines the air channel
92
B, and side regions
94
D,
94
E which contact the upper ground plane regions
64
B. The circuit board
62
remains continuous at the sides beyond the air channel
92
B to preserve the board's stiffness and avoid the cost of board cutting along the channel route. Ground connection between the top cover
94
, the base plate
90
and the ground planes
64
B and
66
is accomplished through the use of ground vias
68
when the assembly is clamped together with screws (not shown).
The center conductor contact of the coaxial line onto the microstrip pad can use either a “hard” contact (such as a solder or conductive epoxy) or use a compressible center such as densely packed wire bundles (fuzz buttons), bellows or pogo pins. The use of the compressible center allows for re-usable blindmate connections.
The outer conductor ground shield formed by the metal plate portion
94
B is fabricated as an integral structure with the top cover
94
. The outer conductor of the coaxial transition is continuous with no cutouts or opening when it contacts the top ground plane of the secondary microstrip transition line. This metal ground contact can be accomplished by either pressure with or without the addition of RF gaskets to prevent RF leakage. Another method to realize a continuous metal ground contact between the coax and microstrip is with either conductive epoxy or solders.
Two connecting vias
70
,
72
transition the coaxial line to the primary microstrip without the need to open the outer conductor of the coaxial line, which would result in RF leakage into the air dielectric of the primary microstrip transmission line. These connecting vias are separated by the secondary microstrip line
66
A
1
at a distance designed to assure negligible mismatch interactions. The two connecting vias utilized in this invention allows additional degrees of freedom for routing since the primary and secondary microstrip lines do not necessarily have to be connected in a straight line but can run at varying angles with respect to each other along the circuit board as illustrated in
FIGS. 4A and 4B
.
FIG. 4A
illustrates the top surface of an alternate embodiment of a circuit board
60
′ comprising a transition in accordance with the invention, and
FIG. 4B
the bottom surface of the circuit board. Here, the primary microstrip conductor
64
A′ extends along a linear axis
641
, and the secondary microstrip conductor
66
A
1
′ extends along an axis
661
. In this exemplary embodiment, the primary line axis
641
extends at a 90 degree angle with respect to the secondary line axis
661
, although the angle need not be a right angle.
It is understood that the above-described embodiments are merely illustrative of the possible specific embodiments which may represent principles of the present invention. Other arrangements may readily be devised in accordance with these principles by those skilled in the art without departing from the scope and spirit of the invention.
Claims
- 1. A coaxial-to-microstrip vertical transition, comprising:a dielectric substrate having formed on a first surface thereof a primary microstrip conductor, and on a second surface a secondary microstrip conductor, wherein the primary microstrip conductor extends along a first linear axis, and the secondary microstrip conductor extends along a second linear axis, and wherein the first linear axis is not parallel to the second linear axis, such that the secondary microstrip conductor extends at an angle with respect to the primary microstrip conductor; a first conductive via extending through the dielectric substrate and electrically connecting the primary conductor to the secondary conductor; second conductive via spaced from the first conductive via and extending through the dielectric substrate to electrically connect the secondary conductor to a coaxial center conductor; a bottom microstrip ground plane layer defined on said second substrate surface; a conductive base plate structure having a cavity formed therein, the substrate positioned relative to the base plate structure such that the base plate structure is in contact with the bottom ground plane layer, and the secondary conductor is positioned over the cavity so that the secondary conductor is not in electrical contact with the base plate structure; a conductive cover structure disposed such that the substrate is positioned between the cover structure and the base plate structure, the cover structure disposed in spaced relation with respect to the first surface of the substrate; and a coaxial transmission line structure having an outer shield, said coaxial center conductor disposed within the outer conductor and transverse to the substrate, the coaxial center conductor passing through an opening in the cover structure to contact the second via.
- 2. The transition of claim 1, further comprising a conductive plate structure having an opening formed therein and positioned next to the cover structure, the plate providing shielding surrounding the coaxial center conductor in a space between the cover and the substrate.
- 3. The transition of claim 1 wherein the conductive cover structure has a channel defined therein, the channel defining a cavity through which the primary microstrip conductor extends, with conductive sidewalls providing side shielding of a primary microstrip transmission line comprising the primary microstrip conductor.
- 4. The transition of claim 1, wherein the coaxial center conductor includes a rigid solid conductor portion and a compressible contact structure positioned between a tip of the rigid solid center conductor portion and the second via.
- 5. The transition of claim 1, further comprising a plurality of conductive ground vias extending through the substrate between the first and second surfaces, the plurality of conductive ground vias positioned so as to contact the base plate structure and the cover structure.
- 6. The transition of claim 1, further comprising a contact pad formed on the first surface of the substrate in electrical contact with the second via, the center conductor structure in electrical contact with the first contact pad.
- 7. The transition of claim 1, wherein a portion of the outer shield of the coaxial transmission line structure extending from the cover structure is threaded.
- 8. The transition of claim 1 wherein the coaxial transmission line structure further includes a dielectric sleeve disposed in a space between the cover structure and the substrate, the sleeve surrounding the tip region of the center conductor.
- 9. The transition of claim 8 wherein the coaxial center conductor includes a compressible conductive contact structure disposed within the sleeve structure and positioned between the tip region and the second via.
- 10. A coaxial-to-microstrip vertical transition, comprising:a dielectric substrate having formed on a first surface thereof a primary microstrip conductor, and on a second surface a secondary microstrip conductor; a first conductive via extending through the dielectric substrate and electrically connecting the primary conductor to the secondary conductor; a second conductive via spaced from the first conductive via and extending through the dielectric substrate to electrically connect the secondary conductor to a coaxial center conductor; a bottom microstrip ground plane layer defined on said second substrate surface; a conductive base plate structure having a cavity formed therein, the substrate positioned relative to the base plate structure such that the base plate structure is in contact with the bottom ground plane layer, and the secondary conductor is positioned over the cavity so that the secondary conductor is not in electrical contact with the base plate structure; a conductive cover structure disposed such that the substrate is positioned between the cover structure and the base plate structure, the cover structure disposed in spaced relation with respect to the first surface of the substrate so that an air cavity is defined about the primary microstrip conductor; a coaxial transmission line structure having an outer conductor, said coaxial center conductor disposed within the outer conductor and transverse to the substrate, the center conductor structure passing through an opening in the cover structure to contact the second via, the center conductor structure including a rigid solid conductor portion and a compressible contact structure positioned between a tip of the rigid solid center conductor portion and the second via; and a conductive plate structure having an opening formed therein and positioned next to the cover structure, the plate providing shielding surrounding the center conductor in a space between the cover and the substrate.
- 11. The transition of claim 10, further comprising a plurality of conductive ground vias extending through the substrate between the first and second surfaces, the plurality of conductive ground vias positioned so as to contact the base plate structure and the cover structure.
- 12. The transition of claim 10, further comprising a contact pad formed on the first surface of the substrate in electrical contact with the second via, the center conductor structure in electrical contact with the first contact pad.
- 13. The transition of claim 10 wherein the primary microstrip conductor is parallel to the secondary microstrip conductor.
US Referenced Citations (2)
Number |
Name |
Date |
Kind |
5872550 |
Quan et al. |
Feb 1999 |
|
5886590 |
Quan et al. |
Mar 1999 |
|