Apparatus for providing a ground for circuits on carriers

A ground path for microwave circuits positioned on carriers is provided. A ground path is provided between a first and second carrier coupled to respective microwave circuit substrates. The first and second carriers are coupled to a base, forming an opening between the first and second carriers. A conductive element is coupled to a first microstrip transmission line on the first substrate and a second microstrip transmission line on the second substrate. A conductive spring is then positioned in the opening in order to create a ground path. The microstrip transmission line may consist of alumina or a high-frequency plastic, such as Teflon or Duroid.RTM.. The conductive spring may consist of beryllium copper. A conductive rubber material may also be positioned in the opening, contacting the conductive spring and the base. The conductive spring and conductive element form a transmission having an impedance of between approximately 45 ohms and 60 ohms, thereby substantially improving microwave signal quality transmitted on a substrate coupled to relatively thick carriers.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to grounding circuits, and in particular, 
grounding circuits positioned on carriers. 
2. Description of the Related Art 
Traces are used to electrically connect electronic devices and/or circuits 
formed on a substrate. For example, trace 12 connects circuit 15 to 
circuit 14 on substrate 10, as shown in FIG. 1a. Likewise, trace 11, 
connected to circuit 14, may also be used to connect circuit 14 to other 
circuits which are not on substrate 10. FIG. 1b illustrates a 
cross-sectional view of substrate 10, trace 11 and ground 13. Typically, 
the traces have an impedance of approximately 50 ohms. Circuits 14 and 15 
may be microwave circuits, such as an amplifier, antenna, filter or 
circulator. When using microwave circuits, traces are typically referred 
to as microstrip transmission lines. 
In designing electrical systems, and in particular microwave systems, 
multiple substrates having electronic devices and/or circuits may be 
combined to form the microwave system. FIG. 2 illustrates a microwave 
system having, but not limited to, two substrates. Both substrate 22 and 
substrate 20 have at least one electronic device and/or circuit. Circuits 
formed on substrate 22 and substrate 20 are electrically connected by 
jumper 25. In particular, jumper 25 electrically couples microstrip 
transmission line 28 to microstrip transmission line 29. These microstrip 
transmission lines are then in turn coupled to the various circuits and/or 
electronic devices. 
The impedance corresponding to jumper 24 is not the typical 50 ohm trace 
impedance, but considerably higher. The typical impedance of jumper 24 is 
approximately 100 ohms. This higher impedance will cause reflection in a 
transmitted signal. In particular, this reflection will increase when 
using high frequency signals due to the longer phase length. At 
frequencies above 20 gHz, appreciable reflection may be observed. 
Often, substrates must be mounted on carriers. For example, substrate 22 is 
mounted on carrier 23, and substrate 20 is mounted on carrier 21, 
respectively. Carriers 23 and 21 are then coupled to base 26 acting as a 
ground plane. 
Generally, a carrier is a piece of metal used for a variety of reasons. For 
example, a carrier may be coupled to a substrate to increase heat sink 
characteristics. A carrier may also provide a better ground for electronic 
devices and/or circuits on a substrate. A carrier may be used to 
strengthen a relatively weak substrate. Using a carrier may also be 
required if the substrate is coupled to a base by a screw. A carrier may 
also be used in order to allow for ease in replacing the substrate. If 
different substrate thicknesses are used, carriers may be used to create a 
common plane between substrate surfaces. 
While the use of carriers solves many of the problems outlined above, 
carriers may also undesirably extend the ground path of a circuit to such 
an extent that signal quality is degraded. In certain applications, a 
ground path may be lengthened by twice the thickness of a carrier. A 
ground path may have to be extended down a first carrier to a ground plane 
and then back up a second carrier. This lengthening of the ground path is 
further complicated when using higher frequency microwave signals. 
For example, if a 75 gHz microwave signal is transmitted on microstrip 
transmission line 28, jumper 25 and microstrip transmission line 29, and 
carrier 23 and carrier 21 have an approximate thickness of 0.5 mm (0.020 
inches), the ground path length down carrier 23 to the top of base 26 and 
then up carrier 21 is equal to approximately a quarter of the 75 gHz 
wavelength. This will result in a short at a circuit on substrate 22 and 
an open circuit at a circuit on substrate 20. 
Therefore, it is desirable to provide an apparatus including multiple 
substrates coupled to relatively thick respective carriers which preserves 
signal quality when using jumpers between substrates. Signal quality 
should be preserved while the electrical system benefits from the numerous 
advantages outlined above in regard to using carriers. In particular, a 
microwave system having a plurality of substrates including microwave 
circuits, coupled to relatively thick carriers, which preserve microwave 
signal quality is desirable. 
SUMMARY OF THE INVENTION 
An apparatus for providing a ground path for a circuit is provided. The 
apparatus includes a first carrier which is coupled to a first substrate 
having a first trace. A second carrier is coupled to a second substrate 
having a second trace. A base is coupled to the first carrier and the 
second carrier, forming an opening between the first and second carriers. 
A conductive element couples the first trace to the second trace. A 
conductive spring is positioned in the opening for creating a circuit 
ground. 
According to another aspect of the present invention, the first and second 
traces are microstrip transmission lines. 
According to another aspect of the present invention, the first and second 
traces consist of alumina. 
According to another aspect of the present invention, the first and second 
traces consist of high-frequency plastic. 
According to another aspect of the present invention, the first and second 
traces consist of Teflon fiberglass. 
According to another aspect of the present invention, the first and second 
traces consist of Duroid.RTM.. 
According to yet another aspect of the present invention, the conductive 
spring and jumper form a transmission line having an impedance of 
approximately between 45 and 60 ohms. 
According to yet another aspect of the present invention, the conductive 
spring consists of beryllium copper. 
According to yet another aspect of the present invention, a conductive 
rubber material is positioned in the opening contacting the conductive 
spring and the base. 
According to another aspect of the present invention, an apparatus for 
providing the ground path for microwave circuits is provided. The 
apparatus comprises a first carrier coupled to a first substrate having a 
first microwave circuit including a first microstrip transmission line. 
The first carrier includes a first upper portion for positioning the first 
substrate and a lower portion. A second carrier is also coupled to a 
second substrate having a second microwave circuit including a second 
microstrip transmission line. The second carrier includes a first upper 
portion for positioning the second substrate and a lower portion. A base 
is coupled to the lower portions of the first and second carriers, forming 
a first opening between the upper portions of the first and second 
carriers. A second opening is also formed between the lower portions of 
the first and second carriers. A jumper is coupled to the first and second 
microstrip transmission lines. A conductive spring is positioned in the 
first opening for creating a circuit ground path. A conductive rubber 
material is positioned in the second opening. The conductive rubber 
material contacts the conductive spring and the base. 
According to another aspect of the present invention, the conductive spring 
is flexible in order to be positioned into the first opening and contact 
the first and second substrates. 
Other aspects and advantages of the present invention can be seen upon 
review of the figures, the detailed description, and the claims which 
follow.

DETAILED DESCRIPTION OF THE INVENTION 
FIGS. 3a-b illustrate an electrical system, and in particular, a microwave 
system 70 according to the present invention. Microwave system 70 includes 
a plurality of microwave circuits. For example, block diagrams of 
microwave circuit 60 and 61 positioned on substrates 34 and 35 are shown 
in FIG. 3b. It should be understood that in complex microwave systems, 
many more microwave circuits may be present and only block diagrams 
representing microwave circuits 61 and 60 are shown. 
For example, microwave circuit 61 may be an amplifier and microwave circuit 
60 may be an antenna. Other microwave circuits may include, for example, 
switches, circulators and filters. Microwave circuit 60 may be coupled to 
other circuits and/or electronic devices on substrate 34 or on other 
substrates by a microstrip transmission line 41. Likewise, circuit 61 may 
be coupled to other circuits and/or electric devices on substrate 35 by 
microstrip transmission line 42. Substrate 36 is shown with a microstrip 
transmission line 43. Typically, microstrip transmission lines 41, 42 and 
43 have an impedance of approximately 50 ohms. 
In an embodiment, microstrip transmission lines 41, 42 and 43 consist of 
alumina or a high frequency plastic which is able to carry microwave 
signals, such as a 75 gHz signal. Typically, the microstrip transmission 
lines have an impedance of approximately 50 ohms. In other embodiments, 
microstrip transmission lines 41, 42 and 43 may be made of Teflon 
fiberglass or Duroid.RTM.. 
As can be seen by FIG. 3a, electronic circuits and/or electrical devices 
are positioned on substrates 34, 35 and 36 which are then coupled to 
carriers 31(a-b), 32(a-b) and 33(a-b), respectively. In an embodiment, 
carriers 31, 32 and 33 consist of a ceramic. In the preferred embodiment, 
carriers 31, 32 and 33 consist of a metal, such as brass. 
As described above, carriers 31, 32 and 33 are used for: 1) increasing heat 
sink characteristics; 2) providing a better ground for circuits and/or 
electronic devices; 3) strengthening the substrate (microwave circuits 
tend to be small and fragile); 4) allowing for a screw to be positioned 
between a substrate and a base; 5) allowing ease of replacement; and 6) 
allowing for common height if different substrate thicknesses are used. 
As can be seen in FIG. 3a, carriers 33 and 32 are coupled to base 30, 
forming an opening 38 between carrier upper portion 33a and carrier upper 
portion 32a. In an embodiment, base 30 may be a part of a housing for 
microwave system 70. Likewise, opening 37 is formed between base 30 and 
carrier lower portions 33b and 32b. Similarly, opening 40 is formed 
between carrier upper portions 32a and 31a, while opening 39 is formed 
between carrier lower portions 32b and 31b and base 30. Base 30 acts as a 
ground plane for microwave system 70. 
In an embodiment, the depth 71 of carriers 33, 32 and 31 is approximately 
0.5 mm. 
Microwave system 70 is manufactured by positioning the carriers 33, 32 and 
31 on base 30. The various microwave circuits and/or electrical devices on 
substrates 34, 35 and 36 are coupled electronically between substrates by 
a jumper. For example, jumper 44 couples microwave circuit 60 on substrate 
34 to microwave circuit 61 on substrate 35. 
In an embodiment, jumper 44 is soldered to microwave transmission line 42 
on substrate 35 and to microwave transmission line 41 on substrate 34. In 
an embodiment, jumper 44 consists of gold. Typically, jumper 44 has an 
impedance higher than the microwave transmission lines 41, 42 and, 43. 
Jumper 44 has an impedance of greater than approximately 100 ohms. 
As described above in regard to the related art, the high impedance jumper 
between substrates and relatively thick carriers may cause undesirable 
signal quality degradation when transmitting a high frequency signal 
between carriers. For example, the relatively thick carriers cause a 
ground path between carriers 32 and 31 equal to twice the distance 71. 
FIG. 5 illustrates the forward reflection when transmitting microwave 
signals in microwave system 70, as illustrated in FIG. 3a. As can be seen 
by the curve 100 in FIG. 5, substantial forward reflection occurs for high 
frequency signals transmitted on microwave transmission lines 42 and 41 
and jumper 44. Forward reflection was measured for microwave signals 
between approximately 0.13 gHz and 64 gHz. Forward reflection generally 
increases as frequency increases. Similarly, curve 101 illustrates forward 
transmission loss as frequency increases between approximately 0.13 gHz 
and 64 gHz. 
FIGS. 4a-b illustrate an apparatus for improving signal transmission 
quality for microwave systems having relatively thick carriers. In 
particular, the apparatus reduces forward reflection and increases forward 
transmission of microwave signals. FIGS. 4a-b illustrate the microwave 
system 70 having carriers 31, 32 and 33 coupled to base 30 acting as a 
ground plane. The carriers are used to position substrates 34, 35 and 36. 
A conductive spring 50 is positioned in opening 38 between upper carrier 
portion 33a and upper carrier portion 32a. Likewise, a conductive spring 
59 is positioned in opening 40 between upper carrier portion 32a and upper 
portion carrier 31a. 
Conductive springs 50 and 59 are positioned such that the tops of 
conductive springs 50 and 59 are almost at the top of substrates 36, 35 
and 34. Conductive springs 50 and 59 generally contact the edges of 
carriers 33, 32 and 31. However, it is not necessary that conductive 
springs 50 and 59 contact carriers 33, 32 and 31 because a ground is 
established by the pressure of conductive rubber materials 51 and 80 
against base 30. In an embodiment, materials 51 and 80 may not be 
conductive. 
In an embodiment, conductive springs 50 and 59 consist of beryllium copper. 
Other conductive spring-type material may also be used in an embodiment. 
Conductive springs 50 and 59 are adjusted to the optimum shape to be 
positioned in the openings 38 and 40. Conductive springs 50 and 59 and 
conductive rubber materials 51 and 80 allow for a low impedance when 
transmitting signals between substrates. 
The carriers 31, 32 and 33 may be positioned on base 30 in order to form a 
specified distance for openings 38 and 40, allowing for positioning 
conductive springs 50 and 59. In the preferred embodiments, the distance 
of openings 38 and 40 between upper portion of carriers 33a, 32a and 31a 
is between approximately 0.006 and approximately 0.012 inches. The 
distance between substrates may vary by 20% with little degradation of 
signal quality. 
The distance between conductive springs 50 and 59 and jumpers 52 and 53, 
respectively, can be adjusted slightly by bending jumpers 52 and 53. 
Ideally, the spaces between conductive springs 50 and 59 and jumpers 52 
and 53 would provide an approximately 50 ohm impedance, respectively, (air 
dielectric microstrip transmission line), although, an impedance created 
between approximately 45 and 60 ohms would provide good results. 
FIGS. 5 and 6 illustrate the improved signal quality using the apparatus 
illustrated in FIGS. 4a-b. Curve 102 represents forward transmission when 
using the apparatus illustrated in FIGS. 4a-b, while curve 101 represents 
forward transmission when not using the apparatus. In particular, FIG. 5 
shows a curve 102 which illustrates very little forward transmission loss 
as frequency increases from approximately 0.13 gHz to 64 gHz. A comparison 
of curve 102 with curve 101 shows a dramatic improvement in forward 
transmission when using the apparatus illustrated in FIGS. 4a-b. An 
approximately 21 db loss is observed at approximately 64 gHz when not 
using conductive springs 38 and 40 along with materials 51 and 80. 
Likewise, FIG. 6 illustrates an improved forward reflection curve 105 when 
using the apparatus shown in FIGS. 4a-b as compared to not using the 
apparatus as seen by curve 100 in FIG. 5. An approximately 3 db loss can 
be observed at approximately 62 gHz when not using the ground apparatus. 
Thus, an apparatus for providing a ground path for circuits used with 
substrates having relatively thick carriers enables improved signal 
quality while achieving the benefits of carriers. 
The foregoing description of the preferred embodiments of the present 
invention has been provided for the purposes of illustration and 
description. It is not intended to be exhaustive or to limit the invention 
to the precise forms disclosed. Obviously, many modifications and 
variations will be apparent to practitioners skilled in the art. The 
embodiments were chosen and described in order to best explain the 
principles of the invention and its practical applications, thereby 
enabling others skilled in the art to understand the invention for various 
embodiments and with the various modifications as are suited to the 
particular use contemplated. It is intended that the scope of the 
invention be defined by the following claims and their equivalents.