Hermetically sealed millimeter waveguide launch transition feedthrough

A hermetically sealable millimeter waveguide launch transition feedthrough for channelling high frequency electrical signals in a circuit which includes at least one waveguide. An aperture is formed in one wall of the waveguide and a conductive pin passes through the aperture. The pin is sealed therein by a dielectric material which surrounds the pin so as to isolate it from the waveguide wall. A probe head is disposed at one end of the conductive pin and within the waveguide. The opposite end of the pin contains a second probe head or other circuit connector. The transition may be effectively implemented in waveguide-to-waveguide or waveguide-to-device applications as well as in colinear device housing applications.

BACKGROUND OF THE INVENTION 
1. Technical Field 
This invention relates generally to millimeter wave electronic devices and, 
more particularly, to a hermetically sealable millimeter waveguide 
transition for channelling high frequency signals through a waveguide 
wall. 
2. Discussion 
The demand for Gallium Arsenide (GaAs) Monolithic Microwave Integrated 
Circuit (MMIC) devices is expected to drastically increase during the next 
few decades. This technology is capable of ever increasing operating 
frequencies and is frequently utilized in radars, electronic warfare, 
missiles and array weapons as well as a wide variety of non-military 
applications. As the monolithic circuitry in these devices becomes 
increasingly dense, and as operating frequencies increase to W-band (94 
GHz) and beyond of the millimeter wave range, signal capture loss becomes 
a progressively more severe problem. This places a heavy burden on 
existing millimeter wave packaging technology, and especially on radio 
frequency (RF) input/output transitions, often the source of this loss. 
Transitions for channelling high frequency signals from one waveguide to 
another formed by mechanically joining the waveguides together are 
typically bulky and incapable of operating at very high frequencies. 
Teflon.RTM. (tetrafluoroethylene or (TFE) pin feedthroughs sometimes used 
as a transition to interconnect two waveguides solve the space problem but 
have a number of inherent weaknesses. Many are found to have electrical 
mismatching, discontinuity problems, high RF losses and the ability only 
to operate below 30 GHz in frequency. This type of conventional RF 
transition arrangement also has a limited scope of usage because it is 
generally non-hermetic and unable to withstand elevated temperatures of 
125.degree. C. and beyond, often required in high RF device packaging and 
a must for space flight applications. 
Millimeter wave housing packages, in which a GaAs MMIC chip or other high 
frequency electronic device is to be sealed for communication with a 
waveguide, without built-in hermeticity may require waveguide sealing 
windows used to hermetically seal off open ends of the waveguides. This, 
however, requires incoming and/or outgoing RF signals to pass through the 
sealed waveguide window, thereby creating signal distortion and loss. 
Additionally, extensive waveguide window development and reliability 
testing of the resulting package is often necessary, thereby leading to 
higher overall costs for these types of millimeter wave housings. 
Current waveguide-to-device transition designs for millimeter wave housing 
packages which have addressed hermeticity generally have not been 
applicable to colinear applications in which the incoming and outgoing 
waveguide channels are disposed along a common linear axis orthogonal to 
the RF transition feedthrough. With non-colinear transition designs, 90 
degree adapters are necessary and must be fabricated, thereby creating 
taller fixture housings and increasing material and machining costs. 
Increased volume and weight of the resulting housings makes their use 
undesirable for typical colinear system applications wherein size and 
weight are restricted. 
There is therefore a need for a compact and hermetically sealable 
millimeter wave transition capable of operating in elevated temperatures 
and having low loss in the 30 GHz to 150 GHz frequency range. It would 
also be advantageous for this transition to be useful in providing an 
inexpensive low-profile colinear millimeter wave package with built-in 
hermeticity, useful in a variety of military and commercial applications 
from smart weapons to spaceborn RF payloads. 
SUMMARY OF THE INVENTION 
The present invention provides a signal transition feedthrough for 
channelling high frequency electrical signals in a circuit which includes 
at least one waveguide. An aperture is formed in one wall of the waveguide 
and a conductive pin passes through the aperture. The pin is sealed 
therein by a dielectric material which completely surrounds the pin so as 
to isolate it from the waveguide wall. A conductive probe head is attached 
to one end of the pin and is disposed within the waveguide. The opposite 
end of the pin contains a second probe head or is otherwise connected to 
another circuit element, depending upon the application in which the 
transition is to be implemented. 
The transition of the present invention may be used in a variety of 
waveguide-to-waveguide or waveguide-to-device applications and may be 
effectively applied to colinear applications. Additional objects, 
advantages, and features of the present invention will become apparent 
from the following description and appended claims, taken in conjunction 
with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring generally to FIG. 1, an integrated millimeter waveguide and 
launch transition feedthrough (MWLTFT) according to the teachings of the 
present invention is indicated generally at 10. Transition feedthrough 10 
includes a generally cylindrical conductive collar or ring 12 through 
which passes an elongated solid cylindrical conductive pin 14. Ring 12 and 
pin 14 are preferably concentric to within a close tolerance and may be 
formed of any suitable conductive material or metal but are preferably 
made of Kovar.RTM., a trade name for an alloy of iron, cobalt and nickel, 
or any of a similar group of alloys which have an expansivity similar to 
that of glass. 
Conductive pin 14, intermediate a first end 14a and a second end 14b, 
passes perpendicularly through a controlled-thermally matched dielectric 
glass seal 16 which is disposed within conductive ring 12 to substantially 
fill the interior space defined by ring 12 and to thereby hermetically 
seal pin 14 within ring 12. The glass composition used form glass seal 16, 
such as Corning 7070, is preferably thermally matched to that of the 
material of conductive pin 14 and ring 12 so as to prevent cracking of the 
glass, and therefore destruction of the hermetic seal, upon thermal 
expansion or contraction of any of the glass 16, ring 12 or pin 14. 
Disposed at a first end 14a of pin 14 is a probe head 18. Probe head 18 is 
preferably a solid cylinder having a diameter larger than that of pin 14 
but smaller than that of ring 12. Probe head 18 may alternately be of 
another suitable configuration or shape such as a bullet shape or double 
bullet shape. Probe head 18 may be formed separately for attachment to pin 
14 by brazing, soldering or swaging or may be integrally formed therewith. 
Probe head 18 is preferably formed of the same conductive material as pin 
14 but also may be made of any other suitable material. Probe head 18 is 
preferably spaced from ring 12 and glass seal 16 along pin 14. Depending 
upon the application in which it is to be implemented, a transition 10 may 
additionally include a second probe head 20, similar to probe head 18 and 
disposed at a second end 14b of pin 14. 
Probe heads 18 and 20 act as antennas to receive or transmit high frequency 
signals to or from a waveguide channel. The dimensions of the conductive 
pin and the size of the probe heads are directly related to the unique 
frequency range of the signals to be channelled and therefore the size of 
the waveguide selected. Smaller sized waveguides are generally used for 
higher frequencies. After an initial size estimate, the precise size of 
the various component parts of transition 10 may be enlarged or decreased 
based upon test results. Based on such results, and on various theoretical 
design concepts and predictions commonly know to those having skill in the 
art, the transition proportions can be readily scaled to allow transitions 
to operate in the frequency range from 60 to 94 GHz, depending upon 
application. By further scaling the proportions, a 150 GHz frequency range 
is possible. 
An exemplary application of transition 10 in a waveguide-to-waveguide 
configuration is illustrated in the perspective view of FIG. 2. As shown 
in the figure, a package or housing 22 defines a pair of waveguide 
cavities 24 and 26, between which high frequency electrical signals are to 
be channelled. Housing 22 is preferably formed of a conductive metallic 
material but may alternately be made of any other suitable material 
including a nonconductive material which has been coated or plated with a 
conductor. 
Waveguide channels 24 and 26 shown in FIG. 2 each have four surrounding 
walls used to contain electrical signals, with one wall of each channel 24 
and 26 cooperating to define a transition wall 28 through which high 
frequency electrical signals are to be transmitted. Alternately, however, 
waveguide channels 24 and 26 may be of any other suitable cross-sectional 
shape. Waveguide channels 24 and 26 may be integrally formed in housing 22 
or may be formed separately and joined together to define a transition 
wall 28. 
Transition wall 28 of housing 22 preferably includes a suitably sized 
aperture 30 into which ring 12 of transition 10 fits and may be 
hermetically sealed, such as by soldering. Preferably, the width of ring 
12 is substantially equal to the thickness of wall 28 along the periphery 
of aperture 30. Alternately, however, ring 12 may be eliminated by 
providing a suitable conductive inside surface of aperture 30 or ring 12 
may be integrally formed therewith. 
Transition 10 in the configuration of FIG. 2 includes a pair of probe heads 
18 and 20 which, in this preferred embodiment, act as excited cylindrical 
antennas in rectangular waveguide channels 24 and 26. Probe head 18 is 
disposed within waveguide opening 24 and receives high frequency signals 
channelled therethrough. The signals pass from probe head 18 through pin 
14 into probe head 20. Probe head 20 then serves to transmit signals into 
waveguide 26 for channelling. 
Shown in FIG. 3 is an alternative implementation of the present invention 
in a waveguide-to-device-to-waveguide configuration. This type of 
application is intended to facilitate hermetic sealing of an 
environmentally sensitive electronic device such as a GaAs MMIC chip 
within a device housing 34. Housing 34 in this exemplary embodiment 
includes an interior cavity 36 into which a device 32 is to be sealed. 
Cavity 36 is preferably completely hermetically sealable, with the 
exception of a pair of apertures 38 and 40 which extend through transition 
walls 42 and 44. Housing 34 may be formed of two like portions joined 
together to create sealed cavity 36 or may alternately include a 
separately attachable cover similarly directed toward this purpose. 
Adjacent cavity 36 are waveguide channels 46 and 48. Channels 46 and 48 may 
be integrally formed in housing 34 or may be separately formed and 
attached thereto. Apertures 38 and 40 extend through transition walls 42 
and 44 which separate cavity 36 from waveguide channels 46 and 48. 
Disposed in each of apertures 38 and 40 is a ring 12 of a transition 10. 
Each transition 10 has a probe head 18 at a first end 14a of pin 14 but not 
at second end 14b. Each transition 10 is positioned such that the probe 
head 18 of each is disposed in one of waveguide openings 46 and 48. The 
second end 14b, of each conductive pin 14 is electrically connected 
directly to electronic device 32 or to an interface or other connector, 
such a device interface often being a microstrip line disposed on a 
dielectric substrate. This connection is preferably made using a method 
know to those skilled in the art such as microsolder, wire or a ribbon 
bond. 
Each transition 10 is preferably preassembled prior to placement in 
apertures 38 and 40 whereby probe head 18 is disposed at end 14a of pin 14 
and wherein pin 14 has been sealed by dielectric glass 16 into ring 12. 
Each ring 12 is then hermetically sealed into one of apertures 38 and 40, 
such as by soldering or other suitable method. End 14b of each pin 14 may 
then be electrically attached to device 32, or to an interface thereof, by 
a suitable method such as microsolder, wire or a ribbon bond. 
Application of transition 10 to a low-profile and compact device housing 
having colinear waveguide channels is illustrated generally at 50 in FIG. 
4. As shown therein, a housing 52 includes an internal cavity 54 into 
which a device 56 such as a low noise amplifier may be dropped. Housing 52 
also has defined therein a pair of waveguide openings 58 and 60 which are 
disposed such that they are colinear or have a common longitudinal axis. 
While this common axis in the configuration shown in FIG. 4 is below 
device 56, it should be apparent to one having skill in the art that it 
could also lie above or beside device 56. 
Between waveguide channels 58 and 60 and cavity 54, housing 52 further 
defines transition walls 62 and 64 which separate waveguide channels 58 
and 60 from cavity 54. Through each of walls 62 and 64 is formed an 
aperture, 66 and 68, respectively, each preferably circular in cross 
section. Sealed within each aperture is a conductive ring 12 of a 
transition 10 according to the present invention. Each ring 12 is 
hermetically sealed into apertures 66 and 68, preferably by soldering. A 
conductive pin 14 is preferably already hermetically sealed in each ring 
12 by a dielectric glass as discussed above. This forms an impermeable 
hermetic seal between waveguide channels 58 and 60 and cavity 54. 
Each pin 14 has disposed at first end 14a a probe head 18 formed in the 
manner discussed above. A second end 14b of each pin 14 protrudes from 
ring 12 into cavity 54. Second ends 14b of each pin 14 are connected 
electrically to device 56, preferably by conductive ribbon bonds 70. Each 
transition 10 has preferably been preassembled as discussed above and then 
ring 12 thereof is soldered or otherwise hermetically sealed into each of 
apertures 38 and 40. 
This configuration allows device 56 to be sealed in a compact, low-profile 
housing having colinear waveguide channels without requiring bulky 
90.degree. adapters. High frequency RF signals coming into waveguide 
channel 58 are received by probe head 18 disposed therein and passed 
through pin 14 and ribbon bond 70 to the device 56. After these signals 
are processed by device 56 they are passed in a like fashion into 
waveguide channel 60 as outgoing signals. 
This integrated MWLTFT transition of the present invention as disclosed 
above provides enhanced electrical integrity at a reduced cost. The 
materials selected for use in the transition of the present invention 
allow for use in elevated temperatures as well as the ability to handle an 
ever increasing range of frequencies and power with a high degree of 
reliability. Theoretical and experimental test results on these 
transitions have demonstrated the ability to extend the operating 
frequencies from 35 to 60 to possibly 150 GHz. 
Manufacturing cost reductions and system integration are made possible by a 
standardized transition configuration which may be preassembled for 
implementation into a variety of application configurations. For the high 
frequency millimeter package, it will allow unpackaged devices to drop 
into the cavities for interconnection, thus reducing cost as well as 
improving electrical performance. Each module may be integrated, 
minimizing mechanical interconnections and hardware, thereby improving 
system performance and greatly reducing size, volume, weight and 
manufacturing costs. Available low cost integrated fabrication techniques 
minimize electrical discontinuities and consequently reduce VSWR and 
transmission losses. 
Implemented as an orthogonal transition as shown in FIG. 4, RF performance 
with an insertion loss of less than 1 dB across a measured bandwidth of 6 
GHz has been achieved. The return loss was shown to be better than 15 dB, 
limited mainly by discontinuities between Kovar.RTM. pin 14 and the device 
interface substrate. An alumina substrate 0.4 inches in length was shown 
to have a measured insertion loss of 0.4 dB at 44 GHz, indicating a 
waveguide-to-microstrip transition less in this design configuration of 
less than 0.3 dB at 44, GHz. 
It may be appreciated by one skilled in the art that the main advantage of 
this design, however, is its diversity and flexibility. It can be easily 
reproduced and scaled to a variety of waveguide sizes and frequency bands. 
This design is applicable to all millimeter-wave component products 
requiring a colinear and hermetic package for both industrial and 
commercial uses. This transition can be utilized in a number of military 
and commercial applications including low noise amplifiers, 
downconverters, mixers, power amplifiers, filters, test fixtures, single 
and multiple chip high frequency module packages and 
waveguide-to-microstrip transitions. 
The foregoing discussion discloses and describes merely exemplary 
embodiments of the present invention. One skilled in the art will readily 
recognize from such discussion, and from the accompanying drawings, that 
various changes and modifications can be made therein without departing 
from the spirit and scope of the invention as defined in the following 
claims.