General technique for the integration of MIC/MMIC'S with waveguides

A technique for packaging and integrating of a microwave integrated circuit (MIC) or monolithic microwave integrated circuit (MMIC) with a waveguide uses a printed conductive circuit pattern on a dielectric substrate to transform impedance and mode of propagation between the MIC/MMIC and the waveguide. The virtually coplanar circuit pattern lies on an equipotential surface within the waveguide and therefore makes possible single or dual polarized mode structures.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to the packaging and interconnection of microwave 
integrated circuits or monolithic microwave integrated circuits with a 
waveguide structure. (As used herein microwave integrated circuit will be 
taken generically to mean both microwave integrated circuits and 
monolithic microwave integrated circuits.) At both the transmit and 
receive ends of a microwave communications or radar system, energy 
radiators in the form of horns or slots are provided. It is necessary to 
transfer energy efficiently between microwave integrated circuits and 
these radiators. In order to achieve efficient energy transfer, it is 
necessary to transform the waveguide impedance and mode of propagation to 
that of the microwave integrated circuits and vice versa. 
2. Description of the Prior Art 
The prior art describes two techniques for providing impedance matching and 
mode conversion to effect energy transition between a microwave integrated 
circuit and a waveguide. 
A first technique uses a coaxial connector element between a waveguide and 
a microwave integrated circuit. This technique has the disadvantages of 
relatively large size and weight, narrow bandwidth and considerable 
insertion losses of the circuit, especially at high frequencies. 
Consequently, it is of little or no use for certain applications such as 
direct broadcast satellite transmission. 
The second technique uses a ridged waveguide transformer inserted in the 
waveguide between a full height section of the waveguide and the microwave 
integrated circuit. This technique has the disadvantage of using a device 
requiring highly complex and precise machining steps during fabrication. 
In addition, positioning the transformer in the waveguide requires 
difficult assembly procedures. 
The introduction of monolithic microwave integrated circuits (MMIC's) has 
caused several additional problems directly related to their small size 
and fragility. With microstrip, it is possible to contact the substrate 
with a coaxial center conductor or a flat metal tab. Establishing a 
reliable contact is very difficult, if not impossible, with an MMIC 
circuit due to its fragility. Consequently, it is necessary to package 
MMIC's in a way which maximizes performance and reliability and minimizes 
size and weight. 
SUMMARY OF THE INVENTION 
The invention relates to the incorporation of a microwave integrated 
circuit (MIC) or a monolithic microwave integrated circuit (MMIC) with a 
waveguide by attaching the circuit on a dielectric substrate having a 
predetermined electrical conductor pattern thereon and then locating the 
substrate within a section of the waveguide. In a preferred embodiment, an 
MMIC is either soldered or epoxied directly onto a metallized surface of a 
ceramic substrate, with the substrate surface parallel to the electric 
field and approximately centered within the waveguide. On the metallized 
surface of the substrate, the structure includes a unilateral finline 
transition from the waveguide to a slotline and a broadband balun for 
converting the balanced slotline mode to the unbalanced microstrip or 
coplanar waveguide (CPW) on the MMIC. 
It is an object of the present invention to couple an MMIC to a waveguide 
while transforming the impedance and mode of propagation between the 
waveguide and the MMIC so that energy may be transferred efficiently 
between the structures. 
It is another object of the invention to integrate MMIC's with waveguides 
in a small lightweight package which can be removed easily for adjustment 
or repairs and which allows reproducible and non-invasive measurement of 
the MMIC chips. 
It is a further object of the invention to provide a device which may be 
used with rectangular, square or circular waveguides to accommodate dual, 
or orthogonally polarized electric fields. 
It is a yet further object of the invention to provide an MMIC waveguide 
transition device that is reliable, less expensive and is simple to 
fabricate, with little or no machining. 
The aforementioned and other objects and features of the invention will 
become more apparent from the following description taken in conjunction 
with the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
FIG. 1a shows a conventional waveguide transition device consisting of a 
stepped ridge transformer 1. It can be seen that the device which, of 
necessity, must be very small, has a highly complex geometry requiring 
precise, sophisticated machining with critical tolerances. 
Now with reference to FIG. 1b, the transformer of FIG. 1a is shown inserted 
within a rectangular waveguide 3 which may be bolted to a radiator or 
other component (not shown) of a microwave system by means of flange 4. 
The transformer is electrically connected in circuit by means of a 
spring-loaded contact 2 which engages microstrip transmission line 5. The 
microstrip transmission line runs along the length of a ceramic substrate 
6 which has a metallic base plate (not shown) forming a ground plane. 
The position of the transformer must be such that it is in perfect 
alignment with the electric field present within the waveguide. The 
stepped ridge transformer disallows its use in dual polarized applications 
since one polarization will be cut off in the plane for which the device 
has not been aligned. 
As used herein, a finline is a general term for a type of microstrip 
transmission line comprising a very thin metallized section on a substrate 
which forms a wall that runs down the length of a waveguide wherein two 
opposing walls form a gap therebetween and the electric field is 
concentrated on the edges or "fin" of the walls forming this gap. Ideal 
thin conductors are assumed for the fins. It is a "balanced" device in 
that at any point along the finline a voltage +V will be present on one 
wall edge and a voltage -V will be present directly across from it while 
zero voltage will be present at the center of the finline. A finline must 
be positioned within a waveguide in order for the electric field to 
propagate within the gap. This is in contrast to an unbalanced device 
which requires a conductor and a ground plane wherein the conductor has 
some potential difference +V or -V with respect to the ground plane. 
Finline may be unilateral, i.e., both fins on the same side of the 
substrate or antipodal, i.e., one fin on one side of the substrate and 
another fin on the opposite side of the substrate. A slotline is similar 
to a finline in that it is a balanced device, however, it need not be 
positioned within a waveguide to propagate the electric field. A balun is 
a passive circuit for transmitting energy from a balanced system or device 
to an unbalanced system or device. A waveguide is a device wherein the 
electric field is found everywhere within the cross-section of the 
waveguide. 
FIG. 2a shows an MMIC waveguide transition device 10 according to a 
preferred embodiment of the present invention. For purposes of 
simplification, the following discussion will refer to the left side of 
the transition device. As can be readily seen, the device is symmetrical 
with respect to the center line. A metallized surface 12 is deposited on a 
dielectric substrate 30 which may be typically a ceramic, such as alumina 
or beryllia. The substrate is metallized on only one surface. The 
metallization is removed, such as by etching, to cut a tapered, unilateral 
finline impedance transformer 14. The taper begins at both edges of the 
structure and converges inwardly. The taper may be sinusoidal, 
exponential, or stepped, as dictated by bandwidth, size limitations, or 
other requirements. The most common realization of a transformer according 
to the present invention would use a sinusoidal or exponential taper 
rather than a stepped, quarter-wavelength "taper" due to the uncertainties 
associated with the characteristic impedances and discontinuities in a 
stepped transformer. A disadvantage of the curved taper, however, is that 
it requires a greater length than the stepped version for a given return 
loss. A general advantage of unilateral finlines over antipodal finlines, 
is that in the presence of two orthogonal fields, the former will couple 
almost exclusively to the field with which it is aligned while the latter 
will couple to both fields. This property makes possible dual polarized 
structures such as phased array elements and dual mode filters. The 
present invention utilizes unilateral finlines. 
The finline connects to one end 16 of a semi-circular slotline 18, the 
other end 20 of which comprises a short circuit. A coplanar waveguide 
(CPW) structure 22, etched into the metallized surface 12, is shown 
positioned on a side opposite of the slotlines 18 from an MMIC chip 24 
which is soldered or epoxied directly on a portion of metal surface 12. 
Wire 26 passes over the slotline 18 and connects the CPW 22 with the MMIC 
chip 24 to effect energy transfer between the chip 24 and the slotline 
through the magnetic and electric fields present at this junction. The 
electric field across the slotline 18 produces a magnetic field 
perpendicular to the transition plane, which couples to the magnetic field 
of the wire 26. The balun according to the present invention comprises CPW 
22, the bottom half of semi-circular slotline 18, dimensioned to equal a 
one-quarter wavelength of the center of the operating frequency of the 
system in which the device will be used, and wire bond 26. The device may 
be tuned by adjusting the length of the short-circuited slotline. 
It readily can be seen that structures 14', 16', 18', 20', 22', and 26' are 
configured similarly to their counterparts discussed hereinabove. 
Two additional CPW structures 28 and 28' are connected to the MMIC by means 
of wire bonds 29, 29' to provide DC power to the MMIC device. 
By way of example, the left side of the Figure would be the input side of 
the device wherein a propagated field in a waveguide is transformed to an 
input of the MMIC and the right side of the Figure would be the output 
side of the device transforming the output of the MMIC to a field for 
propagation in another section of waveguides. Thus, assuming RF power 
enters at the left end of the device, it will depart from the right end. 
FIG. 2b is an illustration of the device taken along section A--A' of FIG. 
2a showing the MMIC waveguide transition device 10 positioned within a 
rectangular waveguide 50. A pair of spring-fingered beryllium-copper rails 
(not shown) are mounted on the edges of the substrate to make contact 
between the gold metallization of the substrate and the waveguide walls. 
The metallized surfaces 12 and the edges of finline 14 are seen and the 
dielectric substrate 30 is also shown. It should be recognized that FIG. 
2b is "not-to-scale" as the metallized surface has virtually no thickness 
relative to the substrate. Further, FIG. 2b does not depict the MMIC chip 
as its inclusion in the Figure is not necessary to understand the 
invention. Since the circuit is virtually coplanar, it lies on an 
equipotential surface for a horizontally polarized electric field and 
appears transparent to that polarization. 
FIG. 2c shows a variation comprising a stepped finline-slotline 
arrangement. The first three sections are quarter wavelength sections in 
the finline 52 and the fourth is a quarter wavelength section of the 
slotline 54. Tuning is achieved as above by adjusting the short-circuited 
stub 56 of the slotline. 
FIG. 3 shows a simplified close-up of the balun structure used in FIG. 2c, 
including the required wire bond and quarter wavelength short-circuited 
slotline section. The other components and connections shown in FIG. 2a 
would be required to fully implement the invention. A microstrip 40 is 
positioned on a ceramic substrate 42 which is adhered to the metallized 
surface 12 of the device. Wire bond 26 connects the microstrip to a 
portion of the metalized surface on a side opposite to slotline 18. Again, 
the length of the slotline from the bond wire to the short circuit end of 
the slotline is dimensioned to equal one-quarter wavelength of the center 
of the operating frequency. The open circuit side of the slotline connects 
to the finline similar to that of the FIG. 2a arrangement. The structure 
of FIG. 3 is simpler than that of FIG. 2a in that CPW 22 is not required 
but the bandwidth of the FIG. 3 balun is not as great as that of the FIG. 
2a. 
FIG. 4 shows another balun structure comprising an extended CPW 60 crossing 
through the slotline 18 which may be substituted for the baluns described 
hereinabove. As above, it is understood that the Figure only shows a 
variation on the balum structure and the other components and connections 
shown in FIG. 2a are required to implement the invention. Metallized areas 
12 are connected by means of air bridges 61 and 62 to establish a DC 
connection between sides of the metallized areas opposite the CPW 60. This 
embodiment is useful if an MMIC is not to be mounted proximate to the 
slotline on the transition structure since the CPW can extend to any 
desired length. 
FIG. 5 shows insertion loss curves and return loss curves as a function of 
frequency for an MMIC chip integrated with a waveguide transition device 
according to the present invention. The insertion loss is a measure of the 
power lost between the input and the output and the return loss is a 
measure of the power reflected by the input port. Optimal results are 
achieved by making the insertion loss as low as possible and the return 
loss as high as possible for a given frequency. Curve (a) shows the return 
loss of more than 25 dB for an operating frequency of 19 GHz. Curve (b) 
shows that at 19 GHz, there is an insertion loss of approximately 1 dB. Of 
this, approximately 0.4 dB is due to losses in the short microstrip 
section and 0.3 dB is due to each transition. The latter figure may be 
further reduced by use of a substrate with a smoother surface. These 
performance figures are comparable to or better than many conventional 
waveguide transition devices which in many cases are impractical or 
impossible to use in given applications as discussed above. 
Electrically, the present invention will couple, with very little loss of 
power, between a waveguide and an MIC/MMIC in single or dual polarized 
systems. Mechanically, it provides a single-piece, rugged and easily 
reproduced module that can be produced inexpensively. Finally, the use of 
beryllia for the substrate material allows for a low thermal resistance 
structure. 
Although the invention has been described and shown in terms of preferred 
embodiments thereof, it will be understood by those skilled in the art 
that changes in form and details may be made therein without departing 
from the spirit and scope of the invention.