Hybrid microwave circuit

A coaxial type microwave filter having an outer conductor and at least one inner conductor supported in the outer conductor has a strip transmission line extending from an external strip-line circuit to the inner conductor through an opening in the outer conductor. The strip transmission line includes a dielectric member forming an integral part of the dielectric substrate of the external circuit and a thin layer of conductive strip also forming an integral part of the conductive film pattern of the external circuit. The extending strip transmission line is electromagnetically coupled with the inner connector to permit the two microwave circuits to be interconnected with no use of a connector.

FIELD OF THE INVENTION 
The present invention relates to a microwave filter of the type having at 
least one inner conductor mounted coaxially in an outer conductor, and 
more specifically to a connector for connecting such microwave filters to 
a printed circuit, particularly microwave integrated circuit. 
BACKGROUND OF THE INVENTION 
Microwave filters of the coaxial type are usually provided with coaxially 
structured terminals for connection with external circuit modules and such 
connection is provided by a coaxial cable or the like. Each terminal is 
provided with a probing electrode which projects into the filter interior 
for energy transfer. In cases where a system is desired to meet low-loss 
requirements, a hybrid construction is often employed in which such 
microwave filters are connected to strip line circuit modules such as 
integrated circuits. This requires a specially designed connector that 
provides mode conversion between different field configurations of the 
dominant mode. However, an abrupt structural change tends to occur at the 
connecting point, so that impedance mismatch is introduced. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the invention is to permit connection between a 
coaxial type microwave filter and a strip-line circuit without introducing 
impedance mismatch between them. 
Another object of the invention is to provide a microwave filter of coaxial 
type comprising at least one inner conductor mounted in an outer conductor 
and a strip-line forming an integral part of an external strip-line 
circuit, such as microwave integrated circuit, the strip line comprising a 
dielectric substrate extending from said external circuit to the inner 
conductor through an opening in the outer conductor and a conductive film 
secured to and extending along the dielectric substrate to a point which 
is located a short distance from the inner conductor to establish a 
reactive coupling therewith. 
A further object of the invention is to provide a hybrid microwave circuit 
comprising a microwave filter of coaxial type and a strip-line circuit 
including a dielectric substrate having a portion extending into said 
filter and secured to the inner conductor of the filter and a printed 
circuit having a portion secured to and extending along said substrate 
portion to a point located a short distance from the inner conductor to 
establish a reactive coupling therewith. 
Therefore, a still further object of the invention is to eliminate the use 
of a separate connector between different circuit modules of a hybrid 
microwave circuit.

DETAILED DESCRIPTION 
In FIGS. 1 to 4 of the drawings, a preferred embodiment of the invention is 
illustrated. In FIG. 1, a coaxial type microwave filter 1 and a strip-line 
circuit 2 such as integrated circuit are separately shown. The microwave 
filter comprises an outer conductor of a box-shaped configuration having 
side walls 3, 4 and end walls 5, 6 and top and bottom walls 7 and 8. A 
plurality of inner conductors 9 (only two of which are illustrated) is 
supported within the outer conductor spaced from the top and bottom walls 
extending parallel with each other between side walls 3 and 4. A capacitor 
10 is preferably provided for each inner conductor between its free end 
and side wall 3. 
The strip-line circuit, or microstrip 2 includes a printed circuit 12a of a 
mixer, for example, formed by etching a thin layer of metal of a 
dielectric substrate 11a preferably having a small dielectric loss 
(particularly when frequency is relatively high in the microwave region). 
Strip-line circuit 2 further includes an extension 2a formed by a 
dielectric substrate 11b which is an integral part of substrate 11a and a 
conductive film strip 12a which extends from film 12a to a point a little 
distance away from the end of substrate 11b. Near the end of extending 
substrate 11b is provided a screw hole 14. 
On the end wall 5 of microwave filter 1 is provided an insertion hole 15 as 
shown in FIG. 2 through which extension 2a of strip-line circuit 2 is 
inserted into filter 1. When the two circuit modules 1 and 2 are assembled 
together to form a hybrid circuit the extension 2a of strip-line circuit 2 
is inserted through hole 15 and substrate portion 11b is secured to inner 
conductor 9a by means of a screw 16 as illustrated in FIG. 4. In this 
illustrated position, the end of strip film 12b is spaced a distance from 
inner conductor 9a to establish a reactive coupling between them. 
Preferably, the two circuit modules are grounded as at 17 and 18 after 
assemblage. A connecting path is thus established between strip-line 
circuit 2 and microwave filter 1. If circuit 2 is an input external 
circuit, microwave energy is injected through extending portion 2a into 
the interior of filter 1 and propagates through the parallelly arranged 
inner conductors to the output side. Although not shown in the drawings, 
an output external circuit similar to circuit 2 may be connected to inner 
conductor 9b through an opening 16 formed on end wall 6. 
It is appreciated that, since the external circuit directly extends into 
the microwave filter, extension 2a serves not only as the probing 
electrode of the conventional microwave filter but as a connecting path 
for microwave energy, so that smooth energy transfer is achieved with the 
result that no impedance mismatch occurs at the coupling point. Therefore, 
the interconnected circuit modules can be considered as a single, 
impedance-matched circuit. This permits frequency adjustment of filter 1 
with the use of a terminating load resistor having a resistance value 
other than 50 ohms, the impedance of microstrip 2 looking from the strip 
126 toward the strip 12a. 
A further advantage of the present invention is that the two circuit 
modules of different field configuration can be interconnected with 
greater ease and precision than is possible with conventional hybrid 
circuits and that a specially designed connector is not required. 
FIG. 5 is an illustration of modification of the present invention which 
differs from the previous embodiment in that the stripline-to-coaxial 
transition point is achieved by a capacitive coupling formed by a 
conductive film 20 and a microstrip line 21 having an enlarged portion 22 
spaced from the film 20. The microstrip line 21 extends from microstrip 2 
which may be formed by triplate-type transmission lines. 
It is to be noted that the coaxial-stripline transition point may be 
provided at any point along the length of inner conductor 9a by 
appropriately forming the conductive pattern of strip 12a according to the 
field configuration of the connecting point. Formation of microstrip lines 
can be made with ease if the dielectric substrate is formed, for example, 
of a Teflon glass laminate.