Canonical dual mode filter

A plural cavity waveguide filter comprising a plurality of cascaded waveguide cavities each resonating in first and second independent orthogonal modes is disclosed. The cavities may be either square and/or circular resonating in the TE.sub.101 or TE.sub.111 modes, respectively. The filter makes it possible to realize the general class of coupled cavity bandpass transfer filter functions by providing that the input and output couplings to the filter be physically connected to the first physical cavity. A general set of canonical couplings can then be achieved within the structure.

BACKGROUND OF THE INVENTION 
The present invention generally relates to waveguide filters of the type 
using plural dual mode resonant cavities, and more particularly to an 
improvement in such filters which makes it possible to realize the general 
class of coupled cavity transfer functions. 
U.S. Pat. No. 3,697,898 to Blachier and Champeau describes a dual mode 
circular and/or square waveguide filter having input and output ports 
located at the two physical ends of the filter. More specifically, the 
Blachier and Champeau filter uses N physical waveguide cavities which 
resonate in two independent orthogonal modes and are coupled together to 
provide the filtering capacity of n=2N electrical cavities resonating in a 
single mode. Intra cavity coupling is provided by a structural 
discontinuity such as a screw mounted in the cavity wall. Inter cavity 
coupling is provided by means of selective polarization discriminating 
couplings between the N cavities to transfer energy between identical 
modes in the coupled cavities. A particular feature of the Blachier and 
Champeau filter is the use of a phase inversion means in coupled cavities 
to provide a subtraction capability between identical modes in the coupled 
cavities. This substration capability can provide steep response skirts 
for the passband of the filter. 
The Blachier and Champeau filter has significant advantages not only in an 
improved passband response but also in economies in weight and volume and 
in the ease of fabrication which results from the realization of two 
electrical cavities in one physical cavity. However, the Blachier and 
Champeau filter structure will not realize the general class of coupled 
cavity transfer functions since no provision to couple electrical cavities 
1 and n, 2 and n-1 and so forth are provided. For a development of a 
synthesis procedure for the general class of canonical waveguide filters, 
the reader is referred to the article by Atia, Williams and Newcomb 
entitled "Narrow-Band Multiple-Coupled Cavity Synthesis" published in the 
IEEE Transactions on Circuits and Systems, Vol. CAS-21, No. 5, September 
1974, at pages 649 to 655. The Blachier and Champeau filter is not capable 
of generating the optimum response and, therefore, the full potential of 
this particular waveguide cavity structure is not realized. 
SUMMARY OF THE INVENTION 
The present invention is an improvement in the Blachier and Champeau filter 
using either square and/or circular dual mode waveguide cavities. As in 
the Blachier and Champeau filter, intra cavity coupling is provided by 
structural discontinuities, and inter cavity coupling is provided by 
selective polarization discriminating irises or circular holes between the 
cavities. The improvement according to the invention is the provision of a 
reflective plate in one end cavity and both input and output ports in the 
other end cavity of the cascaded waveguide cavities. More specifically, 
the input and output filter ports are connected to the same physical 
cavity, but the input port is coupled to electrical cavity No. 1 and the 
output port is coupled to electrical cavity No. n. As a result of the 
improvement according to the invention, it is possible to couple 
electrical cavities 1 and n, 2 and n-1 and so forth thereby permitting the 
realization of the general class of coupled cavity transfer functions. 
Moreover, all of the advantages associated with the Blachier and Champeau 
filter are retained.

DETAILED DESCRIPTION OF THE INVENTION 
Since the present invention may be generally considered as an improvement 
in the basic Blachier and Champeau dual mode filter, this filter will be 
first described with reference to FIG. 1 of the drawings. The filter 
comprises a plurality of cascaded waveguide cavities 11, 12 and 1N. As 
illustrated in FIG. 1, these cavities may be cylindrical waveguide 
sections connected end to end. It will be understood, of course, that the 
filter could comprise a plurality of square waveguide sections connected 
in line. Each of the cylindrical waveguide cavities is capable of 
resonating at its resonant frequency in first and second independent 
orthogonal modes. Thus, if the filter comprises N physical cavities, then 
there are n=2N electrical cavities. If cylindrical waveguide cavities are 
used, each of the electrical cavities resonates in the TE.sub.111 mode, 
whereas if square waveguide cavities are used, each electrical cavity 
resonates in the TE.sub.101 mode. 
Coupling between the electrical cavities within each physical cavity is 
provided by a physical discontinuity such as a screw in the side wall. The 
intra cavity coupling screws 21, 22 and 2N are mounted in their respective 
cavities at an angle of 45.degree. between the two orthogonal modes 
supported by the cavity. In addition, certain ones of the coupling screws 
such, for example, coupling screws 21 and 22, may be shifted by 90.degree. 
with respect to one another. As a result, the coupling provided by screw 
22 has an opposite sign to the coupling provided by the screw 21. This 
difference in sign in the coupling is a particular feature of the Blachier 
and Champeau filter which permits the achievement of the particular 
function response of the filter. 
Inter cavity coupling is provided by the plates 31, 32 and 3(N-1) which 
define the common end walls of successive cavities in the filter. Each of 
these plates is provided with a cross slot iris which couples like 
oriented modes in the successive cavities. These irises are polarization 
discriminating to transfer energy between identical modes in the coupled 
cavities. Energy is coupled into the filter by means of a slot in the 
exposed end wall 41 of the first cavity 11, and energy is coupled out of 
the filter by means of a similar slot in the exposed end wall 4N in the 
last cavity 1N. The slots in the end walls 41 and 4N are oriented to 
maximize the coupling of any incoming and outgoing waves having the proper 
polarization but to minimize the coupling of waves of all other 
polarizations. 
With the Blachier and Champeau filter as illustrated in FIG. 1 as 
background, the improvement according to the invention will be readily 
understood with reference to FIG. 2 wherein like reference numerals 
designate identical or corresponding parts. According to the invention, 
the exposed end wall of the last cavity 1N is a reflecting plate 5N. At 
the other end of the filter, the exposed end wall of the first cavity 11 
is shown as a plate 51 with a cross slot iris. The purpose here is to 
illustrate that input and output energy is taken from and coupled to the 
same end physical cavity 11 of a cascaded set of dual mode cavities. The 
manner in which the input and output ports of the filter may be actually 
implemented will be described in more detail hereinafter. The cavity 
couplings which are made possible by the filter structure shown in FIG. 2 
can be represented in the matrix form known as a canonical coupling 
matrix. A development of this matrix form is provided in the article by 
Atia, Williams and Newcomb referenced hereinabove. A similar filter 
composed of square waveguide cavities 61, 62 and 6N is illustrated in FIG. 
3. 
As in the Blachier and Champeau filter, inter cavity coupling is provided 
by the plates 31, 32 and 3(N-1) having cross slot irises. For most 
practical applications, a symmetrical filter structure can be used. In 
such cases the cascade couplings between electrical cavities 1 and 2 
equals that between cavities n-1 and n, the couplings between electrical 
cavities 2 and 3 equals that between cavities n-2 and n-1, and so forth. 
These cascade couplings are provided by the cross slot irises in plates 
31, 32 and 3(N-1). Because of the symmetry of these slots in a symmetrical 
filter structure, the cross slot irises can be replaced by circular holes. 
The circular holes have great advantages in the manufacturing and 
machining processes of those filters as contrasted to the cross slot 
irises. FIG. 2A illustrates a plate 3i having a circular hole. Obviously, 
the same modification can be made in the structure shown in FIG. 3. 
A cross slot in the exposed end wall of the first waveguide cavity by 
itself does not provide a practical means for independently coupling the 
input and output ports to the filter. FIGS. 4, 5 and 6 illustrate three 
different ways in which the input and output ports may be coupled 
independently to the two orthogonal electrical cavities within the first 
physical cavity of the filter structure. In FIG. 4, the exposed end wall 
of the first cavity 11 is blank and two coaxial probes 71 and 72 are 
connected to the side wall of the cavity. More specifically, the coaxial 
probes 71 and 72 are mutually perpendicular with respect to one another 
and with respect to the longitudinal axis of the filter. Moreover, these 
coaxial probes are oriented to be in line with the two orthogonal modes in 
the cavity 11. 
In the arrangement shown in FIG. 5, the first cavity 11 is provided with a 
coaxial probe 71 as before, but instead of a second coaxial probe, the 
exposed end wall 51 of the cavity 11 is provided with a slot 73. The slot 
73 is oriented so as to couple energy orthogonally to the energy coupled 
by the coaxial probe 71. 
In FIG. 6, the cross slot 74 in the end wall 51 of the first cavity 11 as 
shown in FIG. 3 is retained. An orthogonal mode transducer 75 is connected 
to this end wall 51. As is well known in the art, an orthogonal mode 
transducer is capable of independently coupling orthogonally polarized 
waves. To this end, the orthogonal mode transducer 75 is provided with a 
shunt port 76 and a series port 77. 
In FIG. 7, the first cavity 11 is provided with a shunt port 77 in the 
sidewall, and the end wall 51 is provided with a slot 73. In this case the 
slot 73 is oriented so as to couple energy orthogonally to the energy 
coupled by the shunt port 77. 
In yet another variation shown in FIG. 8, two shunt ports 77 and 78 are 
provided in the sidewall of the first cavity 11. The shunt ports 77 and 78 
are oriented about the axis of cavity 11 90.degree. with respect to one 
another so as to couple energy in two orthogonal modes. 
Having described the invention in terms of a preferred embodiment, it will 
be understood that the invention is an improved waveguide bandpass filter 
which makes it possible to realize the general class of transfer filter 
functions. Experimental verification of the fourth and eighth order 
elliptic function bandpass filter transfer functions has been made at 
4GHz, and the results are shown in FIGS. 9 and 10, respectively. As will 
be appreciated from those figures, theory and experiment were in excellent 
agreement. The improvement according to the invention is the realization 
of optimum filter responses by allowing the input and output ports to be 
coupled to the same end physical cavity of a cascaded set of dual mode 
cavities.