LOW INSERTION LOSS WAVEGUIDE FILTER

FILTRE DE GUIDE D'ONDES A FAIBLE PERTE D'INSERTION

English Abstract

ABSTRACT OF THE DISCLOSURE:
A low insertion loss waveguide bandpass filter, having
a high unloaded Q factor, is achieved using cascaded cavities of
a length which is n times the half guide wavelength, and being
the third index of a TE mode microwave to be transmitted and at
least larger than one. Volume optimization for high Q performance
is obtained by cascading cylindrical or square cavities of the
same cross-sectional area whereas two different techniques are
used to eliminate spurious responses in the stop-band. In the
first technique, exclusively square or cylindrical cavities of
different cross-sectional areas are cascaded whereas the second
technique is based upon an alternate arrangement of square and
cylindrical cavities. Also, even order filters as well as odd
order filters having high Q performances are realized.

Claims

The embodiments of the invention in which an exclusive
property or privilege is claimed, are defined as follows:
1. A low insertion loss waveguide band pass filter,
comprising M waveguide resonant cavities provided with inter-
coupling means, M being an integer, each of said cavities having
a physical length equal to "n" times the half guide wavelength,
where "n" corresponds to the third mode index of a TE mode
microwave, to be transmitted and is at least larger than one.
2. A filter as claimed in claim 1, wherein said filter
is a dual mode filter and said cavities are of a same cross-
sectional area, each cavity having a length equal to n times the
half guide wavelength at the centre frequency of the filter.
3. A filter as claimed in claim 1, wherein at least one
of said cavities is of a different cross-sectional area than the
other cavities, each cavity being of a length equating n times the
half-guide wavelength thereof.
4. A filter as claimed in claims 2 or 3, wherein all
of said cavities have a square cross-sectional area.
5. A filter as claimed in claim 2 or 3, wherein all
said cavities are cylindrical.
6. A filter as claimed in claims 2 or 3, wherein at least
one of said cavities has a square cross-sectional area and the
remaining cavities are cylindrical.
7. A filter as claimed in claims 1, 2 or 3, further
characterized in that said filter is of order N, N being an even
integer, and that the number M of cavities is equal to ?.
8. A filter as claimed in claims 1, 2 or 3, further
characterized in that said filter is of order M, N being an odd

integer, and that the number M of cavities is equal to <IMG>
cavities operating in dual mode plus at least one single mode
cavity.
9. A filter as claimed in claims 1, 2 or 3, wherein
said filter further comprises a rectangular cross-sectional cavity
mounted in cascade with said M cavities.

Description

i~O50~27
The present invention relates in general to waveguide
bandpass filters for Transverse Electric (TE) waves and concerns
in particular a low-loss waveguide band-pass filter having a~high
unloaded Q factor, the latter being a quality factor which deter~
mines, and is inversely proportional to, the mid-band insertion loss
of a speclfic filter design.
Known rectangular waveguide band-pass filters use s~nchro-
nously tuned rectangular cavities in cascade. These filters arecons~
tructed out of a straight waveguide section in which the cavities
are formed through the use of inductive posts or irises. A more
recent development of waveguide type microwave filters i5 the dual
mode filter. Such filters employ square or cylindrical resonators
operating in TElol and TElll modes respectively. The length of each
cavity is equal to half guide'wave'length at the filter centre fre-
quency, and its unloaded Q is approximately equal to the Q of the
rectangular waveguide filters. For those types of filters the un-
loaded Q decreases with increasing frequency.
The advantages and the construction details of the dual
mode filter are described, for example, in Canadian Patent No.
896,116 issued on March 21, 1972, to Blachier et al. It is much
lighter than the conventional filters and also its mechanical struc-
ture permits to achieve elliptic filter functions.
The types of filter described above can, with care, be
realized to exhibit unloaded ~'s of 10,000 at 4 GHz and 5,000 at
12 GHz. This means that the mid-band insertion loss of a filter at
12 GHz, with the same percentage bandwidth and other passband cha-
racteristics as one at 4 GHz, will be twice larger. In modern com-
munication systems, the realization of filters with higher unloaded
Q's, particularly at frequencies above 10 GHz, would be advantageous.
In satellite systems, a second advantage would accrue i~ the volume,
and hence the weight of such filters could be minimized.
A prime o~ject of the present invention resides in a
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105~27
waveguide bandpass filter which exhibits significantly higher un-
loaded Q's than conventional ones, and which therefore promotes a
substantially low mid-band insertion loss therein.
In accordance with the present invention, the low-loss
bandpass waveguide filter comprises M waveguide resonant cavities
provided with intercoupling means, M being an integer, and wherein
each cavity has a physical length of "n" times the haLf guide wave-
length, where "n" is the third mode index of a TE mode microwave to
be transmitted and is larger than one.
Preferred emb~diments of the present invention ~ill be
hereinafter described with reference to the accompanying drawings,
wherein
Figures lA and lB show views, in perspective, of TElo and
T~ dual mode bandpass microwave filters in accordance with the
lln
present invention, which filters are respectively constituted of
square and cylindrical cavities of the same cross-sectional area;
Figures lC and lD are cross-sectional views taken along
line C-C and line D-D, respectively, of figures lA and lB;
Figures 2A and 2B respectively show square and cylindrical
filters provided with cavities of different cross-sectional areas,
following another embodiment of the present invention;
Figure 3 depicts a bandpass filter having cascaded cylin-
drical and squa~ cavities of cross-sectional areas in accordance
with a further embodiment of the present invention.
Figures 4A and 4B illustrate a bandpass filter wherein a
single rectangular cavity is cascaded with other square or cylindri-
cal cavities, respectively, according to a further embodiment of the
present invention.
Figures lA and lB illustrate, in perspective, high Q, low
insertion loss, dual mode filters provided with M cascaded cavities
each bein~ of a length "1", M ~einq an inte~er.The ilter of figure lA
is a TElon filter macle up of cavities Sl ..Sm of square cross-

~ )5C~127
sectional area whereas the ilter of figure lB is a TE ~ filter madeup of cylindrical cavitiesC~ . The cavities composina each filter
have the ~ame cross-sectional area and ~ach cavit.v is n ~ g long,
where ~ g is the guide wavelength at the centre frequency of the
filter. Also, the height or diameter of each cavity is carefully
selected from known mode lattice charts for optimum ~ and spurious
response elimination. - ~
The reSonance modes TE10 and TElln' where n ls the
third mGde index and larger than 1, have been selected in order to
accomplish effectivély a reduction in both the insertion loss
and the volume of the filter. In the embodiments of figures lA and
lB, each increase in the index "n" corresponds to an additional
increase of.half a guide wavelength in the length of the cavity.
According to the present invention, by choosing n larger than 1,
in connection with the length of each cavity, the unloaded Q of the
filter thereby obtained for the TE modes is approximately 60% lar-
ger than that of conventional filters, wherein n _ 1. Therefore,
with the illustrated filters, unloaded Q optimization in the filter
is achieved through a volume optimization of each cavity, which
volume is proportional to the length thereof.
Turning again to figures lA and lB, two adjacent cavities
of the filters are separated by an intercoupling reElective plate
1 or ~ provided with a slot or iris 1' or 2', and both filters
are limited by input ports 3 and 5 and output por~s 4 and 6, res-
pectively.
Three tuning screws per cavity are used, these screws
being disposed along a predetermined cross-sectional plane in a
given cavity. The frequency tuning screws Tl and T2 are 90 apart
and are in line with the two orthogonal E field components of the
cavity, as better seen from figures lC and lD. The coupling screw .
C can be located at any multiples of 45 with respect to Tl and
T2. The length location Ls of the screws is a function of the
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5~7
mode index "n" and is equal to Ls = (2K~- 1) Lj where K is any
integer ranging from 1 to n. The choices of Ls = Lj/2 for n odd,
and Ls close to Lj/2 for n even are preferable, Lj being the
length of the cavity.
Two dual mode filters of the type shown in Fig~ lA and lB
were realized in the 11.7-12.2 GHz communications band. Both filters
were four-section elliptic function type. The square waveguide
type was operated in TE103 mode and i~s side was of 0.860 inch.
The cylindrical waveguide type was operating in TE112 mode and its
diameter D was of 1.275 inches. Both filters have a bandwidth of
80 MHz centred on 12 GHz and a measured unloaded Q of 8000. The
intercavity coupling was obtained via inductive irises, and the
..
in-band spurious signals at a level of 35 dB below that of the use-
ful signal transmitted in the passband.
Figures 2A and 2B show further arrangements of the bandpass
of the present invention, wherein the various cascaded cavities
Si...S or C ...C ,composing each filter are of different cross-
sectional areas for improved elimination of spurious waves. Bv
doing so, the spurious of any one cavity do not coincide with the
spurious o~ the other cavities, and are therefore attenuated. The
attenuation level of the spurious passbands is dependent on the
number of filter sections, the communication band, and the numher
of non-identical ~ilter cavities.
The filters of Figures 2A and 2B are constructed out of
square or cylindrical cavities with gradually or successively increa-
sing and/or decreasing cross-sectional areas. Such cavities may be
cascaded having the cavity with the minimum or maximum cross-sec-
tional area at the centre of the filter and repeated pairs of
square or cylindrical cavities in such a way that its symmetry is
maintained.
In the arrangements of Figures 2A and 2B, by eliminating
the spurious responses the cavity volume can be optimized for higher

~5~1~,7
values of unloaded Q's. The location of the tuning screws (not
shown) is the same as in the filters of Fig. 1, but the cavity
length is equal to n 2 gc, where ~ gc is the cavity guide wave-
length and n ls the mode index.
One square four-section elliptic function prototype fil-
ter using cavities of heights 0.860 inch and 0.900 inch,and one
filter havin~ cylindrical cavities at diam~ters of 1.275 inches and
1.225 inches were both experimented in the 11.7-12.2 G~z communica-
tions band. ~oth filters had a bandwidth of 80 M~z and a measured
unloaded Q of 8300. The in-band spurious ~ere at a level of 50 dB
below that of the transmitted signal in the passband.
In most communication systems, we are usually concerned
with having a 500 MHz band free of any spurious responses. The
above-described technique in accordance with the present invention
satisfies this requirement. But, if spurious have also to be
eliminated at frequencies outside of that band, then the filter
may be constructed out of mixed square and cylindrical cavities S'
and C', as shown in Fi~. 3. That illustrated arran~ement is even
more powerful hecause the behaviour of the two types of cavities is
entirely different. ~ combination of both techni~ues ~i.e., a cascade
of s~uare and cylindrical cavities with different cross sections),
along with modes of different index "n" will give excellent spurious
eliminationand unloaded Q performance. The dimensions o the
cavities can be calculated using known square or cylindrical mode
lattice charts, the frequency location o each spurious can also
be evaluated.
In Fig. 3, the square and cylindrical cavities of diffe-
rent cross sections are cascaded, in conformity with the methods
described in Figs. 2A and 2B above. Such cavity arrangements pro-
vide a ilter near its optimum Q at the frequency of interest and,at the same time, eliminate the spurious passbands. In this case,
mixed mode cavities can also be employed (i.e., TE103 and TE
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2'7
or TE103 and TE113). The construction de-tails of Figs. lA and ls
as well as 2A and 2B remain valid in such fil~er embodiments.
In the above-described bandpass filters, the filter order
"N" is also of importance in the selection of filter cavities.
Where "N" is even, the filter can be carried out by using N/2 dual
mode physical cavities. For "N" odd, the filter is made up of
(N-1)/2 cavities operating in dual mode, and an additional cavity
operating in single mode. The symmetry of the filter may be pre-
served by placing the single mode cavity at the centre of the filter.
Using the above technique, every filter order can be carried out
with the restriction that a gO rotation exists between input-
output ports for some "N" values (i.e., N = 6, 7, 10, 11, etc.).
-
If port rotation is undesirable, then additional single mode cavi-
ties are used to bring the input and output ports in line.
~ igures 4A and 4B show the interconnection of a single
mode cavity R in odd ord~,r filters and filters of n~n-rotated input-
output ports. A rectangular aavity with a suitably chosen height-
~o-width ratio is cascaded between square or cylindrical cavities.
In this case, only one frequency tuning screw ~not shown) is re~ui-
red. Cavities of different cross sections, cylindrical or ~qu~re,or both ~an also be used.
A five-section Chebychev function filter incorporating
two square cavities of height 0.860 inch operating in dual mode
and a single mode centre rectangular cavity having a height of 0.900
inch and a width of 0.800 inch was experimented in the 11.7-12.2 GHz
communication band. The filter's unloaded Q was 8000 and its in-
band spurious responses at a level of 45 dB below that of the trans-
mitted signal in the passband. A further attenuation of the spurious
passbands may be achieved when using filter cavities of the type
discussed in Figs. 2A, 2B and 3. Bandpass filters having maximally
flat or Chebychev or quasi-elliptic responses can be realized using
the configurations of Figs. 4A and 4B.

i~ 27
It is to be understood that modifications may be drawn
to the above-described, preferred embodiments c the present inven-
tion without departing from the ambit thereof which is solely limi-
ted by the scope of claims which follow.
. . ,