Note: Descriptions are shown in the official language in which they were submitted.
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RCA 70,724
BACKGROUND OF THE INVENTION
1 Field of Invention
This invention relates to microwave filters
and more particularly to an array of directional filters.
Description of the Prior Art
A man made satellite that orbits about the
earth is often used to transmit a message to the earth.
Typically, the message is transmitted by a transponder
that is aboard the satellite.
In one type of transponder, the message is a
modulated signal that has a frequency within one of twelve
signal channels. The channels are bands of freq~encies
of approx1mately 36 MHz within a broad band that extends
from 3.7 GHz to 4.2 GHz. There is usually a guard band
of approximately four MHz hetween adjacent channels.
.
The transponder additionally includes twelve
traveiling wave tube amplifiers that respectively
- amplify message signals within the twelve channels. The
outputs of the amplifiers are connected to an antenna
through twelve band pass filters, respectively. The
pass bands of the twelve filters are substantially equal
~ to the twelve channels whereby the filters reject noise - ;
,~ generated by the amplifiers. Therefore, each filter
corresponds to a channel.
When an amplified message signal passes
through a filter with a phase shift that is linearly -
proportional to the frequency of the amplified message
signal, the filter provides an undistorted output. The
rate of change of the phase shift with respect to the
' frequency is known as the group delay of the amplified
message signal. When the phase shift is linearly
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proportional to the frequency, the group delay is
constant.
When two filters, corresponding to adjacent
channels, have their outputs connected together, there is
usually an undesired interaction between the two filters.
The i~raction occurs because the adjacent channels are
separated by only the four MHz guard band. The interaction
causes a variation of the group delay of amplified message
signals within the corresponding adjacent channels.
Additionally, the interaction causes the connected filters
to havé distorted pass bands. To obviate the variation
of the group delay and the distortion of the pass bands, -
the twelve filters are formed into first and second groups
- of six filters that are connected to first and second ports,
respectively, of the antenna.
The first group of filters correspond to six
alternate adjacent channels, whereby the second group also
corresponds to six alternate adjacent channels. Accordingly,
both of the antenna ports receive signals of alternate
adjacent channels, thereby obviating the variation of the
group delay and the distortion of pass bands. However,
because the antenna has two ports, the design of the antenna
is complex.
SUMMARY OF THE INVENTION
According to the present invention, at least
three directional filters are each operable to filter a
corresponding channel of frequencies. All of the
filters are connected to a wall of a waveguide at differing
distances from an end of the waveguide. The filters
closest to the end are in a group of the filters that
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correspond to consecutive alternate adjacent channels.
The number of fllters in the group equals the greatest
number of consecutive alternate adjacent channels.
BRIEF DESCRIPTION OF TE~E DE~AWING
Figure 1 is a block diagram of a preferred
embodiment of the present invention;
Figure 2 is a graphic representation of
frequency channels of a transponder in the embodiment of
Figure l;
Figure 3 is a schematic diagram of an array of
directional filters in the embodiment of Figure l;
Figure 4 is a perspective view with parts broken
away of some of the filters of Figure 3;
Figure 5 is a perspective view, with parts
broken away, of one of the filters of Figure 4; and
Figure 6 is a schematic diagram of an alternative
array of directional filters.
~ESCRIPTION OF THE PREFERRED EMBODIMENT
As shown in Figure 1, a transponder includes
twelve travelling wave tube amplifiers 10-21 that have
their outputs connected to a filter array 22 through ,
signal lines 24-35, respectively. The output of filter
array 22 is connected to an input port of antenna
system 36 through a rectangular waveguide ~ff~. Array
22 comprises twelve directional filters.
The transponder additionally includes a - -
receiver 40 that has an input connected to antenna system
36 through a signal path 42. Receiver 40 has an output
connected through a manifold and input filter system 44
to the inputs of amplifiers 10-21. A suitable filter
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1 system 44 is described in the copending Canadian
patent application number 273,863 filed March 14, 1977 ;;
entitled "Dual Mode Filter" based on the invention of
Chuck Kng Mok.
In response to antenna system 36 receiVing a
signal from a ground station (not shown), receiver 40
provides a message signal to one of the amplifiers 10-21
via system 44, thereby causing an amplified message
signal to be provided through array 22 to antenna
system 36. The amplified message signal causes a
corresponding radiation of electromagnetic energy by
antenna system 36.
As shown in Figure 2, frequencies of all
amplified message signals are within one of twelve
contiguous signal channels 46c-57c which are bands of
frequencies within a broad band that extends from 3.7 GHz
to 4.2 GHz. Each of the channels 46c-57c has a nominal
bandwidth of 40 MHz. Channels 46c-47c are not separated
by guard bands. In this embodiment, message signals
within channels 46c-57c are provided by system 44
(Figure 1) to the inputs of amplifiers 10-21, respectively.
Amplified message signals within channels 46c-57c are
provided to array 22 by amplifiers 10-21, respectively.
Amplifiers 10-21 may introduce distortion into an
amplified message signal. Array 22 rejects the
distortion and provides all amplified message signals to
antenna 36 via waveguide 38 as explained hereinafter. --
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1 As shown in Figures 3-5, array 22 is
comprised of directional filters 46f-57f (Figure 4)
which are all of generally similar construction.
Exemplary of filters 46f-57f, filter 46f (Figure 5)
is a circular ~.
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waveguide with end walls 60 and 62 that are perpendicular
to a central axis 63 of filter 46f. End wall 60 has
passing therethrough a round ended slot 64 and a round
ended slot 66. Slots 64 and 66 are both offset from
axis 63. Additionally, slot 64 is perpendicular to slot ;~
66 and at a known distance therefrom. Slots 64 and 66
form a hybrid slot pair of a type that is well known ;
in the microwave art. ~ -
' 'Similar to slots 64 and 66, a round ended
slot 6~'and a round ended slot 70 form
a hybrid slot pair which pass through end wall 62. '~
Slots 68 and 70 are respectively parallel to slots 64
and 66. Therefore, end plates 60 and 62 include hybrid
slot pairs that have similar orientations about axis 63.
Filter 46f additionally includes a disc shaped
metal couplin~ obstacle 72 that has a circular central
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hole 74 therethrough. Alternatively, filter 46f
may include a coupling obstacle with slots of equal
len~th that lntersect perpendicularly to form a single
slot in the'shape of a cruciform, well known in the art ' ~-'
but not shown here. As explained hereinafter, filter
46f has two modes of operation. In one mode, filter
46f is a band pass filter. In the other mode, filter
46f is a band stop filter. Moreover, the bandwidth
of the stop and the pass bands are the same. The
bandwidth of the pass and stop bands is determined by
the size of hole 74 and the shape of slots 64, 66, and 70
in a manner well known in the art. The pass and stop
bands of a directional filter are collectively referred
to hereinafter as the resonant frequency band of the filter.
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1~841~8
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I Coupling obstacle 72 is mounted midway
between end walls 60 and 62, thereby forming cavities
76 and 78. The axial lengths of cavities 76 and 78
are equal to one half of the wavelength associated with
the center frequency 46m (Figure 2) of channel 46c.
~ecause of the axial lengths of cavities 76 and 78,
the center of the resonant fre~uency band of filter 46f
is substantially equal to frequency 46m. rlOreOver,
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because filter 46f includes two cavities, it is a second
order filter that is conceptually similar to a second
order low pass prototype filter. An alternative
` embodiment may-include directional filters of any
desired order. As known in the art, the order of a
filter equals the number of singularities in a transfer
function that is representative of the filter. Thus,
a second order low pass filter may be represented by a
transfer function having two singularities.
As stated hereinbefore, waveguide 38a is ~ ~-
connected to the output of array 22 to antenna system 36.
End wall 60 is integrally connected to waveguide 38b
(Figure 4) through a port 80 within a wall 82 of waveguide
- 38b. Additionally, filter 46f is oriented with
axis 63 perpendicular to wall 82 and with slot 64
parallel to the top~ bottom, and side walls of waveguide
38b.
End wall 62 is integrally connected (in the
same manner as the connection of end wall 60) to a
waveguide 84 through a port (not shown), similar to
port 80, within a bottom wall (not shown) of waveguide 84.
Filter 46f is oriented with axis 63 perpendicular to the
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; 1 bottom wall of waveguide 84 and with slot 68 parallel
to the top, bottom, and side walls of waveguide 84.
; Waveguide 84 has a closed end 84a that is in
the general shape of a wedge extending into the cavity
of waveguide 84. End 84a is comprised of a solid solution
oE iron oxide powder in epoxy, thereby forming a well ;~
known type of waveguide termination. The concentration
of the solution causes the impedance of the
termination to be the characteristic impedance of
waveguide 84. Thus, there can be no reflection of
signals from end 84a; all signals propagated within
waveguide 84 to end 84a are dissipated therein. In
addition to end 84a, waveguide 84 has an open end 84b.
In the description of this embodiment, the
direction of arrow 86 is referred to as a downstream
direction. Additionally, antenna system 36 has a port
connected in any suitable manner to what is referred to
as a downstream end 88 of waveguide 38a (Figure l).
However, as explained hereinafter, in an alternative
embodiment signals are provided to array 22 from a
source connected to end 88.
The operation of filter 46f is understood by
considering propagation thereto of an exemplary signal
rece1ved via receiver 40. The exemplary signal is
comprised of electromagnetic field components that have
substantially all of the frequencies within the broad
band (3.7 - 4.2 ~,Hz). When the exemplary signal is
propagated through waveguide 38b in the downstream
direction towards filter 46f, the orientation of slots
64, 66, 68, and 70 and the resonant frequency band of
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filter 46f causes substantially the entire component
of the exemplary signal within channel 46c to be
propagated via filter 46f and waveguide 84 to end 84a
where it is dissipated; all other components of the
exemplary signal are propagated downstream through end 88.
Therefore, filter 46f operates as a band stop filter
that rejects signals within channel 46c that are
propagated thereto via waveguide 38b.
It should be understood that the input
impedance of antenna system 36 substantially equals the
characteristic impedance of waveguide 38a. Therefore,
there is substantially no reflection of signals
propagated through end 88.
- When the exemplary signal is propagated
through waveguide 84 from end 84b, the orientation
slots 64, 66, 68, and 70 and the resonant frequency ~ -
band of filter 46f causes substantially the entire signal
component within channel 46c to be propagated through
filter 46f and downstream through end 88; all other
components of the exemplary signal are propagated
through waveguide 84 to end 84a where they are dissipated.
Therefore, filter 46f operates as a band pass filter
that passes signals within channel 46c that are-propagated
thereto from end 84b. Thus, filter 46f functions as ~; -
a band stop filter for signals received via port 80
and functions as a band pass of signals received via
end 84b.
Corresponding to filter 46f, filters 47f-57f,
each include two cavities that have axial lengths equal
0 to one half of the wavelength associated with the
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1 center frequencies of channels 47c-57c, respectively.
Accordingly, filters 46f-57f are respectively
assoclated with channels 46c-57c.
Filters 47f-57f additionally have slots within
end walls thereof corresponding to slots 64, 66, 68,
and 70. Additionally, filters 47f-57f have coupling
obstacles corresponding to coupling obstacle 72.
The bandwidths of the resonant frequency bands of filters
58b-581 is described hereinafter.
In a manner similar to that described in
connectlon with filter 46f, filters 47f-57f all have one
end connected to waveguide 38b. The placement of
filters 46f-57f relative to each other
and to end 88 is as shown in Figure 3. The other
end of filters 47f-57f are connected to waveguides 90-100,
respectively. Waveguides 90-100 are each similar to wave-
guide 84. The connection of waveguides 90-100 to
filters 47f-57f is similar to the connection of filter
46f to waveguide 84.
Waveguides 90-100 have ends 90a-lOoa, respectively,
that are similar to end 84a. Additionally, waveguides
; 90-100 have ends gob-loob, respectivley, that are similar
to end 84b. Ends 84~ and gob-loob are connected
to amplifiers 10-21 through signal lines 24-35, respectively.
Because the input impedance of antenna system 36 substantially
equals the characteristic impedance of waveguide 38a,
the spacing between filters 46f-57f is not critical.
It should be appreciated that when the
exemplary signal'is propagated through waveguide 38b
to filter 47f, there may be an undesired rejection of a
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l portion of the component that includes the lowest
frequencies within channel 47c. The undesired
rejection is a result of the resonant frequency
band of filter 46f undesirably extending into channel
47c, which is the channel adjacent to channel 46c.
The undçsired rejection may be compensated for by a
network at the ground station that receives radiation
from antenna system 36. Since most r,etworks have a
symmetrical response to applied signals, it is desirable
that filter 47f respond symmetrically to the amplified
message signals. Therefore, when a low fre~uency
portion of a message signal within channel 47c is
- undesirably rejected, it is desirable to reject a
corresponding high frequency portion thereof.
Rejection of corresponding portions of message signals
is provided as explained hereinafter.
According to the present invention, in an
array of directional filters, the filters most
downstream correspond to consecutive alternate
adjacent channels. The number of filters most
downstream equals the greatest number of consecutive
alternate adjacent channels. Since array 22 is
comprised of twelve filters corresponding to twelve
adjacent channels, in this embodiment the greatest
- ! 25 number of "consecutive alternate adjacent channels"
equals six. Filters 46f, 48f, 50f, 52f, 54f, and 56f,
which correspond to "consecutive alternate adjacent
channels," are the most downstream of the filters of
array 22.
Because filters 46f, 48f, 50f, 52f, 54f, and 56f
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are the most downstream of the twelve filters of
array 22, there is substantially no rejection of portions
of amplified message signals within the corresponding
channels 46c, 48c, 50c, 52c, 54c, and 56c. However,
when an amplified message signal within channel 47c
is propagated through filter 47f and downstream towards
end 88, the highest and lowest frequency portions of the
amplified message signal may be rejected by filters 48f
and 46f, respectively. In a simiIar manner, amplified
message signals that pass through filters 49f, 51f, 53f,
and 55f have high and low frequency portions that may be
rejected. In summary, there is substantially no
rejection of any portion of an amplified message signal
that pass through six of the filters of array 22. There
may ke a rejection of corresponding high and low
frequency portions of message signals that pass through
five of the filters of array 22.
The most constant group delays of amplified
message signals are attained when the resonant frequency
bands of filters 46f-57f have as large a bandwidth as
feasible. The bandwidths are as large as feasible
when each of the filters 46f-57f has a resonant frequency
band that includes the channel corresponding thereto
and substantial portions of adjacent channels that
correspond to those of filters 46f-57f that are downstream
therefrom. Downstream from filter 47f, for example,
are filters 46f, 50f, 54f, 48f, 52f, and 56f.
Therefore, to attain the most constant group delay,
filter 47f has a resonant frequency band that includes
channel 47c, and substantial portions of channels 46c and
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48c; undesired signals within channels 46c and 48c
that pass through filter 47f are rejected downstream
by filters 46f and 48f, respectively.
Usually, a travelling wave tube amplifier
causes intermodulation distortion of an amplified signal,
' thereby introducing distortion at its output. Amplifier -
21 (Figure 1), for example, may introduce a distortion
signal that has a frequency within any of the signal -~
- channels 46c-57c. Although the band pass characteristics
of filter 57f causes a rejection of components of the
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distortion signal having frequency within channels 46c-56c, ~-
the band stop characteristics of filters 46f-56f
causes an additional rejection of the components of
the distortion signal during their propagation downstream.
; 15 ~herefore, because of the band stop characteristics of
filters 46f-56f, there is a rejection of distortion signals
that may be caused by intermodulation distortion.
Mechanical imperfection of components that
comprise array 22 may cause portions of an amplified
message signal to be propagated upstream through
. - wàveguide 38b. In this embodiment, waveguide 38b has
an upstream end 102 comprised of a termination similar
to that described in connection with end 84a (Figure 4).
Therefore, when a portion of an amplified message
signal propagated upstream to end 102, it is dissipated.
Mechanical imperfections of hybrid slots-
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" may cause undesired reflections of portions of amplified
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message signals from hybrid slots of filters 46f-57f,
thereby causing a build-up of standin~ waves within
waveguide 38b. As shown in Figure 6, in an array 22a,
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such a build-up of standing waves is reduced by including
a circulator 104 of any suitable type in series with
waveguide 38b. Circulator 104 provides a unidirectional
signal path in the direction of arrow 86.
It should be understood that array 22 (Figure 1)
is a linear bilateral network. Therefore, in an alternative
embodiment, end 88 may be connected to a signal source
that provides input signals having frequencies within
channels 46c-57c. In response to the input signals,
waveguides 84 and 90-100 provide output signals having
frequencies within channels 46c-57c, respectively.
Thus, there is described hereinbefore an
array of twelve directional filters that provides
amplified message signals through one port, e.g.,
end 88. The filters are for filtering
twelve contiguous channels,without guard bands,within a
broad~band. Although the channels are contiguous, there
is substantially no rejection of amplified message
signals propagated through six of the filters and a
symmetric rejection of amplified message signals
propagated through five of the filters.
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