Output Multiplexer
Description
The present invention relates to an output multiplexer (OMUX). More particularly,
5 the present invention relates to an OMUX comprising a plurality of hybrid-coupled
filters.
Communications satellites are widely used for providing telecommunications links
between different locations on the Earth's surface. Figure 1 illustrates an example
10 of a conventional satellite communication system. A communications satellite 101
receives a frequency-multiplexed signal from an uplink antenna 102. The satellite
101 demultiplexes the received signal into a number of channels, and amplifies the
signal on each channel. An output multiplexer (OMUX) is used to recombine the
amplified signals into a high-power multiplexed output signal, which is transmitted
15 to a plurality of ground-based receivers 103.
Figure 2 illustrates a conventional OMUX for use on a communications satellite.
The OMUX 200 comprises a plurality of filters 201 arranged along a manifold 202,
and hence may be referred to as a manifold multiplexer. The OMUX 200 receives a
20 plurality of input signals 203, 204, which are combined in the manifold 201 and
outputted as a multiplexed output signal 205. However, as there is considerable
interaction between filters on different input channels, manifold multiplexers are
complicated to design and tune. Designing and tuning the OMUX 200 becomes
progressively more complex as additional input channels are added, making it
25 impractical to produce a manifold multiplexer with more than ~20 input channels.
Figure 3 illustrates a conventional hybrid-coupled OMUX 300, which is commonly
used in ground-based applications. The hybrid-coupled OMUX 300 comprises a
plurality of hybrid-coupled filters 310, 320, 330, 340. Each hybrid-coupled filter
30 310 comprises an input hybrid coupler 311 which splits an input signal into two
half-power signals. These half-power signals are passed through two filters 312, 313
and recombined by an output hybrid 314. Therefore, as two filters are required per
input channel, the conventional hybrid-coupled OMUX has a significantly increased
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mass in comparison to a manifold multiplexer. As a result, hybrid-coupled OMUXs
are unsuitable for use in communications satellites, where any increase in weight
may significantly increase the launch cost.
5 The present invention aims to address the drawbacks inherent in known
arrangements.
According to the present invention, there is provided an output multiplexer OMUX
comprising a plurality of hybrid-coupled filters, each arranged to receive a first
10 input signal via a first input port and a second input signal via a second input port,
and output a first output signal via a first output port and a second output signal via
a second output port, wherein the hybrid-coupled filters are connected to combine a
plurality of said first output signals into a first multiplexed signal output from a first
output port of the OMUX, and combine a plurality of said second output signals
15 into a second multiplexed signal output from a second output port of the OMUX.
20
Each one of the hybrid-coupled filters may comprise first and second hybrid
couplers, with first and second bandpass filters connected between the first and
second hybrid couplers.
The first and second input ports may comprise input ports of the first hybrid
coupler, and the first and second output ports may comprise output ports of the
second hybrid coupler.
25 The first and second bandpass filters of any one of the plurality of hybrid-coupled
filters may be arranged to have substantially similar transfer functions.
30
The first and second bandpass filters may be arranged to be controllable so as to
tune at least one of a centre frequency and a passband width.
The plurality of hybrid-coupled filters may comprise a number N of hybrid-coupled
filters, wherein the first OMUX output port is an output port of an Nth one of the
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hybrid-coupled filters, and the second OMUX output port is an output port of a
first one of the hybrid-coupled filters.
The OMUX may further comprise at least one bidirectional connection between
5 adjacent ones of the hybrid-coupled filters, the bidirectional connection being
arranged to carry the first and second output signals in opposite directions, and
preferably, the bidirectional connection may be arranged to connect the first output
of one of the hybrid-coupled filters to the second output of another one of the
hybrid-coupled filters.
10
The first multiplexed signal may be arranged to be transmitted as a vertically
polarised signal, and the second multiplexed signal may be arranged to be
transmitted as a horizontally polarised signal.
15 The first and second input signals of each one of the plurality of hybrid-coupled
filters may either correspond to downlink channels having the same frequencies, or
may correspond to downlink channels which are adjacent in frequency.
The plurality of first input signals and the plurality of second input signals may
20 comprise microwave signals having frequencies in the Ku band.
The OMUX may be configured for use in a communications satellite.
According to the present invention, there is also provided apparatus comprising the
25 OMUX and means for combining the first and second multiplexed signals.
The means for combining may comprise an orthogonal mode transducer OMT, the
OMT being arranged to receive the first and second multiplexed signals and output
a combined signal to a reflector antenna, wherein the combined signal comprises
30 the first multiplexed signal as a vertically polarised signal and the second
multiplexed signal as a horizontally polarised signal.
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The means for combining may comprise first and second feed horns of a reflector
antenna, the first feed horn being arranged to receive the first multiplexed signal
and the second feed horn being arranged to receive the second multiplexed signal,
and wherein the first and second feed horns and the reflector antenna are arranged
5 to combine the first and second multiplexed signals in space.
According to the present invention, there is further provided a hybrid-coupled filter
for use in the OMUX, the hybrid-coupled filter comprising a first hybrid coupler
having first and second input ports, a second hybrid coupler having first and second
10 output ports, and a plurality of filters connected between the first and second
hybrid couplers, wherein the hybrid-coupled filter is arranged to receive the first
input signal via the first input port and the second input signal via the second input
port, and output the first output signal via the first output port and the second
output signal via the second output port.
15
Embodiments of the invention will now be described, by way of example, with
reference to the accompanying drawings, in which:
Figure 1 illustrates a conventional satellite communications system, according to the
prior art;
20 Figure 2 illustrates a manifold multiplexer for use on a communications satellite,
according to the prior art;
Figure 3 illustrates a hybrid-coupled OMUX for use in ground-based applications,
according to the prior art;
Figures 4a and 4b illustrate a hybrid-coupled filter according to an example of the
25 present invention;
Figure 5 illustrates a hybrid-coupled OMUX according to an example of the present
invention;
Figures 6a and 6b illustrate the allocation of frequencies within the Ku band;
Figure 7 illustrates frequency-shifting of blocks within the A band for inputting into
30 a hybrid-coupled OMUX, according to an example of the present invention;
Figure 8 illustrates a hybrid-coupled OMUX according to an example of the present
invention;
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Figure 9 illustrates an output section of a communications satellite, according to an
example of the present invention;
Figure 10 illustrates an output section of a communications satellite, according to
another example of the present invention;
5 Figure 11 illustrates an output section of a communications satellite, according to a
further example of the present invention;
Figure 12 illustrates an output section of a communications satellite, according to a
further example of the present invention;
Figure 13 illustrates the allocation of frequencies within the input signals of Fig. 12,
10 and the corresponding passbands of each hybrid-coupled filter, according to an
example of the present invention; and
Figures 14a and 14b illustrate a hybrid-coupled OMUX comprising tunable
bandpass filters, according to an example of the present invention.
15 Referring now to Figs. 4a and 4b, a hybrid-coupled filter for use in a hybrid-coupled
OMUX is illustrated according to an example of the present invention. The hybridcoupled
filter 400 comprises a first hybrid coupler 401 and a second hybrid coupler
402, with a first bandpass filter 403 and a second bandpass filter 404 connected
between the hybrid couplers 401, 402. Both the first and second bandpass filters
20 403, 404 are arranged to have similar transfer functions, i.e. the similar passband
widths and centre frequencies.
In Fig. 4a the path taken by a signal A through the hybrid-coupled filter 400 is
illustrated in bold. The signal A is input to a first port 410 of the first hybrid
25 coupler 401, which splits the signal into two signals, each having half the power of
the original input signal A. A signal emerging from one output port (the
"transmitted" port) is in-phase with the original input signal. In Fig. 4a, this signal
is shown as A0 - 3 dB, the subscript indicating that the signal is phase-shifted by
zero degrees (i.e. in phase), and the- 3 dB indicating that the signal is reduced in
30 power by 3 dB (i.e. 50%). A signal emerging from the other output port (the
"coupled" port) is phase-shifted by ninety degrees with respect to the original input
signal A. In Fig. 4a, this signal is shown as A90 - 3 dB.
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As shown in Fig. 4a, each of the half-power signals (A0 - 3 dB; A90 - 3 dB) passes
through one of the bandpass filters 403, 404. The signals are substantially
unchanged by the bandpass filters 403, 404, since the input signal A is arranged to
only contain frequencies lying within both bandpass filters 403, 404. The half-
5 power signals are then input to the second hybrid coupler 402.
A first output port 412 of the second hybrid coupler 402 acts as the coupled port
for the in-phase signal, i.e. A0 - 3 dB, hence this signal is phase-shifted by ninety
degrees and outputted as A90 - 3 dB. The first output port 412 acts as the
10 transmitted port for the phase-shifted signal, i.e. A90 - 3 dB, hence this signal is
unchanged and outputted as A90 - 3 dB. Therefore, at the first output port 412,
these signals are in-phase and add together, the overall result being that the signal
outputted from this port is A90, i.e. phase-shifted by ninety degrees with respect to
the input signal A, and with substantially the same power as the input signal A.
15
Similarly, at a second output port 413 of the second hybrid coupler 402, the signals
are out-of-phase (i.e. A0 - 3 dB and A180 - 3 dB). Therefore the signals cancel, and
no signal is outputted from the second output port 413.
20 In prior art examples of hybrid-coupled filters, a second port of the input hybrid is
unused and is terminated by a matched load (cf. matched load 315 of Fig. 3).
However, in the present example as shown in Figs. 4a and 4b, a second port 411 of
the first hybrid coupler 401 is used as an input port for a second signal B. The
operation of the hybrid-coupled filter 401 on this second signal B will now be
25 described with reference to Fig. 4b.
In Fig. 4b the path taken by a signal B through the hybrid-coupled filter 400 is
illustrated in bold. The first and second hybrid couplers 401, 402 operate on the
second signal B in a similar manner to that described previously for the first signal
30 A, and so a detailed description will be omitted. In brief, the first hybrid coupler
401 splits the second signal B into two half-power signals, B0 - 3 dB and B90 - 3 dB.
These pass through the first and second bandpass filters 403, 404, are recombined
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by the second hybrid coupler 402, and outputted from the second output port 413
as the output signal B90 .
The first signal A and the second signal B may be simultaneously input to the first
5 hybrid coupler 401. Therefore, in the present example, the hybrid-coupled filter
400 is able to simultaneously receive and output two separate signals, unlike prior
art hybrid-coupled filters which may only receive a single input signal.
Furthermore, in the present example, the first bandpass filter 403 and the second
bandpass filter 404 are each used for both input signals A and B. Therefore it is
10 only necessary to provide two bandpass filters for two input signals, unlike prior art
hybrid-coupled filters which require two bandpass filters for a single input signal.
Referring now to Fig. 5, a hybrid-coupled OMUX is illustrated according to an
example of the present invention. The hybrid-coupled OMUX 500 comprises first,
15 second, third and fourth hybrid-coupled filters 510, 520, 530, 540, each of which is
substantially similar to the hybrid-coupled filter 400 of Figs. 4a and 4b. To avoid
confusion, in the present example subscripts are used to denote signals input to a
particular one of the hybrid-coupled filters, rather than to denote any specific phase
relationship. For example, A1 and B1 denote signals input to the first hybrid-
20 coupled filter 510, A2 and B2 denote signals input to the second hybrid-coupled
filter, and so on. In Fig. 5, a path taken by a second signal B4 input to a second
input port of the fourth hybrid-coupled filter, is shown in bold.
The first hybrid-coupled filter 510 receives a first signal A1 via an input hybrid
25 coupler, and outputs the first signal A1 via the corresponding output port of an
output hybrid coupler. Once on the output side of the hybrid-coupled OMUX 300,
the first signal A1 is unable to pass through the bandpass filters of any of the
remaining hybrid-coupled filters 520, 530, 540, since these bandpass filters are
arranged to reject any frequencies within the first signal A1 . Specifically, the
30 bandpass filters within each hybrid-coupled filter are arranged to pass wanted
frequencies within the first and second input signals, and reject other frequencies.
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The bandpass filters within a hybrid-coupled filter therefore effectively act as oneway
gates, allowing an input signal through to the output side of the hybrid-coupled
OMUX 500 but preventing other signals from exiting. In this way, a plurality of
first input signals A1, A2, A3, A4 are combined on the output side of the hybrid-
S coupled OMUX 500, and outputted as a first multiplexed signal via a first output
port 541 of the hybrid-coupled OMUX 500.
The plurality of second signals B1, B2 , B3, B4 are similarly combined on the output
side of the hybrid-coupled OMUX 500, but travel through the output side in an
10 opposite direction to the plurality of first signals A1 , A2, A3 , A4• Therefore, the
plurality of second signals B1, B2, B3, B4 are outputted as a second multiplexed signal
via a second output port 511 of the hybrid-coupled OMUX 500.
As shown in Fig. 5, adjacent ones of the plurality of hybrid-coupled filters are
15 connected by bidirectional connections which carry the first and second signals in
opposite directions. Here, 'adjacent' refers to hybrid-coupled filters which are
sequentially adjacent in the OMUX 500, i.e. adjacent in terms of a sequence in
which the hybrid-coupled filters are connected in the OMUX 500. It is not
necessary that the sequentially adjacent hybrid-coupled filters are physically adjacent
20 to one another. Preferably, each bidirectional connection is arranged to connect the
first output of one of the hybrid-coupled filters to the second output of another one
of the hybrid-coupled filters, as in the embodiment shown in Fig. 5.
Referring now to Figs. 6a and 6b, the allocation of frequencies within the Ku band
25 is illustrated. As shown in Fig. 6a, the Ku band is subdivided into low band and
high band frequencies, with low-band being used for fixed satellite services (FSS)
and high-band being used for broadcast satellite services (BSS). The low-band is
further divided into A, B, C and D bands, whilst the high-band is further divided
into E, F and G bands.
30
As shown in Fig. 6b, the A band is further subdivided into sixteen frequency blocks
01-16, of which the odd-numbered blocks are transmitted with a vertical (V)
polarisation, and the even-numbered blocks are transmitted with a horizontal
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polarisation (H). Each block is 27 MHz wide, with a guard interval of 4.25 MHz
between adjacent blocks.
An orthogonal mode transducer (OMT) may be used in order to transmit the
5 horizontally polarised and vertically polarised signals via the same antenna.
Specifically, a first input port of the OMT is arranged to vertically polarise an input
signal, whilst a second input port is arranged to horizontally polarise an input signal.
Therefore, the OMT may allow two input signals of the same frequency to be
transmitted via the same antenna, by polarising the two signals with respect to one
10 another.
Examples of the present invention will now be described in which one or more
hybrid-coupled OMUXs are used to provide multiplexed signals for transmission as
either horizontally polarised signals or vertically polarised signals. The skilled
15 person will appreciate that in these examples, the first and second input signals are
not actually polarised with respect to one another as they travel through the
OMUX, since they pass through the same waveguide and filters. The horizontal
and vertical polarisation may be applied later, by inputting the first and second
multiplexed signals to respective inputs of an OMT. However, for clarity, signals
20 which are intended to be transmitted with a horizontal polarisation will hereinafter
be denoted by an 'H', whilst signals which are intended to be transmitted with a
vertical polarisation will be denoted by a 'V'.
According to an example of the present invention, the first multiplexed signal from
25 a hybrid-coupled OMUX may be arranged to be transmitted as a vertically polarised
signal, and the second multiplexed signal may be arranged to be transmitted as a
horizontally polarised multiplexed signal. This will now be described with reference
to Figs. 7 and 8.
30 Figure 7 illustrates frequency-shifting of blocks within the A band for inputting into
the hybrid-coupled OMUX 800 of Fig. 8, according to an example of the present
invention. The hybrid-coupled OMUX 800 comprises first, second, third and
fourth hybrid-coupled filters 810, 820, 830, 840, and functions in a substantially
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similar manner to the hybrid-coupled OMUX SOO of Fig. S. As such, a detailed
description will be omitted in order to maintain brevity.
Before the H signals (i.e. signals to be transmitted with a horizontal polarisation) are
5 input into the hybrid-coupled OMUX 800, they are shifted down in frequency by
1S.62S MHz in order to align with the V signals (i.e. signals to be transmitted with a
horizontal polarisation). Two H signals and two V signals are then allocated to one
of four S8.2S MHz channels, CH1, CH2, CH3, or CH4.
10 In Fig. 8, CH1 (V) denotes an input signal containing frequencies within blocks 01
and 03 of Fig. 7, whilst CH1 (H) denotes an input signal containing frequencies
within blocks 02 and 04. Similarly, CH2 (V) contains blocks OS and 07, CH2 (H)
contains blocks 06 and 08, CH3 (V) contains blocks 09 and 11, CH3 (H) contains
blocks 10 and 12, CH4 (V) contains blocks 13 and 1S, and CH4 (H) contains blocks
15 14 and 16.
Taking the first hybrid-coupled filter 810 as an example, the bandpass filters within
the first hybrid-coupled filter 810 are arranged to have a passband covering all
frequencies within the first channel of Fig. 7, i.e. CH1. Therefore, signals having
20 frequencies within blocks 01, 02, 03 and 04 are permitted to pass through the
bandpass filters, whilst any other frequencies (e.g. blocks OS to 16) are rejected.
Similarly, the bandpass filter of the second hybrid-coupled filter 820 are arranged to
pass frequencies within blocks OS to 08, the bandpass filter of the third hybridcoupled
filter 830 are arranged to pass frequencies within blocks 09 to 12, and the
25 bandpass filter of the fourth hybrid-coupled filter 840 are arranged to pass
frequencies within blocks 13 to 16.
As the V signals are input into the first input port of each hybrid-coupled filter, the
V multiplexed signal is outputted via an output port of the fourth hybrid-coupled
30 filter 840, i.e. a first output port 841 of the OMUX 800. Conversely, as the H
signals are input into the second input port of each hybrid-coupled filter, the H
polarised multiplexed signal is outputted via an output port of the first hybridcoupled
filter 810, i.e. a second output port 811 of the OMUX 800.
5
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Various exemplary output section architectures of a communications satellite will
now be described with reference to Figs. 9 to 13, according to examples of the
present invention.
Referring now to Fig. 9, an output section of a communications satellite is
illustrated according to an example of the present invention. The output section
900 comprises first, second and third hybrid-coupled OMUXs 901, 902, 903, each
of which may be similar in structure to the hybrid-coupled OMUX shown in Fig. 8.
10 In the present example, the first hybrid-coupled OMUX 901 is arranged to receive
input signals within the C-band of the Ku band (cf. Fig. 6a), the second hybridcoupled
OMUX 902 is arranged to receive input signals within the A-band of the
Ku band, and the third hybrid-coupled OMUX 903 is arranged to receive input
signals within the E-band of the Ku band. Therefore the first hybrid-coupled
15 OMUX 901 covers the frequency range 10.95- 11.20 GHz, the second hybridcoupled
OMUX 902 covers the frequency range 11.20- 11.45 GHz, and the third
hybrid-coupled OMUX 903 covers the frequency range 11.70- 12.10 GHz.
In the present example, not all channels of the hybrid-coupled OMUXs 901, 902,
20 903 are utilised at the same time. For example, some channels may not be required
during normal operation of the communications satellite, but may be provided for
redundancy, i.e. to back up channels of another satellite in the event of a failure on
that satellite. Therefore, a switching block 907 is provided to route a plurality of
input signals 911,912,913,914,915,916,917,918,919,920 to appropriate
25 channels of the hybrid-coupled OMUXs 901, 902, 903.
The output section 900 further comprises a first manifold multiplexer 904 and a
second manifold multiplexer 905. The first manifold multiplexer 904 is arranged to
receive the H multiplexed signals from each of the first, second and third hybrid-
30 coupled OMUXs 901, 902, 903. Specifically, a first filter of the first manifold
multiplexer 904 is arranged to have a passband from 10.95 GHz- 11.20 GHz, and
to receive the H multiplexed signal from the first hybrid-coupled OMUX 901.
Similarly, the second and third filters of the first manifold multiplexer 904 are
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arranged to have pass bands from 11.20 - 11.45 GHz and 11.70 - 12.10 GHz
respectively, and receive the H multiplexed signals from the second and third
hybrid-coupled OMUXs 902, 903, respectively.
5 The second manifold multiplexer 905 is arranged to receive the V multiplexed
signals from each of the first, second and third hybrid-coupled OMUXs 901, 902,
903. Specifically, a first filter of the second manifold multiplexer 905 is arranged to
have a passband from 10.95 GHz- 11.20 GHz, and to receive the V multiplexed
signal from the first hybrid-coupled OMUX 901. Similarly, the second and third
10 filters of the second manifold multiplexer 905 are arranged to have pass bands from
11.20-11.45 GHz and 11.70-12.10 GHz respectively, and receive the V
multiplexed signals from the second and third hybrid-coupled OMUXs 902, 903,
respectively.
15 Output signals from the first and second manifold multiplexers 904, 905 are then
passed to an orthogonal mode transducer (OMT) 906. The OMT 906 horizontally
polarises the H signals and vertically polarises the V signals, and outputs the
horizontally polarised H signals and vertically polarised V signals to a feed horn of a
downlink reflector dish (not shown).
20
The output section 900 illustrated in Fig. 9 may offer a substantially advantage over
prior art arrangements, since the majority of the multiplexing is performed by
hybrid-coupled OMUXs. As discussed above, hybrid-coupled OMUXs according to
examples of the present invention only require a single filter per input signal, and
25 therefore may not have a significantly increased weight in comparison to a manifold
multiplexer. However, because there is little or no interaction between filters of a
hybrid-coupled OMUX, the time and effort required to design and tune the
OMUXs of Fig. 9 may be significantly reduced, and hence the overall cost may also
be reduced.
30
Referring now to Fig. 10, an output section of a communications satellite is
illustrated according to another example of the present invention. The output
section 1000 is substantially similar in many respects to the output section 900 of
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Fig. 9, and so a detailed description will be omitted in order to maintain brevity.
However, the output section 1000 of the present example differs in that instead of
combining the H multiplexed signals and V multiplexed signals in manifold
multiplexers, they are combined in another hybrid-coupled OMUX 1001. This
5 arrangement may offer a further cost saving over the arrangement shown in Fig. 9,
since the hybrid-coupled OMUX 1001 may be simpler to manufacture than the
manifold multiplexers.
Referring now to Fig. 11, an output section of a communications satellite is
10 illustrated according to a further example of the present invention. The output
section 1100 is substantially similar in many respects to the output sections 900,
1000 of Figs. 9 and 10, and so a detailed description will be omitted in order to
maintain brevity. However, the output section 1100 of the present example differs
in that connections 1104, 11 OS are provided between the first, second and third
15 hybrid-coupled OMUXs 1101, 1102, 1103. This allows the first, second and third
hybrid-coupled OMUXs 1101, 1102, 1103 to perform the full multiplexing
operation, providing a single H multiplexed signal and a single V multiplexed signal
to the OMT.
20 Although examples of the present invention have been described in which a hybridcoupled
OMUX is provided for separately multiplexing H signals and V signals,
other arrangements are possible. For example, Figs. 12 and 13 illustrate an output
section of a communications satellite, according to a further example of the present
invention, in which all input channels are intended for transmission with the same
25 polarisation.
As shown in Fig. 12, a hybrid-coupled OMUX is provided which comprises eight
hybrid-coupled filters, arranged to receive a total of sixteen input signals. Figure 13
illustrates the allocation of frequencies within the input signals of Fig. 12, and the
30 corresponding passbands of each hybrid-coupled filter. Adjacent ones of the input
channels 1 to 16 are input into each hybrid-coupled filter. Specifically, oddnumbered
channels are input to the first input port of each hybrid-coupled filter,
and even-numbered channels are input to the second input port of each hybridwo
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coupled filter. Accordingly, the hybrid-coupled OMUX 1201 outputs a first
multiplexed signal comprising the odd-numbered channels, and a second
multiplexed signal comprising the even-numbered channels. The first and second
multiplexed signals are then sent to first and second feed horns 1202 respectively,
5 and combined in space by primary and secondary reflectors 1203, 1204.
Referring now to Figs. 14a and 14b, a hybrid-coupled OMUX 1400 comprising
tunable bandpass filters is illustrated, according to an example of the present
invention. The filters operating on a pair of input channels (i.e. the first and second
10 inputs of a hybrid-coupled filter) may be tuned independently of the filters for
different input channels, as there is minimal interaction between the different filters
of a hybrid-coupled OMUX. As shown in Fig. 14a, a control unit 1401 may be
provided for controlling the filter pairs of the OMUX 1400. When the OMUX
1400 is provided for use on a communications satellite, the control unit 1401 may
15 be configured to allow the filters to be tuned remotely by a ground-based operator
whilst the satellite is in orbit.
Figure 14b illustrates a tunable bandpass filter for use in the hybrid-coupled OMUX
1400 shown in Fig. 14a, according to an example of the present invention. The
20 tunable bandpass filter 1410 comprises four interconnected resonant cavities having
moveable end plates 1411, which can be remotely adjusted so as to adjust a centre
frequency at which the filter operates. The tunable bandpass filter 1410 is one
example of a tunable bandpass filter which may be suitable for use in a tunable
hybrid-coupled OMUX, and other filter designs are possible.
25
The hybrid-coupled OMUX 1400 of Fig. 14a offers a substantial technical
advantage over conventional manifold multiplexers, in which the interaction
between different filters means that it is not possible to tune one filter without
affecting the behaviour of the remaining filters. This may be particularly
30 advantageous, for example, when the OMUX 1400 is provided to back-up channels
of another satellite in the event of failure. Because a conventional manifold
multiplexer cannot be retuned when in orbit, it would be necessary to provide large
manifold multiplexer with a separate input channel for every channel which is
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required to be backed-up. However, a tunable hybrid-coupled OMUX 1400 such as
the one shown in Fig. 14a may be provided with fewer input channels, which may
then be retuned as necessary in the event of a failure of another satellite. Therefore,
a tunable hybrid-coupled OMUX 1400 with relatively few input channels may allow
5 a single satellite to provide back-up for multiple other satellites.
10
Whilst certain embodiments of the present invention have been described above, it
will be clear to the skilled person that many variations and modifications are
possible while still falling within the scope of the invention as defined by the claims.
For example, although hybrid-coupled OMUXs have been described in which each
hybrid-coupled filter comprises a single input hybrid coupler and a single output
hybrid coupler with two bandpass filters connected therebetween, other
arrangements are possible. In some examples, input and output hybrid networks
15 comprising a plurality of hybrid couplers may be provided, with the number of
bandpass filters being increased accordingly.
Additionally, in some examples of the present invention, each bandpass filter may
be replaced with tunable low-pass and high-pass filters connected in series. This
20 arrangement may allow a passband width of the hybrid-coupled filter to be adjusted,
by tuning one of the low-pass of high-pass filters accordingly.
Furthermore, although examples of the present invention have been described in
relation to multiplexing microwave Ku band signals, the skilled person will
25 appreciate that the present invention is not limited thereto. In other examples of
the present invention, hybrid-coupled OMUXs may be provided for multiplexing
signals of other frequencies.

Claims
1. An output multiplexer OMUX (500) comprising:
a plurality of hybrid-coupled filters (510, 520, 530, 540), each arranged to
5 receive a first input signal via a first input port (410) and a second input signal via a
second (411) input port, and output a first output signal via a first output port (412)
and a second output signal via a second (413) output port;
wherein the hybrid-coupled filters are connected to combine a plurality of
said first output signals into a first multiplexed signal output from a first output
10 port of the OMUX, and combine a plurality of said second output signals into a
second multiplexed signal output from a second output port of the OMUX.
2. The OMUX of claim 1, wherein each one of the hybrid-coupled filters
comprises first and second hybrid couplers, with first and second bandpass filters
15 connected between the first and second hybrid couplers.
3. The OMUX of claim 2, wherein the first and second input ports comprise
input ports of the first hybrid coupler, and
wherein the first and second output ports comprise output ports of the
20 second hybrid coupler.
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30
4. The OMUX of claim 2 or 3, wherein the first and second bandpass filters of
any one of the plurality of hybrid-coupled filters are arranged to have substantially
similar transfer functions.
5. The OMUX of claim 4, wherein the first and second bandpass filters are
arranged to be controllable so as to tune at least one of a centre frequency and a
passband width.
6. The OMUX of any preceding claim, wherein the plurality of hybrid-coupled
filters comprises a number N of hybrid-coupled filters, and
5
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wherein the first OMUX output port is an output port of an N'h one of the
hybrid-coupled filters, and the second OMUX output port is an output port of a
first one of the hybrid-coupled filters.
7. The OMUX of any preceding claim, further comprising at least one
bidirectional connection between adjacent ones of the hybrid-coupled filters, the
bidirectional connection being arranged to carry the first and second output signals
in opposite directions, and preferably,
wherein the bidirectional connection is arranged to connect the first output
10 of one of the hybrid-coupled filters to the second output of another one of the
hybrid-coupled filters.
8. The OMUX of any preceding claim, wherein the first multiplexed signal is
arranged to be transmitted as a vertically polarised signal, and the second
15 multiplexed signal is arranged to be transmitted as a horizontally polarised signal.
9. The OMUX of any one of claims 1 to 8, wherein the first and second input
signals of each one of the plurality of hybrid-coupled filters either correspond to
downlink channels having the same frequencies, or correspond to downlink
20 channels which are adjacent in frequency.
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10. The OMUX of any preceding claim, wherein the plurality of first input
signals and the plurality of second input signals comprise microwave signals having
frequencies in the Ku band.
11. The OMUX of any preceding claim, wherein the OMUX is configured for
use in a communications satellite.
12. Apparatus comprising:
the OMUX of any preceding claim; and
means for combining the first and second multiplexed signals.
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13. The apparatus according to claim 12, wherein the means for combining
comprises an orthogonal mode transducer OMT, the OMT being arranged to
receive the first and second multiplexed signals and output a combined signal to a
reflector antenna, and
5 wherein the combined signal comprises the first multiplexed signal as a
vertically polarised signal and the second multiplexed signal as a horizontally
polarised signal.
14. The apparatus according to claim 12, wherein the means for combining
10 comprises first and second feed horns of a reflector antenna, the first feed horn
being arranged to receive the first multiplexed signal and the second feed horn
being arranged to receive the second multiplexed signal, and
15
wherein the first and second feed horns and the reflector antenna are
arranged to combine the first and second multiplexed signals in space.
15. A hybrid-coupled filter for use in the OMUX of any one of claims 1 to 11,
the hybrid-coupled filter comprising:
a first hybrid coupler (401) having first (410) and second (411) input ports;
a second hybrid coupler (402) having first (412) and second (413) output
20 ports; and
a plurality of filters (403, 404) connected between the first and second hybrid
couplers,
wherein the hybrid-coupled filter is arranged to receive the first input signal
via the first input port and the second input signal via the second input port, and
25 output the first output signal via the first output port and the second output signal
via the second output port.