WO2012164264A1 - A band combining filter - Google Patents
A band combining filter Download PDFInfo
- Publication number
- WO2012164264A1 WO2012164264A1 PCT/GB2012/051183 GB2012051183W WO2012164264A1 WO 2012164264 A1 WO2012164264 A1 WO 2012164264A1 GB 2012051183 W GB2012051183 W GB 2012051183W WO 2012164264 A1 WO2012164264 A1 WO 2012164264A1
- Authority
- WO
- WIPO (PCT)
- Prior art keywords
- filter
- coupler
- band combining
- band
- cascade
- Prior art date
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Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01P—WAVEGUIDES; RESONATORS, LINES, OR OTHER DEVICES OF THE WAVEGUIDE TYPE
- H01P1/00—Auxiliary devices
- H01P1/20—Frequency-selective devices, e.g. filters
Definitions
- the present invention relates to a band combining filter. More particularly, but not exclusively, the present invention relates to a band combining filter comprising a plurality of filter sections connected together in cascade along with a phase shifter, the filter sections including resonators and at least one of the filter sections being a high Q filter section.
- Band combining filters are known. Such band combining filters can include a plurality of resonators, in the case of a rapid transition from passband to stopband the resistive loss of the resonators causes a roll off of the insertion loss into the passband.
- unloaded Qs of greater than 20,000 are required resulting in the necessity, at microwave frequencies to use dielectric resonators for all of the cavities resulting in a physically large heavy and expensive filter.
- the present invention seeks to overcome the problems of the prior art.
- the present invention provides a band combining filter for filtering a microwave signal, the band combining filter having at least one band edge at a band edge transition frequency, the filter comprising a plurality of filter sections, each filter section comprising first and second 3dB hybrid couplers, each 3dB hybrid coupler comprising first and second input ports and first and second output ports; a first resonator connected between the second input port of the first coupler and the first input port of the second coupler; and, a second resonator connected between the second output port of the first coupler and the first output port of the second coupler; each filter section comprising first and second input ports defined by the first Input port of its first coupler and the second input port of its second coupler respectively; each filter section comprising first and second output ports defined by the first output port of its first coupler and second output port of its second coupler respectively; the filter sections being connected in cascade with the first and second outputs of one filter section being connected to the first and second inputs of the next filter section in
- the band combining filter according to the invention requires only two high Q resonators per band edge and still has low loss across the entire passband.
- the coupled phase shifter can be the last element of the cascade with the inputs of the phase shifter receiving the outputs from the final filter section of the cascade.
- the coupled phase shifter can be arranged between filter sections in the cascade.
- the Q values of the resonators in the subset are at least four times, more preferably five times, that of each of the remaining resonators.
- the Q value of the first resonator in the filter section is equal to the Q value of the second resonator in the same filter section.
- the number of high Q filter sections is equal to the number of band edges.
- the band combining filter according to the invention can have one band edge.
- the band combining filter according to the invention can comprise two filter sections connected in cascade.
- the band combining filter according to the invention can comprise at least three, preferably four, filter sections in cascade.
- the band combining filter further comprises an electrical signal generator.
- Figure 1 shows a first embodiment of a band combining filter according to the invention
- Figure 2 shows a second embodiment of a band combining filter according to the invention
- Figure 3 shows a third embodiment of a band combining filter according to the invention
- Figure 4 shows a fourth embodiment of a band combining filter according to the invention.
- Figure 5 shows a practical design of a band combining filter according to the invention
- Figure 6 shows the performance of the filter of figure 5
- Figure 7 shows a symmetrical four port structure
- Figure 8 shows a 3dB hybrid with reactive admittances connected to two of the ports.
- Figures 9(a) to 9(c) show a sections which can be connected together in cascade to produce the filter of the invention.
- the filter 1 is a third order filter having a single band edge at a band edge transition frequency.
- the band combining filter 1 comprises a plurality (in this case three) filter sections 2 connected in cascade.
- Each filter section 2 comprises first and second input ports 3,4 and first and second output ports 5,6.
- the first and second output ports 5,6 of one filter section 2 are connected to the first and second input ports 3,4 of the next filter section 2 in the cascade as shown.
- the first and second input ports 3,4 of the first filter section 2 comprise the input ports 7,8 of the filter 1.
- the output ports 5,6 of the last filter section 2 are connected to a coupled phase shifter 9.
- the signals received at the input ports 10,11 of the coupled phase shifter 9 are presented at the output ports 12,13 of the coupled phase shifter 9 with a phase difference introduced therebetween.
- the output ports 12,13 of the coupled phase shifter 9 are the output ports 14,15 of the filter 1. The function of the coupled phase shifter 9 is explained in more detail below.
- Each filter section 2 comprises first 16 and second 17 3dB hybrids.
- Each hybrid 16,17 has first and second input ports 18,19,20,21 and first and second output ports 22,23,24,25.
- the second input port 19 of the first hybrid 16 is connected to the first input port 20 of the second hybrid 1 by a first resonator 26.
- the second output port 23 of the first hybrid 16 is connected to the first output port 24 of the second hybrid 17 by a second resonator 27.
- the first and second resonators 26,27 have the same value.
- One of the filter sections 2 is a high Q filter section.
- the Q values of the resonators 26,27 in this section are a factor of four higher than the Q values of the resonators 26,27 in the remaining filter sections 2.
- the band combining filter 1 has only two high Q value resonators 26,27 the combining filter 1 shows low loss across the entire passband.
- the Q values of the resonators 26,27 of the high Q filter section 2 are a factor of four higher than the Q values of the resonators 26,27 of the remaining filter sections 2. More generally speaking, it is preferred that the Q values of the resonators 26,27 of the high Q filter sections 2 have values which are at least a factor of three, more preferably at least a factor of four, more preferably at least a factor of five larger than the Q values of the resonators 26,27 of the remaining filter sections 2.
- the low Q value resonators 26,27 are typically realised as combline resonators.
- High Q resonators 26,27 are typically realised as ceramic resonators.
- FIG 2 Shown in figure 2 is a second embodiment of a band combining filter 1 according to the invention.
- This embodiment is similar to that of figure 2 except the coupled phase shifter 9 is included between filter sections 2 in the cascade.
- the high Q filter section 2 is the last filter section 2 in the cascade.
- the coupled phase shifter 9 and the filter sections 2 can be arranged in any order in the cascade.
- FIG 3 Shown in figure 3 is a further embodiment of a band combining filter 1 according to the invention.
- This filter 1 is a fourth order filter and as such has four filter sections 2.
- the filter 1 has two band edges at band edge transition frequencies and accordingly has two high Q filter sections 2.
- the number of high Q filter sections 2 is equal to the number of band edges.
- FIG 4 Shown in figure 4 is a further embodiment of a band combining filter 1 according to the invention.
- the filter 1 is a second degree filter having a single band edge.
- One of the two filter sections 2 is a high Q filter section.
- the Q values of the resonators 26,27 of this section 2 are a factor of 8 higher than the Q values of the resonators 26,27 of the other filter section 2.
- Shown in figure 5 is a practical design of a second degree band combining filter 1 according to the invention.
- the Q values for the high Q filter section are set at 25,000 whilst those for the low Q filter section are set at 6000.
- Shown in figure 6 is the reflection and transmission performance of the filter as a function of frequency. The operation of the band combining filter according to the invention described with reference to figure 7 and subsequent figures.
- Each all pass section can be realised with two equal reactive admittances connected to two of the ports of a 3dB hybrid as shown in figure 8.
- the resonant part of the even mode realisation is as shown in figure 9(a) and the odd mode is shown in figure 9(b) and the phase shifters required in the even and odd mode functions can be combined to form a single coupler shown in figure 9(c).
- the whole band combining filter 1 is produced from the cascade of the sections shown in figures 9(a) to 9(c) which can be cascades in any order.
- the impedance ration between Yi and Y 2 is (V2 + l) 2 thus enabling the resonator Yi to be realised with a Q factor considerably less than the resonator Y 2 .
- the Q values of the resonators 26,27 of one filter section 2 are sufficiently high then the loss of the filter 1 across the passband is determined by that of the high Q resonators 26,27 only.
- the synthesis process is similar in that the transfer functions of the even and odd mode networks can be factorised as unity degree all pass factors as and where N e and N 0 are within one degree of each other and Y er , Y 0 r are of unity degree, Y e o and Y 0 o result in the frequency independent coupler 9.
- the overall realisation is the cascade of the independent filter sections 2 and the overall performance is independent of the order of the cascade.
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Abstract
A band combining filter (1) for filtering a microwave signal, the band combining filter having at least one band edge at a band edge transition frequency, the filter comprising a plurality of filter sections (2), each filter section comprising first and second 3dB hybrid couplers (16,17), each 3dB hybrid coupler comprising first and second input ports (18,21) and first and second output ports (22,25); a first resonator (26) connected between the second input port of the first coupler and the first input port of the second coupler; and, a second resonator (27) connected between the second output port of the first coupler and the first output port of the second coupler; each filter section comprising first and second input ports (3,4) defined by the first input port of its first coupler and the second input port of its second coupler respectively; each filter section comprising first and second output ports (5, 6) defined by the first output port of its first coupler and second output port of its second coupler respectively; the filter sections being connected in cascade with the first and second outputs of one filter section being connected to the first and second inputs of the next filter section in the cascade; the band combining filter further comprising a coupled phase shifter (9) in the cascade having first and second inputs adapted to receive microwave signals and provide them at output ports with a phase shift therebetween; characterised in that a subset of the filter sections are high Q filter sections with the Q values of the resonators of those filter sections having values each of which are at least a factor of three higher than the Q values of the resonators of the remaining filter sections.
Description
A BAND COMBINING FILTER
The present invention relates to a band combining filter. More particularly, but not exclusively, the present invention relates to a band combining filter comprising a plurality of filter sections connected together in cascade along with a phase shifter, the filter sections including resonators and at least one of the filter sections being a high Q filter section.
Band combining filters are known. Such band combining filters can include a plurality of resonators, in the case of a rapid transition from passband to stopband the resistive loss of the resonators causes a roll off of the insertion loss into the passband. In order to meet typical rejection requirements unloaded Qs of greater than 20,000 are required resulting in the necessity, at microwave frequencies to use dielectric resonators for all of the cavities resulting in a physically large heavy and expensive filter.
The present invention seeks to overcome the problems of the prior art.
Accordingly, in a first aspect, the present invention provides a band combining filter for filtering a microwave signal, the band combining filter having at least one band edge at a band edge transition frequency, the filter comprising a plurality of filter sections, each filter section comprising first and second 3dB hybrid couplers, each 3dB hybrid coupler comprising first and second input ports and first and second output ports; a first resonator connected between the second input port of the first coupler and the first input port of the second coupler; and, a second resonator connected between the second output port of the first coupler and the first output port of the second coupler;
each filter section comprising first and second input ports defined by the first Input port of its first coupler and the second input port of its second coupler respectively; each filter section comprising first and second output ports defined by the first output port of its first coupler and second output port of its second coupler respectively; the filter sections being connected in cascade with the first and second outputs of one filter section being connected to the first and second inputs of the next filter section in the cascade; the band combining filter further comprising a coupled phase shifter in the cascade having first and second inputs adapted to receive microwave signals and provide them at output ports with a phase shift therebetween; characterised in that a subset of the filter sections are high Q filter sections with the Q values of the resonators of those filter sections having values each of which are at least a factor of three higher than the Q values of the resonators of the remaining filter sections.
The band combining filter according to the invention requires only two high Q resonators per band edge and still has low loss across the entire passband.
The coupled phase shifter can be the last element of the cascade with the inputs of the phase shifter receiving the outputs from the final filter section of the cascade.
Alternatively, the coupled phase shifter can be arranged between filter sections in the cascade.
Preferably, the Q values of the resonators in the subset are at least four times, more preferably five times, that of each of the remaining resonators.
Preferably, for each filter section the Q value of the first resonator in the filter section is equal to the Q value of the second resonator in the same filter section.
Preferably, the number of high Q filter sections is equal to the number of band edges.
The band combining filter according to the invention can have one band edge.
The band combining filter according to the invention can comprise two filter sections connected in cascade.
The band combining filter according to the invention can comprise at least three, preferably four, filter sections in cascade.
Preferably, the band combining filter further comprises an electrical signal generator.
The present invention will now be described by way of example only at not in any limitative sense with reference to the accompanying drawings in which
Figure 1 shows a first embodiment of a band combining filter according to the invention;
Figure 2 shows a second embodiment of a band combining filter according to the invention;
Figure 3 shows a third embodiment of a band combining filter according to the invention;
Figure 4 shows a fourth embodiment of a band combining filter according to the invention;
Figure 5 shows a practical design of a band combining filter according to the invention;
Figure 6 shows the performance of the filter of figure 5;
Figure 7 shows a symmetrical four port structure;
Figure 8 shows a 3dB hybrid with reactive admittances connected to two of the ports; and,
Figures 9(a) to 9(c) show a sections which can be connected together in cascade to produce the filter of the invention.
Shown in figure 1 is a band combining filter 1 according to the invention. The filter 1 is a third order filter having a single band edge at a band edge transition frequency. The band combining filter 1 comprises a plurality (in this case three) filter sections 2 connected in cascade. Each filter section 2 comprises first and second input ports 3,4 and first and second output ports 5,6. The first and second output ports 5,6 of one filter section 2 are connected to the first and second input ports 3,4 of the next filter section 2 in the cascade as shown. The first and second input ports 3,4 of the first filter section 2 comprise the input ports 7,8 of the filter 1.
The output ports 5,6 of the last filter section 2 are connected to a coupled phase shifter 9. The signals received at the input ports 10,11 of the coupled phase shifter 9 are presented at the output ports 12,13 of the coupled phase shifter 9 with a phase difference introduced therebetween. The output ports 12,13 of the coupled phase shifter 9 are the output ports 14,15 of the filter 1. The function of the coupled phase shifter 9 is explained in more detail below.
Each filter section 2 comprises first 16 and second 17 3dB hybrids. Each hybrid 16,17 has first and second input ports 18,19,20,21 and first and second output ports 22,23,24,25. The second input port 19 of the first hybrid 16 is connected to the first input port 20 of the second hybrid 1 by a first resonator 26. Similarly, the second output port 23 of the first hybrid 16 is connected to the first output port 24 of the second hybrid 17 by a second resonator 27. In this embodiment within each filter section 2 the first and second resonators 26,27 have the same value.
One of the filter sections 2 is a high Q filter section. The Q values of the resonators 26,27 in this section are a factor of four higher than the Q values of the resonators 26,27 in the remaining filter sections 2.
Even though the band combining filter 1 according to the invention has only two high Q value resonators 26,27 the combining filter 1 shows low loss across the entire passband.
In this embodiment the Q values of the resonators 26,27 of the high Q filter section 2 are a factor of four higher than the Q values of the resonators 26,27 of the remaining filter sections 2. More generally speaking, it is preferred that the Q values of the resonators 26,27 of the high Q filter sections 2 have values which are at least a factor of three, more preferably at least a factor of four, more preferably at least a
factor of five larger than the Q values of the resonators 26,27 of the remaining filter sections 2.
The low Q value resonators 26,27 are typically realised as combline resonators. High Q resonators 26,27 are typically realised as ceramic resonators.
Shown in figure 2 is a second embodiment of a band combining filter 1 according to the invention. This embodiment is similar to that of figure 2 except the coupled phase shifter 9 is included between filter sections 2 in the cascade. In this embodiment the high Q filter section 2 is the last filter section 2 in the cascade. More generally speaking, the coupled phase shifter 9 and the filter sections 2 can be arranged in any order in the cascade.
Shown in figure 3 is a further embodiment of a band combining filter 1 according to the invention. This filter 1 is a fourth order filter and as such has four filter sections 2. The filter 1 has two band edges at band edge transition frequencies and accordingly has two high Q filter sections 2. Generally speaking it is preferred that the number of high Q filter sections 2 is equal to the number of band edges.
Shown in figure 4 is a further embodiment of a band combining filter 1 according to the invention. In this embodiment the filter 1 is a second degree filter having a single band edge. One of the two filter sections 2 is a high Q filter section. The Q values of the resonators 26,27 of this section 2 are a factor of 8 higher than the Q values of the resonators 26,27 of the other filter section 2.
Shown in figure 5 is a practical design of a second degree band combining filter 1 according to the invention. The Q values for the high Q filter section are set at 25,000 whilst those for the low Q filter section are set at 6000. Shown in figure 6 is the reflection and transmission performance of the filter as a function of frequency.
The operation of the band combining filter according to the invention described with reference to figure 7 and subsequent figures.
Consider a symmetrical four port structure 28 as shown in figure 7 defined by its even and odd mode reflection and transmission coefficients.
For a balanced structure
Pe = Po = 0 and
defining
T„ =
Yn
T =
° 1 + 0
Where Ye and Y0 are obtained from a single two part filter one has -
Which is the reflection coefficient of the equivalent two port filter and
•-Ί4 -
Ϊ (i + Ye) i + Yo
Which is the transmission coefficient of the equivalent 2 port filter. Hence, signals in the passband emerge at port 4 and signals in the stopband emerge at port 3. Since the structure is reciprocal then the device acts as a combiner with signals in the passband applied at port 4 and signals in the stopband applied at port 3 both emerge at port 1 which would normally be connected to an antenna.
Considering the specific example given for the filter, in this case one has to realise two all pass networks, the first being
1-Y,
1 + YP
Which becomes
With ψ = 2 tan"1^ + 1) and,
1- (V2 + 1) P-CV2-1)
T - 1~Y°
0 1 + Y0 1 + ;(V2 + 1). p + (V2-l)J
P-CV2-1)
p + (V2 - 1)
Each all pass section can be realised with two equal reactive admittances connected to two of the ports of a 3dB hybrid as shown in figure 8.
Hence, the resonant part of the even mode realisation is as shown in figure 9(a) and the odd mode is shown in figure 9(b) and the phase shifters required in the even and odd mode functions can be combined to form a single coupler shown in figure 9(c).
Hence, the whole band combining filter 1 is produced from the cascade of the sections shown in figures 9(a) to 9(c) which can be cascades in any order.
The impedance ration between Yi and Y2 is (V2 + l)2 thus enabling the resonator Yi to be realised with a Q factor considerably less than the resonator Y2. In other words, with a band combining filter 1 having a structure according to the invention, provided the Q values of the resonators 26,27 of one filter section 2 are sufficiently high then the loss of the filter 1 across the passband is determined by that of the high Q resonators 26,27 only.
For higher degree networks the synthesis process is similar in that the transfer functions of the even and odd mode networks can be factorised as unity degree all pass factors as
and
where Ne and N0 are within one degree of each other and Yer, Y0r are of unity degree, Yeo and Y0o result in the frequency independent coupler 9. The overall realisation is the cascade of the independent filter sections 2 and the overall performance is independent of the order of the cascade.
Key for figure 5
R5 R= 12741 Ohm
(A resistor)
B5 B= 0.0057 mho
(A susceptance)
Line 5 Z= 50 Ohm
(A transmission line) L = 76.4 mm
Kf5 Zref= Zhy2 Ohm
(A frequency dependent impedance Zf - 0 Ohm/Hz inverter) f0= 0 Hz
Kf11 Zref= Zhy1
(A frequency dependent impedance Zf= 0 Ohm/Hz inverter) f0= 0 Hz
Line 11 Z= 0.400274 Ohm
(A transmission line) L= 38.1969 mm
R2 R = 3000 Ohm
(A resistor)
B2 B = 0 mho
(A susceptance)
Kf3 Zref= Zhy2 Ohm
(A frequency dependent impedance Zf= 0 Ohm/Hz inverter) fo= o Hz
Kf1 Zref = 50 Ohm
(A frequency dependent impedance Zf = 0 Ohm/Hz inverter) f0= 0 Hz
Kf8 Zref = Zhy4 Ohm
(A frequency dependent impedance Zf = 0 Ohm inverter) f0= 0 Hz
Kf2 Zref = 50 Ohm
(A frequency dependent impedance Zf = 0 Ohm inverter) f0= 0 Hz
Kf6 Zref = Zhy2 Ohm
(A frequency dependent impedance Zf = 0 Ohm
inverter) f0= 0 Hz
Line 12 Z = 0.400274 Ohm
(A transmission line) L = 38.1969 mm
R3 R = 3000 Ohm
(A resistor)
B3 B = 0 mho
(A susceptance)
Kf4 Zref = Zhy2 Ohm
(A frequency dependent impedance Zf = 0 Ohm
inverter) fo= 0 Hz
X1 K = 0.32 (coupling value)
(A coupled phase shifter) Phi = 90 degrees
P1 Z = 50 Ohm (source/load impedance)
(A power source/load)
P3 Z = 50 Ohm (source/load impedance)
(A power source/load)
Zinl = 335 Ohm
Zinl = 97 Ohm
Zinl
Zhyl = IT
Zinl
Zhyl = Ί
(Zinl)2
Zhy3 =
100
(Zinl
ZhyA =
100
Claims
1. A band combining filter for filtering a microwave signal, the band combining filter having at least one band edge at a band edge transition frequency, the filter comprising a plurality of filter sections, each filter section comprising first and second 3dB hybrid couplers, each 3dB hybrid coupler comprising first and second input ports and first and second output ports; a first resonator connected between the second input port of the first coupler and the first input port of the second coupler; and, a second resonator connected between the second output port of the first coupler and the first output port of the second coupler; each filter section comprising first and second input ports defined by the first input port of its first coupler and the second input port of its second coupler respectively; each filter section comprising first and second output ports defined by the first output port of its first coupler and second output port of its second coupler respectively; the filter sections being connected in cascade with the first and second outputs of one filter section being connected to the first and second inputs of the next filter section in the cascade; the band combining filter further comprising a coupled phase shifter in the cascade having first and second inputs adapted to receive microwave signals and provide them at output ports with a phase shift therebetween; characterised in that a subset of the filter sections are high Q filter sections with the Q values of the resonators of those filter sections having values each of which are at least a factor of three higher than the Q values of the resonators of the remaining filter sections.
2. A band combining filter as claimed in claim 1 , wherein the coupled phase shifter is the last element of the cascade with the inputs of the phase shifter receiving the outputs from the final filter section of the cascade.
3. A band combining filter as claimed in claim 1 , wherein the coupled phase shifter is arranged between filter sections in the cascade.
4. A band combining filter as claimed in any one of claims 1 to 3, wherein the Q values of the resonators in the subset are at least four times, more preferably five times, that of each of the remaining resonators.
5. A band combining filter as claimed in any one of claims 1 to 4, wherein for each filter section the Q value of the first resonator in the filter section is equal to the Q value of the second resonator in the same filter section.
6. A band combining filter as claimed in any one of claims 1 to 5, wherein the number of high Q filter sections is equal to the number of band edges.
7. A band combining filter as claimed in claim 6, having one band edge.
8. A band combining filter as claimed in claim 7, comprising two filter sections connected in cascade.
9. A band combining filter as claimed in any one of claims 1 to 7, comprising at least three, preferably four, filter sections in cascade.
10. A band combining filter as claimed in any of claims 1 to 9, further comprising an electrical signal generator.
11. A band combining filter substantially as hereinbefore described.
12. A band combining filter substantially as hereinbefore described with reference to the drawings.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US13/151,029 US8686808B2 (en) | 2011-06-01 | 2011-06-01 | Band combining filter |
| GB1109201.2A GB2491379B (en) | 2011-06-01 | 2011-06-01 | A band combining filter |
| GB1109201.2 | 2011-06-01 | ||
| US13/151,029 | 2011-06-01 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| WO2012164264A1 true WO2012164264A1 (en) | 2012-12-06 |
Family
ID=46319796
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| PCT/GB2012/051183 WO2012164264A1 (en) | 2011-06-01 | 2012-05-25 | A band combining filter |
Country Status (1)
| Country | Link |
|---|---|
| WO (1) | WO2012164264A1 (en) |
Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR1442904A (en) * | 1964-08-07 | 1966-06-17 | Marconi Co Ltd | Device combining radio frequency signals |
| JPH08250905A (en) * | 1995-03-13 | 1996-09-27 | Tokin Corp | Dielectric filter circuit and its configuration method |
| US20090231056A1 (en) * | 2006-12-15 | 2009-09-17 | Isotek Electronics Limited | Band Combining Filter |
-
2012
- 2012-05-25 WO PCT/GB2012/051183 patent/WO2012164264A1/en active Application Filing
Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR1442904A (en) * | 1964-08-07 | 1966-06-17 | Marconi Co Ltd | Device combining radio frequency signals |
| JPH08250905A (en) * | 1995-03-13 | 1996-09-27 | Tokin Corp | Dielectric filter circuit and its configuration method |
| US20090231056A1 (en) * | 2006-12-15 | 2009-09-17 | Isotek Electronics Limited | Band Combining Filter |
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