Band combining filter
A band combining filter for filtering a microwave signal having at least one band edge at a band edge transition frequency. The filter comprises a plurality of filter sections. Each filter section comprising 3dB hybrid couplers having input ports and output ports and resonators connected between the input ports and the output ports of the couplers. The filter sections are connected in cascade such that the outputs of one filter section are connected to the inputs of the next filter section in the cascade. 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.
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 3 dB hybrid couplers, each 3 dB 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 the first coupler and the second input port of the second coupler respectively;
each filter section comprising first and second output ports defined by the first output port of the first coupler and second output port of the 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 microwave signals at output ports with a phase shift therebetween;
wherein 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 claim 1 , wherein the Q values of the resonators in the subset are at least four times that of each of the remaining resonators.
5. A band combining filter as claimed in claim 1 , 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 claim 1 , 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 claim 1 , comprising at least three filter sections in cascade.
10. A band combining filter as claimed in claim 1 , further comprising an electrical signal generator.
11. A band combining filter as claimed in claim 4 , wherein the Q values of the resonators in the subset are at least five times that of each of the remaining resonators.
12. A band combining filter as claimed in claim 9 , comprising at least four filter sections in cascade.