CN101803108A - A tuneable bandpass filter - Google Patents

Abstract

A tuneable bandpass filter comprising a plurality of coupled resonators, the filter comprising a common structure comprising at least one common coupling (44) or one common resonator (7); an upper loop (8) comprising first and second end resonators coupled together by a signal path, the loop further comprising at least one further signal path extending between end resonators, the further signal path comprising at least one further resonator, the end resonators being coupled to the common structure; a lower loop (18) comprising first and second end resonators coupled together by a signal path, the loop further comprising at least one further signal path extending between end resonators, the further signal path comprising at least one further resonator, the end resonators being coupled to the common structure; the resonators being coupled together such that the bandpass filter can be divided into a low pass sub filter and a high pass sub filter, one of the sub filters being arranged to receive the output of the other.

Description

Tunable Bandpass Filter

technical field

The present invention relates to tunable bandpass filters. In particular but not exclusively, the invention relates to a tunable bandpass filter comprising a plurality of coupled resonators arranged in first and second loops, each of said first and second loops connected to a common structure, the resonators are arranged so that the filter can be decomposed into low-pass and high-pass sub-filters, one sub-filter being arranged to receive the output of the other sub-filter.

Background technique

There are many needs to use control over operating bandwidth and frequency to tune filters constructed from coupled resonators while retaining a fixed absolute rejection requirement relative to the band edges. Using known synthesis and implementation methods, tuning a bandpass filter with variable frequency and bandwidth will require tuning the resonant frequency of the resonators and the coupling between resonators, and will only satisfy relative rejection requirements . This results in filters that are complex to implement.

Contents of the invention

The tunable bandpass filter according to the present invention seeks to overcome this problem.

Accordingly, the present invention provides a tunable bandpass filter comprising a plurality of coupled resonators, said filter comprising:

a common structure comprising at least one common coupling or one common resonator;

an upper loop comprising first and second terminal resonators coupled together by a signal path, said loop further comprising at least one additional signal path extending between the terminal resonators, said additional signal path comprising at least an additional resonator, said terminating resonator coupled to said common structure;

a lower loop comprising first and second terminal resonators coupled together by a signal path, said loop further comprising at least one additional signal path extending between the terminal resonators, said additional signal path comprising at least an additional resonator, said terminating resonator coupled to said common structure;

The resonators are coupled together such that the bandpass filter can be divided into a low-pass sub-filter and a high-pass sub-filter, one of the sub-filters being arranged to receive the output of the other sub-filter.

The adjustable bandpass filter according to the present invention has a bandpass response bandwidth and a center frequency, and the bandpass response can be tuned only by tuning the resonators without adjusting the coupling between the resonators.

The common structure may include a single common resonator to which the terminal resonators of the upper and lower loops are connected.

The low-pass sub-filter may include the common resonator and a resonator of the lower loop.

The high-pass sub-filter may include the common resonator and a resonator of the upper loop.

Optionally, the common structure may comprise a plurality of common couplings coupled between the upper loop and the lower loop.

The common coupling may be arranged such that each end resonator of one loop is coupled to two end resonators of the other loop.

The upper loop may include an even number of resonators.

Optionally, the upper loop may include an odd number of resonators.

The lower loop may include an even number of resonators.

Optionally, the lower loop may include an odd number of resonators.

At least one of the upper and lower loops may include a plurality of resonators coupled together.

At least one of the loops may comprise a plurality of resonators coupled together in a cascaded manner such that there are multiple signal paths between terminal resonators.

At least some of said resonators of said at least one loop may be coupled together in a cross-coupled ladder configuration.

At least some of the resonators configured in a cross-coupled ladder may further include at least one diagonal cross-coupling to a resonator on an adjacent cross-coupled ladder rung.

The filter may further include a non-resonant input circuit node coupled to a terminal resonator of one of the loops.

The filter may further include a group delay equalization network also coupled between the input circuit node and a terminal resonator connected to the node.

The filter may further include an output circuit node coupled to a terminal resonator of another loop.

The filter may further include a group delay equalization network connected between the output circuit node and a terminal resonator connected to the node.

Description of drawings

The invention is now illustrated by way of example only and not in any limiting sense with reference to the accompanying drawings, in which:

Figures 1(a) and 1(b) show known coupled resonator bandpass filters with even and odd numbers of coupled resonators, respectively;

Fig. 2 shows the first embodiment of the adjustable bandpass filter according to the present invention;

Fig. 3 shows the equivalent circuit of the filter of Fig. 2;

Figure 4 shows a graphical representation of insertion loss versus frequency for the filter of Figure 2;

Fig. 5 shows the second embodiment of the adjustable bandpass filter according to the present invention;

Fig. 6 shows the equivalent circuit of the filter of Fig. 5;

Fig. 7 shows the equivalent circuit of another embodiment of the filter according to the present invention;

Figure 8 shows a filter according to the invention with the equivalent circuit of Figure 7; and

Fig. 9 shows yet another embodiment of a filter according to the invention.

Detailed ways

Bandpass filters at radio frequency RF and microwave frequencies are usually implemented as a network of resonant circuits coupled to each other by resonant circuits. Such a known bandpass filter 1 is shown in Figures 1(a) and 1(b). Filters 1 each include a plurality of resonators 2 . Resonators 2 are coupled together in the cross-coupled ladder configuration shown. The input and output connections to the bandpass filter 1 are via non-resonant circuit nodes 3,4.

It is known to synthesize these networks to produce arbitrary asymmetric responses. The coupling elements shown as diagonals can be produced in either direction.

The center of the passband of filter 1 is determined by the resonant frequency of resonator 2, and the general form of the response is determined by the ratio and topology of the coupling. Changing the center frequency of filter 1 can be achieved by changing only the resonator frequency of resonator 2 . Changing the relative bandwidth of filter 1 requires modifying the coupling between resonators 2 and the frequency of resonator 2 . Such filters 1 tend to be too complex to manufacture and use, and will generally only retain relative selectivity requirements if the coupling is not modified.

An infinite number of variations of the structure in FIG. 1 can be realized by applying a rotation transformation on the coupling matrix of filter 1 .

A tunable bandpass filter 5 according to the invention is shown in FIG. 2 . The filter 5 comprises a common structure 6 comprising a single common resonator 7 . The filter 5 comprises an upper loop 8 comprising first and second terminal resonators 9 , 10 coupled together by a signal path 11 . The terminal resonators 9 , 10 are each also coupled to the common resonator 7 . The upper loop 8 also includes a plurality of further resonators 12 - 15 coupled along a plurality of signal paths 16 between the terminal resonators 9 , 10 . The resonators 9, 10, 12-15 of the upper loop 8 are coupled in a cross-coupled ladder configuration. Diagonal coupling 16 between resonators on different rungs can be produced in either direction. Dotted lines show repeating units. In this embodiment the upper loop 8 comprises 6 resonators, however other embodiments with 8, 10, 12 etc. resonators are also possible. The non-resonant output signal node 17 is coupled to a terminal resonator 10 of the upper loop 8 .

The filter 5 also comprises a lower loop 18 comprising first and second terminal resonators 19 , 20 coupled together by a signal path 21 . The terminal resonators 19 , 20 are each coupled to the common resonator 7 . The lower loop 18 also includes a plurality of further resonators 22 - 25 coupled along a plurality of signal paths 26 between the terminal resonators 19 , 20 . In this embodiment, the topology of the lower loop 18 is such that the filter 5 has complex conjugate symmetry with respect to the common resonator 7 . The non-resonant input signal node 27 is coupled to a terminal resonator 20 of the lower loop 18 .

An equivalent circuit 28 of the adjustable bandpass filter 5 of FIG. 2 is shown in FIG. 3 . In the equivalent circuit 28 , the tunable bandpass filter 5 is divided into two sub-filters 29 , 30 connected together at a non-resonant node 31 . Like reference numerals designate like resonators in the tunable bandpass filter 5 and its equivalent circuit 28 . One of the sub-filters 29 comprises a low-pass sub-filter 29 having a low-pass band edge at frequency wl. Another sub-filter 30 comprises a high-pass sub-filter 30 having a high-pass band edge at frequency w2. The equivalent circuit 28 includes one more resonance node than the adjustable bandpass filter 5 . This is because the common resonator 7 is formed by combining the first resonator 7 of the low-pass sub-filter 29 with the first resonator 7 of the high-pass sub-filter 30 .

The insertion loss of the tunable bandpass filter 5 is a combination of the insertion losses of the low-pass and high-pass sub-filters 29 , 30 of the equivalent circuit 28 . Figure 4 shows the insertion loss of the tunable bandpass filter 5 and of the sub-filters 29, 30 of its equivalent circuit 28, which takes into account the interaction of the two. When the band-edge of the low-pass sub-filter 29 is at a higher frequency than the band-edge of the high- _{pass sub-filter 30, the combined insertion loss includes a band-pass region centered on w 0 which} respectively have ₁ and the edge of w ₂ .

The low-pass and high-pass sub-filters 29, 30 can be varied substantially independently of each other. Therefore, in order to change the lower frequency band edge, the resonant frequencies of the resonators 7, 9, 10, 12, 13, 14, 15 of the adjustable bandpass filter 5 comprising the equivalent circuit 28 are adjusted. High pass sub-filter 30 . In order to change the upper band edge, the resonant frequency of the resonators of the tunable bandpass filter 5 comprising the low-pass sub-filter 29 of the equivalent circuit 28 is adjusted. By adjusting the upper and lower frequency band edges, the center and width of the bandpass region of the tunable bandpass filter 5 can be adjusted only by adjusting the resonant frequency of the resonators without adjusting the coupling between the resonators. This greatly reduces the complexity and increases the ease of use of the tunable bandpass filter 5 compared to known bandpass filters, while retaining the same absolute selectivity.

The tunable bandpass filter 5 of this embodiment generally has a high degree of symmetry. Because of the symmetry of the filter 5, the common resonator 7 is usually tuned to the center of the filter passband. In order to change the bandwidth (constant center frequency), all resonators except the common resonator 7 are tuned.

In an alternative embodiment of the invention (not shown), the tunable bandpass filter 5 is asymmetrical and can be decomposed into n _L -level low-pass sub-filters 29 and n _H -level high-pass sub-filters device 30. For such a filter 5, n _L resonators have to be tuned to move the lower edge, and n _H resonators have to be tuned to move the upper edge (including the common resonator 7). In order to move the center frequency of the response, all resonators must be changed.

Another embodiment of a tunable bandpass filter 5 according to the invention is shown in FIG. 5 . In this embodiment, the upper and lower loops 8, 18 comprise an odd number of resonators. In the equivalent circuit 28 shown in Fig. 6, the two sub-filters 29, 30 have an even number of resonators. Furthermore, the two sub-filters 29, 30 are low-pass 29 and high-pass 30 sub-filters, respectively.

In all the above-described embodiments of the invention, the low-pass and high-pass sub-filters 29, 30 of the equivalent circuit 28 have two transmission zeros at infinity. However, this is not necessary. FIG. 7 shows an equivalent circuit 28 of the tunable filter 5 , in which the low-pass and high-pass sub-filters 29 , 30 have only one transmission zero at infinity. Moving one of the transmission zeros from infinitely yields additional flexibility when design specifications are met. The low-pass and high-pass sub-filters 29 , 30 are now connected together via two non-resonant nodes 42 , 43 .

The topology of the tunable bandpass filter 5 is shown in FIG. 8 . As in the previous embodiment, the filter 5 comprises upper and lower loops 8, 18 each comprising a plurality of resonators. However, public structure 6 is more complex. In this embodiment the common structure 6 comprises a plurality of common couplings 44 arranged for fully coupling the terminal resonators 45, 46, 51, 51, 52. Each terminal resonator 45 , 46 , 51 , 52 of one loop 8 , 18 is coupled to two terminal resonators 45 , 46 , 51 , 52 of the other loop 8 , 18 .

The tunable bandpass filter 5 of Fig. 8 retains the same basic properties of the other tunable bandpass filter 5 according to the invention. In particular, independent adjustment of the bandwidth and center frequency of the bandpass filter can be achieved by adjusting the associated resonators. There is no need to adjust the coupling between the resonators to adjust the center frequency or bandwidth.

Another embodiment of a tunable bandpass filter 5 according to the invention is shown in FIG. 9 . An ideal filter would pass signals in the desired frequency band while attenuating all unwanted signals, with only a constant change in amplitude and constant time delay for the desired signals. If different frequency components of a composite signal arrive at different times, the signal will be distorted. The requirement for constant delay is equivalent to the need for constant group delay. The absolute level of delay is usually not important, what matters is the variation of delay over the bandwidth of the signal.

The embodiment of the filter 5 of FIG. 9 also includes a compensated group delay equalization network 57, in addition to the direct coupling between the input non-resonant electrode 27 and the resonator 37 connected to the input node 27, between the two A compensated group delay equalization network 57 is coupled. The compensated group delay equalization network 57 includes a plurality of resonators 58, 59 connected in series. Another compensated group delay equalization network 60 is coupled between output non-resonant node 17 and resonator 32 coupled to output node 17 . Furthermore, it comprises a plurality of resonators 61, 62 connected in series. Furthermore, it is in parallel with the direct coupling between the output node 17 and the resonator 32 . The responses of the compensating group delay equalization networks 57, 60 are offset from the center frequency of the desired bandpass response so that each compensating network 57, 60 affects one half of the bandwidth more than the other half. The group delay peaks of the compensated group delay equalization networks 57, 60 occur at the nominal center frequency of the compensated design. To compensate for the group delay tilt at the center of the bandpass response, the nominal center frequency of the compensator occurs in the passband of the bandpass response. In order to apply correct compensation to the response, they must be arranged symmetrically around the center frequency of the bandpass response to satisfy the symmetrical amplitude selection requirement.

The compensating group delay equalization networks 57, 60 may be tuned. The compensated group delay equalization networks 57, 60 are adapted to track the tuning of the filter 5 by applying the compensated group delay equalization The frequencies of the networks 57, 60 are synchronized to a correctly offset center frequency.

If the center frequency of the compensated group delay equalization network 57, 60 is changed relative to the overlap of the low pass and high pass sub-filters 29, 30, the amount of compensation and the relative position of the group delay ripple can be changed.

Compensated group delay equalization networks 57, 60 may be used to reduce group delay variation for all adjustable bandwidths. In addition to reducing group delay variation, the variation in insertion loss in the compensation networks 57, 60 also reduces insertion loss variation across the passband, although at the expense of increased minimum insertion loss.

The low-pass sub-filter 29 of the above equivalent circuit 28 must be a band-pass filter whose performance is tailored to favor stop-band rejection above the pass-band of the tunable band-pass filter 5, with little regard for Performance below the passband of the tunable bandpass filter 5. Such a filter 29 is often referred to as a "quasi" low-pass filter.

Similarly, the high-pass sub-filter 30 of the equivalent circuit 28 must be a band-pass filter whose performance is tailored to favor stop-band rejection below the pass-band of the tunable band-pass filter 5 with little Consider the performance above the passband of the tunable bandpass filter 5 . Such a filter 30 is often referred to as a "quasi" high-pass filter.

The tunable bandpass filter 5 according to the invention generally has a resonator which resonates at microwave frequencies, resulting in a passband in the microwave region. Typically, the resonators comprise cavity resonators (not shown), which are tuned by displacing tuning members in the cavity. The two tunable cavity resonators are typically coupled through a hole extending through a common wall of the two cavities.

In all the above-described embodiments, the resonators of the tunable bandpass filter are connected together in an arrangement such that the filter can be considered as a quasi-high-pass sub-filter and a quasi-low-pass sub-filter connected together devices, one of which receives the output of the other. In these embodiments, the output of one sub-filter is connected to the input of the other through a number of non-resonant nodes, the number of which depends on the order of the sub-filter.

Claims (20)

1. A tunable bandpass filter comprising a plurality of coupled resonators, said filter comprising: a common structure comprising at least one common coupling or one common resonator; an upper loop comprising first and second terminal resonators coupled together by a signal path, said loop further comprising at least one additional signal path extending between the terminal resonators, said additional signal path comprising at least an additional resonator, said terminating resonator coupled to said common structure; a lower loop comprising first and second terminal resonators coupled together by a signal path, said loop further comprising at least one additional signal path extending between the terminal resonators, said additional signal path comprising at least an additional resonator, said terminating resonator coupled to said common structure; The resonators are coupled together such that the bandpass filter can be divided into a low-pass sub-filter and a high-pass sub-filter, one of the sub-filters being arranged to receive the output of the other sub-filter. 2. The tunable bandpass filter of claim 1, wherein said common structure comprises a single common resonator to which said terminal resonators of said upper and lower loops are connected. 3. The tunable bandpass filter according to claim 2, wherein said low-pass sub-filter comprises said common resonator and respective resonators of said lower loop. 4. The tunable bandpass filter according to claim 2 or 3, wherein the high-pass sub-filter comprises the common resonator and each resonator of the upper loop. 5. The tunable bandpass filter of claim 1, wherein said common structure comprises a plurality of common couplings coupled between said upper and lower loops. 6. A tunable bandpass filter according to claim 5, wherein said common couplings are arranged such that each terminal resonator of a loop is coupled to two terminal resonators of another loop. 7. A tunable bandpass filter according to any one of claims 1-6, wherein said upper loop comprises an even number of resonators. 8. A tunable bandpass filter according to any one of claims 1-6, wherein said upper loop comprises an odd number of resonators. 9. A tunable bandpass filter according to any one of claims 1-8, wherein said lower loop comprises an even number of resonators. 10. A tunable bandpass filter according to any one of claims 1-8, wherein said lower loop comprises an odd number of resonators. 11. A tunable bandpass filter according to any one of claims 1-10, wherein at least one of said upper and lower loops comprises a plurality of resonators coupled together. 12. The tunable bandpass filter of claim 11, wherein at least one of said loops includes a plurality of resonators coupled together in cascade so that there are multiple signal paths between terminal resonators. 13. The tunable bandpass filter of claim 12, wherein at least some of said resonators of said at least one loop are coupled together in a cross-coupled ladder configuration. 14. The tunable bandpass filter of claim 13 , wherein at least some of the resonators configured in a cross-coupled ladder further comprise: at least one diagonal cross-coupled to a resonator on an adjacent cross-coupled ladder rung couplet. 15. A tunable bandpass filter according to any one of claims 1-14, further comprising a non-resonant input circuit node coupled to a terminal resonator of one of said loops. 16. The tunable bandpass filter of claim 15, further comprising a group delay equalization network also coupled between the input circuit node and a terminal resonator connected to the node. 17. A tunable bandpass filter according to claim 15 or 16, further comprising an output circuit node coupled to a terminal resonator of the other loop. 18. The tunable bandpass filter of claim 17, further comprising a group delay equalization network connected between the output circuit node and a terminal resonator connected to the node. 19. A tunable bandpass filter substantially as hereinbefore described. 20. A tunable bandpass filter substantially as hereinbefore described with reference to the accompanying drawings.

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