CORRECTION UNIT FOR RADIO FREQUENCY FILTER

Abstract

The present invention relates to a filter correction unit (203a) as well as an RF filter including the correction unit for use in radio frequency transmission lines including a band pass filter (201) having input and output interfaces mounted in the signal transmission line. The filter (201) is chosen so as to transmit signals within a predetermined frequency range, the quality factor of the filter having predetermined limitations generating a known distortion to the signal. The correction unit (203a) has a first bus connected to said transmission line and to said filter, wherein the correction unit comprises at least one surface acoustic wave (SAW) transducer (204a-n), each transducer having two electrodes on a piezoelectric substrate where a first electrode is connected to said first bus and the other electrode connected to a second bus, the SAW transducer being adapted to distort a transmitted signal with a factor being the inverse of said known distortion of the filter (201).

Claims

1. A filter correction unit for use in radio frequency transmission lines, the transmission lines including a band pass filter having input and output interfaces mounted in the signal transmission line, the band pass filter being chosen so as to transmit signals within a predetermined frequency range having a filter passband response, the quality factor of the filter's resonators having predetermined limitations generating a known passband distortion of the filter to the signal, wherein the correction unit has a first bus connected to the transmission line and to the filter, wherein the correction unit comprises at least one surface acoustic wave (SAW) transducer, each transducer having two electrodes on a piezoelectric substrate where a first electrode is connected to the first bus and the other electrode connected to a second bus, the SAW transducer being adapted to distort the amplitude of a transmitted signal with a factor being the inverse of the known distortion of the filter.

2. The filter correction unit according to claim 1, wherein the second bus is connected to ground.

3. The filter correction unit according to claim 1, wherein the band pass filter is produced on a piezoelectric substrate material, and the SAW transducers are produced on the same substrate.

4. The filter correction unit according to claim 1, wherein the first bus is connected to a first input and output interface of the filter correction unit, where one interface is connected to a first interface of the transmission line and the other interface to a first filter interface.

5. The filter correction unit according to claim 1, wherein the second bus is connected to a second input and output interface of the filter correction unit, where one interface is connected to a second interface of the transmission line and the other interface to a second filter interface, the filter being a balanced four port band pass filter.

6. The filter correction unit according to claim 1, wherein the dominant acoustic wave mode to which the SAW transducer couples is a pseudo-SAW mode.

7. The filter correction unit according to claim 1, wherein the dominant acoustic wave mode to which the SAW transducer couples is a leaky wave mode bound to the surface.

8. The filter correction unit according to claim 1, including inductive and capacitive elements, the capacitive elements having one end connected to a first transmission line or filter interface and the other end to ground, whereas the inductive elements are connected between the first bus and the transmission line or filter interface, the elements being chosen so as to improve the impedance matching of the correction unit to the transmission line and filter.

9. The filter correction unit according to claim 1, including inductive and capacitive elements, the capacitive elements being connected between a first and second transmission line or filter interface, whereas the inductive elements are connected between the first or second bus and the first or second transmission line or filter interface, the elements being chosen so as to improve the impedance matching of the correction unit to the transmission line and filter, the filter being a balanced four port band pass filter.

10. The filter correction system including two filter correction units according to claim 1, being connected on both sides of the band pass filter.

11. A radio frequency filter including a filter correction unit according to claim 1, wherein the band pass filter is constituted by grounded capacitive filter elements, the capacitive filter elements being realized through at least one of the SAW transducers, each having two electrodes on a piezoelectric substrate where the first electrode is connected to the interior of the band pass filter and the second electrode to ground, the SAW transducer being adapted both to distort a transmitted signal with a factor being the inverse of the know distortion of the filter, and also realizing the capacitance required to define the band pass filter response.

12. The radio frequency filter according to claim 11, wherein the filter is a LC filter.

13. The radio frequency filter according to claim 11, wherein the filter comprises coupled transmission lines in a comb-line structure.

14. The radio frequency filter according claim 11, wherein transmission lines incorporated in the radio frequency filter are microstrip lines.

15. The radio frequency filter according claim 11, wherein transmission lines incorporated in the radio frequency filter are strip lines.

16. The radio frequency filter according to claim 11, wherein the SAW transducers and the transmission lines incorporated in the band pass filter are located on a single piezoelectric substrate.

17. The radio frequency filter according to claim 11, wherein the dominant acoustic wave mode to which the SAW transducer couples is a pseudo-SAW mode.

18. The radio frequency filter according to claim 11, wherein the dominant acoustic wave mode to which the SAW transducer couples is a leaky wave mode bound to the surface.

Description

[0017] The invention will be described in more detail below with reference to the accompanying drawings, illustrating the invention by way of examples.

[0018] FIG. 1a illustrates the basic electrode shape of a SAW transducer according to the prior art.

[0019] FIG. 1b illustrates one possible embodiment of an arbitrary number of SAW transducers connected through two buses according to the prior art.

[0020] FIG. 2a illustrates a preferred embodiment of the invention.

[0021] FIG. 2b illustrates an alternative embodiment of the invention.

[0022] FIG. 2c illustrates the function of the invention.

[0023] FIG. 3: illustrates the response of a SAW impedance element filter (IEF) before connecting it to the correction unit.

[0024] FIG. 4: illustrates the response of a SAW correction unit with inductive and capacitive matching components

[0025] FIG. 5: illustrates the responses of the SAW IEF after connecting correction units on both sides of the filter.

[0026] As can be seen form FIG. 2a a correction unit 203 is connected to a filter 201 which in practice will be a band pass filter with known characteristics and distortions. As is illustrated in the lower part of FIGS. 2a and 2b, the correction unit 203a-b may include at least one SAW transducer 204a-n. In addition, inductive elements 208,209 and capacitive elements 210,211 may be included in the correction unit 203 in order to reduce the impedance mismatch of the correction unit in the signal line. As stated above the construction of the SAW-transducer is known to the person skilled in the art and will not be discussed in detail here.

[0027] In FIG. 2a the correction units 203a and filter 201 are grounded while FIG. 2b illustrates a balanced embodiment of the invention.

[0028] In FIG. 2c the function of the invention is illustrated where the amplitude response of the filter is shown and its insertion loss IL1 indicated. The SAW transducer in the correction unit 203a,b is arranged in a per se manner known to add a correction signal to the input or output of the filter 201 corresponding to the inverse of the known errors of the filter 201. The resulting, combined signal is shown in the last drawing where the error is removed from the signal. As indicated the correction is achieved through a degradation in insertion loss of the connected network, as indicated by IL2IL1+IL.sub.SAW. As illustrated in FIGS. 2a and 2b correction units may be positioned on both sides of the filter.

[0029] As is known in the field the acoustic admittance bandwidth of a SAW transducer is limited by the length of the transducer finger pattern and substrate material properties. In applications where the needed dissipation profile bandwidth exceeds the maximum achievable bandwidth of one transducer, the needed bandwidth may be achieved by employing multiple SAW transducers coupled to the filter, each correcting a separate section of the passband. The magnitude of the induced correction is limited by the aperture of the transducer, here defined as the length over which electrode fingers overlap, and substrate material properties. If the needed correction magnitude exceeds the maximum achievable magnitude of one transducer, the correction magnitude may be enhanced by coupling multiple SAW transducers with overlapping dissipation profiles in parallel. Alternatively, the correction magnitude may be diminished by either reducing the transducer aperture or by coupling multiple similar SAW transducers in series. Consequently, by connecting one or more SAW transducers to a RF filter in a manner described above, an arbitrary correction of the total network passband response may be achieved through the sacrifice of insertion loss. To minimize the insertion loss degradation, losses may be introduced to primarily affect passband ripple peaks, thereby flattening the passband to a level near the passband ripple minima. The resulting insertion loss degradation will here be given by the intrinsic loss of the transducer outside the acoustically active frequency range, given by the capacitance of the transducer, and the peak-peak passband ripple of the connected RF filter. This type of loss profile may be realized by connecting the filter to multiple loss inducing SAW transducers, each introducing losses at a ripple peak, or through multi-band transducers with multiple passbands and stop bands, where each passband coincides with a ripple peak. This process is illustrated in FIGS. 2c and 5.

[0030] One limitation of the invention is that it will impose degradations in return loss of the network due to impedance mismatch between the filter and the SAW transducers. This mismatch may be reduced by including inductive 208,209 and capacitive 210,211 components as illustrated in FIG. 2a,b. The capacitive elements 210,211 may be realized through the parasitic capacitance of the capsule connector pin to which the SAW transducer bus is connected, in combination with optional capacitive components connected to the ground plane of the capsule. The inductive elements 208,209 can be realized through the bonding wire connecting the SAW transducer bus to the capsule pin in combination optional inductive structures realized on the piezoelectric substrate of the SAW transducer, on a separate PCB or on a ceramic substrate mounted in the capsule. Inductive elements may also be realized through alternate inductive elements known to a person skilled in the art, such as chip inductors or toroid components.

[0031] Amplitude perturbations may in practice be induced by the SAW transducers and introduce minor perturbations in the phase of the connected network. The magnitude of the introduced phase dispersion is correlated to the magnitude of the induced amplitude correction through the Kramer-Kronig relations, and may be noticed for severe passband corrections. For the simulation results presented in FIGS. 3-5 a correction of approximately 1 dB was applied to correct the passband amplitude response. The corresponding change in group delay variation and linear phase ripple was 1.2 ns and 0.4 degrees respectively, a respective change of 9% and 3%. The observed perturbation of the phase response will depend on the magnitude and bandwidth of the applied amplitude correction, and is expected to be small for moderate correction magnitudes on the order of a few dB.

[0032] Yet another limitation to the invention is that the power handling capabilities of the network may become limited by the SAW transducers if connected to a filter with power handling capabilities exceeding that of the SAW transducers. It is noted that the power handling capabilities of a loss inducing SAW transducer will not be the limiting factor when combined with a SAW impedance element filter (IEF) on the same substrate since the power handling capabilities of a SAW transducer will exceed that of the SAW resonators incorporated in the SAW IEF. A SAW IEF here refers to a filter network composed of connected SAW resonator elements, the filtering response being defined by the topological configuration and frequency dependent impedances of constituent SAW resonators. The power handling capabilities of a SAW resonator will be inferior to that of a SAW transducer of comparable aperture, as the finger pattern in the SAW resonator is subject to a standing wave of increased magnitude in the resonator cavity, whereas the SAW transducer is subject to a traveling SAW of lesser magnitude.

[0033] Still yet another limitation to the invention is related to the frequency regime to which passband corrections through loss inducing SAW elements can be applied. This frequency range is limited by the processing techniques used to define the SAW transducers and the SAW velocity of the used piezoelectric substrate, where the approximate applicable frequency range spans from 100 MHz to 10 GHz. In one embodiment of the invention, where SAW correction units are implemented on the same substrate as a connected SAW filter, the aforementioned frequency constraints of the SAW correction unit is not the limiting factor, since the limitations imposed by the processing techniques defining the transducer geometry will affect the SAW transducers and SAW filter equally.

EXAMPLE OF INVENTION EMBODIMENTS

[0034] In one embodiment of the invention an electroacoustic filter, examples include but are not limited to transversal SAW filters, SAW impedance element filters (IEF), bulk acoustic wave (BAW) filters and film bulk acoustic resonator (FBAR) filters defined on a piezoelectric substrate, with one input and output terminal, where the input and or output terminal of the filter is connected to one or more loss inducing SAW transducers 204a-n having two electrodes 101,102 defined on the same piezoelectric substrate, where each loss inducing SAW transducer 204a-n has one electrode 101a-n connected to a first bus 103, this bus being connected to both the input or output interface of the filter 207a, and the input or output pin of the capsule housing the filter network 212a, whereas the other electrode 102a-n is connected to a second bus 104 connected to ground. The first ungrounded bus 103 may be connected to the filter 207a and pin interface 212a either through a bonding wire, or through inductive 208,209 and capacitive 210,211 elements. Inductive elements 208,209 may be realized through chip inductors, inductive PCB or ceramic substrates mounted in the capsule or through inductive metallization patterns on the piezoelectric substrate defining the SAW transducers. Capacitive elements 210,211 may be realized through capacitive structures on a printed circuit board (PCB), ceramic substrate or piezoelectric crystal or through chip capacitors connected to the ground plane in the capsule. When integrated on the same piezoelectric substrate, the ungrounded bus may interface the electroacoustic filter directly.

[0035] In another embodiment of the invention an arbitrary planar compatible RF filter, examples include but are not limited to electroacoustic filters as exemplified above, lumped-element LC filters, stripline filters and microstrip filters, with one input and output terminal, where the input and or output terminal of the filter is connected to one or more loss inducing SAW transducers 204a-n having two electrodes 101,102, defined on a separate piezoelectric substrate mounted in the capsule housing the filter network, wherein the SAW transducer electrodes 101a-n,102a-n are connected to the RF filter 201, ground plane and input or output terminals of the capsule in a similar fashion as described in the first described embodiment of the invention.

[0036] In a further embodiment of the invention one or more loss inducing SAW transducers 204a-n are mounted in a capsule to form a filter correction module. Each module consists of one or more SAW transducers 204a-n with two electrodes 101,102 on a piezoelectric substrate mounted in the capsule, where each transducer has one electrode 102a-n connected to a grounded bus 104 and the other electrode 101a-n connected to a bus 103 interfacing the input 205a and output pin 206a of the capsule via a bonding wire, or via an inductive element 208,209 realized either through a bonding wire, through an inductive component such as a chip inductor or inductive PCB or ceramic substrate mounted in the capsule or through an inductive metallized pattern defined on the piezoelectric substrate. The input and or output pin could also be connected to a grounded capacitive element 210,211 such as a chip capacitor in the capsule. One or more loss inducing SAW modules 203a may be connected to the input or output terminal of an external arbitrary RF filter module 201 to form a filter network with improved passband frequency selectivity, at the expense of increased insertion loss.

[0037] In an alternative embodiment of the invention an arbitrary planar RF filter incorporating shunt connected capacitive elements, examples include but are not limited to lumped-element inductor-capacitor (LC) filters and any planar transmission line filter architecture known to a person skilled in the art, is combined with one or more loss inducing SAW transducers, where the SAW transducers are used to both realize shunt connected capacitive elements in the RF filter, and to selectively introduce losses in the passband of the connected network. Each SAW transducer has two electrodes 101,102, defined on a piezoelectric substrate mounted within the capsule housing the filter network, where one electrode 101 is connected to the interior of the RF filter through a bonding wire and the other electrode 102 to ground through a bonding wire. Loss inducing SAW transducers can be defined on a shared piezoelectric substrate or on separate piezoelectric substrates.

[0038] Yet another embodiment of the invention comprises a balanced four port electroacoustic filter, examples include but are not limited to transversal SAW filters, SAW impedance element filters (IEF), bulk acoustic wave (BAW) filters and film bulk acoustic resonator (FBAR) filters defined on a piezoelectric substrate, with two input terminals and two output terminals, where the input and or output terminals are connected to one or more loss inducing SAW transducers 204a-n having two electrodes 101a-n,102a-n, defined on the same piezoelectric substrate, where each loss inducing SAW transducer 204a-n has one electrode 101a-n connected to a first bus 103, this bus being connected to both the input or output interface of the filter 207a, and the input or output pin of the capsule housing the filter network 212a, and the other electrode 102a-n connected to a second bus 104, this bus being connected to the other input or output terminal of the filter 207b, and the other input or output pin of the capsule 212b, wherein the two buses 103,104 are connected to the RF filter 202 and input or output terminals of the capsule 212a-b in a similar fashion as described in the first described embodiment of the invention.

[0039] Still yet another embodiment of the invention comprises an arbitrary planar compatible balanced four port RF filter, examples include but are not limited to electroacoustic filters as exemplified in the embodiment above, lumped-element LC filters, stripline filters and microstrip filters, with two input terminals and two output terminals, where the input and or output terminals are connected to one or more loss inducing SAW transducers 204a-n having two electrodes 101,102, defined on a piezoelectric substrate mounted in the capsule housing the filter network, wherein the SAW transducer electrodes 101a-n,102a-n are connected 207a-b to the RF filter 202, and input or output terminals of the capsule 212a-b in a similar fashion as described in the embodiment described above.

[0040] The present invention has been shown to provide appropriate correction magnitudes for SAW transducers with realizable apertures on common SAW substrate materials, such as Quartz, LiTaO3 and LiNbO3. A dip in transmission on the order of several dB relative to transmission outside the acoustically active region of the SAW transducer was found to be achievable. Materials and rotations supporting other acoustic wave modes are also considered suitable for the realization of correction elements, examples including but not being limited to pseudo-SAW modes and leaky wave modes bound to the surface.

[0041] Simulations have been performed with one loss inducing SAW transducer 204a connected to the input and output of a SAW impedance element filter, where inductive and capacitive elements were included at the input- and output-interface of the connected network to reduce impedance mismatch. FIG. 2c,3,4,5 illustrates the effects and improvements in the filter frequency response that may be obtained according to the invention.

[0042] The response of the SAW impedance element filter shown in FIG. 3, where at the top left the SAW IEF passband amplitude response is illustrated. At the top right the SAW IEF return loss response is shown. At the bottom right the overall SAW IEF amplitude is shown and at the bottom right the SAW IEF group delay response is shown.

[0043] The electrical response of a correction unit 203a comprising one SAW transducer 204a connected to inductive 208,209 and capacitive elements 210,211 only at the input or output terminal is shown in FIG. 4. FIG. 4, Top left shows the amplitude response of the correction unit, whereas the top right Figure shows its return loss response. The bottom left Figure shows the overall correction unit amplitude, while the bottom right Figure shows its group delay response.

[0044] The response of the connected network with one correction unit 203a at the input and output 207a of the SAW impedance element filter is shown in FIG. 5. The top left Figure shows the passband amplitude response of the connected network, whereas the top right Figure shows the return loss response. The bottom left Figure shows the overall amplitude response, while the bottom left shows the group delay response.

[0045] As shown in the Figures discussed above, the insertion loss variation in the passband is significantly improved for the connected network. The average insertion loss over the passband of the network is degraded by 1.3 dB. Minor perturbations in the return loss response and group delay variation are also observed for the connected network.

[0046] To summarize the present invention relates to a filter correction unit 203a-b for use in RF transmission lines including a band pass filter 201,202 having input and output interfaces mounted in the signal transmission line 213. The filter is a band pass filter chosen so as to transmit signals within a predetermined frequency range, and has a quality factor with known limitations generating a known distortion to the signal, such as amplitude ripple and loss. The correction unit 203a-b is connected between the RF transmission line 213 and the input-or output-interface of the filter, thus allowing it to correct a transmitted signal either before entering, or after passing through the filter.

[0047] The correction unit comprises at least one SAW-transducer 204a-n, the transducer or transducers having two electrodes 101,102 on a piezoelectric substrate where a first electrode 101a-n is connected to a first bus 103 and the other electrode 102a-n connected to a second bus 104, wherein the first bus 103 connects said transmission line 213 and RF filter input- or output-interface 207a. The second bus 104 is either connected to ground or used to connect said transmission line 213 with the second input or output filter interface 207b when combined with a balanced four port RF filter 202. The SAW-transducers are preferably provided with electrode finger patterns being adapted or similar, being chosen so as to distort a transmitted signal with a factor being the inverse of said know distortion of the filter.

[0048] The second bus 104 may be connected to ground directly or through a common conductor, or may be used to connect a second interface 212b of said transmission line 213 with the second input or output interface 207b of the RF filter when combined with a balanced four port RF filter 202.

[0049] The filter correction unit will preferably also include capacitive elements 210,211 between the two buses 103,104, at the input- 205a-b and or output-interface 205a-b of the correction unit 203a-b, and inductive elements 208,209 between the buses 103,104 connecting the SAW transducers 204a-n and the input- 205a-b and output-interface 206a-b of the filter correction unit 203a-b, so as to provide impedance matching of the correction unit 203a-b in the signal line.

[0050] The correction unit 203a-b and band pass filter 201,202 may be produced on a common piezoelectric substrate material providing a unitary structure with common interfaces 207a-b.

[0051] According to another aspect of the invention the RF filter is provided including a filter correction unit as described above. In this case the band pass filter may be constituted by grounded capacitive filter elements, the capacitive filter elements being realized through at least one of said SAW transducers, each having two electrodes 101,102 on a piezoelectric substrate where the first electrode 101 is connected to the interior of the band pass filter and the second electrode 102 to ground. The SAW transducer or transducers are then both adapted both to distort a transmitted signal with a factor being the inverse of said know distortion of the filter, and to realize the capacitance required to define the band pass filter response, thus having a combined functionality. The filter and correction part being variations of the same type of elements.

[0052] The filter may be realized in several different ways, e.g. as a LC filter, or as coupled transmission lines, microstrip transmission lines positioned in a comb-line structure being one potential embodiment. The SAW transducers may be realized on the same substrate as said transmission lines, or on a different substrate.

REFERENCES

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