Transformer balun for high rejection unbalanced lattice filters

A transformer balun for high rejection unbalanced lattice filters includes two symmetrical coils separated by a shielding element. In a further exemplary aspect, the transformer may include a conductor, which may be a third coil that operates with a capacitor to form a resonant circuit that enhances mutual coupling. Using either of the exemplary transformers provides improved performance in the passband while concurrently providing out-of-band rejection at levels exceeding seventy decibels (70 dB).

FIELD OF THE DISCLOSURE

The present disclosure relates to a transformer for use with a lattice filter.

BACKGROUND

Computing devices abound in modern society, and more particularly, mobile communication devices have become increasingly common. The prevalence of these mobile communication devices is driven in part by the many functions that are now enabled on such devices. Increased processing capabilities in such devices means that mobile communication devices have evolved from pure communication tools into sophisticated mobile entertainment centers, thus enabling enhanced user experiences. With the advent of the myriad functions available to such devices, there has been increased pressure to improve the speed with which data is transferred to and from such mobile communication devices.

The pressure to improve speed has led to the evolution of the cellular standards commonly used by the mobile communication devices from the third generation (3G) to the fourth generation (4G or long-term evolution (LTE)) and more recently fifth generation new radio (5G-NR). With the advent of LTE, the concept of carrier aggregation was introduced, with a commensurate demand for high-performance wideband filters.

Filters based on coupled resonator filters (CRFs) provide large bandwidths, but have spurious modes, which degrade filter rejection. While solidly mounted resonator (SMR) configurations can mitigate spurious modes, suppression remains imperfect and is sensitive to variation in reflector layer thicknesses. Accordingly, there remains room for improved filters for use by wireless networks.

SUMMARY

Aspects disclosed in the detailed description include a transformer balun for high rejection unbalanced lattice filters. In particular, exemplary aspects of the present disclosure provide a transformer coupled to a lattice filter where the transformer includes two symmetrical coils separated by a shielding element. In a further exemplary aspect, the transformer may include a conductor, which may be a third coil that operates with a capacitor to form a resonant circuit that enhances mutual coupling. Using either of the exemplary transformers provides improved performance in the passband while concurrently providing out-of-band rejection at levels exceeding seventy decibels (70 dB).

In a first exemplary aspect, a transformer is disclosed. The transformer comprises a first coil comprising an input. The transformer also comprises a second coil inductively coupled to the first coil. The second coil comprises an output. The transformer also comprises a resonant circuit comprising a third coil and a third capacitor. The third coil is inductively coupled to the first coil and the second coil.

In another exemplary aspect, a filter system is disclosed. The filter system comprises a lattice filter comprising a lattice output. The filter system also comprises a transformer. The transformer comprises a first coil comprising an input coupled to the lattice output. The transformer also comprises a second coil inductively coupled to the first coil. The second coil comprises an output. The transformer also comprises a resonant circuit comprising a third coil and a third capacitor. The third coil is inductively coupled to the first coil and the second coil.

In another exemplary aspect, a filter system is disclosed. The filter system comprises a lattice filter comprising a filter output. The filter system also comprises a transformer. The transformer comprises a first coil comprising an input coupled to the filter output. The first coil has an axis of symmetry and is symmetrical across the axis of symmetry. The transformer also comprises a second coil inductively coupled to the first coil. The second coil comprises an output. The second coil has a second axis of symmetry and is symmetrical across the second axis of symmetry. The transformer also comprises a shielding trace proximate the first coil and the second coil. The shielding trace is configured to reduce cross-capacitance between the first coil and the second coil.

DETAILED DESCRIPTION

It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present disclosure. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. It will be understood that when an element such as a layer, region, or substrate is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, there are no intervening elements present. Likewise, it will be understood that when an element such as a layer, region, or substrate is referred to as being “over” or extending “over” another element, it can be directly over or extend directly over the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly over” or extending “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present.

Aspects disclosed in the detailed description include a transformer balun for high rejection unbalanced lattice filters. In particular, exemplary aspects of the present disclosure provide a transformer coupled to a lattice filter where the transformer includes two symmetrical coils separated by a shielding element. In a further exemplary aspect, the transformer may include a conductor, which may be a third coil that operates with a capacitor to form a resonant circuit that enhances mutual coupling. Using either of the exemplary transformers provides improved performance in the passband while concurrently providing out-of-band rejection at levels exceeding seventy decibels (70 dB).

Before addressing exemplary aspects of the present disclosure, an overview of a lattice filter is provided with reference toFIGS.1A through2B, along with an analysis of how such a lattice filter may be coupled to a transformer to provide unbalanced operation with reference toFIG.3. The limits of adding a conventional transformer are explored with reference toFIGS.4-7C. A discussion of exemplary aspects of the present disclosure begins below with reference toFIG.8A.

In this regard,FIG.1Aillustrates a block diagram of a lattice filter100. The lattice filter100includes a positive input102and a negative input104that collectively form a Vin input. The lattice filter100further includes a positive output106and a negative output108that collectively form a Vout output. A first impedance110(Za) is located serially between the positive input102and the positive output106. A second impedance112(Zb) is located serially between the positive input102and the negative output108. Likewise, a third impedance114, which is identical to the first impedance110, is located serially between the negative input104and the negative output108. Also, a fourth impedance116, which is identical to the second impedance112, is located serially between the negative input104and the positive output106. Normally, Za and Zb are reactive to reduce filter loss.

Frequently, for simplicity, illustration of the third impedance114and the fourth impedance116is omitted or referenced using dotted lines.FIG.1Ashows the elements for completeness, but subsequent representations (e.g., the lattice filter302ofFIG.3) omits explicit illustration of these impedances.FIG.1Billustrates the same lattice filter100but presented in a balanced bridge form100′ It should be appreciated that the impedances Za, Zb are frequency dependent and may be expressed as Z(ω)=jX(ω).FIG.2Aillustrates a lattice filter200operating in a stopband when the impedances Za and Zb are equal (i.e., Za=Zb=jX) and Vout equals zero (Vout=0). Conversely,FIG.2Billustrates the lattice filter200operating in a passband when Za=−Zb=jX.

In either case, stopband or passband, the filters100,200are balanced. Most radio frequency systems are unbalanced since unbalanced systems are simpler in terms of routing compared to balanced systems. A balun may be used to convert the lattice filter100,200to an unbalanced system. One possible balun is a transformer.

In this regard,FIG.3illustrates a filter system300having a lattice filter302coupled at Vout to an ideal transformer304. The lattice filter302includes a positive input306coupled to a port P1. The lattice filter302also includes a negative input308coupled to a ground310. The lattice filter302also includes a positive output312and a negative output314. A first coil316of the ideal transformer304serially connects the positive output312to the negative output314. The first coil316is inductively coupled to a second coil318of the ideal transformer304. The second coil318is coupled to the ground310and to an output320with port P2.

While an ideal transformer304is desired, the ideal transformer304is more likely to be instantiated as a real (as opposed to ideal) transformer400with two coupled inductors402,404as illustrated inFIG.4. Additional capacitors406,408may be used to compensate for self-inductances. It should be appreciated that the combination of the inductor402with the capacitor406and of the inductor404with the capacitor408form two resonators, and thus are technically a second order filter. The transformer400has a coupling coefficient K between the first inductor402and the second inductor404. Higher coupling coefficients K are desirable as such allows for larger bandwidths and lower loss. At the frequencies of interest, magnetic materials are usually lossy and laminates are typically non-magnetic. The inability to use magnetic materials effectively limits the mutual coupling (K) to those achievable through physical geometries.

For example,FIG.5Aillustrates via graph500how a transformer having K=0.4 responds (line502) in the passband region. The graph500also illustrates how a transformer having K=0.5 responds (line504). It should be apparent that line504has better passband characteristics including a broader passband range and less attenuation in the passband than line502. Graph520inFIG.5Bshows performance across a wideband range, and particularly shows the rejection range. Specifically, line522shows the transformer with K=0.4 and line524shows the transformer with K=0.5. While line522has greater rejection, line522also has a smaller passband area, and thus, the performance of the transformer with K=0.5 (line524) is deemed more desirable.

When the transformer400is used in a lattice filter system in place of an ideal transformer, parasitic capacitances between transformer windings may influence the in-band loss and out-of-band rejection of the filter system. Specifically, as illustrated inFIG.6A, there may be a parasitic capacitance600present between the inductors402,404of the transformer400.FIG.6Bshows graph610comparing line612corresponding to the response of the lattice filter system without parasitic capacitance to line614corresponding to the response of the lattice filter system with parasitic capacitance600. While there is a slightly steeper rejection around 6.1 GHz (area616), the overall rejection outside of this range is generally inferior to that shown by line612(area618or620). At some of the frequencies of interest, the detrimental effect on filter rejection is greater than thirty decibels (>30 dB).

FIGS.7A-7Cillustrate other possible sources of parasitic capacitance and the corresponding impact on both passband and out-of-band performance. Thus,

FIG.7Aillustrates the parasitic capacitance700that may exist between an input of the first inductor402and ground702. Graph704compares the wideband response for line612(no parasitic) to the line706(added parasitic capacitance700). The presence of the parasitic capacitance700reduces the rejection in zones708and710. Likewise, as illustrated by graph720, in the passband, line706has a notch722reflecting unwanted attenuation in the passband.

FIG.7Billustrates the parasitic capacitance730that may exist between an output of the first inductor402and ground702. Graph732compares the wideband response for line612(no parasitic) to the line734(added parasitic capacitance730). The presence of the parasitic capacitance730reduces the rejection in zones736and738. Likewise, as illustrated by graph740, in the passband, line734has a notch742reflecting unwanted attenuation in the passband, although notch742is relatively small compared to notch722.

FIG.7Cillustrates both the parasitic capacitance700and730. Graph750compares the wideband response for line612(no parasitic) to the line752(added parasitic capacitances700and730). The presence of the parasitic capacitance700and730makes the lines612and752essentially identical across the rejection frequencies. However, as illustrated in graph754, in the passband, the parasitic capacitances negatively impact the performance with increased attenuation and notch756.

As noted above, because magnetic materials are lossy at the frequencies of interest, coupling is controlled by the physical geometries, and particularly by a separation between inductor traces. Current manufacturing limitations may place design constraints on minimum trace separation and layer thickness, which places an upper limit on K. Further, the design of the transformer and its nonidealities should be carefully considered to preserve the high rejection of the lattice filter. Likewise, to preserve desired performance, the transformer should have a high common mode rejection (CMR), which is negatively impacted by parasitic capacitances.

To implement an effective balun at the frequencies of interest, exemplary aspects of the present disclosure add electrostatic shielding between the transformer traces. One approach is to put the shielding traces between the transformer windings and tie them to ground. Tying to ground reduces any cross-capacitance between the windings. An exemplary implementation of this shielding is provided with reference toFIGS.8A and8B.

Specifically,FIGS.8A and8Billustrate a planar transformer800configured to be coupled to a lattice filter. The transformer800includes a first coil802(sometimes referred to as a primary coil) corresponding to the inductor402. The transformer800also includes a second coil804(sometimes referred to as a secondary coil) corresponding to the inductor404. The first coil802and the second coil804are symmetrical across an axis of symmetry806. As illustrated, the first coil802and the second coil804are symmetrical across a single axis of symmetry. However, there may be situations (not illustrated) where the second coil804is symmetrical across a second axis of symmetry. Alternatively, it is possible to think of the coils802,804being symmetrical across respective first and second axes of symmetry, but as illustrated the first and second axes of symmetry are identical. The first coil802includes a first input808and a second input810. Vout from the lattice filter may be applied across the inputs808,810. The second coil804includes a first output812and a second output814. The second output814may be coupled to ground816.

A shielding trace818is positioned between the first coil802and the second coil804. That is, the coils802,804may be formed from flat conductive metal traces positioned in a plane and having a physical gap820therebetween. The shielding trace818may be a flat conductive metal or conductive trace (e.g., a conductor) that forms a coil that is positioned in the same plane within the physical gap820. The shielding trace818is coupled to ground816. Further, the shielding trace818is cut822at any point within the coil of the shielding trace818. The planar positioning of the two coils802,804relative to the shielding trace818(also illustrated inFIG.8B) is helpful to reduce parasitic capacitance from the coil802to ground816and thus helps to reduce the notches722,742,756seen inFIGS.7A-7C.

The presence of the shielding trace818reduces mutual capacitance of the coils802,804. While not shown, to reduce capacitive coupling further, additional traces may be placed in layers above and/or below the plane of the coils802,804. The cut822is present to minimize current flow in the shielding trace818so as not to impact the mutual coupling of the coils802,804.

WhileFIGS.8A and8Billustrate a planar transformer800, the present disclosure is not so limited. Thus,FIGS.8C and8Dillustrate a similar transformer850with a first coil852positioned in a first plane, a second coil854positioned in a second plane directly above the first coil852with a shielding trace856positioned in a third plane between the first and second plane. The shielding trace856may include a cut858similar to cut822. Likewise, the second coil852and the shielding trace856are coupled to ground.

FIG.9provides a graph900that shows the response of the lattice filter system using transformer800in line902compared to a similar transformer without the shielding in line904. As is readily apparent, the rejection at the frequencies of interest is greater with the shielding trace818than without the shielding.

There are other ways to reduce parasitic capacitance. One such method is to use laminate stacks with lower electrical permittivity, although the number of commercially available materials is currently limited. Another possible approach is to reduce the width of the first coil802and/or the shielding trace818. Such reduction will also reduce the Q of the first coil802and/or reduce the effectiveness of the shielding. Additionally, the traces are generally at the smallest spacing permitted by the manufacturing process to achieve the largest possible coupling, so there is likely little room for improvement using this technique. Still another possible approach is to remove (at least partially) any dielectric material between the first coil802and the second coil804. This approach poses substantial manufacturing challenges using current technologies.

FIGS.10A and10Bprovide an alternate exemplary transformer1000that shields the secondary coil vertically. This approach may provide some benefit because the secondary coil is relatively insensitive to parasitics to ground. The transformer1000includes a first coil1002. The transformer1000also includes a second coil1004. The first coil1002and the second coil1004are symmetrical across an axis of symmetry1006. The first coil1002includes a first input1008and a second input1010. Vout from the lattice filter may be applied across the inputs1008,1010. The second coil1004includes a first output1012and a second output1014. The second output1014may be coupled to ground1016. A first shielding trace1018(e.g., a third coil) is positioned above the second coil1004and a second shielding trace1020(e.g., a fourth trace) below the second coil1004(better seen inFIG.10B). Laminate or substrate layers1022may be positioned between the shielding traces1018,1020and the second coil1004.

Again, the coils1002,1004may be formed from flat conductive metal traces positioned in a plane and having a physical gap1024therebetween. The shielding traces1018,1020may be flat conductive metal or conductive traces that form respective coils that are positioned parallel to, but in different planes from the second coil1004. The shielding traces1018,1020are coupled to ground1016. Further, the shielding traces1018,1020have cuts1026at any point within the coil of the shielding traces1018,1020.

FIGS.11A and11Billustrate graphs1100and1110that compare the performance of the lattice filter system using the transformer800ofFIG.8A(line1102) and the lattice filter system using the transformer1000ofFIG.10A(line1104) in the wideband and passband, respectively. As can be seen, performance is similar although the line1104shows better performance in the passband and a smaller notch1106. As illustrated, the notch1106is about 0.1 dB less using the transformer1000.

If the notch1106is intolerable, and use of any of the above-identified techniques do not sufficiently suppress the notch1106, the notch1106may be suppressed by increasing separation of the coils1002,1004, albeit at the expense of decreased mutual coupling.

It should be appreciated that there are other ways to increase the mutual coupling. One such way is through the use of an additional resonant inductor as better seen in the transformer1200ofFIG.12. The method is applicable to general coupled inductor systems and not necessarily restricted to balun structures. The transformer1200includes a first coil1202and a second coil1204. The first coil1202includes a first input1206and a second input1208. The second coil1204includes a first output1210and a second output1212. A first capacitor1214may be coupled across the inputs1206,1208. A second capacitor1216may be coupled across the outputs1210,1212. The second output1212may be coupled to a ground1218. A third coil1220is provided adjacent to the second coil1204. The third coil1220is coupled to a third capacitor1222to form a resonant circuit1224. There are now three coupling coefficients K12, K23, and K13. The value of the third capacitor1222may be varied to set the center frequency for the resonant circuit1224.

FIGS.13A and13Bshow graphs1300,1310which compare the performance of a conventional transformer (line1302) to the performance of the transformer1200ofFIG.12(line1304) in the passband and wideband, respectively. As can be seen by line1302, the performance in the passband attenuates the frequencies of interest less than the conventional transformer. Likewise, the rejection for the transformer1200is steeper at higher frequencies with a strong rejection at notch1306.

The method described above may be utilized to enhance mutual coupling of transformer baluns in lattice filter systems.FIG.14Aillustrates a transformer1400with a third coil and shielding. Specifically, the transformer1400includes a first coil1402and a second coil1404. The first coil1402includes a first input1406and a second input1408. The second coil1404includes a first output1410and a second output1412. The second output1412may be coupled to a ground1418. A third coil or conductor1420is provided vertically on top of the second coil1404. The third coil or conductor1420is coupled to a capacitor1422to form a resonant circuit1424. The third coil1420is coupled to the ground1418to serve as both a resonant circuit which enhances mutual coupling and a shield. The transformer1400may be symmetrical across an axis of symmetry1426.

As illustrated inFIG.14B, the different planes of the coils1402,1404relative to the third coil or conductor1420is highlighted. Also, it should be appreciated that a fourth coil or conductor1428may be provided to provide additional shielding. While not illustrated, the fourth coil or conductor1428may also include a capacitor like the capacitor1422to form a resonant circuit. Likewise, the fourth coil or conductor1428is coupled to ground.

While not shown, it is also possible to put the first coil1402and the second coil1404in different planes as shown inFIGS.8C and8D. Likewise, it is possible to put the resonant circuit1424of the third coil or conductor1420in the same plane with the first coil1402and the second coil1404.

FIGS.15A and15Bshow graphs1500and1510that compare performance of a lattice filter system using the transformer1000ofFIG.10A(line1502) to a lattice filter system using the transformer1400ofFIG.14(line1504) over the passband and wideband, respectively.

With the improved coupling, it may be feasible to separate the coils1402,1404to reduce the capacitance to ground1418of the first coil1402. Such separation may reduce the notch in the passband. Accordingly,FIGS.16A and16Bshow graphs1600and1610, substantially similar to graphs1500,1510but with line1602that has increased trace separation.