Description

The invention refers to an improved RF circuit comprising a combination of RF filters, and to a method of reducing losses of the circuit.

Within an RF frontend circuit, different electric components like filters, switches, lines and other useful components are coupled to one another to allow operation of the RF frontend circuit in one or more frequency bands each assigned to one of the filters. The RF frontend circuit must be designed to allow multiple band selection to support single band modes wherein a single filter is operating stand-alone, as well as carrier aggregation (CA) modes, where multiple filters are working at the same time while being connected to the same antenna.

However, when different filters are combined, each filter may load the other ones in a capacitive and resistive manner such that losses arise. Designing a matching network that is able to perfectly match all the filters when operating alone or in any possible CA mode is a very complex problem. In addition, since the out-of-band reflectivity of the filters is not perfect, a resistive loading is also present which degrades the insertion loss of the other filters. When using SAW technology filters, bulk-wave excitation causes one of the loading effects. This effect becomes more severe when the frequency gap between the filters increases. Another one of the loading effects would be e.g. finger resistance.

Currently used RF frontend circuits use a band-select switch to set a desired band combination or to switch between different single bands or band combinations while shunt-matching elements, typically inductances, are implemented to compensate the capacitive loadings of the other filters. This provides acceptable performances if the frequency gap between different filters is rather small that is lower than 30% of a middle frequency of a filter. Otherwise, the dispersion introduced by the loading of the filters becomes too strong to be effectively compensated by use of this type of matching elements. Therefore, in the case of a greater frequency gap between filter bands it is impossible to get all filter combinations matched very well at the same time. Any solution to reduce or overcome the problem of resistive loading and therefore minimize the additional insertion loss that a certain filter introduces to the other filters is not known so far. Hence, it is an object of the present invention to provide an RF frontend circuit showing an improved matching over all used bands and band combinations. A further object is to provide a method of operating the frontend circuit and a method of reducing losses when operating the frontend circuit.

These and other objects are met by an RF frontend circuit according to claim 1. A method of operating the frontend circuit and a method of reducing losses when operating the frontend circuit as well as advantageous embodiments of the invention are given by further claims.

An RF frontend circuit comprises a switch, at least a first and a second signal path with a first and a second frequency filter arranged therein respectively. Both signal paths are coupled to the switch. The switch allows coupling one of first and second signal path separately to an antenna, or to couple first and second signal path in parallel to the antenna. The first frequency filter is assigned to a first frequency band. The second frequency filter, assigned to a second frequency band higher than the first frequency. At out-of-band frequencies of each filter, a first and a second parasitic capacitance respectively as well as a first and a second parasitic resistance are assigned to the filters. A parasitic switch capacitance is assigned to the switch.

According to the invention, the possible losses of the RF frontend circuit can be greatly reduced by inserting a first series coil into the first signal path and coupling the coil between the first filter and the switch. When connected to the antenna, the most relevant loss of a frontend circuit in general occurs at frequencies of the second filter due to resistive loading by the filter having the lower frequency. When connecting first and second path simultaneous to the antenna the first path forms a shunt arm of the second path. In this shunt arm, the series connection of the coil and the parasitic capacitance of the first filter form a band-stop filter whose stop band can be set at or near the first frequency band, which is out-of-band relative to the second filter. Hence, the second filter is not influenced by frequencies of the first frequency band of the first filter anymore.

When looking at the first filter this filter sees the series coil and the parasitic capacitance in a shunt arm. These two elements can be supplemented by a further shunt capacitance connected to a node in the first signal path between the first filter and the series coil. As a result, a phase shifter is formed by the pi member like arrangement of shunt capacitance, series coil and the further parallel parasitic capacitance of the second filter. By properly selecting the inductance value of the series coil and the capacitance value of the shunt capacitance an ideal phase shifter for the frequencies of the first band can be created. The ideal phase shifter has to take into account the parasitic capacitance of the switch that adds to the parasitic capacitance of the second filter. Best results achieves a symmetric pi member with capacitances on both sides of the coil/inductance having nearly the same value.

According to an embodiment a third frequency filter, assigned to a third frequency band is arranged in a third signal path and coupled to the switch. The frequencies of the third frequency band are greater than the frequencies of the first frequency band but lower than the frequencies of the second frequency band. If no additional elements like the series coil were introduced in the circuit simultaneous coupling the three paths to the antenna via the switch would normally introduce some problems to the behavior of the single filters i.e. produce additional losses. In this case, for example the passband of the first filter would suffer ripple due to matching conditions that are not good. The insertion loss of the third filter would suffer resistive losses due to the influence of the lower frequency characteristics of the first frequency band. At last, the second filter suffers the greatest loss due to the influence of the two lower frequency characteristics of the first and third frequency band. Moreover, a ripple may also occur in the passband of the second filter due to bad matching conditions.

With the series coil arranged between the switch and a common node of first and third signal path the transfer of all three filters is improved relative to a known circuit showing the above-mentioned problems and disadvantages. The passband of the first filter is without any influence by the two other filters and shows a good matching. The passband of the third filter shows a similar performance as before.

The greatest effect of this circuit can be seen at the passband of the second filter: It has no additional losses caused by the two other paths and filters, and shows a good matching. The effect is improved if the band gap between the second frequency band having the highest frequency and the other bands gets larger. Hence, this embodiment of the invention provides a triplexer that has two filters with improved behavior and a third filter with no additional loss or ripple.

According to another embodiment, the first and the third frequency filter are coupled to a common node of the first and the third signal path. The node is arranged between the filters and the first series coil. The parasitic capacitance of the third filter, the series coil and the sum of second parasitic capacitance and switch capacitance form a pi member, which is acting as a phase shifter for frequencies within the first frequency band. Designing the phase shifter as an ideal phase shifter for frequencies of the first frequency band by proper selecting the inductance value thereof will provide no negative impact on the behavior of the second filter. The proposed solution works perfectly if both parallel impedance elements (capacitances) present the same or at least a similar parasitic impedance (capacitance).

However, even if the phase shifter is not quite ideal or not quite symmetric an asymmetry of up to about 20% between the capacitances on both sides of the pi member provides less than 10% variation (degradation) of the impedance, which is still a good matching.

This embodiment provides an additional positive effect on the behavior of the second filter by eliminating the resistive loading and the losses resulting therefrom.

According to an embodiment, any imbalance in the parasitic impedance elements can be compensated by a supplemental element, which maybe in the case of the switch and the filters a supplemental capacitance. This supplemental capacitance is circuited in parallel to a selected parallel parasitic capacitance where the selected parasitic capacitance is the one that shows the smaller capacitance. The value of the supplemental capacitance is chosen so that the pi-member gets symmetrical, i.e. the capacitances in the parallel branches at both sides of the pi-member are equal or at least similar.

According to an embodiment, the series connection of the series coil and the two parallel parasitic capacitances of first and third frequency filter form a branch in parallel to the second frequency filter. This branch is acting as a band-stop filter. Selecting filters having proper parasitic capacitances combined with a coil having an optimized inductance value makes the band-stop effective on frequencies of first and third frequency band. Best effect is yielded if the band-stop frequency is set between first and third frequency band and the bandgap between first and third frequency band is relative small.

Further improvements of the frontend circuit are provided by inserting further series coils in one ore of the signal paths between a respective filter and the switch. These coils can be used to compensate asymmetries in the circuit and further improve the matching.

A useful supplementation of the frontend circuit can be provided when the output of a second switch is coupled to one of the signal paths or to a common signal path connecting the first switch and the antenna. To a number of inputs of the second switch a respective matching impedance element is coupled that can be selectively connected to the signal path. Dependent on the possible switching positions switch also a combination of impedance elements can be connected to the signal path. These impedance elements allow switchable matching within the frontend circuit. By these switchable impedance elements, the parasitic capacitance of the switch for example may be compensated or adapted otherwise.

When the second switch is present, a selected impedance element can be coupled to the arrangement via the second switch dependent on a selected frequency band or a selected combination of frequency bands. In an embodiment, the second switch is arranged between the switch for band selection and one of the signal paths or between the first switch and the antenna.

In the following, the invention and a method of operating the frontend circuit is explained in more detail with respect to exemplary embodiments and the

accompanying figures.

The figures are drawn schematically only and may be simplified for better

understanding.

FIG. 1A shows a signal path with a filter in a parallel branch,

FIG. 1 B shows an equivalent circuit for the arrangement of FIG. 1 A that is valid for lower out-of-band frequencies

FIG. 1C shows an equivalent circuit for the arrangement of FIG. 1A that is valid for higher out-of-band frequencies

FIG. 2A shows an equivalent circuit of a band-stop filter having a band-stop frequencies f _B s

FIG. 2B shows an equivalent circuit of the band-stop filter of figure 2A at a

frequency f ₀ « fBs

FIG. 2C shows an equivalent circuit of the band-stop filter of figure 2A at the band-stop frequency f _BS FIG. 2D shows an equivalent circuit of the band-stop filter of figure 2A at a frequency f ₀ » f _B s

FIG. 3 shows a phase shifter embodied as a pi member

FIG. 4 shows in first embodiment a diplexer

FIG. 5 shows an equivalent circuit of the circuit shown in figure 4 as seen from filter F1

FIG. 6A shows the circuit of figure 4 with a supplemental parallel capacitance FIG. 6B shows an equivalent circuit of the circuit shown in figure 6A as seen from filter F1

FIG. 7 shows in second embodiment a triplexer

FIG. 8 shows an equivalent circuit of the circuit shown in figure 7 as seen from filter F1

FIG. 9 shows an equivalent circuit of the circuit shown in figure 7 as seen from filter F3

FIG. 10A shows an equivalent circuit of the circuit shown in figure 7 as seen from filter F2

FIG. 10B shows the equivalent circuit of figure 10A after simplification of the circuit FIG. 10C shows the equivalent circuit of figures 10A and 10B after further

simplification of the circuit

FIGs. 11 A to 1 1C show a comparison each of a filter transfer curve of a filter of figure 7 each and a transfer curve of a respective filter of a known circuit as shown in figure 13

FIG. 12 shows in a further embodiment a circuit supplemented with switched matching elements

FIG. 13 shows a conventional triplexer.

FIG. 1A shows a signal path SP connecting a first terminal T1 to a terminal T2. In a path circuited in parallel to the signal path a filter Fx having a center frequency f _0x is arranged.

FIG. 1 B shows an equivalent circuit for the arrangement of FIG. 1A that is valid for small out-of-band frequencies f ₀ that is for frequencies in the lower stop band. For a frequency f ₀ the filter Fx behaves like a parasitic capacitance C _Fx that produces a capacitive loading of the signal line SP at a frequency f ₀ « f _0x

FIG. 1C shows an equivalent circuit for the arrangement of FIG. 1A that is valid for high out-of-band frequencies that is for frequencies f ₀ in the upper stop band of the filter Fx. For these frequencies f ₀ the equivalent circuit of filter Fx comprises a shunt parasitic capacitance C _Fx and a shunt resistor R _Fx. This is because the center frequency f _ox of the filter Fx is lower than the frequency f ₀ of the regarded signal in the signal line SP. Thus, filter Fx produces a resistive loading at f ₀ » fox.

FIG. 2A shows an equivalent circuit for a band-stop formed by a series connection of an inductance L _BS and a capacitance C _B s arranged in a shunt arm that is coupled in parallel to a signal line connecting terminals T1 and T2.

At a frequency f ₀ at band-stop frequency f _B s the shunt arm shows ideally zero impedance and forms a shunt as shown in figure 2C where f ₀ =

At a frequency f ₀ < f _B s the band stop filter behaves only capacitive and the equivalent circuit of the shunt arm forms only a capacitance C > C _BS .as shown in figure 2B. The value C tends to the value of C _BS as the frequency decreases.

At a frequency f ₀ > f _B s the band stop filter behaves only inductive and the equivalent circuit of the shunt arm forms only an inductance L < L _BS as shown in figure 2D. The value of L tends to the value of L _BS as the frequency increases.

FIG. 3 shows a pi-member comprising a series coil L _s in a signal line and two capacitive impedance elements C _P that are both arranged in shunt branches on both sides of the series coil L _s in parallel to the signal line. The signal line is terminated at both sides with a reference impedance Z ₀ .

An ideal phase shifter is symmetric and works optimally if C _P and L _s are properly selected dependent on the reference impedance Z ₀ and the desired phase shift value Φ ₀ according to the formulae l-cos(<B ₀ ) Z ₀ sin(O ₀ )

CP = LS =

2π ₀ Ζ ₀ sin(<B ₀ ) 2π ₀

Figure 4 shows as a first embodiment a frontend circuit with two filters F1 and F2 that are both arranged in a respective signal path that is connectable via a switch SW to an antenna or another terminal with a reference impedance Z ₀ . Different switching positions allow to connect a single one of the filters, both filters simultaneous or one or two filters together with another signal path or one or more other RF circuit element coupled to an input respectively of the switch SW (no such element is shown in the figure). A series coil L _s is arranged in the signal path between first filter F1 and switch SW. The respective center frequencies of the two filters are fO _! and f0 ₂ where f0 ₂ > fO^ Such a frontend circuit can work as a diplexer or a higher order multiplexer.

When operating the first filter F1 and the respective signal path with its center frequency the signal sees an equivalent circuit like shown in FIG. 5. The series coil LS is within the signal path while second filter F2 acts like a shunt capacitance C _F2 at f ₀ i because f ₀ i _< f ₀ 2. Same is true for the switch SW that has a parasitic capacitance C _S w that is mostly caused by open contacts of the switch SW.

This equivalent circuit of FIG 5 can easily be supplemented such that the coil and the two parasitic capacitances form a low pass phase shifter. This needs a supplemental capacitances C _s in parallel to the first filter as shown in FIG. 6B. Now, a pi member is formed by the parallel supplemental capacitance Cs, the series coil LS and the effective capacitance C _e . These two capacitances CF2 and Csw act like one capacitance having an of effective capacitance value C _eff = C _F2 + C _S w- Such a capacitance may be smaller than the filter capacitance and may be formed as an interdigital metallization on top the according SAW filter e.g. on the substrate of first filter F1. FIG. 6A shows the according structure of the frontend circuit of this embodiment. Now, when acting as diplexer with the two signal paths and their filter F1 and F2 simultaneously coupled to the antenna, filter F1 is not influenced by filter F2 anymore. Hence, no additional losses are created in the diplex mode relative to a single filter mode where only one filter is active and coupled to the antenna.

In a further embodiment the diplexer can be supplemented to a triplexer or a higher multiplexer by coupling one or more further signal paths to the antenna. When third filter F3 having a center frequency f ₀ 3 is connected to a common node in the signal path between first filter F1 and series coil LS such that f ₀ i < f ₀ 3 < f ₀ 2 the third filter F3 can contribute its parasitic capacitance C _F 3 to complete a pi-member forming a phase shifter.

Figure 7 shows a frontend circuit according to this embodiment. The switch SW can selectively couple one or more signal paths simultaneously to the antenna. For the shown embodiment, two switch inputs are sufficient. But further inputs to couple further signal paths or other RF components to the antenna are possible. Moreover, a matching inductance L _m as shown in the first signal path of the first filter F1 can optionally be inserted in each signal path near the respective filter.

The results of an analysis of the behavior of the circuit dependent on the frequency are shown in the equivalent circuits of figures 8 to 10 (here it is assumed that the optional series coil Lm is not used).

The equivalent signal path of filter F1 at its respective center frequency f1 is shown in FIG. 8. A low pass pi member comprises shunt capacitance C _F 3 of filter F3, the series coil LS and the second effective shunt capacitance C _ef r where

C _eff = C _F 2 + Csw- If this phase shifter is made ideal, neither any influence on the signal transfer nor any losses are produced by the parasitic elements of the phase shifter. This is shown in the lower part of figure 8.

An equivalent network representation as seen by third filter F3 at its respective center frequency f3 is shown in FIG. 9. As f3 > f1 the first filter F1 produces a resistive loading in addition to its capacitive loading. This loading can be depicted as a shunt resistance R _F parallel to the signal path of filter F3. The other components are like those of the equivalent circuit of figure 8. Hence, there is similar small influence of first and second filter F1 , F2 at the frequency f3 plus some resistive loading by filter f1 causing minor losses at F3 and f3 respectively.

An equivalent network representation as seen by second filter F2 at its respective center frequency f ₀₂ is shown in FIG. 10A. The switch capacitance C _S w is also parallel to the second signal path. As f ₀₂ is the highest operation frequency in the system, both filters F1 and F3 are showing resistive and capacitive loading as well. Thus, four shunt elements R _F1 , C _F i, _F 3 and C _F3 are present circuited in series with series coil LS that is parallel to the second signal path of filter F2. These five elements R _F i, C _F i , R _F 3, C _F3 and LS form a band stop BS having a center frequency f ₀ Bs:

When foes is chosen between f ₀ i and f ₀₃ then the resistive loading of filter F1 and F3 on filter F2 is minimized. For a band stop BS having another center frequency f ₀ Bs _, the resistive loading is still reduced but not optimally minimized. This is because the impedance of C _F i and C _F3 at the frequency f ₀₂ are small and much smaller than R _F and R _F3 . The circuit of FIG. 10A can then be simplified to that of FIG: 10B, where the shunt arm is composed of an inductance L _eff (of a value similar to L _BS but always smaller) and an effective resistance R _eff . Since the value of R _eff is small, at the end, the shunt branch can be seen just as a parallel inductance (see FIG. 10C). Effectively, the resistive loading of filter F2 is not present anymore.

The positive effect of the invention can best be shown when comparing the transfer curves of the three filter F1 , F2 and F3 of FIG. 7 and the transfer curves of respective filters of a frontend circuit known from the art. Depicted are different curves assigned to different operation modes that is when the filters are operated alone or simultaneously with one or two of the other filters.

A frontend circuit known from the state of the art is shown in FIG. 12. Similar to FIG. 7 three signal paths with a respective frequency filter F1 to F3 arranged in each path are coupled to a switch SW. Two of the filters F1 , F3 are coupled to a common node that is coupled to the switch SW. A first matching coil L ₂ is coupled to the second signal path to compensate for the capacitive loading of second filter F2. A further matching coil l_i ₊₃ is coupled to the common signal path between the common node and the switch SW to compensate for the capacitive loading of first and third filter F1 , F3. The first filter in this example has a pass band complying with band B3. The pass band of second filter F2 complies with band B7 while the pass band of the third filter complies with band B1.

In FIG. 11 , the filter transfer curves of the frontend circuit of FIG. 13 are depicted on the left hand side and compared with respective filter transfer curves of the frontend circuit of FIG. 7 on the right hand side where the same filters are used.

Figure 11A shows the insertion loss of filter F1. In the known circuit, the pass band shows a ripple when operating the three filters simultaneously in parallel. No ripple occurs when operating only first and third filter F1 , F3 simultaneously. The same two cases are shown on the right hand side for the new circuit. It can be seen that no ripple occurs anymore while the insertion loss remains comparable. Different to the known circuit the transfer curves for the two operation modes are identical.

Figure 11 B shows the insertion loss of filter F3. In the known circuit an ideal transfer can only be regarded when filter F3 is operated alone. In case of simultaneous operation with first filter F1 or together with both other filters F1 and F2 additional loss occurs relative to single mode operation in band B1 only. In this case, the new circuit yields no remarkable improvement. Both curves are comparable to those of the known circuit and show similar performance.

Figure 11 C shows the insertion loss of filter F2. In the known circuit an ideal transfer can only be regarded when filter F2 is operated alone. In case of simultaneous operation of all three filter F1 , F2 and F3 enhanced loss and ripple occurs in the pass band. When looking at the curves of the new circuit the most remarkable improvement can be found for the behavior of second filter F2. As this filter is assigned to the highest frequency in the circuit, resistive and capacitive losses degrade the properties of the filter in the known circuit but no degradation of filter F2 properties is visible anymore in the inventive circuit! The resistive losses and the ripple are removed and the performance of the filter is the same in all operation modes as if the second filter would be operated alone. Hence, the matching and insertion of this filter is improved and good.

FIG. 12 shows a possibility to realize a switchable matching. To do so a further switch SW _m is coupled to the signal line between the already described switch SW _Band for band selection and the antenna, or is coupled directly to a signal path between band select switch SW _Band and one or more of the filters.

By the further switch SW _m, different matching elements can be circuited in parallel to the respective signal path. These matching elements may be selected from

inductances and capacitances. Moreover, different matching elements may have different impedance values. Such a switchable matching allows further improving the matching dependent on the operation mode or dependent on the external environment that also has influence on the antenna impedance.

The matching elements can be used e.g. to compensate for some asymmetries in the multiplexing due to an irregular environment, that is for example differences in module routings. This can also be used to gain flexibility if even more filters are combined and therefore the number of operating modes is increased.

Wth a circuit according to one of the embodiments and preferably according to figure 4 or 7, a method of minimizing the resistive loading on the highest frequency filters in a filter combination as shown or in any other filter combination is possible. The filter combination respectively the frontend circuit comprising this combination should comprise a first and a second frequency filter, a series inductance and a common node. The series inductance is arranged in the signal path between the first filter and the common node. The frequency of the second filter is higher than the frequency of the first filter. In a first step of the method, the parasitic capacitance of the first frequency filter is determined. Then, the value of the series inductance is set so that the second filter sees a band stop filter formed by a combination of the series inductance and the parasitic capacitance of the first filter. The center frequency of the band-stop is set to be around the center frequency of the first filter. By doing this the restive loading of the second filter can be improved remarkably.

A method of operating a frontend circuit comprising multiple signal paths with a frequency filter arranged in each path is given. In the circuit, the signal paths are connectable to an antenna by a switch the paths are coupled to. The parasitic capacitances of some of the filters are used together with the parasitic capacitance of the switch to form a pi-member acting as a phase shifter that matches different operation modes comprising simultaneous operation of different filter combinations or single mode operation. By inserting a series coil in one of the signal paths a pi-member is realized and the existing parasitic capacitances are accordingly supplemented. By properly setting the values of the components of the pi-member by designing the filters and the switch accordingly to provide a respective capacitance value the phase shifter can be optimized to a desired frequency that is the preferably the frequency of the filter and in the respective signal path the phase shifter is realized in. Hence, this filter is free of capacitive loading.

In spite of a limited number of embodiments, the invention is not restricted to the explained embodiments and figures. The invention is determined by the wording of claim 1 and comprises further embodiments that can be achieved when combining new elements used in the embodiments and the sub-claims.

List of reference symbols

BS band-stop filter

C _B s capacitance of BS

C _eff effective capacitance

C _F x out-of-band equivalent capacitance of filter Fx

C _P shunt capcitancecapacitance

Csw capacitance of SW

F1 , F2, F3 first, second and third filter

f ₀ i, fo2.. frequency of filter F1 , F2 ..

Fx filter

LBS inductance of BS

L _eft effective inductance

LS series coil

R _eff effective resistance

RFX out-of-band equivalent resistance of Fx

SP signal path

SW switch

T1 , T2 terminals of a circuit

Z ₀ impedance