Cross Reference to Related Application

This application claims priority to German Application N 102016125014.7, filed on December 20, 2016, which is express incorporated by reference in its entirety.

Description

In wireless devices, a front-end circuit serves to direct and filter signals received from or transmitted by an antenna. A front-end circuit comprises an RX branch that is coupled between an antenna and a receiver unit. A TX branch is coupled between a transmitter and the antenna. Both branches usually comprise filters that only signals of a desired frequency can pass the respective branch. Usually, the components of a front-end are integrated on a module .

Due to the low amplitude of signals received at the antenna, high attention is required to eliminate spurious signals and to select a desired band. Frequency bands other than a desired one have to be blocked or substantially attenuated. Further, the signal needs amplifying before reaching the receiver unit. A low-noise amplifier is used that produces low noise to keep the signal quality high.

Together with a growing number of frequency bands used in wireless communication, respective devices operating in these frequency bands need an enhanced number of components. Embedded or integrated in a module, the size of the components should be minimized to keep volume and weight of the module low.

The insertion loss of filters used in modern RF front-end circuits is critical as higher insertion loss causes higher energy consumption which is critical due to limited accumulator capacity. Further, higher energy consumption causes lower operation time of the mobile device before a new load of the accumulator is required. Low insertion loss is also mandatory in order not to reduce the complete reception chain sensitivity.

Hence, it is an object of the invention to provide a front- end circuit that is further improved with respect to insertion loss and out of band attenuation.

This object is met by a front-end circuit according to claim 1. Further improvements and embodiments of the invention are covered by sub-claims .

Central idea of the invention is to provide an improved signal routing and signal processing in an RX branch of the front-end circuit. The RX branch is coupled between an antenna and a receiving unit. The receiving unit may be a section of a transceiver. This is an semiconductor component able to process transmit signals and receive signals as well .

The RX branch comprises an amplifier and an RX filter, the RX filter being coupled between the amplifier and the antenna. While an antenna usually delivers a single-ended signal, known RX filters have single-ended inputs and an output that is adapted to the signal processing in the receiver unit. If the receiver unit operates with differential signals, the RX filter may have an integrated balun function to provide a differential signal that is further processed by the amplifier and finally guided to the receiver unit.

Deviating from this standard RX branch design, the invention proposes to use an RX filter which has a single- ended input and a balanced or differential output. This differential output signal is delivered to the amplifier which has a differential input. The receiving unit then receives a single-ended signal from the amplifier.

Using a filter with a balun function provides several improvements in view of signal quality and insertion loss. A commonly used front-end module can operate within a number of frequency bands. Each band requires at least a separate filter or a separate signal path. If each filter produces a balanced output, the number of required output pins is significantly large. Hence, the module becomes complicated and enlarged. With the invention, the number of outputs can be reduced to at most to the half. This reduces costs as well as the size of the module.

The new front-end module shows a reduced coupling of different branches because of differential filtering. The noise is reduced by symmetrical signal processing thereby eliminating unsymmetrical couplings or other deleterious influences. As a result, a better out-of-band attenuation is achieved. The reduced coupling of TX signals into the RX branch makes a better isolation between RX and TX.

A better out-of-band attenuation allows coexistence with WLAN 2.5 and 5 GHz. Due to reduced coupling operating of the module in a carrier aggregation mode (CA mode) is improved.

If the RX filter is a partial filter of a duplexer, a better level of suppression and isolation without any penalty on insertion loss is achieved. By using a filter with single- ended input and balanced output, a better impedance match between the amplifier and the filter is possible. As a result, a better noise figure of the complete branch is achieved as well as improved reception for the user of a mobile phone, for example.

The RX filter of the front-end module can be embodied in any technology that allows single-ended input and balanced output. Examples for such filters are SAW filters, embodied as DMS filters or ladder-type filters, or a ladder-type filter made of BAW resonators .

The amplifier may be a common low-noise amplifier known from the art. In an embodiment, the amplifier is adapted to receive a balanced input signal and to produce a single- ended amplified output signal. In this case, a balanced signal is converted to an unbalanced signal at the output. Another possibility according to a further embodiment comprises an amplifier which is a full differential amplifier adapted to receive a balanced input signal and to produce a balanced amplified output signal. The signal can then be converted to a single-ended or unbalanced signal by a balun coupled between the output of the amplifier and the receiving unit.

According to another embodiment, the front-end circuit comprises two or more different RX branches. In each RX branch, a band pass filter is arranged having a single-ended input and a balanced output. All branches and, accordingly, all band pass filters are assigned to different frequency bands .

In this case, the outputs of the amplifiers can be coupled to a switch that is able to connect one or more of the RX branches simultaneously to the receiving unit. Using such a switch further simplifies the module by reducing the number of output pins to one, which pin is the output pin for all RX branches .

This embodiment does not exclude that separate ones of the RX branches can be coupled to the receiver unit separately without using the switch. Further, it is possible to use more than one switch if the number of RX branches is too high, making the required single switch too complicated and, hence, too cost-intensive.

According to a further embodiment, several RX branches can use a common amplifier. This can be achieved by coupling the signals received from the different RX filters to the input of a respective switch having a single output only, that output being coupled to the common amplifier. In such a front-end circuit, the switch is adapted to couple a balanced input signal to a balanced output and thus, to the common amplifier. This requires a dual throw switch. The different inputs of the switch allow coupling a desired one of the RX branches, a desired pair or a desired group of RX branches to the amplifier at the same time.

In an embodiment, a further switch being embodied as a band- select switch is coupled between the antenna and the multitude of RX branches. This band-select switch is adapted to couple one or more RX branches simultaneously to the antenna .

Instead of the band-select switch, a multiplexer can be coupled between the antenna and the multitude of RX branches. The multiplexer passively assigns each receive signal to a respective RX branch operating with the respective frequency band and having a respective receive filter .

In another embodiment, the front-end circuit comprises a multitude of RX branches, each branch comprising a separate amplifier. Between the multitude of outputs of the multitude of amplifiers and the receiving unit, a multiplexer can be arranged. Such a solution, too, can reduce the number of output pins of the RX branches to one, thereby reducing the complexity of the front-end module.

Preferred filters for realizing the unbalanced/balanced band pass filters are DMS filters, that are SAW filters having one or more tracks circuited in series or parallel. Each track comprises input transducers and output transducers depending on whether the transducer is coupled to the input side or to the output side of the filter. Two of the output transducers of each DMS filter are circuited in series between two terminals forming the balanced output port of the filter. Alternatively, the balanced output signal can be taken from the two busbars of a single output transducer.

By using a different number of input and output transducers or a different number of electrode fingers at input and output transducers or a respective circuiting in series or parallel, an impedance transformation between input and output of the filter can be done. Then, the filter transforms the impedance from a first impedance Zl at the filter input port to a second impedance Z2 at the balanced filter output port. Hence, the output of the filter is matched to a higher impedance than the input of the filter. Higher impedance may be closer to LNA input impedance, leading to a lower mismatch loss and better noise figure and sensitivity.

In the following, the invention is explained in more detail by means of embodiments and according figures. The figures are drawn schematically only and need not be drawn to scale such that no size relations can be taken from the figures nor any other geometric detail that depends on the size of a depicted detail.

FIG. 1 shows an RX branch according to a first embodiment, FIG. 2 shows an RX branch according to a second embodiment,

shows a comparison of single-ended filters and filters having single input and balanced output in view of insertion loss,

shows the same comparison over a greater range of frequencies,

shows the same comparison in view of wideband suppression,

shows a comparison between the path bands of single-ended duplexers and single- ended/balanced output duplexers,

shows a comparison of close-in suppression between a single-ended duplexer and a balanced output duplexer,

shows a comparison between the wideband suppression of the Tx filter of a single-ended duplexer and Tx filter of a single-ended input/ balanced output duplexer,

shows a similar comparison of wideband suppression of the RX filters of the single-ended duplexer and of the single-ended/balanced output duplexer, shows the isolation of the TX section to the RX section of a single-ended duplexer in comparison with the isolation of a respective duplexer as used by the invention,

shows a DMS filter that may be used as RX filter with single-ended input and balanced output,

shows a DMS filter that may be used as RX filter with single-ended input and balanced output and an impedance transformation between input and output,

shows a front-end circuit with three RX branches using a common amplifier, FIG. 14 shows a front-end circuit with three RX branches and an amplifier each that are coupled to the receiving unit by a switch,

FIG. 15 shows the same circuit like figure 14 wherein the outputs of the different amplifiers are connected to the same output pin without any element there between.

FIG. 1 shows an RX branch according to a first embodiment of the invention. A signal received from an antenna is coupled to the input of a filter F having a single-ended input. The filter F has an integrated balun function and thus, provides a differential signal at its output. The differential signal of the filter F is coupled to an amplifier LNA that however produces an unbalanced or single-ended amplified output signal. The output signal is coupled to a receiving unit RU which may be part of a transceiver unit that can operate receive and transmit signals as well. The filter F may be a DMS filter or a ladder-type filter made from SAW resonators or BAW resonators .

FIG. 2 shows an RX branch of the front-end circuit according to a second embodiment of the invention. A receive signal from the antenna is coupled to the single- ended input of the filter F. The filter F has a balanced output and couples a differential or balanced signal to the input of an amplifier LNA. The amplifier has a differential output and is coupled to a balun BN. The output of the balun BN is coupled to a receiver unit RU. As the amplifier LNA of the second embodiment does not involve a balun function it needs the separate balun BN. According to the invention, an unbalanced signal is delivered to the receiving unit RU. FIG. 3 shows a comparison between the insertion loss of a filter F that is single-ended on both sides compared with the insertion loss of a filter F that is single-ended on the input side and balanced on the output side. The curve of the balanced filter is referred to as B while the curve for the both sided single-ended filter is referred to as S. It can be seen that the insertion loss of the balanced filter is improved, especially near the edges of the passband.

FIG. 4 shows the same curves S and B over a wider spectrum of frequencies. It can be seen that the suppression of frequencies near the passband is enhanced at the balanced filter in comparison with the both-sided single-ended filter.

This means that frequencies of other bands near the passband are better suppressed and do not disturb signal processing within the passband that is within the desired band.

FIG. 5 shows the same curves like FIGs. 3 and 4 but depicts a still wider range of frequencies to show the wideband suppression of both filters. It can be seen that the suppression of frequencies up to about the threefold of the passband frequency of the filter is substantially better at the single-ended/balanced filter. As WLAN frequencies are within this frequency range, such a single-ended/balanced filter can be used in coexistence with WLAN without too high coupling of WLAN signals into the passband.

FIG. 6 shows another comparison between the passbands (insertion loss) over filters that are single-ended at both- sides compared to a filter that is single- ended/balanced. In the left part of the figure, the TX filter of a duplexer is depicted while the right side shows two curves for the comparison between the respective RX filters of the duplexer. The figure shows small differences only between the two curves S,B in the TX filter and the RX filter as well as far as only the passband region is regarded. Both comparisons provide only small differences in the passband region such that TX filters and RX filters of the duplexers show nearly the same insertion loss whether embodied as a single-ended filter or a single-ended/balanced filter .

But besides the passband, the close suppression of

frequencies near the passband substantially differs for single-ended RX filters (see curve S) compared to RX filters embodied as single-ended/balanced filters (see curve B) . The curve B for the balanced filter shows a much higher suppression of close frequencies. The both curves for the TX filters, which are shown on the left side, show only small differences and are nearly identical.

FIG. 8 shows the same comparison for wideband suppression over a frequency spectrum up to about the threefold of the base frequency of the passband for the TX filter of a duplexer. FIG. 9 shows the same comparison for the RX filter of a duplexer. In figure 8 it can be seen that in the Tx filter, curves S and B show only small differences in the wideband. But a substantially higher suppression of these frequencies can be achieved at the RX filter especially t frequencies between 3 GHz and 6 GHz.

FIG. 10 shows a comparison between the isolation of TX to RX for a single-ended duplexer compared with a single-ended input/ balanced output duplexer. Over the whole depicted spectrum, an improved isolation can be noted for the balanced filter as shown by curve B.

FIG. 11 shows a DMS filter having a single-ended input I and a balanced output 0+, 0-. The filter has three input transducers circuited in parallel to the input I. Two output transducers arranged alternatingly to the input transducers are connected in parallel to the output 0+, O. On both sides of the acoustic track of the DMS filter, a reflective grid that functions as a reflector for the acoustic waves is arranged.

Due to the nearly symmetrical distribution of transducer fingers to input and output, the same impedance is yielded at input and output side, for example 50 Ω. Besides these five transducer DMS filters with one track, other DMS filters can be used as well. These filters may differ by the number of input and output transducers or by the number of tracks. Two or more tracks may be circuited in parallel or in series to provide a better selection or a better balanced output.

FIG. 12 shows another DMS filter that can be used as RX filter for an inventive front-end circuit. This filter again has three input transducers connected to a single-ended input I and two output transducers connected in series to the balanced output 0+/0-. Due to the series connection of the output transducers, the impedance at the balanced output side is enhanced. Hence, this filter shows an input impedance of about 50 Ω and an converted output impedance of about 200 Ω.

Beside the DMS filter depicted in FIG. 12, such impedance transformation with a higher or a lower factor can be embodied by a lot of variation of the DMS structure that may differ by the number of transducers or by the circuiting of transducers in series or parallel.

FIGs. 11 and 12 are showing an example of realization of an acoustic component that can convert a signal from single- ended input to balanced output. It can be combined with other acoustic tracks to improve a particular specification. Many different realizations of such a filter are possible and known from the art. Beside this illustrative example, a great flexibility is given how to design such a filter. Further, the filters can be matched with the impedance of the amplifier connected to the filter .

In FIG. 11, output transducers are branched in parallel. The total output impedance is then half times the individual transducer impedance. In this case, if a transducer impedance of 100 Ω is given the output impedance equals the input impedance of 50 Ω.

In FIG. 12, output transducers are branched in series. The total output impedance is two times the individual transducer impedance. In this case, when starting from a transducer impedance of 100 Ω, the output impedance is 2 x 100 Ω = 200 Ω.

In order to simplify the explanation, 50 Ω input is assumed but any other input impedance is possible. The output impedance can rank from the input impedance or slightly lower up to four times the input impedance or higher with a proper distribution of electrode counts .

Flexibility in the choice of output impedance is not possible or possible to a lesser extent only when using a device that is single-ended at input and output. The additional flexibility of balanced output devices allows a better impedance matching between the amplifier and the filter, leading to a better noise figure of the complete branch and improves the reception for the user of the mobile phone .

FIG. 13 shows a front-end circuit with three RX branches, each comprising a bandpass filter Fl to F3 assigned to a respective frequency band. For example, Fl is assigned to band 41, filter F2 is assigned to band 7, and filter F3 is assigned to band 40. A first switch SW1 is connected between the inputs of the RX branches that are identical to the inputs of the RX filters Fl to F3 and the antenna. By the first switch SW1, a desired one of the RX branches can be connected and, thus, be coupled to the antenna.

At the output side of the RX filters, a second switch SW2 is connected to the RX branches that can connect the balanced output of the respective RX filter to the input of a common amplifier LNAc. In this example, the amplifier LNAc is adapted to amplify a range of frequencies according to the range that comprises the center frequencies of the three RX branches, respectively the center frequencies of the three RX filters. It is preferred that the common amplifier LNAc is connectable by the second switch SW2 only to those RX branches that operate with frequencies within the same frequency range, that is, for example, low-band, mid-band, or high-band.

If a front-end circuit or a front-end module operates with frequencies that are assigned to different frequency ranges chosen from low-band, mid-band, high-band or any other band, it is preferred that the common amplifier LNAc is made connectable only with RX branches of the same frequency range. Other frequency ranges are connectable to another common amplifier. An amplifier operating within a limited range of frequencies can operate with higher efficiency in terms of gain and noise figure.

FIG. 13 is a schematic example according to the first embodiment as shown in FIG. 1. The example proposes to use as many DP2T switches as filters or a group of filters are connected together and routed to a common particular amplifier LNA. Amplifier LNA has a balanced input and is designed to convert the balanced input signal into a single- ended signal in the active domain which single- ended output signal is connectable to the module output. Lumped or embedded matching components can be placed in front, after or in-between filter, switch and amplifier.

FIG. 14 shows another embodiment of a front-end circuit having three RX branches. Here, each RX branch comprises an RX filter F and an amplifier LNA. A first switch SW1 couples the signal received from an antenna to a desired one of the inputs of the RX branches. The three outputs of the three amplifiers of the three RX branches are coupled to the input of a second switch SW2 that couples the single-ended signals delivered from one of the RX branches, respectively, one of the amplifiers LNA to the receiver unit. Like in the embodiment of FIG. 13, it is preferred that the two switches SW1 and SW2 are switching synchronously the same RX branch.

The embodiment of FIG. 14 has the advantage that the amplifier is assigned to one frequency band only and, hence, can be optimized to the operating frequency band. As each RX branch has its own amplifier, the center frequencies of the RX bands need not be within the same frequency range and can be assigned to different frequency ranges. FIG. 14 is a schematic example and proposes to use as many amplifiers LNA as filters or groups of filters connected together. Each amplifier LNA is designed to convert a signal from balanced to single-ended in the active domain. An SPNT switch is used to direct the output of the selected RX branch to the module output. Compared to FIG. 13, the noise figure is better because of a lesser attenuation within the signal path (RX branch) before amplification. FIG. 15 is another embodiment similar to the embodiment of FIG. 14 but without the second switch. The outputs of the amplifiers LNA of the three RX branches are coupled to a common node CN which is coupled to the receiver unit. Hence, a selection of a desired RX branch is done only by switching the first switch SW1.

FIG. 15 proposes to use a combination of balanced output filters or duplexers and a respective LNA and means to convert the signal to single-ended. It is proposed to use as many amplifiers LNA as filters F or groups of filters that have to be connected together. The amplifiers LNA are designed to convert a signal from balanced to single-ended in the active domain and should be able to be branched together. Compared to the example before, the noise figure is equivalent and better than the noise figure in the first embodiment of FIG. 13. The number of components and then, complexity and size are lower compared to embodiments of FI.13 and FIG. 14.

For each of the embodiments according to FIGs. 14 to 15, the input switch (first switch SW1) can be replaced by a multiplexer connection. In these realizations, two or more inputs or filters are connected together via a matching network. This feature is mandatory for carrier aggregation but can also be used in any other case to reduce the number of switch throws . Changing the scheme of band selection via switch or a multiplexer does not change the principle of the proposed invention.

The invention has been explained by means of few embodiments and figures only. Hence, the invention is not limited to the shown embodiments but is only defined by the claims . In all embodiments, other components, other or more RX branches can be present without departing from the scope of the invention. Further, it is possible to combine two or more front-end circuits to a common front-end module. Each front- end circuit can be assigned to a given frequency range. Accordingly, each front—end circuit may be coupled to separate antenna or to a common antenna. Further, each front-end circuit or each front-end module can comprise a respective number of TX branches. Further, each RX filter can be part of a duplexer operating in the respective Rx band and Tx band.

List of reference symbols

B signal of balanced filter

BN balun

F bandpass filter

I Input of filter

LNA amplifier

LNAc common amplifier

0+,0- balanced output of filter

RU receiver receiving unit/ receiving section Ssignal of single-ended filter

SW switch

SW1 band-select switch

Zl first impedance

Z2 second impedance