DE102017117755A1 - Bandpass filter and filter circuit with it - Google Patents

Abstract

A bandpass filter comprises a series circuit comprising a first parallel resonator (RP), a series capacitor (CS) and at least one fundamental resonator section (BS). Each fundamental resonator section includes a parallel resonator (RP) and a series resonator.

Description

New front-end modules for mobile communications equipment operating on LTE and WiFi frequencies must be aware of competition between LTE-Tx / Rx and WiFi-Tx / Rx if both channels are operating simultaneously. The currently available generation of front-end modules that use a single extractor component does not meet these requirements.

In order to achieve an OOB (Out-of-Band) OOB suppression for simultaneous LTE and Wi-Fi operation, there is a need to cascade the extractor with a stand alone bandpass filter. However, this additional bandpass filter results in unacceptably high levels of passband insertion losses, which may amount to a loss greater than 4.5 dB at the band edges. At the same time, the additional cascaded bandpass filter increases the footprint of the overall solution by about 45%.

It is an object of the present invention to provide a band pass filter which can reduce insertion losses and which can be used in an extractor circuit, for example for filtering out WiFi frequencies and separating them from LTE frequencies in the 2 GHz range.

These and other objects are achieved by a bandpass filter according to claim 1. An extractor circuit comprising the bandpass filter is the subject of another independent claim.

A bandpass filter may be a reactance filter, e.g. is designed as a ladder type arrangement of resonators. Such a ladder type filter comprises a number of basic resonator sections, each having a series resonator in an array path and a shunt resonator in a shunt path branching from the row path. Each fundamental resonator section provides a complete function, but the selectivity becomes better with each additional fundamental resonator section. Unfortunately, insertion losses increase in parallel. Therefore, an optimized filter is a compromise between loss and selectivity. The invention proposes a ladder type filter wherein at least one fundamental resonator section is replaced by a capacitor. The resulting effect is that the filter characteristics do not deteriorate compared to a known filter. The proposed filter has a reduced number of fundamental sections and the capacitor connected between different fundamental resonator sections. Furthermore, the bandpass filter has an improved OOB and a potential improvement in linearity.

The new bandpass filter comprises a first parallel resonator which forms a first fundamental resonator section, a series capacitor and at least one second parallel resonator. In another version, the bandpass filter comprises a series circuit of first fundamental resonator sections, a capacitor, and at least one second fundamental resonator section. Each of the basic resonator sections comprises a series resonator and a parallel resonator. The parallel resonators are arranged in a respective bypass path parallel to the row path. The capacitor is connected between two fundamental resonator sections and may have a capacitance greater than the static capacitance of a series resonator. It is preferred that the series capacitor is located behind the first parallel resonator or before the last parallel resonator.

More preferably, there are exactly three first fundamental resonator sections and exactly one second fundamental resonator section.

When such a filter with four basic sections is compared with a corresponding filter with five basic resonator sections, but without the series capacitor, the out-of-band rejection and the insertion loss are almost the same. As an advantage, however, the left edge of the pass band is steeper, and thus the new filter has a smaller transition portion where low attenuation changes to high suppression (attenuation). The corresponding right edge of the passband is comparable in both filters.

The left flank is steeper due to additional poles in the transfer function introduced by the series capacitor. These poles are on the left flank, sharpening the edge and improving the steepness of the left flank.

OOB suppression improves at lower frequencies and maintains the standard at higher frequencies.

The effect of the invention also depends on the capacitance Ccs of the capacitor, which according to a preferred embodiment is higher than the static capacitance C _{0 of} a series resonator in a first or a second basic resonator section. In a specific example, optimum filter performance was achieved when C _cs ~ 2.0 pF.

This effect can be further improved if the ground sides of the shunt paths of both the first and second internal fundamental resonator sections directly adjacent to the capacitor are directly on the chip level, inside a package carrier, or on a common ground pad on the ground plane or on a PCB Level are connected.

According to a further improvement, the series frequencies fs of the two series resonators directly adjacent to the capacitor are set to a value which differs from the series frequencies of the other series resonators by an amount of +/- Δf. For Δf, the following relationship may apply: 10 MHz <Δf <20 MHz.

The proposed filter may advantageously be used as a standalone bandpass filter, in the extractor path of an extractor circuit, in a duplexer or in a multiplexer.

• - einen ersten Signalpfad, der ein Bandpassfilter mit mindestens einem Übertragungsbereich umfasst;
• - einen zweiten Signalpfad, der ein Bandsperrfilter mit einem entsprechenden Sperrgebiet umfasst; und
• - wobei der erste und der zweite Signalpfad mit einem ersten Knoten parallel geschaltet sind;
• - wobei der Übertragungsbereich das Sperrgebiet zumindest teilweise überlappt.

Such extractor circuit is for example from the US patent US 7,583,936 B2 known and includes

• a first signal path comprising a bandpass filter having at least one transmission area;
• a second signal path comprising a band rejection filter having a corresponding forbidden area; and
• - Wherein the first and the second signal path are connected in parallel with a first node;
• - wherein the transmission area at least partially overlaps the restricted area.

Due to the improved filter edges, the new bandpass filter - when used in the extractor path - can extract the extractor frequencies in the transmission range better and more clearly with lower losses. Therefore, even if weak signals are to be extracted from a signal comprising high amplitude signals in other and even adjacent bands, this can be done with high efficiencies. This is advantageous over normal multiplexers, where selectivity is best given two different frequencies at comparable signal strengths.

According to one example, the first signal path is coupled to an antenna port and the first node is disposed between the antenna port and the bandstop filter.

According to one embodiment, the first signal path is assigned to WLAN frequencies. Therefore, the transmission range of the band-pass filter is between 2.40 GHz and 2.484 GHz.

An extractor circuit may comprise two individual chips, each having its own base area. However, the bandpass filter and the bandstop filter can be implemented on the same and therefore common filter chip. This saves PCB area since a single filter chip requires less mounting area (footprint) on a PCB than two individual chips.

The first signal path is connected to the first terminal. The second signal path may include filters, diplexers, duplexers, and / or other extractors for all other bands other than the restricted area. By such a branch, a multiplexer circuit for separating a desired number of frequency bands.

Other filter elements such as e.g. Diplexers may be located between the antenna port and the first node or at the extractor's OUT1 and / or OUT2 nodes.

The new bandpass filter is therefore a ladder type filter that uses fewer resonators than known bandpass filters that are comparable in passband. In addition, the left flank is steeper, the insertion loss in the pass band is improved, and better matching and OOB attenuation are achieved.

When used in a duplexer as an Rx filter or when used as a stand-alone Rx filter, the new band pass filter improves linearity in the Rx path. Furthermore, just as when used as a self-contained filter, the pass-band insertion loss and the left-hand edge of the band-pass filter are improved. The advantage of the latter is significant for all bands in which Tx frequencies are below the Rx band. This is true for most LTE tapes except the tapes B13 . B14 . B20 or B24 ,

When there is a Tx filter in duplexers for the tapes B13 . B14 . B20 or B24 is used, better pass-band insertion loss and left-hand flank are improved.

Compared to a DMS track, which is often used as an alternative bandpass filter in cell applications, the right flank, power processing and linearity are improved.

• 1 stellt ein Blockdiagramm des Bandpassfilters gemäß dem Stand der Technik dar
• 2 stellt ein Blockdiagramm des Bandpassfilters gemäß der ersten Ausführungsform dar
• 3 stellt die Transferkurven der Ausführungsform von 2 im Vergleich mit dem Filter von 1 dar
• 4 stellt die gleichen Kurven im Gebiet des Durchlassbands vergrößert dar
• 5 stellt die gleichen Kurven im Breitband abgebildet dar
• 6 stellt ein Blockdiagramm des Bandpassfilters gemäß der zweiten Ausführungsform vereinfacht dar
• 7 stellt ein Blockdiagramm der dritten Ausführungsform dar
• 8A bis 8C stellen Extraktorfilterschaltungen dar, die ein Notchfilter und ein entsprechendes neues Bandpassfilter umfassen
• 9 stellt die Transferkurven der beiden Pfade des Extraktors von 8 dar
• 10A bis 10C stellen andere Extraktorfilterschaltungen dar, die ein Notchfilter und eine entsprechende Ausführungsform des neuen Bandpassfilters umfassen
• 11 stellt die Transferkurven der beiden Pfade des Extraktors von 10 dar
• 12A bis 12E stellen eine vereinfachte partielle Struktur eines Bandpassfilters dar.

In the following the invention will be explained in more detail with reference to the accompanying figures. The figures are only schematic and not scale. For better understanding, some details may be enlarged.

• 1 FIG. 12 illustrates a block diagram of the bandpass filter according to the prior art. FIG
• 2 FIG. 12 is a block diagram of the band-pass filter according to the first embodiment. FIG
• 3 FIG. 12 illustrates the transfer curves of the embodiment of FIG 2 in comparison with the filter of 1 is
• 4 represents the same curves enlarged in the area of the pass band
• 5 represents the same curves shown in the broadband
• 6 FIG. 10 is a block diagram of the band-pass filter according to the second embodiment. FIG
• 7 FIG. 12 is a block diagram of the third embodiment. FIG
• 8A to 8C represent extractor filter circuits comprising a notch filter and a corresponding new bandpass filter
• 9 represents the transfer curves of the extractor's two paths 8th is
• 10A to 10C illustrate other extractor filter circuits that include a notch filter and a corresponding embodiment of the new bandpass filter
• 11 represents the transfer curves of the extractor's two paths 10 is
• 12A to 12E represent a simplified partial structure of a bandpass filter.

1 FIG. 12 illustrates a block diagram of a commonly used bandpass filter. The filter is a ladder type array of resonators RS . RP and includes five basic sections BS which are connected in series between a filter input and a filter output. The static capacitance and resonant frequency of the resonators may be the same or may be different.

Each basic section includes a series resonator RS arranged in an array path and a parallel resonator RP which is arranged in a parallel path to ground. This filter can be used as a bandpass filter with low insertion loss and good OOB suppression.

2 FIG. 12 illustrates a block diagram of a bandpass filter according to a first embodiment of the invention. Similarly to FIG 1 The filter includes basic sections BS1 to BS4 , But in contrast to the known filter are three basic sections BS1 to BS3 to a series capacitor CS and a fourth basic section BS4 connected in series. Therefore, the first embodiment saves from the filter of FIG 1 the fifth basic section BS5 and replace it with the series capacitor CS which requires much less area on the filter chip. If the capacitor in one PCB or integrated at another location other than the surface of the filter chip, more area can be saved. Replacing two directly adjacent parallel resonators with a new single parallel resonator having the same series frequency but doubled static capacitance compared to one of the replaced resonators in this or other Ladder Type filter circuits provides an equivalent but simplified filter structure. Similarly, replacing two directly adjacent series resonators with a single series resonator has the same series frequency and half the static capacitance compared to one of the replaced resonators.

In 3 become the transfer properties (S-matrix elements S (2,1) and S (2,2)) of the filter of 2 with those of the known filter according to 1 compared. Despite the fact that it has a basic section less than the known filter, the curves of the S (2,2) transfer curve 2a of the first embodiment are very similar to those of the filter of FIG 1 , Surprisingly and moreover, an improved steepness of the left filter edge and an improved pass-band insertion loss (cf. 4 , which shows a passband behavior of the two filters increased) achieved by the new filter. The OOB suppression is significantly improved in the frequency range below the passband, as best of all 5 which represents the broadband transfer curves of the filter.

In an example according to the curves in FIGS 3 to 5 the circuit diagrams are identical for all basic sections. The resonators are SAW resonators, but could be made in any other technique such as BAW or HQ LC resonators. In this example, every second basic section is mirrored such that in two directly adjacent basic sections BSn, BSn + 1 the respective parallel resonators RPn, RPN + 1 (here: RP1 . RP2 ) are opposite to each other. Accordingly, this applies to the series resonators RS (here: RS2 . RS3 ) too. But other configurations are possible as well.

On both sides of the series capacitor CS begins the corresponding adjacent basic section BS3 . BS4 with a parallel resonator RP3 . RP4 ,

6 FIG. 12 illustrates the second embodiment that satisfies the requirements for a cell filter when used as a stand-alone filter or as a filter in a duplexer or a multiplexer or extractor circuit.

The filter has three basic sections BS1 . BS2 . BS3 that with a series capacitor CS between the first basic section BS1 and the second basic section BS2 are connected in series. The first and the third parallel resonator RP1 . RP3 are with a parallel capacitor CP1 . CP3 connected in parallel to reduce the bandwidth of the resonator. CP1 and CP3 are also used for optimizing / improving the filter matching and for clarifying the resonance and thus the left edge of the pass band of the filter. The parallel capacitor CP1 reduces the pole-zero distance of the resonator.

An inductance LP2 is with the second parallel resonator RP2 connected in series. This will change the bandwidth of the resonator RP2 extended and the filter adaptation is improved.

As already mentioned above with reference to 2 is explained, the filter circuit of 6 equivalent to the filter circuit of 2 in terms of the number of basic sections, if static capacitances of respective resonators are set as explained.

7 Figure 3 illustrates a third embodiment of a bandpass filter for a filter when used as a stand-alone filter or as a filter in a duplexer or a multiplexer or extractor circuit. The filter has four basic sections BS1 to BS4 and a series capacitor CS is connected in series between the first and second basic sections BS1 / BS2. The first and the third parallel resonator RP1 and RP3 are with a respective parallel capacitor CP1 . CP3 connected in parallel. Furthermore, an inductance LP2 with the second parallel resonator RP2 connected in series.

An important and useful application of the new bandpass filter is in an extractor circuit. Such an extractor circuit comprises a notch filter FN (= Bandstop filter) in a row signal path that is connected to an extractor path with a bandpass filter FB is connected in parallel. The frequencies of the passband of the bandpass filter FB can route the row signal path through the filter FB but are prevented from going through the row signal path since the notch filter FN only suppresses these frequencies to be extracted.

The 8A to 8C illustrate variations of a first embodiment of such an extractor filter circuit using a bandpass filter as proposed by the invention. Any notch filter FN can work together with the bandpass filter FB be used. In the illustrated embodiments, the notch filter comprises FN a first exit OUT1 , a first row coil LS1 , a first parallel coil LP1 , a first series resonator RS1 , a first parallel resonator RP1 , a second series resonator RS2 and a second parallel coil LP2 that have a series capacitor CS _N with the entrance IN is coupled. Optional is a series capacitor CSP1 with the second parallel coil LP2 connected in series. The bandpass filter FB is between the entrance IN and a second exit OUT2 coupled to the extractor path. In the embodiment of 8A becomes the bandpass filter FB the third embodiment of 7 used and therefore need not be further described.

In the bandpass filter FB the extractor circuit of 8B are the positions of the second series resonator RS _B 2 and the series capacitor CS _B in comparison with 8A reversed.

In the bandpass filter FB the extractor circuit of 8C becomes the series capacitor CS _B in comparison with any one of 8A and 8B replaced by a series resonator.

9 sets the power of the extractor circuit of 8A based on the respective response, as between the input IN and the first and second outputs OUT1 . OUT2 measured, dar. The curve 8N forms the signal transfer of IN to the (first) exit OUT1 of the notch filter FN from. The flanks of the restricted area of the notch are steep and the insertion loss adjacent to the restricted area is low. The curve 8B forms the signal transfer of IN to the (second) output OUT2 the bandpass filter FB from. At the second exit OUT2 the bandpass filter FB indicates the pass band of the filter FB steep flanks and also shows a low insertion loss. The transmission range of the bandpass filter FB the restricted area overlaps perfectly so that the flanks intersect each other. Therefore, such a new extractor circuit can be used for perfectly extracting a frequency range from the row signal line. All frequencies minus the restricted area may pass and be routed to other filter elements for selecting further bands from the lines.

The 10A to 10C illustrate variations of a second embodiment of an extractor filter circuit using a bandpass filter proposed by the invention. A notch filter FN and a bandpass filter FB are with an entrance IN connected in parallel. Both filters have their own separate output OUT1 . OUT2 on. In this embodiment, the signal line of the notch filter comprises FN , starting from the exit OUT1 of the filter, a first parallel inductor, a first series inductor, a first parallel resonator, a second series inductor, a first series resonator, a second parallel resonator, a second series resonator, a second parallel inductor and a series capacitor CS _N that with the entrance IN is coupled.

In 10A includes the signal line of the bandpass filter FB , starting from the entrance IN , a first series resonator RS1 , a first parallel resonator RP1 , a series capacitor CS _B , a second parallel resonator RP2 , a second series resonator, a third parallel resonator, a third series resonator and a fourth parallel resonator. Parallel to the first and the third parallel resonator are a first and a second parallel capacitor CP1 . CP2 connected. A first parallel inductance LP2 is coupled in series with the second parallel resonator.

This extractor circuit has superior OOB suppression, improved filter edges of the bandpass filter FB and the notch filter FN on, like in 11 can be seen that the performance, as in the signal lines of FN and FB measured, represents. The upper curve shows the notch with a very small insertion loss on both sides of its restricted area. On both sides of the restricted area is clearly delimited against the transmission areas. According to the very good characteristics of the bandpass filter FB Both the passband suppression and the OOB are perfect, and the transition from passband to high attenuation is clear. The restricted area of the notch filter FN and the passband of the bandpass filter FB are perfectly adapted.

The bandpass filter FB from 10B has a sequence of elements (series resonators, series capacitors, and series inductances in a parallel branch) which are in terms of the sequence of the bandpass filter FB from 10A is mirrored. This extractor filter circuit shows almost the same performance as that of 8A ,

In 10C becomes the series capacitor CS _B in comparison with the bandpass filter FB from 10A through a series resonator RS replaced. Furthermore, and according to another variant, not shown, the series capacitor CS _B of the bandpass filter of the extractor filter circuit of 8B through a series resonator RS replaced.

In addition, the last series resonator RS Next OUT2 in any of the bandpass filters FB of the 10A to 10C may be omitted to provide a corresponding further embodiment of an extractor filter circuit which is slightly modified (not shown in the figures).

The above-mentioned omission of the last series resonator in any of the described bandpass filters provides further structures of extractor filter circuits which perform well. Such variations can be started with a bandpass filter, as in any of 2 . 6 and 7 represented, performed.

Furthermore, a bandpass filter represents FB as in any of the extractor circuits of 8A to 8C and 10A to 10C or a variation thereof, a new bandpass filter that can be used as a stand-alone filter.

The in the 8th and 10 illustrated embodiment is optimized to WLAN frequencies (2400 MHz - 2484 MHz) at the output OUT2 to extract. Other bands and in particular adjacent LTE bands B40 (2300 MHz - 2400 MHz) and B41 (2496 MHz - 2690 MHz) can be the signal line of the notch filter FN go through with very little weakening. Therefore, B40 and B41 frequencies can be filtered out of the row signal at a later time.

The 12A to 12E are simplified embodiments showing the required elements of a bandpass filter as described above to achieve a very steep left flank and to improve matching when used in a ladder type bandpass filter. The figures differ in their respective bypass path and show different ways of connecting a parallel branch to ground. As already explained, a series capacitor CS replace a basic section in a ladder type filter. Therefore, in the 12A to 12E depicted filter structures equivalent to the structures, each comprising two basic sections. However, to meet true filter requirements, additional filter elements, such as another basic section, may be needed to supplement and further enhance the filter.

In 12A become the shunt paths of two parallel resonators RP1 . RP2 directly to the series capacitor CS connected before being connected to a ground terminal.

In 12B The same two shunt paths are individually connected to a ground terminal.

In 12C The same two shunt paths are in series with an inductor LS before they are individually connected to a ground connection.

In 12D For example, one of the same two shunt paths is directly connected to a ground terminal while the other shunt resonator is connected in series with an inductor LS is coupled before it is connected to a ground terminal.

12E is a variant of 12D , Here is the inductance LS with a shunt capacitor CP coupled in parallel.

A series inductance connected to the bypass path may have an inductance value LS _GND where 0.01 nH <LS _GND <2.0 nH. This is at least as far as the filter is optimized for WLAN frequencies.

All mentioned bandpass filters can be used independently or in filter circuits, such as the mentioned extractor circuits, or in multiplexers. They can be used for any desired frequency band.

The invention is not to be limited by the embodiments and the accompanying figures, but only by the claims. Each disclosed filter device may include other elements that are not explicitly mentioned or illustrated. Thus, individual elements used in any embodiment described or illustrated may also be combined with disclosed filter structures of other embodiments.

LIST OF REFERENCE NUMBERS

1a

Transfer curve of the filter of 1

1b

Mirror image of the filter of 1

2a

Transfer curve of the filter of 2

2 B

Mirror image of the filter of 2

8B

Transfer curve of the bandpass filter of 8th

8N

Transfer curve of the notch filter of 8th

BS

basic section

CP

parallel capacitor

CS

series capacitor

Department

Bandpass filter

FN

Notch filter

IN

entrance

LP

Parallel inductance / parallel coil

LS

series inductance

OUT1, OUT2

Output from FN and FB

RS

series resonator

RP

parallel resonator

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US Pat. No. 7,538,936 B2

Claims (11)

Bandpass filter (BP) comprising a series connection comprising a first basic resonator section, a series capacitor (CS) and at least one second basic resonator section (BS), wherein each fundamental resonator section comprises a parallel resonator (RP). Bandpass filter of the preceding claim, wherein at least one of the fundamental resonator sections comprises a series resonator (RS) and a parallel resonator (RP). The bandpass filter of the preceding claim, wherein each of the fundamental resonator sections comprises a series resonator (RS) and a parallel resonator (RP). A bandpass filter as claimed in any one of the preceding claims, comprising only three first fundamental resonator sections and exactly one second fundamental resonator section. A bandpass filter as claimed in any one of the preceding claims, wherein the series resonators in all the fundamental resonator sections are identical in static capacitance and series frequency. A bandpass filter as claimed in any one of the preceding claims, wherein the capacitance Ccs of the capacitor (CS) is higher than the static capacitance C _{0 of} a series resonator (RS) in the first or second fundamental resonator section and ranges from 1 C ₀ to 25 C ₀ . The bandpass filter of the preceding claim, wherein the ground sides of the shunt paths of both the inner first and second fundamental resonator sections are connected directly adjacent to the capacitor (CS) directly on a chip plane, within a package carrier, on a common pad on the ground plane, or on a PCB plane are. Extractor circuit comprising: a first signal path comprising a bandpass filter (BP1) with at least one transmission range; a second signal path comprising a band rejection filter (BS) with a corresponding stop band and a second band pass filter (BP2) in series; and - Wherein the first and the second signal path are connected in parallel to a first node; - wherein the transmission area at least partially overlaps the restricted area; - wherein the band-pass filter (BP1) is a ladder-type filter according to one of the preceding claims. Extractor circuit of the preceding claim, wherein the first signal path is assigned to WLAN frequencies and wherein the transmission range of the bandpass filter (BP1) is between 2.40 GHz and 2.484 GHz. Extractor circuit of one of the preceding claims, - Wherein the first signal path is coupled to an antenna port - Wherein the first node between the antenna terminal and the band stop filter (BS) is arranged. Extractor circuit of one of the preceding claims, wherein the bandpass filter and the bandpass filter are implemented on the same filter chip.

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