WO2019228751A1

WO2019228751A1 - Radio frequency filter

Info

Publication number: WO2019228751A1
Application number: PCT/EP2019/061432
Authority: WO
Inventors: Andreas Bergmann
Original assignee: RF360 Europe GmbH
Priority date: 2018-05-29
Filing date: 2019-05-03
Publication date: 2019-12-05
Also published as: DE102018112839A1

Abstract

A radio frequency filter comprises a series path including a plurality of in series connected resonators (110, 121, 122, 131, 132) between an antenna port (ANT) and another port (RX). At least one parallel path is connected between one of the resonators and a terminal (170) for ground potential. The series path comprises a cascade of a first and a second resonator (131, 132) wherein a capacitance (133) is connected in parallel to one of the first and second resonators (132). The capacitance generates an additional damping pole so that the selection of the filter is increased.

Description

Radio frequency filter

Technical Field

The present disclosure relates to a radio frequency filter. Specifically, the present disclosure relates to a radio frequency filter including a series path connected between an antenna port and another port, wherein the series path comprises a plurality of in series connected resonators. At least one parallel path is connected between one of the resonators of the series path and a terminal for ground potential .

Background

Radio frequency (RF) filters are widely used in electronic communication systems at the antenna frontend to select the wanted bandwidth from the antenna signal or provide the RF signal to the antenna. The send (Tx) passband may be located above the receive (Rx) passband, and often the send and receive passbands are located adjacent to one another. In this case it is desired that the upper skirt of the Rx filter has sufficient isolation in the portion that extends into the Tx band. For example, in band 71 used in the 4G (LTE) standard the Rx band ranges from 617 MHz to 652 MHz and the Tx band ranges from 663 MHz to 698 MHz.

It is an object of the present disclosure to provide a radio frequency filter that has an increased selectivity at the upper skirt region. It is yet another object of the present disclosure to provide a receive filter that has an increased selection in the adjacent send passband.

It is another object of the present disclosure to provide a radio frequency filter that has an increased isolation in the adjacent band.

Summary

According to the present disclosure, a radio frequency filter that achieves one or more of the above-mentioned objects comprises an antenna port to be connected to an antenna and another port; a series path connected between the antenna port and the other port, the series path comprising a

plurality of resonators connected in series; at least one parallel path connected between one of the resonators of the series path and a terminal for ground potential; the series path comprising a cascade of a first resonator connected in series with a second resonator; a capacitance connected in parallel to one of the first and second resonators of the cascade .

An RF filter according to the present disclosure comprises an antenna port that is to be connected to an antenna of an electronic device. Another port may be an Rx or Tx port that is connected to the internal signal processing circuitry of the electronic device. A series path of the filter connected between the antenna port and the other port comprises a plurality of resonators. The resonators are connected in series and may include one or more cascade connected

resonators. Within a cascade of two resonator devices, the resonators match each other so that they have substantially the same impedance or generate substantially the same voltage drop across each resonator of the cascade during the

operation of the RF filter at the resonance frequency.

The RF filter comprises one or more parallel paths that are connected between one of the resonators and a terminal for ground potential. A parallel path may include a single resonator or a cascade connection of at least two resonators. The parallel path connects to the terminals of a cascade. Usually, no such connection of a parallel path is made to the node between the two resonators within one cascade connection of resonators.

According to the present disclosure, a capacitance is

connected in parallel to one of the resonators of a cascade connection of resonators. The capacitance generates an additional damping pole of the admittance curve of the cascade. If the additional damping pole is located in the upper near stop band of the filter, the selectivity of the filter and the isolation against the adjacent passband is increased. The admittance curve of the cascade including the capacitance connected in parallel to one of the resonators of the cascade includes a resonance pole and two damping poles to the right side of the resonance pole wherein the left sided damping pole is caused by the capacitance.

The cascade is characterized in that no parallel path is connected to the node between the two resonators of the cascade. Parallel resonators may be connected to the other outer terminals of the cascade connection.

In an embodiment, it may be useful to connect the cascade connection including the additional capacitance to the other port of the filter which may be the Rx port. In this case, the terminals of the capacitor are connected to the Rx port and to the node between the two resonators of the cascade, resp .

In an embodiment, the resonators of the cascade each have an acoustic track having an aperture. The aperture of that resonator to which the additional capacitance is connected is selected smaller than the aperture of the acoustic track of that resonator to which no additional capacitance is

connected. The apertures are selected such that the same or an equal voltage drop occurs at each resonator of the

cascade. On the other hand, the apertures may be determined such that the impedances of the resonator without and the resonator with additional capacitance are the same or equal. The equal voltage drop and the equal impedances are

considered at the resonance frequency of the respective devices .

In an embodiment, the resonators used in the RF filter according to the present disclosure may be surface acoustic wave (SAW) resonators. SAW resonators comprise electrodes that are disposed on a piezoelectric substrate. The

electrodes may have fingers or are disposed in a comb

structure wherein the fingers of corresponding electrodes forming an input or output of the resonator are interleaved with each other thus forming an interdigital transducer

(IDT) . The piezoelectric substrate may be a crystalline substrate such as a lithium niobate substrate or a lithium tantalate substrate. The presently disclosed approach is particularly useful for a substrate that has a sufficient coupling such that it allows a wide pole-zero distance of the admittance curves of the resonators. The pole-zero distance must be such that the additional damping pole which is located at a lower frequency than the original damping pole is in the stop band and still outside of the passband of the concerned filter. In the case of a lithium niobate

piezoelectric substrate, the cut angle of the substrate relative to the lithium niobate crystal is of 170 degrees to achieve a suitable pole-zero distance.

The present disclosure may be useful to increase the

isolation within a duplexer wherein the receive passband is immediately below the send passband. The duplexer has three terminals such as an antenna port, the Rx port and the Tx port. The additional pole generated by the additional

capacitor connected in parallel to one of the resonators of a cascade is inserted close to the upper skirt of the Rx filter .

A filter useful for band 71 of a 4G (LTE) communication service includes a series path including at least two

resonator cascades wherein the cascade connected to the Rx port includes the additional capacitor connected to one of the resonators. Another cascade of resonators includes two identical resonators wherein a capacitor is connected in parallel to the cascade, that is in parallel to the in series connected resonators of the cascade. The outer terminals of the cascade may be connected to parallel paths each including at least one resonator or another cascade of resonators connected to ground potential.

It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or

framework to understand the nature and character of the claims. The accompanying drawings are included to provide a further understanding and are incorporated in, and constitute a part of, this description. The drawings illustrate one or more embodiments, and together with the description serve to explain principles and operation of the various embodiments. The same elements in different figures of the drawings are denoted by the same reference signs.

Brief Description of the Drawings

In the drawings:

Figure 1 shows a schematic diagram of an Rx filter for band 71 according to the principles of this disclosure;

Figures 2A and 2B show the admittance curves of the

resonators and transfer functions of the Rx and Tx filters for band 71 without and with the additional capacitor

connected to one of the resonators of a resonator cascade;

Figure 3 shows the isolation of the Rx filter with respect to the Tx filter of band 71; and

Figure 4 shows a layout of a portion of the Rx filter of Figure 1.

Detailed Description of Embodiments

The present disclosure will now be described more fully hereinafter with reference to the accompanying drawings showing embodiments of the disclosure. The disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that the disclosure will fully convey the scope of the disclosure to those skilled in the art. The drawings are not necessarily drawn to scale but are configured to clearly illustrate the

disclosure .

Figure 1 shows a schematic diagram of a Rx filter for band 71 of the 4G (LTE) communication standard. The Rx filter may be part of a duplex filter including the receive (Rx) path and the send (Tx) path. The filter depicted in Figure 1 comprises an antenna terminal ANT and a terminal RX at which the received filtered signal is forwarded to the signal

processing circuitry in the electronic equipment such as a mobile communication device or a smartphone. A series path of resonators includes a resonator 110, a cascade of resonators 121, 122 and another cascade 131, 132 connected in series between antenna and receive ports ANT, RX. Parallel paths are connected between one of the resonators and ground terminal 170. A resonator 141 is connected to a node between

resonators 110, 121 and terminal 170 for ground potential. A cascade of resonators 151, 152 is connected between the node between resonators 122, 131 and ground potential. A resonator 161 is coupled between resonator 132, receive terminal RX and ground potential. An inductor is connected in series to resonator 161.

Capacitors are connected in parallel to the series cascades and the parallel paths. Capacitor 123 is connected in

parallel to the cascade of resonators 121, 122. Capacitor 142 is connected in parallel to resonator 141. Capacitor 153 is connected in parallel to the cascade of resonators 151, 152. Another parallel capacitor is connected between resonator 161 and the node between resonators 122, 131. The described capacitors are connected to the outer terminals of cascade connected resonators. That is that capacitor 123 is connected to the outer terminals of the cascade of resonators 121, 122. No capacitor nor any parallel path is connected to the node between resonators 121, 122.

Turning now to Figure 2A, the admittance curves of the resonators and the transfer functions of the filters are shown. Curve 210 depicts the transfer function of the Rx filter of Figure 1 as described above (that is without capacitor 133) , and curve 211 depicts the transfer function of the corresponding Tx filter of a duplexer handling LTE band 71. The admittance curves of the resonators and the parallel connected capacitors of the above-described filter elements are also depicted in Figure 2A, the interconnection of which achieve the filter transfer functions 210, 211. Of specific interest is the area 212 where the right skirt of transfer function 210 is situated in the passband 211 of the corresponding Tx filter.

The selection in the close upper stop band is caused by the number and the frequency of the entire resonance frequencies of the resonators of the series paths. Each series resonator contributes to the selection with its damping pole.

Resonators are cascaded such as resonators 121, 122 or 131, 132 to achieve sufficient power durability. Another benefit to cascade resonators is to achieve a sufficient number of the fingers of the IDTs in order to achieve sufficient pole-zero distance and to reduce the undulation of the filter characteristic below the resonance frequency. The admittance curves are labelled with the reference numerals of the resonators/capacitors to which they belong. Specifically, the cascade of resonators 131, 132 shows a damping pole 220 at the frequency of 680 MHz.

Now turning back to Figure 1, according to the principles of the present disclosure, an additional capacitor 133 is connected in parallel to resonator 132 of the resonator cascade 131, 132. The capacitor 133 is connected to the node 134 between resonators 131, 132 and the outer node of the cascade 131, 132 which is connected to the Rx port RX. It is to be noted that the node between cascade resonators 131, 132 is not connected to a parallel path. Instead, node 134 is connected only to the cascade resonators 131, 132 and the parallel connected capacitor 133.

Turning now to Figure 2B, the admittance curves of the resonators and the transfer functions 270, 211 of the filters are shown for the filter of Figure 1 including capacitor 133. The admittance curve of the resonator cascade 131, 132 including the parallel connected capacitor 133 is shown in Figure 2B. As can be gathered from Figure 2B, the admittance curve of elements 131, 132, 133 includes an additional damping pole 252, wherein the damping pole 251 of that admittance curve corresponds to damping pole 220 of Figure 2A. The additional damping pole 252 is situated at a lower frequency, i.e. 665 MHz, than original damping pole 220. The modified admittance curve of elements 131, 132, 133 achieves a larger damping in the areas depicted at 261, 262. The difference is in the range of about 1 dB more damping which is a considerable factor since it is located in the frequency area of the Tx filter 211. The additional level of damping is achieved with the parallel connected capacitor 133 which generates the additional damping pole 252. With this solution an additional selection can be achieved in the nearby upper stop band without increasing the insertion loss and only with a relatively small increase in area occupied by the

components .

Turning now to Figure 3, comparative curves of the isolation for the Rx filter shown in Figure 1 (including capacitor 133) with respect to the corresponding Tx filter for band 71 and the isolation for the corresponding filters without capacitor 133 are depicted. Specifically, curve portions 310, 311 show the isolation for a filter without capacitor 133. Curve portions 320, 321 show the isolation for the filter of Figure 1 including capacitor 133. It is to be noted that the local maxima of curve portions 320, 321 are slightly below the local maxima of curve portions 310, 311. The difference is in the range of about -1 dB . As mentioned above, this is in fact a considerable increase of isolation as it concerns the damping of the Rx filter in the Tx band. The isolation is roughly determined as the product of the transfer functions of the Tx and the Rx filters which is the sum of the

corresponding curves in the depicted logarithmic scale.

Figure 4 shows a layout diagram of a portion of the Rx filter circuit of Figure 1. The same reference numerals are used for the elements of Figure 1 and the corresponding layout areas of Figure 4. Specifically, Figure 4 depicts the resonators 131 and 132 wherein each resonator comprises the IDT active area portion and reflector portions to the left-hand and the right-hand sides. For example, resonator 132 comprises active area portion 132a and reflector portions 132b, 132c located on both sides of the acoustic track of resonator 132.

Capacitor 133 is connected in parallel to resonator 132 and is located between the two metallizations RX, 134. In the same way, resonators 122, 121 are disposed having the active area in the middle and two reflector areas confining the acoustic track regions.

The acoustic track of resonator 132 has a width 412 and the acoustic track of resonator 131 has a width 411. In order to establish the same voltage across resonators 132 and 131 at the resonance frequency, the width of the acoustic track of resonator 132 is modified to take into account capacitor 133. The width or aperture of resonator 132 is adjusted such that the voltage drop across resonator 132 and the power

dissipation in resonator 132 is equal to the voltage drop across resonator 131 and the power dissipation in resonator 131, resp., during operation of the device. In other words, the aperture of resonator 132 is modified such that the impedance of the parallel connection of resonator 132 and capacitor 133 is equal to the impedance of resonator 131 at the resonance frequency. In practice, the aperture 412 of resonator 132 is slightly smaller than the aperture 411 of resonator 131 to achieve the above-mentioned effects

concerning voltage drop and/or impedance. By contrast, the resonators 122, 121 have apertures 422, 421, respectively, which are substantially equal.

According to the present disclosure, a capacitor is connected to cascaded resonators in the series path of a RF filter so that the anti-resonating frequencies of both cascaded

resonators are shifted. The serial connection means an additional damping pole in the filter. If the aperture of the resonator without capacitor is selected higher than the aperture of the parallel connection of capacitor and

resonator so that both acoustic tracks exhibit the same impedances, the equal distribution of voltage and power of both serially connected tracks are maintained. It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit or scope of the disclosure as laid down in the appended claims. Since modifications, combinations, sub combinations and variations of the disclosed embodiments incorporating the spirt and substance of the disclosure may occur to the persons skilled in the art, the disclosure should be construed to include everything within the scope of the appended claims.

Claims

Claims :

1. A radio frequency filter, comprising:

- an antenna port (ANT) to be connected to an antenna and another port (RX) ;

- a series path connected between the antenna port and the other port, the series path comprising a plurality of resonators (110, ..., 132) connected in series;

- at least one parallel path (141, 151, 152, 161) connected between one of the resonators of the series path and a terminal (170) for ground potential;

- the series path comprising a cascade of a first resonator

(131) connected in series with a second resonator (132);

- a capacitance (133) connected in parallel to one of the first and second resonators (132) of the cascade.

2. The radio frequency filter of claim 1, wherein a node (134) between the first and second resonators (131, 132) is not connected to a parallel path.

3. The radio frequency filter of claim 2, wherein the cascade (131, 132) is connected to the other port (RX) and the capacitance (133) is connected to the node (134) between the first and second resonators (131, 132) of the cascade and the other port (RX) .

4. The radio frequency filter of any of claims 1 to 3, the first and second resonators (131, 132) of the cascade each having an acoustic track having an aperture (412, 411), wherein the aperture (412) of the acoustic track of the one

(132) of the first and second resonators is smaller than the aperture (411) of the acoustic track of the other one (131) of the first and second resonators.

5. The radio frequency filter of any of claims 1 to 4, wherein the apertures (411, 412) of the acoustic tracks of the first and second resonators (131, 132) of the cascade are determined such that a voltage drop at the first resonator (131) and a voltage drop at the second resonator (132) during operation are equal.

6. The radio frequency filter of any of claims 1 to 5, wherein the apertures (411, 412) of the acoustic tracks of the first and second resonators (131, 132) of the cascade are determined such that the impedance of the parallel connection of the one (132) of the first and second resonators and the capacitor (133) and the impedance of the other one (131) of the first and second resonators are equal.

7. The radio frequency filter of claim 5 or 6, wherein the apertures (411, 412) are determined such that one of the voltage drop and the impedance of the other one (131) of the first and second resonators and the parallel connection of the one (132) of the first and second resonators and the capacitor (133) are the same at the respective resonance frequencies .

8. The radio frequency filter of any of claims 1 to 7, wherein the resonators (110, 121, 122, 131, 132, 141, 151, 152, 161) are surface acoustic wave resonators comprising electrodes disposed on a piezoelectric substrate.

9. The radio frequency filter of claim 8, wherein the

piezoelectric substrate is made of crystalline lithium niobate having a cut angle of 170 degrees.

10. The radio frequency filter of any of claims 1 to 9, wherein the radio frequency filter is a receive filter and the other port (RX) is configured to output a signal to a signal processing circuit.

11. The radio frequency filter of claim 10, further

comprising a send filter connected between the antenna port (ANT) and another port (RX) , wherein the passband of the send filter (211) is adjacent to the passband of the receive filter (270) .

12. The radio frequency filter of any of claims 1 to 11, further comprising a cascade of a third and a fourth

resonator (121, 122) connected in series with the cascade of the first and second resonators (131, 132), a capacitor (123) connected in parallel to the cascade of the third and fourth resonators (121, 122) and parallel paths including resonators (141, 151, 152) connected from the cascade of the third and fourth resonators (121, 122) to the terminal (170) for ground potential .