WO2019174825A1

WO2019174825A1 - Rf filter with minimized in-band ripple

Info

Publication number: WO2019174825A1
Application number: PCT/EP2019/052993
Authority: WO
Inventors: Roeland Heijna
Original assignee: RF360 Europe GmbH
Priority date: 2018-03-15
Filing date: 2019-02-07
Publication date: 2019-09-19
Also published as: DE102018106030A1

Abstract

RF-filter with a ladder type structure with series reactance elements (RSI, RS4) and parallel reactance elements (RP1,...) in which additional shunt lines (SL1, SL4) with a respective notch element are added to provide additional degrees of freedom for filter design; the notch elements can be SAW resonators which may be optimised in view of static capacitance (Col,..., Co4) or resonance frequence (Fol,..., Fo4). When optimizing such a filter structure impedance variation and hence pass band ripple in a Tx filter (TXF) can be minimized.

Description

RF filter with minimized in-band ripple

In cellular communications a frontend module may be used comprising a power amplifier PA, a PA-matching and a TX filter. The Tx filter may be a single pass band filter or part of a duplexer or a higher multiplexer. Such a module is often called a PAMiD module. It has been found out that in such a PAMiD module a lot of specifications are influenced by a variation of the input impedance of the used Tx filter that is a e.g. a duplexer or a quadplexer.

In such a module a substantial ripple within the Tx frequency band may occur. It has been found out that the total ripple is predominantly determined by a non-constant input impedance of the filter. For example, the TX inband ripple of the used Tx filter can be tenfold worse in a PAMiD by this unwanted impedance variation when compared with the input impedance variation of a single filter measured sepatate from a module.

A filter for a given frequency band such as a SAW filter for a cellular band has an input impedance which is situated, as close as possible, at a constant value, for example 50 ohm.

An deviation from this impedance results in a different transfer in the total system. This is undesirable because a different transfer gives too much or too little power, which is reflected in an undesirable ripple, large ACPR (= Adjacent Channel Power Ratio) etc.

Hence it is an object of the present invention to provide a filter with a minimized in-band ripple. A further object is to provide a method how such a filter can be designed. These and other objects are met by an RF filter and a method of improving a Tx filter circuit according to the independent claims .

The basic idea of the present invention has been to improve a PAMiD for making the input impedance variation of the Tx filter of the module as small as possible. For providing such a filter a computer aided optimization of the filter or a mdodule comprising the filter can be used.

To end with an accordingly improved filter extra degrees of freedom are provided to the filter allowing to adapt an enhanced number of parameters.

As a result an RF filter on the base of a ladder type

structure is proposed comprising basic members circuited in series in a signal line. Each basic member comprises a series reactance element and a shunt line with a parallel reactance element arranged in the shunt line.

In the signal line nodes are provided before the first and after the last basic member of the ladder type structure as well as between each two adjacent basic members. To at least some of the nodes an additional shunt line is coupled

respectively. Each shunt line comprises a notch element. The number of the additional shunt lines as well as the values of all present notch elements are selected and set to yield at a minimized impedance variations at an input of the filter.

According to an embodiment n basic members are provide in the signal line of the ladder type structure. The number of possible nodes is then n+1. Hence, n+1 additional shunt lines may be coupled to the signal line and a notch element is arranged in each of the additional shunt lines. When compared with a common ladder type filter without additional shunt lines n+1 additional degrees of freedom are generated that may be used to optimize the filter. Further, the values of the notch elements may be varied as a further degree of freedom.

A preferred notch element is a capacitive notch element that provides a high pass to ground in the respective additional shunt line. The parameter to be varied is then the

capacitance value of the notch element.

A preferred capacitve notch element is a SAW resonator and the static capacitance thereof can be used as the parameter to be varied. When a SAW resonator as a notch element is used in a filter with a ladder type structure comprising SAW resonators the notch element and the resonators can be manufactured in the same technology and on the same chip. In such a case the additional shunt lines and the notch elements can be manufactured with very low additional effort at and the costs thereof are minimized.

Moreover, a resonator has a resonance frequency which can be used as another additional parameter for optimizing and improving the filter. Hence, using resonators as notch elements provides a further degree of freedom in an

optimizing process and every resonator provides two degrees of freedom.

The proposed Tx filter provides maximum benefit within a frontend module comprising a power amplifier and an impedance matching circuit arranged between power amplifier and filter. According to a preferred embodiment an additional shunt line is coupled to each of the nodes respectively and each shunt line comprises a notch element. Then a maximum number of freedom degrees for optimizing the input impedance of the filters is at disposal.

In an embodiment the reactance elements of the basic members comprise SAW resonators for use as series and parallel reactance element. Moreover, each basic member respectively comprises one series and one parallel reactance element.

Preferably, all reactance elements as well as all notch elements are embodied as SAW resonators.

In case all degrees of freedom have to be used, thereby varying all respective parameters in all notch elements to yield a constant input impedance of the filter, a filter is yielded wherein the reactance elements of the basic members respectively comprise a SAW resonator as series and parallel reactance element and wherein each basic members respectively comprises a series and a parallel reactance element.

According to the invention a method of improving a Tx filter circuit is disclosed. The method comprises the steps of

A) providing a common band pass filter circuit comprising a ladder type structure designed for a given frequency band, the ladder type structure comprising basic members with series and parallel reactance elements and nodes between each two adjacent basic members

B) coupling additional shunt lines to each of the nodes, each shunt line comprising a capacitive notch element C) providing a module comprising the pass band filter as a Tx filter, an power amplifier and an amplifier matching circuit

D) starting optimizing the filter by determining the input impedance of the filter within the module and proofing whether the in-band course of the input impedance is within a desired input impedance tolerance

E) if not, setting or varying a value of the capacitive notch element in a shunt line and

F) proceeding with step D and E thereby again varying the value of the capacitive notch element in the previous shunt line or another shunt line

G) if yes stopping the optimization.

In step A) a filter that is known in principle is provided in a common way with an optimized pass band and sufficient isolation and attenuation in the stop band. Providing

comprises providing a physical entity or a with virtual parameters. The properties of the design have to be

calculated by a computer program. Depending on the band requirements a standard number of basic members is set.

In step B) the design achieved after step A) is extended by introducing shunt lines and coupling same to the nodes.

In step C) the environment of the filter which is a PAMiD frontend module is considered and incorporated into the design and involved into calculation or testing thereof.

In step D) regarded parameter "input impedance" and in-band course therof is determined as a first step of optimization. It is proofed whether said course is within a tolerance or whether the variation and deviation thereof needs to be reduced and improved.

In case of a determined untolerable deviation step E) is performed and a capacitive notch element is introduced in at least one shunt line and a parameter thereof is selected. An probable assumption which parameter may be helpful can be used for the first setting of the at least one new parameter.

Steps D) and E) are then repeated and new notch elements with respective parameters are set or alredy set notch element parameters are varied.

If after repeated iteration of step D) and E) the deviation of the input impedance is small enough and hence tolerable the method of optimizing can be ended. However, the method can be processed until a minimum of deviation of input impedance is reached.

Step E and/or F may comprise varying of the capacitance of the capacitive notch element.

Alternativley the resonance frequency of the notch element can be varied.

In an embodiment step E and/or F comprises varying the static capacitance of the SAW resonator provided as capacitive notch element .

In an embodiment step E and/or F comprises varying the resonance frequency of the SAW resonator provided as

capacitive notch element. In the following the invention will be explained in more detail with reference to specific embodiments and the

accompanying figures. The figures are schematically only and are not drawn to scale. For better understanding some detail may be depicted in enlarged form.

Figure 1 shows impedance variation of a Tx filter with depicted circle size

Figure 2 shows impedance variation of another filter with a large circle size

Figure 3 shows impedance variation of a Tx filter with a small circle size

Figure 4 shows a block diagram of a filter circuit with an Rx part and a Tx part together with a simplified depiction of the transfer curve S21 of the Tx filter

Figure 5 shows a block diagram of a comparable filter circuit with an additional notch element together with a simplified depiction of the transfer curve S21 of the Tx filter

Figure 6 shows a block diagram of an embodiment of the new filter circuit with an additional shunt lines together with a simplified depiction of the transfer curve S21 of the Tx filter

Figure 7 shows impedance variation of an embodiment of the filter with nearly zero circle size together with an enlarged cut-out thereof Figure 8 shows a cut-out of the pass band of a Tx filter where the response of the single device is compared with the response of the filter in combination with a power amplifier

Figure 9 shows the response of the new filter.

Figure 1 shows an impedance variation of a common filter in a Smith diagram. The bold section of the curve accords to the impedance variation within the pass band. A dotted line depicts the so-called circle size that is a circle shown as a dotted line drawn around the desired impedance of e.g. 50 Ohm to enclose the bold curve section in the pass band. The circle size is thus a measure for the impedance variation and specifies a maximum deviation of the desired input impedance within the TX band (Sll) .

Figure 2 shows another impedance variation of a common filter with a large circle size.

Figure 3 shows another impedance variation of a common filter with a small circle size.

In general a well working filter has an impedance variation the the impedance variation that is restricted to said circle size. If there is a choice, the smallest circle size may be selected. Figure 2 is a perfect filter but not designed for the smallest circle size. Figure 3 gives the result for almost the same filter behavior but now with a smaller circle size .

The block dieagram of Figure 4 shows a currently used

structure of a Tx filter TXF with a ladder type arrangement of reactance elements RS, RP that may be embodied as SAW resonators. With the aid of precisely selected series

elements RS and shunt elements RP, the desired transfer is selected. Only indicated is the structure of an according Rx filter RXF that is connected to the same antenna ANT. Tx filter TXF and Rx filter RXF my hence provide a duplexer or a higher multiplexer. In the right part of the figure a typical transfer curve S21 of the Tx filter is shown together with the according Tx and Rx bands .

Sometimes an RX notch is additionally included into the Tx filter structure and may be embodied as a notch element in a shunt line that is coupled to the signal line. Figure 5 shows an exemplary example for a Tx filter with additional notch. This notch is responsible for a little bit more isolation in the RX band as showm by the arrow in the transfer curve S21 at the right part of Figure 5.

To further improve/reduce the circle size as many as possible notch elements NE are added to the Tx filter TXF. The notch elements are selected to respectively produce a notch within the according band of the Rx filter. The left part of Figure 6 shows such an embodiment as a block diagram.

The Tx filter is construed on the base of the ladder type structure shown in Figure 4. Additionally shunt lines SL are coupled to each node of the Tx signal line. Such nodes are present between each two neighbored series elements RS where the shunt elements RP are coupled to the signal line. Further nodes and thus further additional shunt lines SL can possibly be placed between the antenna ANT and the series element RS4 placed next thereto and between the Tx section (at the left end of the signal line) and the following "first" series element RSI . When comparing the transfer curve S21 of this Tx filter TXF with the respective one of a currently used filter structure as shown in the right part of Figure 5 for example improved isolation is achieved in the Rx band.

However the most substantial advantage of this filter

structure is the achieved reduction of circle size. In the example of Figure 6, eight extra degrees of freedom have been introduced by the additional shunt lines SL, namely the four dimensions of four capacitance values Co of the four notch elements NE, and four possible frequencies (F₀₎ of the notch elements NE that are embodied as SAW resonators. These additional degrees of freedom allow further improving circle size of the filter and hence, reduction of in-band ripple of the Tx filter TXF e.g. when arranged in a PAMiD frontend module. The degrees of freedom are used in an optimization process which is usually done by a respective computer program.

Figure 9 displays the result after optimizing the filter structure especially after optimally selecting and setting the parameters of the shunt lines SL and notch elements NE . The Smith diagram of an optimized filter in Figure 7 shows that the circle size is reduced to a point (see left part of the filter) . A magnified depiction of the circle size i.e. of the course of impedance within the pass band is shown in the right part of the figure. The impedance variation corresponds to a perfect behavior. In this figure it is seen that if the input impedance of the center point starts to drift away, one of the extra degrees of freedom is used to tune the impedance levels and causes that the impedance is again bent to the desired 50 ohm (center) impedance level. The positive effect of such a reduced circle size onto the pass band of the Tx filter becomes obvious when comparing Figures 8 and 9.

Figure 8 shows with the dotted line the response (S21) of a TX Filter TXF with a "normal" minimum size circle

corresponding to a filter as shown in Figure 5 that has not yet been optimized ot minimum circle size. The continuous line too indicates the response of the same filter, but now in combination with a power amplifier PA. This line is calculated and measured for a Tx filter that is mounted within a frontend module comprising a power amplifier coupled to the Tx filter.

While the course of the dotted line shows only a small ripple there arises a substantial ripple in the continuous line. This is due to the overall gain that is proportional to the y axis of the S21 diagram is also dependent on the input impedance of the TX filter TXF. Varying input impedance with substantial circle size produces varying gain in the pass band corresponding to the shown ripple.

Figure 9 shows, the same situation again, but now the circle size is reduced to the dot. The input impedance disrupts the transfer no more. Hence, both curves show a similar course and the continuous line does not show additional ripple like that of Figure 8. Such a Tx filter is perfect for use in a frontend module.

One may assume that such an optimized Tx filter may have as a disadvantage that the additional components of the additional shunt lines cost extra surface on a filter chip making the chip greater and producing more costs. However with this new filter solution it is prossible to compensate that

"disadvantage". As the additional RX-notches in the

additional shunt lines are responsible for improved isolation in the Rx band this can be used to reduce the area of the normal filter elements that is the area of the series and shunt elements. At last a reduction of the number of filter elements is possible without unduely deteriorating the Rx isolation. This gives a extra profit in insertion loss as can be seen when comparing the continuous lines of the transfer curves S21 shown in Figures 8 and 9. Figure 9 and the

respective filter structure clearly show smaller insertion loss than the same filter structure without optimization using the additional degrees of freedom.

As a preferred goal RX isolation elements can be used.

However, the invention can be taken for improving the

isolation of any other frequency channel that is

simultaneously operated by the module. With the additional degrees of freedom an optimizer has the ability to use more then the normal number of freedoms to achieve an input impedance of the filter close to the normally required 50 ohm.

In practice this means that the filters structure hat to adapted a bit when optimized for said minimum circle size.

The optimization self is part of a normal optimization routine for the design of these filters, but adapted for this extra possibilities.

A general idea of the new filter structure can be seen by adding additional a substantial number of additional degrees of freedom when optimizing such a filter structure. The filter is not limited to a duplexer. Quadplexers or multiplexers of a higher degree are also possible. Further improvements can be made to reduce impedance variation as much as possible. Then, variations in the transfer functions of the corresponding filters can be reduced. In particular with respect to reflections of power in signal paths that cause undesired ripple, the following is possible.

The impedance optimizations can be made with respect to a transmission filter so that the input impedance of the impedance matching circuit is as close as possible to the load light impedance, i.e. to the intrinsic impedance of the signal line. Thus, considering the specific properties of the signal line itself, it can lead to further optimizations of the filter's electrical properties. Compensation of

variations of the signal line's frequency dependence, power dependence or amplifier gain dependence can be performed at the input side of the corresponding RF filter.

Further, the input side of the filter can be provided such that its input impedance can be varied such that different gains caused by a frequency or power dependence of the circuit elements before the filter can be compensated. This can be obtained by making the filter impedance lie on a constant gain line (in a Smith chart) so that frequency variations do not alter the gain at the specific circuit node .

Another possibility to reduce passband ripple is to provide a small deviation from the circular line of a constant gain around a conjugated impedance to compensate for small errors in the filter transfer in the desired frequency band. Another or additional goal of the optimization of a filter structure can be to enhance the input impedance of the filter at those frequencies where the filter shows the greatest power dissipation.

A SAW filter (duplexer) has a maximum allowable power level for a cellular band. Defects in the duplexer usually occur with excessive power on the high side of the band generally in the smaller series elements. The maximum power depends strongly on the used power source and the duplexer

impedances. It is proposed to deviate from a desired duplexer impedance to achieve a different maximum power in the total system. The input impedance should be made at those frequency at which the respective filter receives too much power and exceeds the maximum power level.

In some saw filter (duplexer) it is desirable to suppress a band close to the TX band. According to an embodiment a proper setting of the filter input impedance is used to increase the suppression in a neighbored channel. The goal is to get more gain in the desired frequency band with the aid of the whole system and to get less gain for the undesirable frequency band. This can be done by intentionally producing a mismatch of the power amplifier PA with the PA-matching circuit at the frequency of the band to be suppressed.

Thereby suppression of undesired bands can be maximized. In practice this means that the filter must be optimized for a given system. Enhancing the reflection Sll for the

undesirable band can be set as a new goal of the filter optimization routine. In a PAMiD module the total gain for harmonics is determined by input impedance of the filter and output impedance of the PA-matching circuit. According to an embodiment it is not a goal to reduce this gain, but to shift the maximum gain from an undesirable place to a place where it is not important. By an adjustment of the PA matching network, or of the TX input impedance gives a different frequency at which maximum gain occurs. So it is possible to shift the frequency of maximal gain to location where it does not matter and where neither a neighbor channel nor a harmonics occur. By doing this less gain is produced in these channels and suppression of same can be improved.

There are four ways proposed to shift this gain peak to a point where the damage is most limited.

1) The output impedance of the PA-matching can be pushed a little bit by choosing the internal impedance a little bit different.

2) The line length of the interconnect between PA or PA matching and filter rotate the output impedance of the PA-matching. Thereby the gain peak can be shifted

3) The input impedance of a filter at high frequencies is capacitive, which means that the dimensions of the first filter element determines the input impedance of the TX filter to a large extent. Hence, by varying the dimension of the first filter element (preferably a series element) input impedance can be varied.

4) The first element of a filter element may be a series or a shunt element. A choice of a proper kind of first filter element can be used to determine the input impedance of the TX filter to a large extent. In practice, this means that all four possibilities of shifting need to be suitably selected and weighted to achieve a proper balance towards the desired goal.

In cellular communications a system consisting of a PA, PA- matching and TX filter (e.g., a SAW duplexer) , the load line should be tuned for each frequency band to the correct impedance. For this purpose, parallel circuited capacities can be switched on or off. In a PAMiD fronted module in some places capacities are used while in other places too much capacity is already present. According to an embodiment a method is disclosed to also use these additional capacities to make more insolation in the RX band. The additional input capacity is replaced by an additional RX notch element with exactly the right capacity value in the TX band, then two problems have been solved. Instead of placing a capacity necessary for matching the power amplifier to the Tx filter in the matching circuit it is proposed to place the capacity at the input (towards PA) of the Tx filter parallel to the signal line. The capacitance value thereof can be selected to compensate for the inductance of the filter. At the same time, this capacitance together with the inductance be can be used to produce an additional notch to improve the

suppression for an unwanted frequency.

The notch has not to be limited to its own RX band. The notch can be used for any frequency. In a carrier aggregation solution, the notch can be used for RX cross isolation.

The invention shall not be limited by the provided

embodiments and figures. The scope of the invention is defined by the claims only in their broadest interpretation. Hence, arbitrary further elements and features may be present in an inventive filter despite being possibly known from the art as such.

List of reference symbols

ANT antenna

Co capacity of notch element (Co of notch resonator)

Fo frequency of notch element (resonance frequency of notch resonator)

NE notch element

RP parallel (reactance) element and

RS series (reactance) element

RXF Rx filter

SL additional shunt line

TXF Tx filter

Claims

1. A RF filter

- comprising a ladder type structure with basic members circuited in series in a signal line

- wherein each basic member comprises a series reactance element and/or a shunt line with a parallel reactance element

- wherein nodes are provided in the signal line before the first and after the last basic member of the ladder type structure as well as between each two adjacent basic members

- wherein additional shunt lines are coupled to at least some of the nodes, each shunt line comprising a notch element

- wherein a number of the additional shunt lines and

values of the notch element respectively arranged therein is set to minimize impedance variations at an input of the filter.

2. The RF filter of claim 1,

wherein the filter is a Tx filter arranged in a frontend module comprising a power amplifier and a matching circuit.

3. The RF filter of one of the foregoing claims,

wherein the notch element comprises a capacitve element.

4. The RF filter of the foregoing claim,

wherein the notch element comprises a SAW resonator.

5. The RF filter of one of the foregoing claims, wherein to each of the nodes an additional shunt lines is coupled respectively, each shunt line comprising a notch element .

6. The RF filter of one of the foregoing claims,

wherein to each node a shunt line is coupled

wherein each shunt line comprises a SAW resonator

wherein all resonators are different in view of resonance frequency and static capacitance.

7. The RF filter of one of the foregoing claims,

- wherein the reactance elements of the basic members

respectively comprise a SAW resonator as series and parallel reactance element

- wherein each basic members respectively comprises a

series and a parallel reactance element.

8. A method of improving an Tx filter circuit, the method comprising the steps of

B) coupling additional shunt lines to each of the nodes, each shunt line comprising a capacitive notch element

C) providing a module comprising the pass band filter as a Tx filter, an power amplifier and an amplifier matching circuit

D) starting optimizing the course of input impedance within the module E) determining the input impedance of the filter within the module and proofing whether the input impedance is within a desired input impedance tolerance

F) if not, varying a value of the capacitive notch element in a shunt line and

G) proceeding with step E and F thereby again varying the value of the capacitive notch element in the previous shunt line or another shunt line

H) if yes stopping the optimization.

9. The method of the previous claim

comprising in step F and/or G varying of the capacitance of the capacitive notch element.

10. The method of one of the previous claims,

wherein a SAW resonator is provided as capacitive notch element

wherein step F and/or G comprises varying the static

capacitance of the SAW resonator.

11. The method of one of the previous claims,

wherein a SAW resonator is provided as capacitive notch element

wherein step F and/or G comprises varying the resonance frequency of the SAW resonator.