Filter circuit with improved isolation and frontend module comprising same

The invention relates to a filter circuit for RF frequencies that has an improved isolation for more suppression of unwanted bands and signals.

In cellular communications a system consisting of a PA, PA- matching and a TX filter (e.g., a SAW or BAW duplexer, quadplexer etc.) or another respective frontend module, the isolation within the stop band may be too bad to comply with given specification of a specified frequency band. Hence additional measures have to be taken to improve the isolation between different bands.

A known method for doing so is to include a notch filter in the signal path that filters out a specific frequency to create a pole in the filter transfer function. Usually one notch filter is filtering out just one single frequency and is thus useful for attenuating just one spurious signal and improves the isolation at one specific frequency only.

Introducing any further notch would unduly enhance the series resistance thereby producing losses what is not desired.

A common method for introducing a notch into a filter circuit is to use an additional notch for additional isolation in the RX band. Starting from a common ladder type filter this notch can be produced by integrating an additional shunt element into the filter circuit. As a result, it is seen that the isolation can be improved. However, the additional shunt element gives a small

disturbance in the TX band.

A further method comprises coupling an additional inductor to a present shunt resonator of the ladder type circuit to shift the respective pole/notch that is produced by the series resonance of the shunt resonator' s mechanical capacitance and the inductor. Such a method is only possible for creating a notch on the high frequency side of the filter pass band.

It is an object of the invention to provide a filter circuit having improved isolation against a frequency band in the stop band without causing too much additional loss.

This and other obejcts are met by a filter circuit according to claim 1. Further embodiments including advantageous features are given by dependent claims.

The invention proposes to use a common band pass filter that is designed to create the desired/required pass band and then couple an additional notch filter element to the filter signal line in series but separate from the band pass filter.

The band pass filter may comprise a ladder type arrangement of resonators embodied in SAW or BAW technology or any other filter technology that is useful for the required pass band.

The notch filter element comprises an inductor arranged in a shunt path that is connected to ground in parallel to the signal line. In a preferred embodiment the band pass filter is a ladder type arrangement of SAW or BAW resonators designed for a given frequency band. A capacitance is formed as a resonator of the same technology. This allows using a single filter chip for band pass filter and notch filter element.

Any other capacitive element is possible too for creating the notch filter element.

The inductor can be a coil. This coil can be planar coil produced on the filter chip. Alternatively, any other

inductive structure like a meandering conductor line or a discrete inductor is possible too.

The proposed inductor that is arranged separate from the filter determines a notch in combination with the filter element. The same combination additionally determines a pass band which can be placed exactly on the frequency of the TX band. This pass band actually has no losses so that the notch can be chosen larger. Also a second notch is created which can be placed on another frequency to be suppressed, e.g. a harmonic frequency. While the formerly used single notch produces big losses and a disruption, the twofold notch now produces negligible losses. Only an extra inductor and a ground terminal are necessary which are small differences that are achieved with low effort but which yields much benefit .

Preferably the given frequency band is a Tx band of a

cellular band of a wireless communication standard according to G4 or G5 generation. Then it is advantageous to adapt the notch filter element to produce a notch at a frequency of the according Rx band of the same cellular band. According to another embodiment the notch filter element is adapted to produce a notch at a harmonic frequency of the given frequency band.

In the filter circuit the signal line couples a Tx section of a communication device to an antenna of the device. The notch filter element is preferably arranged between the Tx section and band pass filter.

An embodiment relates to a frontend module adapted for operation in a carrier aggregation mode and comprising the filter and the filter circuit described above. This frontend circuit is adapted for parallel and simultaneous operation in the given frequency band and a second frequency band. Then, the notch filter element is adapted to produce a notch at an Rx frequency according to the second band. This improves isolation between the aggregated band such that an

simultaneous and undisturbed operation within the two bands is possible.

In the following the invention is explained in more detail with reference to specific embodiments and the related figures. The figures may be drawn schematically only and not to scale.

Figure 1 shows the transmission curve of a common band pass filter with the notches produced by filter elements.

Figure 2 shows the pass band with the Rx isolation

requirement . Figure 3 shows an example where the Rx isolation requirement is met.

Figure 4 shows the effect of a double notch.

Figure 5 shows a reference band pass filter design.

Figure 6 shows some characteristics of the reference filter.

Figure 7 shows the reference filter with an included notch element .

Figure 8 shows some characteristics of the filter of figure

7.

Figure 9 shows the reference filter with another included notch element element.

Figure 10 shows shows some characteristics of the filter of figure 9.

Figure 11 shows a first embodiment of a filter circuit.

Figure 12 shows characteristics and different transfer curves for the filter circuit of figure 11 with varying notch frequencies .

Figure 13 shows characteristics of the filter circuit of figure 11 with a notch having set a notch frequency.

Figure 14 again shows the first embodiment of the filter circuit . Figure 15 shows a second embodiment of a filter circuit wherein the number of resonators has been reduced relative to the reference design.

Figures 16 and 17 show different parts of the transfer curves of filters an the base of Figure 15 according to two new embodiments .

Figure 17 shows a new filter with a RX band below the TX band .

Figure 18 shows two possibilities of a notch filter realized with a ground inductor to create a notch below the TX band.

Figure 19 shows some characteristics of the filter of figure 15 wherein the notch is set to frequency below the pass band.

Figure 20 shows some characteristics of the filter of figure 15 wherein a double notch is set to a frequency below the pass band.

Figure 21 shows the fly back of common filter circuit, and

Figure 22 shows the fly back of a new filter circuit.

Figure 1 shows a normal TX gain plot (S21) of a band pass filter realized in SAW technique. The filter is constructed by means of series elements RS and shunt elements RP

comprising SAW resonators. In the figure an example of the behavior of the two elements (series and shunt elements) is given in two different scales. The dotted line complies with the effect of one single series element and one single parallel element respectively. Figure 2 shows the pass band of a Tx filter circuit together with the Rx isolation requirement the filter has to meet. In the middle is the TX band where the filter circuit should have low insertion loss, and right or left are possible RX bands where optimal isolation is required the depicted filter curve does not yet show.

Figure 3 shows a first common method wherein a notch filter element is used to improve the Rx isolation of a Tx filter. The method uses a (shifted) shunt element for additional isolation in the RX band. As a result, it is seen that the isolation is improved. However, the additional shunt element gives a small disturbance in the desired TX band.

The second common method is to use a ground inductor. The isolated behavior of such a notch is shown in Figure 4. It can be seen that two notches are formed due to the

interaction of the ground inductor with other series and shunt elements. The dotted line shows the behavior of the notch while the continuous line shows the effect of a normal shunt element.

The common methods are only possible on the right side of the TX. There is until now no solution know for additional RX isolation on the left side of the TX band which would be required for some accordingly defined Rx/Tx band

combinations .

In order to compare current solutions and new solutions a reference design for a filter circuit is required. Figure 5 shows the construction of the reference design which is used as a Tx filter TXF in this case. It comprises a ladder type arrangement of resonators typically made of SAW resonators.

In a signal line SL coupling a Tx section to an antenna ANT series elements (resonators) RS alternate with shunt elements RP . Figure 6 shows the behavior of this reference design.

Figure 6 shows some characteristics of this reference filter circuit. Matrix element Sll shown on the left side of the figure has the so called circle size. Curve Sll of a TX filter should be as close as possible around an impedance, in this case 50 ohm. In the present case, the circle of figure 6 has only a small figure showing that the reference filter is well matched.

The S21 value of a filter should be high (low attenuation) as possible in the TX band and low as possible in the RX band and for the harmonic frequencies as well. The depicted dots in the curve illustrate the position of the harmonics. The different depictions of S21 are scaled differently or show cutouts of the total course of S21.

Figure 7 shows the construction of a common filter circuit with an additional Rx notch depicted as a small block denoted with an "n". The notch is placed just inside the ladder type structure of the filter TXF.

Figure 8 shows some characteristics of this common notch solution. The solid arrow stands for a deteriorated course of S21 and the open for an improved transfer curve.

Whether such a solution is advantageous or not is highly dependent on the chosen reference, but in general, the circle size of Sll can be made as small as the reference. The RX isolation is improved by this additional notch element, see open arrow.

This means that the filter can be less effective without detrimental effect on the characteristics of the total filter circuit. For a given isolation, every object that is met by the notch does needs not be solved by the filter itself. The harmonic suppression is slightly worse (see small arrow in the upper left S21 depiction) . The insertion loss is improved because the filter needs to do a little less. However the RX notch gives an additional disturbance in the TX band which is approximately equal to the previous advantage, making the total IL approximately equal.

Figure 9 shows the construction of a filter circuit design TXF with an additional ground notch NE . The design uses a shunt element RP already present in the filter TXF and produces the notch NE by adding an inductor LG in series to the shunt resonator RP . The result of the Rx notch is just visible and indicated by the arrows.

As can be seen from Figure 10 advantages and drawbacks are again highly dependent on the chosen reference, but in general the circle size can be kept as small as the reference design at the expense of potential more insertion loss. With regard to the harmonics this circuit is not beneficial as S21 shows less suppression of the harmonics as indicated in the figure by the open arrow.

The embodiment of an improved filter circuit is shown in Figure 11 and based on the idea of building the filter TXF on a normal way, ideally without making any concessions. And after that a notch element NE is added by placing a separated and additional shunt element (resonator) RP in series with an inductor L in a way that it does not disturb the performance of the filter TXF. When doing this, the frequency value of the additional notch element NE is chosen so that capacitance of the element is canceled by the inductor L in the TX band. The inductor value is set so that it produces a notch at the desired frequency.

Using this concept a simulation according to Figure 12 has been performed for different notch frequencies which are set by accordingly chosen inductor values.

The figure shows that it is possible to place the notch at any desired frequency without affecting the TX performance. The pass band at the Tx frequency is totally unaffected by the notch (see bottom picture) . The Sll is exactly at 50 ohms for the center of the Tx band (see arrow) . For lower or higher frequencies this can be compensated in the filter without affecting the filter performance.

Fig. 13 shows the behavior of a notch element NE as shown in Figure 11, but now for only one selected notch element.

Because each filter element contains a mechanical series capacity and an inductance together with a large shunt capacity, the combination of a filter element and a series of a shunt resonator and a grounding inductors will always gives two resonances frequencies (notch frequencies) , one below the TX band and one above the TX band as indicated in the figure by arrows. The figure further shows that the greater notch effect can be placed just below the Tx band which up to now has been impossible with common notch solution. The zoomed cutout in the bottom part of the figure shows that the Tx band is unaffected by the notch element NE . Figure 14 again shows the proposed filter circuit with the notch element NE and the separated Tx filter TXF.

A further advantage of the proposed filter circuit is that some of the RX isolation is made by the notch element NE and hence outside the Tx filter. Then, the Tx filter can be reduced in size respectivly in number of filter elements. Figure 15 shows an example where two filter elements are omitted when compared with the embodiment of Figure 11. The omitted filter elements comprise a series resonator and a shunt resonator and are indicated in the figure by a

respective X.

Figure 16 shows some characteristics of the filter circuit of Figure 15. The dotted line shows the additional notch

behavior. In general, the circle size of Sll will be smaller because the filter is reduced in size and number of filter elements. The harmonic suppression is therefore

proportionally less. However as a met goal of this new circuit the isolation is the same. Further, the insertion loss is significantly better by about 0.5 dB in this case which is unlikely high.

Moreover the total costs of the filter circuit will also be smaller because the large elements in the filter are replaced by a small additional element.

So far, only the possibilities for notch frequencies above the TX band have been discussed. The existing and common filter circuits with notch elements do not offer

opportunities below the TX band. A simulation of a new embodiment directed to this situation is shown in Figure 17 representing some characteristics of a respective filter circuit which is in the example a filter for band 20. Here, the Rx band is below the Tx band. The figure shows that with the invention the Rx isolation can also be improved for such a band with interchanged location of Tx band and Rx band.

If a ground inductor is coupled to an already present shunt element of the Tx filter itself to make a notch on the RX frequency below the Tx frequency - see arrow down - then the result is that the used shunt element is no longer working on the upper side of the RX band, which causes the isolation to worsen, see arrow upwards. The dotted lines in Figure 18 show as a cutout taken near the Rx frequency the filter's S21 compared with the solid line which is the S21 of a circuit without a ground inductor coupled to the present shunt element .

Figure 19 shows some characteristics of a new proposed filter circuit where the RX band to be suppressed is below the TX band. Here, the notch frequency is set to the Rx frequency. Because the notch element NE produces two notches it is possible to place the second notch with the highest frequency exactly on the 2 ^nd harmonic.

Also in this case the insertion loss at the TX band is significantly better. The dotted line shows the additional notch behavior.

The choice to set the higher notch frequency at the the 2nd harmonic is logical. However, in another embodiment, which is shown as an extreme case in Figure 20, even both notches can be placed at the harmonics. In this case one notch is set at the 2nd harmonic and the seond notch is set at the 3 ^rd harmonic. In this figure too the dotted line shows the additional notch behavior.

A further advantage of the invention which has not yet been mentioned above becomes obvious in the fly back. In a common filter with a notch within the filter circuit the value of S21 is bad in the fly back as indicated by an arrow in Figure 21.

Figure 22 shows the fly back region for a filter circuit with the new notch element. Here, the open arrow shows that the suppression in the fly back is improved which is due to the new solution because the separated notch element makes no interaction with the filter.

The filter circuit of the invention is not limited to the presented embodiments and figures. Further, the notch

frequency in the Tx filter circuit needs not being limited to the respective RX band that is assigned to the Tx band. The notch can be set at an arbitrary frequency and can be used for any frequency band.

In a frontend module that can be operated in a carrier aggregation mode for example, the filter circuit with the new notch can be used for a RX cross isolation that means for isolating against the aggregated Rx band. In all embodiments the filter function and the notch function are separated and optimized independently so that no unwanted interactions occur anymore. List of used reference symbols

ANT antenna

L inductor

n notch

NE notch filter element

RP shunt element

RS series element

SL signal line

TX Tx section

TXF Tx filter