Abstract:
A duplexer for connection with an antenna comprises an antenna port, a transmitting filter comprising bulk acoustic wave (BAW) resonators having a first antenna side impedance coupled with the antenna port, a receiving filter comprising BAW resonators having a second antenna side impedance coupled with the antenna port, and a shunt inductance coupled between the antenna port and ground. The shunt inductance and the first and second antenna side impedances of the transmitting filter and the receiving filter are selected in such a way that the shunt inductance turns the first and second input impedance in a negative direction in a Smith diagram.

Description:
TECHNICAL FIELD 
   The present application relates to duplexer for use in a transmitter, such as a mobile phone. 
   BACKGROUND 
   In many different communications applications, such as mobile phones or any other transceiver, a common signal path is coupled both to the input of a receiver and to the output of a transmitter. In such a transceiver, an antenna may be coupled to the input of the receiver and to the output of the transmitter. A duplexer is, thus, used to couple the common signal path to the input of the receiver and to the output of the transmitter. The duplexer provides the necessary coupling while preventing the modulated transmit signal generated by the transmitter from being coupled from the antenna back to the input of the receiver and overloading the receiver. A duplexer generally consists, thus, of three ports. At the antenna both signals coexist representing incoming and outgoing signals. To avoid collision of these signals, usually different frequency bands are allocated to the Tx and Rx path. The main purpose of a duplexer is to direct incoming Rx signals to the receiver port and to guard TX signals from the TX port to the antenna. To this end, a duplexer usually comprises two highly selective radio frequency (RF) filters for the respective Tx and Rx frequency bands. The RF-filters need to have sufficient rejection for the opposite band. At the antenna port, both filters are connected in parallel. In order to avoid that the two filters degrade each other&#39;s impedance a matching network is required. 
     FIGS. 1 and 2  show exemplary embodiments of such duplexers according to the prior art. In  FIG. 1 , the antenna port is coupled directly to the transmitting filter  110  (Tx) whereas a transmission line  130  is used to couple the antenna port with the receiving filter  120  (Rx). The transmission line is usually a 90° phase shifting device for transforming the impedance of the Rx filter  120 . The Tx filter usually comprises a filter  111  with acoustic resonator devices such as thin film resonators or as applied in the present invention bulk acoustic wave resonators (BAW). An optional shunt matching network.  113  and optional output matching network  112  are provided depending on the respective application as shown in filter  110  of  FIG. 1 . The Rx filter  120  comprises similar elements with BAW filter  121 , and networks  122  and  123 . 
     FIG. 2  shows another exemplary embodiment of a duplexer  200  according to the prior art. Similar elements carry similar numerals. In this embodiment a so-called pi-network  210  is used instead of the transmission line  130 . The pi-network  210  consists of two capacitors each connected on one side with ground and coupled on the other side through an inductor. Both implementations, the transmission line  130  as well as the pi-network  210  provide for a +90° phase shift and impedance transformation. Both function as a delay line, the transmission line  130  comprises a quarter wavelength and the pi-network comprises shunt capacitors and a series inductance. The +90° phase shifting is required to transform the input impedance which the Rx filter presents towards the antenna to high values at the Tx frequency range to effectively block the TX signals. The input impedance of the Tx filter in the Rx frequency range is typically high by default and does not require additional impedance transformation or phase shifting. 
   SUMMARY 
   Conventional duplexers as described above have several drawbacks. For example, the delay line enforces the use of a multilayer substrate and requires a relatively large area. The losses of the delay line or of the pi-network affect an insertion loss for both Rx and Tx signal paths. Furthermore, a +90° phase shift has by its nature already an unwanted low pass characteristic and does not provide a DC current path down to ground. Additional circuitry is, thus, required to ensure electro static discharge (ESD) protection and robustness. 
   According to an embodiment, a duplexer for connection with an antenna comprises an antenna port, a transmitting filter comprising bulk acoustic wave (BAW) resonators having a first antenna side impedance coupled with the antenna port, a receiving filter comprising BAW resonators having a second antenna side impedance coupled with the antenna port, and a shunt inductance coupled between the antenna port and ground, wherein the shunt inductance and the first and second antenna side impedances of the transmitting filter and the receiving filter are selected in such a way that the shunt inductance turns the first and second input impedance in a negative direction in a Smith diagram. 
   According to another embodiment, a duplexer for connection with an antenna according comprises an antenna port, a transmitting filter having an antenna side and a signal input side, the transmitting filter comprising bulk acoustic wave (BAW) resonators having at least a first series BAW resonator coupled with the antenna side, a receiving filter, having an antenna side and a signal output side, the receiving filter comprising bulk acoustic wave (BAW) resonators having at least a second series BAW resonator coupled with the antenna side, and a shunt inductance coupled between the antenna port and ground. 
   According to another embodiment, a duplexer for connection with an antenna comprises means for filtering a transmission signal by bulk acoustic wave (BAW) resonators wherein the means comprise a first antenna side impedance, means for filtering a receiving signal by bulk acoustic wave (BAW) resonators wherein the means comprise a second antenna side impedance, means for coupling the means for filtering the transmission signal and for filtering the receiving signal with an antenna, and a shunt inductance means coupled between the antenna port and ground for shifting the first and second antenna side impedance in a negative direction in a Smith diagram. 
   A rotation of the input impedance can be established by approximately −90°. The transmitting filter may comprise a plurality of series connected BAW resonators and a plurality of shunt BAW resonators. The duplexer may comprise four series BAW resonators and three shunt BAW resonators wherein each shunt BAW resonators is coupled between a node between two series BAW resonators and ground. The duplexer may further comprise bond wire inductances for signal coupling of the series circuit of BAW resonators and for coupling the shunt BAW resonators with ground. The transmitting filter and the receiving filter can be coupled with the antenna port through bond wire inductances. The receiving filter may comprise a plurality of series connected BAW resonators and a plurality of shunt BAW resonators. The duplexer may comprise in, particular four series BAW resonators and four shunt BAW resonators wherein each shunt BAW resonators is coupled with a terminal of one series BAW resonators and ground. The duplexer may further comprise bond wire inductances for signal coupling of the series circuit of BAW resonators and for coupling the shunt BAW resonators with ground. The transmitting filter and the receiving filter each can be implemented on a chip. The transmitting filter chip and the receiving filter chip can be arranged on a substrate. The transmitting filter chip can also be mounted on the substrate by flip-chip technology. 
   According to another embodiment, a method for processing signals through a duplexer for an antenna comprises the steps of filtering a transmission signal by means of bulk acoustic wave (BAW) resonators wherein the means comprise a first antenna side impedance; filtering a receiving signal by means of bulk acoustic wave (BAW) resonators wherein the means comprise a second antenna side impedance; coupling the means for filtering the transmission signal and for filtering the receiving signal with an antenna; and shifting the first and second antenna side impedance in a negative direction by means of a shunt inductance coupled between the antenna port and ground. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1 and 2  show embodiments of duplexers according to the prior art; 
       FIG. 3  shows a principal circuit diagram of one embodiment according to the present invention; 
       FIG. 4  shows a more detailed embodiment of the Tx and Rx filters according to the present invention; 
       FIG. 5  depicts a Smith diagram of the characteristics of the Tx path without the shunt according to the invention; 
       FIG. 6  depicts a Smith diagram of the characteristics of the Tx path with the shunt according to the invention; 
       FIG. 7  depicts a Smith diagram of the characteristics of the Rx path without the shunt according to the invention; 
       FIG. 8  depicts a Smith diagram of the characteristics of the Rx path with the shunt according to the invention; 
       FIG. 9  depicts a Smith diagram of the characteristics of the overall input impedance of the antenna port of an arrangement according to the invention; 
       FIG. 10  shows the pass band characteristics of the Rx and Tx branch and the isolation between Rx and Tx paths; 
       FIG. 11  shows a schematic embodiment of a substrate with a filter chip; and 
       FIG. 12  shows a schematic embodiment of a substrate with a flip chip. 
   

   DETAILED DESCRIPTION 
     FIG. 3  shows one embodiment of an improved duplexer. Again, a Tx filter  300  and a Rx filter  320  are provided. The Tx filter has a specific impedance  350  on the antenna side. Similarly, the Rx has its specific impedance  360  on the antenna side. These antenna side couplings of Tx filter  300  and Rx filter  320  are, for example, coupled through bond wire inductances  301  and  321  with the antenna port. Alternatively, a direct coupling without inductances is possible as indicated by the broken connection lines. Thus, the bond wire connections  301 ,  321  are optional and are only present if the manufacture technology requires bond wire connection for the substrate and the BAW chip. If the manufacture technology allows for a flip-chip mounting, no inductances are necessary. According to  FIG. 3 , the antenna port is provided with a shunt inductance  340  which couples the antenna port with ground. 
   Instead of a transmission line or a pi-network, this embodiment comprises a shunt inductance  340  at the antenna side. Thus, the antenna side of the TX filter  300  and the antenna side of the RX filter  320  are either coupled directly or through respective bond wire inductances  301 ,  321  with the antenna port which is coupled through shunt inductance  340  with ground. This specific arrangement, thus, results in a transformation of the input impedance which results in a high pass characteristic. In a Smith diagram, this shows as a rotation in a negative direction such as a −90° phase rotation. However, the negative rotation angle may be established in a relatively broad range. To achieve appropriate matching for the respective frequency bands of the Tx and Rx paths, in one embodiment the filter topology can be respectively chosen. For example, in a conventional duplexer as shown in  FIGS. 1 and 2 , the Rx filter starts first with a shunt resonator at the antenna side because this results in a shorter delay time requirement. However, with the shunt inductance  340  being “external” of the Rx filter as shown in the embodiment in  FIG. 4 , here, for example, a series resonator  322  can be implemented as the first element of the Rx filter on the antenna side because this results in a shorter transformation path. In one embodiment, the Tx filter can be modified in a way to accommodate the transformation by the shunt inductance  340  and the input capacitance of the Rx filter. 
   The shunt inductance matching scheme as shown in  FIG. 3  has the benefit that the shunt inductor  340  is easier to integrate into a substrate and requires less real estate. Furthermore, inductances are less sensitive to manufacture tolerances than delay lines or pi-networks. Also, in establishing a negative rotation angle, the shunt inductance can be usually smaller as compared to a transmission line or pi-network according to the prior art. Finally, the shunt inductor  340  at the antenna port may provide for a perfect DC path to ground and makes the antenna port inherently ESD robust. 
   The difference between a conventional duplexer matching scheme as shown in  FIGS. 1 and 2  to the proposed duplexer as, for example, shown in  FIG. 3 , is that the input impedances of the Tx and Rx filters are designed to become an integral part of the matching circuitry. In the Tx frequency band, the input impedance  360  of the Rx filter  320  forms a parallel resonance with the shunt inductance  340 . On the other hand, in the Rx frequency band, the Tx filter resonates with the shunt inductance  340 . 
     FIG. 4 , shows an embodiment of such an arrangement as shown in  FIG. 3 . The Tx filter path comprises inductor  301 , four BAW resonators  302 ,  303 ,  304 , and  305 , and inductor  306  coupled in series. Between each node of BAW resonators  302 ,  303 ,  303 ,  304 ; and  304 ,  305  and ground a shunt BAW resonator  307 ,  308 ,  309  is coupled in series with a respective shunt inductor  310 ,  311 ,  312 . The inductors  301  and  306  represent bond wire inductances which again depending on the manufacture technology might not be present as indicated by the broken connection line. 
   The Rx filter path comprises inductor  321 , four BAW resonators  322 ,  323 ,  324 , and  325 , and inductor  326  coupled in series. Between each node of BAW resonators  322 ,  323 ;  323 ,  324 ;  324 ,  325 ; and between BAW resonator  325  and inductor  326  one terminal of a shunt BAW resonator  327 ,  328 ,  329  and  330  is coupled. The other terminals of BAW resonators  327  and  328  are coupled with ground via an inductor  331 . The other terminals of ;BAW resonators  329  and  330  are coupled with ground via an inductor  332 . Again, the inductors  321  and  326  represent bond wire inductances. 
   The Tx filter  300  and the Rx filter  320  can each be implemented on a separate filter chip. The duplexer consists then of these filter chips  300 ,  320  and a substrate on which these filters are located. As shown in  FIG. 4 , both filter chips  300 ,  320  comprise additional inductances  310 - 312  and  331 ,  332  in the ground connections. These inductors allow for additional notches in the stopband. Secondly, they help widening the bandwidth of the filters  300 ,  320 . These shunt inductors  310 - 312  and  331 ,  332  may generally be realized as bond wires to a ground plane, a printed inductor on the substrate or may be a combination of both. Similarly, there are inductors  301 ,  306 , and  321 ,  326  in series to each signal connection of the BAW filters  300 ,  320  which represent the wire bonds to connect these filters as stated above. 
   Both filters  300  and  320  start at the antenna side with a series resonator  302  and  322 , respectively. The Rx filter  320  consists out of four shunt and 4 series resonators, i.e. four stages, while the Tx filter  300  in this embodiment comprises three shunt and four series resonators, i.e. 3.5 stages. The number of stages is a trade off between insertion loss/bandwidth versus stopband performance of the filters. Thus, other designs with more or less stages are possible as long as the above described principles for an antenna port shunt inductor matching scheme are met. 
   As an example, the resonance frequencies of the resonators for a Universal Mobile Telecommunications System (UMTS) duplexer can be chosen as: The Tx filter  300  has a pass band from 1920 MHz to 1980 MHz, while the Rx filter shows a pass band from 2110 MHz to 2170 MHz. In one embodiment, with properly chosen capacitances of the resonators, the topology according to the present invention may allow to match the antenna port with a printed inductor  340  of, for example, 3.3 nH. Thereby it is assumed, that the ground inductance of inductor  340  has a parasitic capacitance of 0.5 pF and a Q factor of 20. 
     FIGS. 5-8  illustrate the effect of the shunt inductor on the input impedance (antenna side) of filters  300 ,  320 .  FIGS. 5-8  show on top the relevant filter coupled with the antenna port. Also a proper input load  401  or output load  402  is connected on the respective input or output side of the Tx or Rx filter  300 ,  320 .  FIGS. 5 and 7 , show the Tx filter and Rx filter without the matching shunt inductances whereas  FIGS. 6 and 8  show the same circuits with the implemented matching shunt inductance  340 . On the bottom of each of  FIGS. 5-8  is a Smith chart showing the effect of the shunt inductance as a polar plot of the complex reflection coefficient, which represents the ratio of the complex amplitudes of the backward and forward waves. In both cases, the shunt inductor turns anti-clockwise (“−90° phase shift”) the input impedance from the capacitive into the inductive region. An inductive impedance of the “parasitic” branch can be easier to match in the pass band of a BAW filter since BAW filters tend to have a more capacitive input impedance in their pass bands. In one embodiment, due to the series resonators at the antenna signal ports of the filters, the starting input impedance lies already in the high ohmic capacitive region of the Smith chart. Therefore, the required phase rotation is small, resulting in a reasonable value for the shunt inductance  340 . 
     FIG. 9  shows the overall input impedance of the antenna port of a duplexer as shown, for example, in  FIG. 4 . It is matched with a return loss better than −12 dB in the pass bands of the filters. Achieving this result implies a co-optimization of both filters with the goal to match the antenna port in both pass bands. Finally,  FIG. 10  shows the pass band characteristics of the Rx- and Tx-branch and the isolation between Rx and Tx filters. 
     FIG. 11  shows a schematic embodiment of a substrate  410  with a filter chip  400  arranged on its surface. In this embodiment, bond wire connections  401  and  402  are shown which are used to establish input and output coupling as mentioned with respect to  FIGS. 3 and 4 . 
     FIG. 12  shows another embodiment with a substrate  410  and a flip-chip  420 . The electrical coupling between the flip chip and for example printed circuit board tracks on the substrate  410  is established, for example, by solder balls  403 ,  404  as mentioned with respect to  FIGS. 3 and 4 .