Antenna filtering arrangement for a dual mode radio communication device

A dual mode radio apparatus has an integrated filtering part (51) which includes an antenna port (51a) for connection to an antenna (21), at least one port (51b, 51c, 51d) for connection to each of the system-specific radio-frequency parts (54, 55) of the dual mode radio apparatus and filtering means for directing the propagation of signals between ports on the basis of the signal frequency. The integrated filtering part replaces earlier separate filters and their impedance matching circuits as well as some of the required radio-frequency switches.

The invention relates to the separation of transmission and reception
 generally in radio transceiver devices and particularly in dual mode
 devices which are designed for operation in multiple radio systems.
 BACKGROUND OF THE INVENTION
 The Global System for Mobile Telecommunications (GSM) is currently the most
 widely used one of the operational digital cellular networks. Because of
 network congestion it has been imperative to change the operating
 frequency of the GSM system from the original 900 MHz, approx., to 1.8
 GHz. Cellular networks complying with other standards are also widely used
 around the world. With the mobility of people and communication between
 people increasing, there is a growing need for general-purpose phones that
 operate in different networks according to network availability and/or
 service prices. In dual mode radio telecommunications, the GSM and DECT
 (Digital European Cordless Telephone), for example, or other systems with
 significantly different specifications, can operate as pairs. In dual band
 radio telecommunications, the systems are very much alike (e.g. GSM and
 PCN, Personal Communication Network), but the operating frequency of the
 higher-frequency system is a multiple of the lower-frequency system. The
 dual mode capability is also taken into account in the so-called third
 generation cellular systems (Universal Mobile Telecommunication System,
 UMTS/Future Public Land Mobile Telecommunications System, FLPMTS).
 A dual mode radio communication device has to accommodate the duplexing and
 multiple access methods of the different systems. Duplexing means
 separation of traffic in the transmit direction from the traffic in the
 receive direction in the communication between two transceiver devices.
 Common methods include time division duplexing, TDD, and frequency
 division duplexing, FDD. Multiple access means sharing the capacity of a
 system or its part (a base station, for instance) between several
 terminals (such as mobile phones, for example). Commonly used methods
 include time division multiple access, TDMA, frequency division multiple
 access, FDMA, and code division multiple access, CDMA. In addition, the
 systems employ various multiplexing methods in which one device directs
 the transmitted information from several sources to a common transmission
 channel, separating the signals by means of, say, time division
 multiplexing, TDM, or frequency division multiplexing, FDM.
 A prior art radio apparatus using full time division or frequency division
 duplexing includes several RF and IF filters both on the transmitter side
 and on the receiver side. FIG. 1 shows a prior art GSM radio. In the GSM
 system, transmission and reception are carried out in different time slots
 and at different frequencies. The radio apparatus 100 includes on the
 receiver side a band-pass filter 12 the input port of which is connected
 to an antenna switch 14. The output port of the filter is connected to a
 low-noise amplifier (LNA) 17 which amplifies the received radio signal. It
 is followed by a second band-pass filter 18 which further filters the
 received signal. The output port of the filter 18 is connected to a mixer
 11 in which the received signal is mixed with a first injection signal
 coming from a synthesizer 22. The mixing result, which is an
 intermediate-frequency signal IF, is taken via a filter 24 to a RF circuit
 in the receiver for further processing.
 The transmitter part of the radio 100 includes a second local oscillator
 signal (LO) 26 which is produced by the transmitter pre-stage (not shown)
 and mixed in the mixer 30 with the first injection signal. The output of
 the mixer 30 is taken to a band-pass filter 13 which is normally found
 prior to the transmitter power amplifier 16. The output of the power
 amplifier 16 is connected to the input of a low-pass or band-pass filter
 15 so as to further filter out undesired components in the signal before
 transmitting it via an antenna 21. In between the power amplifier 16 and
 the low-pass filter 15 there is often a directional coupler (not shown)
 which can be used for measuring the power level of the signal brought to
 the antenna.
 FIG. 2 shows a DECT radio according to the prior art. A radio apparatus 200
 includes a band-pass filter 19 the input port of which is connected to an
 antenna switch 14. The output port of the filter is connected to an
 antenna 21. One output port of the antenna switch is connected to a
 low-noise amplifier (LNA) 17 which amplifies the received radio signal. It
 is followed by a second band-pass filter 18 which further filters the
 received signal. The output port of the filter 18 is connected to a mixer
 11 in which the received signal is mixed with a first injection signal
 coming from a synthesizer 22. The mixing result, which is an intermediate
 frequency signal IF, is taken to a RF circuit in the receiver for further
 processing.
 The transmitter part of the radio 200 includes a mixer 30 in which the
 I/Q-modulated transmission signal is mixed with an injection signal. The
 output of the mixer 30 is taken to a band-pass filter 13 which is normally
 found prior to the transmitter power amplifier 16. The output of the power
 amplifier 16 is connected to a second output port of the antenna switch
 14.
 The antenna switch, which connects the antenna alternately to the
 transmitter and receiver branches, is used in a mobile phone to separate
 the signals if the transmission and reception frequencies are the same. If
 the transmission frequency band is different from the reception frequency
 band, the separating unit may be a filter similar to the duplex filter
 used in analog phones. The latter option can also be used in systems
 employing frequency division multiple access. FIG. 3 shows a prior art GSM
 radio 301 which differs from the radio 100 shown in FIG. 1 in that in this
 apparatus 301 the antenna switch (14), band-pass filter (12) and low-pass
 filter (15) are replaced by a duplex filter 20. The rest of the functions
 of these two radios are identical. A duplex filter is a three-port circuit
 element in which there is a receive branch filter between the antenna port
 and the receiver port, and a transmit branch filter between the
 transmitter port and the antenna port. The operating frequencies of the
 filters are such that a transmission-frequency signal cannot enter the
 receiver port and a reception-frequency signal cannot enter the
 transmission port. The frequency characteristics of the filters may be
 adjustable.
 FIG. 4 shows a prior art dual mode GSM/DECT TDD radio 400 wherein both
 systems use a common antenna. In the radio according to FIG. 4 the antenna
 filtering arrangements in both systems are based on antenna switches and
 separate filters. An antenna switch 41 connects the common antenna either
 to the GSM or to the DECT system. When choosing the DECT system, the rest
 of the functions of the radio are, mainly the same as those shown in FIG.
 2 and comprise a band-pass filter 19, a second antenna switch 14b, a
 receiver chain 17b-18b- 11b-24b and a transmitter chain 13b-16b. When the
 GSM system is used the rest of the functions of the radio are mainly the
 same as those shown in FIG. 1 and comprise a receiver chain 12a-17a
 -18a-11a-24a and a transmitter chain 13a-16a-15a as well as a third
 antenna switch 14a which corresponds to the antenna switch 14 shown in
 FIG. 1. A switch 42 on the receive side and a switch 43 on the transmit
 side operate synchronously with the antenna switch 41, connecting the
 radio-frequency parts of either the DECT or the GSM system shown in FIG. 4
 to the common modulation and demodulation parts of the dual mode phone and
 thence to other parts of the radio apparatus.
 Even if a digital mobile phone using frequency duplex had an antenna switch
 to separate transmission and reception, it also must have filters since
 there has to be selectivity in the receiver input and it has to protect a
 low-noise preamplifier. Harmonic multiples of the output frequency and
 other spurious signals such as mirror frequencies have to be attenuated at
 the transmitter output. In addition, the filters eliminate noise generated
 on the receiver band by the transmitter chain. Also the frequencies below
 the transmission band have to be attenuated by a separate filter. In
 systems employing time duplex, such as DECT, or Digital European Cordless
 Telephone, it has to be made sure, in addition to the above, that spurious
 signals generated in the direction of the antenna by the receiver side
 during the transmission of the signal are sufficiently attenuated.
 The standard impedance at interfaces between discrete components and
 filters is 50 ohms. Filter and semiconductor manufacturers match the input
 and output impedances of their products to the standard value in order to
 make modular design easier. In dual mode radio communications, the
 matching of a GSM duplex filter or transmission and reception filters,
 and, on the other hand, the matching of a DECT band-pass filter to a
 common antenna proves problematic. In prior art arrangements, impedance
 matching requires bulky and lossy separate components.
 Thus, the prior art dual mode phone shown in FIG. 4 has to have as much as
 three separate antenna filters (reference designators 12, 19 and 15) and
 the matching circuits required by them. In addition, the construction
 includes all in all five radio-frequency switches. It is obvious that this
 kind of arrangement takes a lot of space on the printed circuit board of
 the radio apparatus and is expensive to manufacture. Furthermore, a high
 number of separate components increases losses and susceptibility of the
 circuit to electrical interference and to electrical or mechanical
 failure.
 SUMMARY OF THE INVENTION
 An object of the present invention is to provide a compact and low-loss
 antenna filtering construction for a dual mode radio communication device.
 Another object of the invention is that the antenna filtering construction
 according to the invention can be used in a digital cellular radio system
 based on time division multiple access. A further object of the invention
 is to raise the integration level of the radio communication device,
 thereby reducing the need for separate components. A yet further object of
 the invention is that the antenna filtering construction according to the
 invention is suitable for large-scale series production.
 The objects of the invention are achieved by combining the separate
 two-port antennna filters of a dual mode radio communication device into
 one multi-port filter in which the matching circuits between different
 filtering parts are part of the filter structure.
 The antenna filtering arrangement according to the invention is
 characterized in that it comprises an integral filtering part for
 connecting system-specific radio-frequency parts to an antenna, the
 integral filtering part comprising
 an antenna port for connection to the antenna,
 at least one port for connection to a first radio-frequency part,
 at least one port for connection to a second radio-frequency part, and
 filtering means to direct the propagation of signal between ports on the
 basis of signal frequency.
 The invention is also directed to a radio communication device which uses
 the antenna filtering arrangement described above. The radio communication
 device according to the invention is characterized in that it comprises an
 integral filtering part for connecting system-specific radio-frequency
 parts to an antenna, the integral filtering part comprising
 an antenna port for connection to the antenna,
 at least one port for connection to a first radio-frequency part,
 at least one port for connection to a second radio-frequency part, and
 filtering means to direct the propagation of signal between ports on the
 basis of signal frequency.
 The invention is based on that the filter design is given more emphasis in
 the design of the whole radio apparatus. A radio-frequency filter can be
 constructed in such a way that it has several signal ports, in which case
 the propagation of signals at different frequencies from one port to
 another depends on the internal connections of the filter and on control
 signals possibly arriving from outside the filter. A single filtering
 part, which is connected through its ports to the antenna and, on the
 other hand, to the transmission and reception chains that the mobile phone
 has for different systems, replaces separate filters and some of the rf
 switches required by the prior art arrangements. Since the filtering part
 according to the invention is one constructional whole, the parts inside
 it need not be limited to 50-ohm interface impedances but the matchings
 can be optimized so that the need for space, losses and manufacturing
 costs remain low. The radio-frequency filter, in the prior art, too, is
 built on a low-loss substrate and inside a shielding metal cover, which
 factors tend to reduce the susceptibility of the integrated structure to
 electrical interference and faults.

DETAILED DESCRIPTION OF THE
 Above, in connection with the discussion about the prior art, we referred
 to FIGS. 1 to 4, so below, in connection with the description of the
 invention and its preferred embodiments, we will mainly refer to FIGS. 5
 to 10. Like elements in the drawings are denoted by like reference
 designators.
 FIG. 5 shows a radio communication device 500 which includes, connected to
 an antenna 21, a so-called triplex filter 51, or a four-port circuit
 element, the ports of which in this embodiment are: an antenna port 51a, a
 DECT port 51b, a GSM reception port 51c and a GSM transmission port 51d.
 The characteristics of a triplex filter depend in a known manner on how
 many resonators it has, how the resonators are interconnected, what
 capacitive and inductive elements it includes in addition to the
 resonators and to which locations in the filter construction the different
 ports are connected.
 If we consider the transfer function (not shown) of filter 51 between the
 antenna port 51a and the DECT port 51b we can see that it behaves
 essentially like a 1.9-GHz band-pass filter, which in a separate DECT
 radio communication device is located between the antenna and the antenna
 switch (cf. reference designator 19 in FIGS. 2 and 4). Between the DECT
 port 51b and the GSM ports 51c and 51d there is a very high attenuation on
 a broad frequency band, so the DECT port 51b can be said to be separated
 from the GSM ports 51c and 51d at all relevant radio frequencies. The
 transfer functions between antenna port 51a and GSM ports 51c and 51d are
 substantially the same as in the known duplex filter of the GSM system,
 denoted by reference designator 20 in FIG. 3. Since the frequency of the
 DECT system (1.9 GHz) is very far from the frequencies of the GSM system
 (900 MHz, approx.), the antenna port can be said to be separated from the
 GSM ports at the DECT frequency and, correspondingly, separated from the
 DECT port at the GSM frequencies.
 The radio communication device 500 according to FIG. 5 comprises a receiver
 chain according to the DECT system, comprising a low-noise amplifier 17b,
 band-pass filter 18b, mixer 11b and band-pass filter 24b, and a
 transmitter chain according to the DECT system, comprising a band-pass
 filter 13b and a power amplifier 16b. An antenna switch 14 alternately
 connects the input of amplifier 17b and the output of amplifier 16b to the
 DECT port 51b of the triplex filter 51. The entity constituted by parts
 according to the DECT system is denoted by reference designator 54 in FIG.
 5.
 In addition, the radio communication device comprises a receiver chain
 according to the GSM system, comprising a low-noise amplifier 17a,
 band-pass filter 18a, mixer 11a and a band-pass filter 24a, and a
 transmitter chain according to the GSM system, comprising a band-pass
 filter 13a and a power amplifier 16a. The input of the low-noise amplifier
 17a is connected to the GSM receiver port 51c of the triplex filter, and
 the output of the power amplifier 16a is connected to the GSM transmitter
 port 51d of the triplex filter. The entity constituted by parts according
 to the GSM system is denoted by reference designator 55 in FIG. 5. A
 radio-frequency switch 42 connects either the output of the band-pass
 filter 24b last in the DECT receiver chain or the output of the band-pass
 filter 24a last in the GSM receiver chain to the other reception parts in
 the radio apparatus, depicted by block 52. A radio-frequency switch 43
 connects the signal coming from the modulator 53 of the radio apparatus
 either to the band-pass filter 13b first in the DECT transmitter chain or
 to the band-pass filter 13a first in the GSM transmitter chain.
 The present invention sets no limitations as to the technology used to
 realize the triplex filter 51. However, considering the relatively high
 frequencies of the DECT and GSM systems, it is probable that of the known
 filter technologies the filter construction based on dielectric
 resonators, as shown in FIG. 6, is the most advantageous one. In that
 construction, cylindrical holes 61 or grooves or other known resonator
 forms, coated with an electrically conductive material, are created on a
 dielectric body block 60 which can be of a ceramic material, for example.
 Also the greater part of the outer surface of the block is made
 electrically conductive so that the inner conductors formed by the coating
 of the resonator forms and the outer conductor formed by the block coating
 make resonators the electrical lengths of which are a half, a quarter or
 other applicable part of the frequency in question. According to an
 advantageous construction, the body block is attached by one of its sides
 to a low-loss substrate board 62 on the surface of which it is possible to
 create transmission lines and soldering pads to which separate components
 63 are connected. Ports for connecting to the antenna and other parts of
 the radio apparatus are advantageously strips extending to the edge of the
 substrate board. It is also possible to create transmission lines and
 soldering pads (not shown) on the surface of the dielectric body block. A
 complete construction is covered by an electrically conductive shield 64
 which prevents the coupling of electrical interference between the filter
 and its surroundings.
 FIG. 7 shows the internal connections of the filtering part according to
 FIG. 6. The resonators 61 are coupled at their so-called open end mainly
 by means of capacitive coupling to a signal line, which between the GSM
 transmission port GSM Tx and the antenna port ANT comprises inductive
 parts and between the antenna port and the DECT port DECT, capacitive
 parts. The GSM reception port GSM Rx is connected to the latter section
 two resonator stages earlier than the DECT port. The coupling arrangement
 shown in the drawing is not meant to be of limiting nature but a person
 skilled in the art, having read this description, can easily provide other
 filter coupling arrangements that realize the desired triplex function.
 FIGS. 8 and 9 show measurement results representing the frequency response
 of the filter depicted in FIG. 7, wherein the horizontal axis represents
 the frequency in megahertzs starting from 820 MHz and ending at 1020 MHz,
 and the vertical axis represents the attenuation in decibels so that the
 horizontal line which has triangles at its ends represents the 0-dB level.
 Curve 81 in FIG. 8 represents the insertion loss and curve 82 represents
 the return loss between the antenna port and the GSM transmission port.
 Curve 91 in FIG. 9 represents the insertion loss and curve 92 represents
 the return loss between the antenna port and the GSM reception port. In
 FIG. 10, the scale of the vertical axis is the same as above but on the
 horizontal axis the frequency starts from 1700 MHz and ends at 2250 MHz.
 Curve 101 in FIG. 10 represents the insertion loss and curve 102
 represents the return loss between the antenna port and the DECT port.
 FIGS. 8 to 10 show that the integrated filtering part realizes the
 required filtering functions at each operating frequency, ie. the
 insertion loss is at its lowest at the desired operating frequency.
 Other filtering methods that are suitable for implementing the multi-port
 filtering part are filters based on helix, strip line or coaxial
 resonators. In these, too, the construction includes a board-like part
 made preferably of a low-loss substrate which steadies the structure and
 serves as an attachment base for separate components and transmission
 lines. In addition, all filter constructions include an electrically
 conductive protective casing.
 Use of the invention is not limited to the GSM and DECT systems but it can
 be applied in all dual mode radio apparatuses in which the operating
 frequencies of the different systems are so much apart that it is possible
 to arrange, using known filter constructions, a sufficient frequency-based
 separation in a single filtering part. If the operating frequencies of the
 systems are the same, the arrangement shown in FIG. 5 is not applicable
 because there will be no adequate separation between the uppermost port
 51b of the triplex filter and the other two ports 51c and 51d on the radio
 apparatus side. The invention does not restrict the operation of the radio
 apparatus to two parallel systems but a single radio apparatus can also
 include three or more parallel radio-frequency parts designed for
 different systems. If all the parallel systems operate at different
 frequencies, the arrangement according to the invention can be applied in
 the antenna filtering.
 There are several known arrangements according to the prior art for
 changing the frequency response of a radio-frequency filter by means of an
 electrical signal. The multi-port filter according to the invention can be
 made adjustable. For example, the duplex part (the GSM part in the
 drawings) of the filter can be replaced by a switchable band-pass filter
 which at a first value of an electrical control signal passes the
 transmission band signal but attenuates the signals at the reception
 frequency, and at a second value of the electrical control signal passes
 the reception band signal but attenuates the signals at the transmission
 frequency.
 The arrangement according to the invention achieves significant reduction
 in the need for space in the radio apparatus as the filters, which
 formerly were separate, are integrated in one assembly having a common
 protective casing and mechanical attachment. Compared to the prior art
 arrangement shown in FIG. 4 the invention dispenses with two
 radio-frequency switches, dropping manufacturing costs and reducing
 losses. Elimination of separate impedance matching circuits brings more
 savings in costs, need for space and losses. Especially filters based on
 dielectric resonators can be mass-produced with a relatively high
 precision and with a good throughput.