United States Patent
`
`[19]
`
`[11] Patent Number:
`
`5,896,562
`
`[45] Date of Patent: Apr. 20, 1999
`Heinonen
`
`
`
`US005896562A
`
`[54] TRANSMITTER/RECEIVER FOR
`TRANSMITTING AND RECEIVING OF AN
`RF SIGNAL IN TWO FREQUENCY BANDS
`
`[75]
`
`Inventor:
`
`Jarmo Heinonen, Salo, Finland
`
`[73] Assignee: Nokia Mobile Phones, Ltd., Salo,
`Finland
`
`FOREIGN PALFENT DOCUMENTS
`
`0581573 A1
`0594894 Al
`0653851 A3
`91819
`2279519
`
`2/1994
`European Pat. OE. .
`5/1994
`European Pat. Oif. .
`5/1995
`European Pat. Of. .
`Finland .
`4/1994
`1/1995 United Kingdom .
`OI‘HER PUBLICATIONS
`
`[21] Appl. No.2 as/327,323
`
`[22] Filed:
`
`Mar. 26, 1997
`
`[30]
`
`Foreign Application Priority Data
`
`Apr. 1,1996
`
`[FI]
`
`Finland
`
`............................ 961465
`
`Int. Ci.‘ ....................................................... 1104]} 1/50
`[51]
`[52] U.S. Cl. ................................. 455fl6; 455/86; 455/78;
`455/83; 455/553
`[58] Field of Search .................................. 455/76, 78, 82,
`455/83, 73, 88, 180.1, 183.1. 188.1, 189.1,
`190.1. 324, §53', 375/219, 84, 86
`
`[56]
`
`References Cited
`U.S. PATEN'I‘ DOCUMENTS
`
`3/1982 Dimon .................................... 455/203
`4,320,531
`4,965,852 10/1990 Sasaki
`455/82
`5,091,919
`2/1992 Kuisma .
`375/60
`5,123,031
`6/1992 Kuisma .
`........ 375/60
`5,448,762
`9/1995 Ward
`455/67.1
`5,564,076 10/1996 Auvray ..
`........ 455/76
`5,610,559
`3/1997 Dent ..................
`455fl6
`5,715,525
`2/1998 Tarusawa et al.
`.. 455/101
`.
`5,722,053
`2/1998 Kornfeld et al.
`455/86
`5,734,970
`3/1998 Saito ......................................... 455/76
`
`Microwave Engineering Europe, Jan. 1993. pp. 59,60,63.
`Microwave Engineering Europe, May 1993, pp. 53,54,57,
`58.
`
`Primary Examiner—Nguyen Vo
`Assistant Examiner—-Duc Nguyen
`Attomey, Agent, or Fimt—Perman & Green, LLP
`
`,_
`
`[57]
`
`ABSTRACT
`
`The object of the invention is a transmitter/receiver for
`transmitting and receiving of an RF signal in two frequency
`bands. In the solution according to the invention, a
`transmitter/receiver is used which is based on direct con-
`version and in it, when operating in both frequency bands,
`the mixing frequency is formed by means of the same
`synthesizer (340). This is implemented preferably in such a
`way that in the higher operating frequency, the frequency of
`the output signal of the synthesizer is used as such as a
`mixing frequency and when operating in the lower operating
`frequency, a mixing frequency is used which is obtained by
`dividing the frequency of the output signal of the synthesizer
`by two or a larger integer in the divider (311, 361). When the
`solution according to the invention is used, many synthe-
`sizers are not needed in the transmitter/receiver and no
`intermediate frequency components are needed.
`
`11 Claims, 4 Drawing Sheets
`
`
`
`
`TION OF
`OFFSET
`VOLTAGE
`
`CONTROL OF
`A SYNTHESlZER
`
`
`
`BASEBAND
`
`PROCESSING
`OF A TX
`SIGNAL
`DCS/GSM I
`
`__
`SELECT|0iu_ __<
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`
`
`
`APPLE 1004
`
`1
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`APPLE 1004
`
`

`
`U.S. Patent
`
`Apr. 20, 1999
`
`Sheet 1 of 4
`
`5,896,562
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`U.S. Pafent
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`Apr. 20, 1999
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`Sheet 4 of 4
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`5,896,562
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`OUTPUT
`
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`

`
`1
`TRANSMITTER/RECEIVER FOR
`TRANSMITTING AND RECEIVING OF AN
`RF SIGNAL IN TWO FREQUENCY BANDS
`
`BACKGROUND OF THE INVENTION
`
`The object of the invention is a transmitter/receiver for
`transmitting and receiving an RF signal in two operating
`frequency bands.
`Mobfle station systems have developed and expanded at
`an extremely rapid rate, which is the reason why a variety of
`systems using many diiferent standards have been or are
`being constructed in many areas. This has generated the
`need for mobile stations which can use more than one
`
`system. As an example one could mention the digital GSM
`system and DCS that is PCN system which operate in
`different frequency bands but whose specifications are oth-
`erwise similar to each other.
`
`From the published patent application EP 653851, a
`transmitter/receiver arrangement is lmown in which one
`local oscillator is used and its frequency has been selected
`between the lower operating frequency band and the higher
`operating frequency band in such a way that the same
`intermediate frequency can be used when operating in both
`operating frequency bands. The weak point of this solution
`is, however, that due to the need for these intermediate
`frequency stages, the implementation is extremely compli-
`cated and due to the great amount of components it requires,
`the manufacturing costs of such a device are high.
`In a direct conversion receiver. that is in a zero interme-
`diate frequency receiver, a radio frequency signal is con-
`verted directly to a baseband without
`there being any
`intermediate frequency. Since no intermediate frequency
`stages are needed, only ‘a few components are needed in the
`receiver, which makes it a preferable solution for various
`applications. However. in mobile stations, direct conversion
`receivers have so far rarely been used.
`FIG. 1 shows a prior known schematic block diagram of
`a transmitter/receiver of a mobile station and in this block
`diagram the receiver is a so-called direct conversion
`receiver. An RF signal received by an antenna 138 is
`conducted via a duplex filter 102 to a pre-arnpljfier 104. The
`purpose of the duplex filter is to permit the use of the same
`antenna both in transmitting and receiving. Instead of a
`duplex filter, also a synchronous antenna changeover switch
`can be used in a time-division system. The RF signal which
`is received from the amplifier 104 is low-pass pass or band
`pass filtered 106 and demodulated in an I/Q demodulator
`. 108 into an in-phase signal 108a and into a quadrature signal
`108b. Alocal oscillator signal 114b which is needed in the
`demodulation is received from a synthesizer 114. In block
`110. removal of dc-voltage as well as automatic gain control
`AGC are carried out. Block 110 is controlled by a processing
`block 116 which may contain, for example. a microproces-
`sor and/or a digital signal processor DSP. Automatic gain
`control is regulated by a signal 110a and removal of the
`offset voltage is regulated by a signal 11%. Signals received
`from block 110 are converted into digital signals in block
`112 from which the signals are further transferred to digital
`signal processing circuits in the processing block 116.
`The transmitter unit comprises an I/Q modulator 128. This
`takes an in-phase signal 128a and a quadrature signal 128b
`and creates a carrier frequency signal which is low-pass
`filtered andlor high-pass filtered by a filter 130. The carrier
`frequency signal is amplified by an RF amplifier 132 and the
`amplified signal is transferred via a duplex filter 102 to an
`antenna 138. A power control unit 134 of the transmitter
`
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`controls the amplification of the RF amplifier 132 on the
`basis of the measured output power 136 and of the control
`1340 received from the processor.
`FIG. 1 also shows. attached to the processing unit, a
`memory unit 126 and user interface means which comprise
`a display 118, a keyboard 120, a microphone 122 and an
`earpiece 124.
`Practical solutions for the implementation of a direct
`conversion receiver have been described more closely, for
`example. in the following publications:
`[1] Microwave Engineering Europe, January 1993, pages
`59 .
`.
`. 63,
`
`[2] Microwave Engineering Europe, May 1993, pages
`53 .
`.
`. 59 and
`
`[3] published patent application EP 0 594 894 AI.
`FIG. 2 shows a solution for the implementation of a
`transmitter/receiver which operates in two frequency bands.
`An RF signal received by the antenna is connected either to
`the DCS branch or to the GSM branch of the circuit via a
`switch 204. If a DCS frequency band signal
`is being
`received, the received signal is conducted to a band pass
`filter 206, to a low noise amplifier LNA 208 and to a band
`pass filter 210. Thereafter, components which are separated
`by a phase shift of 90 degrees are formed from the signal in
`block 212. The in-phase component I and the quadrature
`component Q are conducted further by switches 214 and 234
`to mixers 216 and 236. A mixing signal for the mixers is
`obtained from a DCS synthesizer 240, the frequency of
`which corresponds to the received carrier frequency and
`then an in-phase and a quadrature component of a complex
`baseband signal are obtained as a result of this mixing
`process. The baseband signal is processed further in a
`processing unit of a received signal, which means an RX '
`signal, block 239.
`Similarly, when a GSM signal is being received. the
`switch 204 controls the received signal to the GSM branch
`in which there are, respectively connected in series, a band
`pass filter 226, a low noise amplifier 228. a band pass filter
`230 and a phase shifter 232 which forms two signals which
`are separated by a phase difference of 90 degrees. The
`signals are conducted f1n1her. controlled by the switches 214
`and 234. to the mixers 216 and 236 - in which a signal
`selected by a switch 261 and obtained from a GSM synthe-
`sizer 250 is now used as mixing frequency. Signals obtained
`from the mixers are conducted further to tire processing unit
`239 of a baseband received signal, which means an RX
`signal.
`The DCS synthesizer is formed, as known. from a phase
`locked loop PLL which comprises a voltage controlled
`oscillator VCO 241. the output signal of which is amplified
`by an amplifier 246 for forming an output signal. The
`frequency of a signal transmitted by the oscillator 241 is
`divided by an integer Y in a divider 242 and the resulting
`signal is conducted to a phase comparator 243. Similarly, the
`frequency of the signal formed by a reference oscillator 258
`is divided by an integer X in a divider 244 and conducted to
`the phase comparator 243. The phase comparator produces
`a signal which is proportional to the phase diflerence of said
`two input signals and which has been conducted to a low
`pass filter LPF 245, and the filtered signal controls further
`the voltage controlled oscillator 241. The above described
`phase locked loop operates in a known manner so that the
`output frequency of the synthesizer becomes locked to the
`frequency which is led to the phase comparator from the
`reference frequency branch. The output frequency is con-
`trolled by changing the dividing number Y.
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`The GSM synthesizer 250 comprises respectively a volt-
`age controlled oscillator 250, an amplifier 256, dividers 252
`and 254, a phase comparator 253 and a low pass filter 255.
`The GSM synthesizer operates in a similar way as the above
`described DCS synthesizer but the output frequency of the
`GSM synthesizer corresponds to GSM frequency bands.
`In the transmitting unit. the baseband complex transmit-
`ting signal, which means the TX signal, is processed in the
`processing unit of a TX signal and from there the in-phase
`and the quadrature component of the signal are conducted to
`mixers 262 and 282 in which a carrier frequency signal is
`formed by multiplying the input signal by the mixing signal.
`Ifthe DCS frequency is used in the transmission, the output
`signal of the DCS synthesizer is selected via a switch 261 as
`a mixing signal. The carrier frequency signal is conducted
`via a switch 264 to the DCS branch in which a phase shift
`"of 90 degrees is formed first between the in-phase compo-
`nent and the quadrature component, and after this,
`the
`received signals are summed, block 266. The formed DCS
`signal is conducted to a band pass filter 268, to an amplifia
`270 and to a band pass filter 272. The formed RF signal is
`conducted further to an antenna 202 via a switch 280.
`If the transmission takes place in the GSM frequency
`band, the output signal of the GSM synthesizer is used as the
`mixing signal. The received carrier frequency signal
`is
`conducted to the GSM branch in which a similar processing
`occurs as in the DCS branch in blocks 286, 288, 290 and
`292. The formed RF signal is conduded to the antenna 202
`via the switch 280. To permit the use of the same antenna
`202 both in transmitting and in receiving, the transmitting
`and the receiving circuits have to be connected to the
`antenna, for example, via a Duplex filter as in the arrange-
`ment shown in FIG. 1. When operating in two frequency
`bands, filters are needed for each frequency band. Instead of
`the Duplex filter, also a synchronized antenna changeover
`switch can be used in a time-division system.
`One disadvantage of the above described circuit arrange-
`ment is that it requires the use of two synthesizers, which
`increases considerably the complexity and the manufactur-
`ing costs of the transmitter/receiver.
`Another problem connected to the above presented solu-
`tion is achieving an adequate phase accuracy. The accuracy
`demand for the phase difference between the I and the Q
`components is only of a few degrees’ magnitude. Since in
`conventional RC phase shifters, factors on which the phase
`shift depends include the frequency and the temperature of
`the components. it is difficult to achieve an adequate phase
`accuracy throughout the entire frequency band and in all
`operating conditions. In addition, operating in two frequency
`bands which are far from each other complicates the con-
`trolling of the phase accuracy.
`One solution is to form signals in dilferent phases of a
`higher oscillator frequency by dividing the signals in which
`case a better phase accuracy is achieved which is indepen-
`dent on the frequency. The disadvantage of this solufion is,
`however. that when operating, for example. in the 2 GHz
`frequency band. one would need a synthesizer with an
`output frequency of 4 GHz which is such a high frequency
`value that the implementation of the synthesizer and the
`frequency dividers would become extremely complicated.
`SUMMARY OF THE INVENTION
`
`The aim of the invention is to devise a simple solution for
`the implementation of a transmitter/receiver which operates
`in two frequency bands so that the above presented disad-
`vantages connected to the soluflons according to the prior art
`can be avoided.
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`One idea of the invention is to use a transmitter/receiver
`which is based on direct conversion and in which the mixing
`frequency is formed by means of the same synthesizer when
`operating in two frequency bands. This is implemented
`preferably in such a way that in the higher, first operating
`frequency band, the frequency of the output signal of the
`synthesizer is used as such as a mixing frequency and when
`operating in the lower. second operating frequency band, a
`mixing frequency is used which is obtained by dividing the
`frequency of the output signal of the synthesizer by at least
`two. When operating in the lower frequency band. two
`mixing signals which are separated by a phase difference of
`90 degrees can be formed in the context of the dividing of
`the synthesizer frequency in which case no RC phase
`shifters in the signal line are needed and an excellent phase
`accuracy is achieved.
`A direct conversion transmitter/receiver according to the
`invention which operates in two separate frequency bands
`and in which the first frequency band comprises the first
`transmitting frequency band and the first receiving fie-
`quency band and the second frequency band comprises the
`second transmitting frequency band and the second receiv-
`ing frequency band, and in which
`said receiver comprises at least one RX mixer for mixing
`the received signal into a baseband signal,
`said transmitter comprises at least one TX mixer for
`mixing the baseband signal into a carrier frequency
`transmitting signal and
`the transmitter/receiver comprises synthesizer means for
`forming the first PX mixing signal to the RX mixer for
`mixing the signal which has been received in the first
`receiving frequency band into a baseband signal and for
`forming the first'I'X mixing signal to the TX mixer for
`mixing the first basebandTX signal into the first carrier
`frequency TX signal in the first transmitting frequency
`band, is characterized in that the transmitter/receiva
`comprises additionally
`the first conversion means for forming the second RX
`mixing signal from the first signal formed by said
`synthesizer means for mixing the signal which has been
`received in the second receiving frequency band to the
`second baseband RX signal and
`the second conversion means for forming the second TX
`mixing signal from the second signal formed by said
`synthesizer means for mixing the second baseband TX
`signal into the second carrier frequency signal in the
`second transmitting frequency band.
`Preferable embodiments of the invention have been pre-
`sented in the dependent claims.
`BRIEF DESCRIP'I'ION OF THE DRAWINGS
`
`The invention is described in the following in more detail
`by means of the attached drawings in which
`FIG. 1 shows a block diagram of a transmitter/receiver
`which is based on direct conversion,
`FIG. 2 shows a block d.iagra.n1 of a novel solution for the
`implementation of a transmitter/receiver which operates in
`two frequency bands,
`FIG. 3 shows a block diagram of a solution according to
`the invention for the implementation of a transrnitterl
`receiver which operates in two frequency bands,
`FIG. 4 shows a circuit diagram of a solution for forming
`signals of ditferent phases in RC circuits via differential
`signals,
`FIG. 5 shows a circuit diagram of a solution for forming
`signals of ditferent phases in RC circuits via signals which
`have one terminal connected to the ground level and
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`FIG. 6 shows a circuit diagram of a solution for forming
`signals of dilferent phases at dividers.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`FIGS. 1 and 2 were already described above. In the
`following. a transrnitter/receiver according to the invention
`is described by means of FIG. 3. Finally, some possible ways
`of implementation for forming signals of different phases in
`a transmitter/receiver according to the invention are
`described by referring to FIGS. 4-6.
`FIG. 3 shows a block diagram of a transmitterlreceiver
`according to the invention. In it, an RF signal received by an
`antenna is connected either to the DCS branch of the circuit
`or to the GSM branch via a switch 304. If a DCS frequency
`band signal is received, the received signal is conducted to
`a band pass filter 306 of the DCS branch, to a low noise
`amplifier LNA 308 and to a band pass filter 310. After this.
`components which are separated by a phase shift of 90
`degrees are formed from the signal in block 312. The
`in-phase component I and the quadrature component Q are
`conducted further via switches 314 and 334 to mixers 316
`and 336. For the parts described above, the circuit arrange-
`ment corresponds to the circuit arrangement shown in FIG.
`2.
`
`A mixing signal for the mixas is obtained from a syn-
`thesizer 340. the frequency of which corresponds to the
`received carrier frequency and then, as a mixing result, an
`in-phase and a quadrature component of a complex base-
`band signal are obtained. The baseband signal is conducted
`further to an automatic gain control block AGC 337 and to
`an offset voltage correcting block 338. After this, the signal
`is processed further in a baseband processing unit of a
`received signal, which means an RX signal, block 339.
`When a GSM signal is being received, the switch 304
`controls the received signal to the GSM branch in which
`there are respectively conneaed in series a band pass filter
`326. a low noise amplifier 328 and a band pass filter 330.
`Thereafter, the signal is conducted cophasal to mixers 316
`and 336. The signal which is received from the synthesizer
`is now selected via switches 315 and 335 as a mixing
`frequency, and the frequency of the signal has been divided
`by two in block 311. In block 311, signals which are
`separated by a phase shift of 90 degrees are formed out of
`the signal to mixers 316 and 336. Thus the phase shift of 90
`degrees which is needed in the mixing is not carried out to
`the received signal but rather to the mixing signal. A
`baseband complex signal which is received from the mixers
`is conducted further to a processing unit 339 of a baseband.
`received signal that is an RX signal.
`The synthesizer 340 operates in a similar way as the DCS
`synthesizer shown in FIG. 2. It comprises thus a voltage
`controlled oscillator VCO 341, the output signal of which is
`amplified by an amplifier 346 to form an output signal. The
`frequency of the signal produced by an oscillator 31 is
`divided by an integer Y in a divider 342 and the resulting
`signal has been conducted to a phase comparator 343.
`Similarly, the frequency of a signal formed by a reference
`oscillator 358 is divided by an integer X in a divider 344 and
`is conducted to the phase comparator 343. The phase com-
`parator produces a signal which is proportional to the phase
`diflerence of said two input signals and which has been
`conducted to a low pass filter 345, and the filtered signal
`controls further the voltage controlled oscillator 341. The
`output frequency is controlled by changing the dividing
`number Y.
`
`In the transmitting unit, the baseband complex transmit-
`ting signal, which means the TX signal, is processed in the
`processing unit 360 of a TX signal and from there, the
`complex components of the signal are conducted to mixers
`362 and 382 in which a carrier frequency signal is formed by
`multiplying the input signal by a mixing signal. If the DCS
`frequency is used in the transmitting, the output signal of the
`synthesizer 340 is selected as a mixing signal via switches
`363 and 383. The formed DCS signal is conducted to a band
`pass filter 368. to an amplifier 370 and to a band pass filter
`372. The formed RF signal is conducted further to an
`antenna 302 via a switch 380.
`
`If the transmission occurs in the GSM frequency band, the
`mixing signal is fonned by dividing the frequency of the
`output signal of the synthesizer 340 by two in the divider
`361 from which mixing signals are obtained which are
`separated by a phase shift of 90 degrees to the firstTX mixer
`362 and to the second TX mixer 382. The carrier frequency
`signal is conducted via switches 364 and 384 to the GSM
`branch in which the in-phase component and the quadrature
`component which have been received from the mixers 362
`and 382 are summed together, block 386. After this, filtering
`and amplification proceed in blocks 388, 390 and 392. The
`formed RF signal is conducted to the antenna 302 via the
`switch 380. In the GSM frequency, the phase shift of 90
`degrees is thus processed to the mixing signal and not to the
`carrier frequency signal which has been obtained as a
`mixing result.
`It should be noted that when operating in the GSM
`frequency band, the mixing signals of the receiving and the
`transmitting can be formed by means of the same frequency
`divider. Then the outputs of different phases of the frequency
`divider can be connected either to the RX mixers of the
`receiver during the receiving or to the TX mixers of the
`transmitter during the transmitting, for example, by using
`controllable switches. In this case, the switches are con-
`trolled by a signal which is in the first state during the time
`slot of the receiving and in the second state during the time
`slot when it is transmitting. Another option, when one
`divider is used, is to control the signals received from the
`divider to the mixers of both the transmitter and the receivers
`during both the transmitting and the receiving. In this case,
`the signals can be routed to said mixers by using a dividing
`means such as a power divider.
`'
`In addition. it is to be noted that instead of the described
`dividers, also other conversion means can be used for
`forming the second RX mixing frequency and the secondTX
`mixing frequency from the signal formed by the synthesizer
`when operating in the second frequency band. The conver-
`sion function of the frequency can thus also be other than
`division by two, depending, for example, on which operat-
`ing frequency bands are in use.
`Change-over switches 314, 334, 315, 335. 363, 383. 364
`and 384 are controlled most preferably by a two-level signal
`BC (Band Control). In the first level of the switch of the
`control signal the change-over switches are in a position in
`which high frequency circuits of the first frequency band are
`used and in the second level of the control signal the
`change-over switches are in a position in which high fre-
`quency circuits of the second frequency band are used. The
`values corresponding to the first and the second level of the
`control signal BC depend, among others, on the irnplemen—
`tation of the change-over switches.
`For the change-over switches 314. 334, 315, 335. 363,
`383. 364 and 384, also some other known method can be
`applied for controlling the path of the high frequency signal.
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`The change-over switches can be substituted, for example,
`by adapter elements which are known per se, in which case
`when the first frequency band is used, the high frequency
`circuits of the second frequency band are of high impedance
`to the signals of the first frequency band. Similarly, when the
`second frequency band is used, the high frequency circuits
`of the first frequency band are of high impedance to the
`signals of the second frequency band. In this case, the high
`' frequency circuits of different frequency bands do not cause
`mutual interference.
`
`The signal BC which controls the change-over switches is
`formed most preferably in the processing block 116 of a
`mobile station, FIG. 1, which comprises preferably a
`processor, such as a microprocessor. The processing block
`116 forms a signal on the basis of a change-over command
`of the system which the user has fed by using the keyboard
`120, FIG. 1. The selection of the system can be, for example,
`menu-based in which case the desired system is chosen by
`a particular keystroke from the menu shown on the display
`118. Then the processing block 116 forms a control signal
`BC which corresponds to the selected system. The change-
`over command of the system can also be transmitted via the
`mobile station system in which case the mobile station
`receives data which has been transmitted by another system.
`The received data may include a system change-over
`command, and on the basis of it, the processing block
`changes the system. Into a memory unit 126 which is
`attached to the processing block and which comprises pref-
`erably an HROM or EEPROM memory, a control program
`has been stored which monitors the received data and as it
`detects a system changerover command in the data, it
`transmits a command to the processing block to convert the
`control signal BC into the state according to the selecting
`command.
`-
`
`The processing block forms additionally a control signal
`of a synthesizer and with this signal, a dividing number is
`given to the divider 342 (FIG. 3) of the frequency
`synthesizer, and this dividing number corresponds to the
`given channel frequency. Then the divider 342 of the syn-
`thesizer forms from the frequency of a voltagecontrolled
`oscillator VCO, 341, a phase comparison frequency to a
`phase comparator 343. For example, in the GSM system, the
`channel spacing is 200 kHz in which case 200 kHz is used
`as a phase comparison frequency.
`In the solution shown in FIG. 3, the first frequency band
`comprises the frequency band of the DCS system and the
`second frequency band comprises the frequency band of the
`GSM system Then the band pass range of the band pass
`filters 306 and 310 of the receiving branch of the first
`frequency band is approximately 1805-1880 MHZ. The
`band pass range of the band pass filters 326 and 328 of the
`receiving branch of the second frequency band is approxi-
`mately 925-960 MHz. Correspondingly. in the transmitter,
`the band pass range of the band pass filters 368 and 372 of
`the first frequency band is approximately 1710-1785 MHz
`and the band pass range of the band pass filters of the second
`frequency band is approximately 880-915 MHz.
`One farther possible method for selecting the signal
`branch is to switch off the supply voltages from that branch
`which is not in use. This method can be applied both to the
`transmitter and to the receiver. The advantage of this alter-
`native is that adual selecting switches are not necessarily
`needed.
`
`FIG. 4 shows a circuit diagram of a solution for forming
`signals of different phases in the DCS receiving branch. In
`the circuit, differential signals are used, that is neither of the
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`signal taminals is connected to the ground level. This circuit
`can be used as block 312 in the circuit arrangement of FIG.
`3. In the circuit arrangement, the differential input signal
`comprises signal lines of 0° and —180° and for each of them
`a phase shift of —45° and a phase shift of +45° will be
`processed for forming two signals which are separated by a
`90° phase shift. Phase shifters of —45° comprise resistors
`401 and 402 and capacitors 403 and 404. Respectively,
`phase shifters of +45° comprise capacitors 405 and 406 as
`well as resistors 407 and 408. The advantage of this solution
`is that the implementation of integrated circuits is often
`economical when differential signals are used.
`FIG, 5 shows a circuit diagram of a solution for forming
`signals which are separated by a 90° phase shift in an RC
`phase shift circuit in which one of the signal conductors is
`connected to the ground level. In it, the —45° phase shift is
`performed by a phase shifter which is formed of a resistor
`S11 and a capacitor 512 and the +45° phase shift is formed
`by a phase shifter which is formed of a capacitor 513 and a
`resistor 514.
`
`Furthermore, it is to be noted that circuits according to
`FIGS. 4 and 5 can be used, on the basis of symmetry, also
`as block 366 to perform the phase shift of 90 degrees
`between the input signals and to sum up the received signals.
`Then the signals move in opposite direction in the circuits.
`that is the input signals are fed to the right-hand terminals of
`the circuits shown in the figures and the output signal is
`obtained from the left-hand terminals.
`
`FIG. 6 shows a phase shifter which divides the input
`frequency by two and which can be used in the implemen-
`tation of blocks 311 and 361. The circuit comprises two
`dividers 601 and 602 in which case the input signal is fed
`into the non-inverting input of the first divider 601 and into
`the inverting input of the second divider 602. This is how
`output signals which are separated by a phase difference of
`90 degrees are formed, as kpown.
`Mixers 362 and 382 have been shown individually in the
`block diagram shown in FIG. 3 but in practice they can be
`produced by integration to the same circuit in which case
`two GSM transmitting signals can be summed up. for
`example, in a common collector resistance of a prior known
`Gilbert Cell type mixer, and his collector resistance thus
`functions as a summer 386.
`
`Another altanative is to have the summing up processed
`in a switching circuit which connects the signal after the
`mixers to the GSM or to the DCS branch. One preferable
`method is the implementation of the switches by using
`parallel transistor stages in which case the selection of
`signals proceeds. for example, by switching the supply
`voltage to that transistor stage through which one wishes the
`signal to pass and by switching the supply voltage off from
`that stage which one wishes to remain open. These same
`transistor stages can be used for summing up the signals.
`A third method is to achieve the summing up by using the
`method shown by FIG. 3 in a separate summer which has
`been connected to the transmitter chain after the mixer and
`the GSM/DCS selecting switches.
`By means of the solution according to the invention, it is
`possible to implement a transmitter/receiver which is con-
`siderably simpler and has lower manufacturing costs com-
`pared to the solutions according to the prior art. In the circuit
`arrangement according to the invention, only one synthe-
`sizer is needed and absolutely no intermediate frequency
`components, such as expensive intermediate frequency
`filters, are needed Thus it is easy to integrate the circuit.
`Since no intennediate frequencies are used in the circuit, the
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`circuit does not either cause intermediate frequency inter-
`ference or become perturbed by po