Ulllted States Patent
`
`[19]
`
`[11] Patent Number:
`
`5,983,081
`
`Lehtinen
`
`[45] Date of Patent:
`
`Nov. 9, 1999
`
`US005983081A
`
`[54] METHOD FOR GENERATING
`FREQUENCIES IN A DIRECT CONVERSION
`TRANSCEIVER OFA DUAL BAND RADIO
`COMMUNICATION SYSTEM, A DIRECT
`CONVERSION TRANSCEIVER OF A DUAL
`BAND RADIO COMMUNICATION SYSTEM
`AND THE USE OF THIS METHOD AND
`APPARATUS IN A MOBILE STATION
`
`[75]
`
`Inventor: Kari T. Lehtinen, Salo, Finland
`
`[73] Assignee: Nokia Mobile Phones, Ltd., Salo,
`Finland
`
`[21] Appl. No.: 08/823,997
`
`[22]
`
`Filed:
`
`Mar. 25, 1997
`
`[30]
`
`Foreign Application Priority Data
`Finland .................................. .. 96 1428
`
`Mar. 29, 1996
`
`[F1]
`
`Int. Cl.6 ..................................................... .. H04B 1/40
`[51]
`[52] U.S. Cl.
`......................... .. 455/76; 455/86; 455/179.1;
`455/188.1; 455/260
`[58] Field of Search ................................ .. 455/76, 84, 86,
`455/552, 553, 179.1, 188.1, 260, 264, 118
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`6/1992 Kuisma ................................... .. 375/60
`5,123,031
`5/1996 Vaisanen ................................. .. 455/76
`5,519,885
`
`10/1996 Auvray . . . . .
`. . . . .. 455/76
`5,564,076
`
`5,584,068 12/1996 Mohindra
`455/324
`5,790,587
`8/1998 Smith etal.
`.......................... .. 455/305
`FOREIGN PATENT DOCUMENTS
`
`0631400 12/1994
`0631400 A1
`12/1994
`81933
`8/1990
`92636
`8/1994
`4338721
`5/1995
`4338721 A1
`5/1995
`WO 92/16078
`9/1992
`
`.
`.
`
`European Pat. Off.
`European Pat. Off.
`Finland .
`Finland .
`Germany .
`Germany .
`WIPO .
`
`Primary Examiner—Nguyen Vo
`Assistant Examiner—Greta J. Fuller
`
`Attorney, Agent, or Firm—Perrnan & Green, LLP
`
`[57]
`
`ABSTRACT
`
`The invention relates to a method for generating frequencies
`in a direct conversion transceiver of a radio communication
`
`system operating in two different frequency bands. In this
`method a first frequency band comprises a first transmission
`frequency band and a first reception frequency band, and a
`second frequency band comprises a second transmission
`frequency band and a second reception frequency band. The
`frequencies are Generated by using one frequency synthe-
`sizer (12) and a reference oscillator (14), which generates an
`essentially constant mixing frequency (L04).
`
`5,091,919
`
`2/1992 Kuisma ................................... .. 375/60
`
`20 Claims, 7 Drawing Sheets
`
`APPLE 1005
`
`1
`
`APPLE 1005
`
`

`
`U.S. Patent
`
`Nov. 9, 1999
`
`Sheet 1 of7
`
`5,983,081
`
`|—RX
`
`Q—RX
`
`PCN
`
`1710-1880
`
`
`
`GSM
`
`880-960
`
`|——TX
`
`Q—TX
`
`FIG.
`
`W
`
`(PRIOR ART)
`
`2
`
`

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`U.S. Patent
`
`Nov. 9, 1999
`
`Sheet 2 of7
`
`5,983,081
`
`SYSSEL
`
`——————————————————————————
`
`3
`
`

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`U.S. Patent
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`Nov. 9, 1999
`
`Sheet 3 of7
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`5,983,081
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`4
`
`

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`U.S. Patent
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`Nov. 9, 1999
`
`Sheet 4 of 7
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`5,983,081
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`5
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`

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`U.S. Patent
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`Nov. 9, 1999
`
`Sheet 5 of7
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`5,983,081
`
` BASEBAND
`
`PROCESSING
`
`33
`
`MEMORY
`
`6
`
`

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`U.S. Patent
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`Nov. 9, 1999
`
`Sheet 6 of7
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`5,983,081
`
`_
`
`Q Q
`0.0-
`
`I-RX
`
`7
`
`

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`U.S. Patent
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`Nov. 9, 1999
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`Sheet 7 of 7
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`5,983,081
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`
`
`'
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`Q
`I-RX
`.A_ X Q-RX
`
`L01
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`F1!
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`28
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`8
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`

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`5,983,081
`
`1
`METHOD FOR GENERATING
`FREQUENCIES IN A DIRECT CONVERSION
`TRANSCEIVER OF A DUAL BAND RADIO
`COMMUNICATION SYSTEM, A DIRECT
`CONVERSION TRANSCEIVER OF A DUAL
`BAND RADIO COMMUNICATION SYSTEM
`AND THE USE OF THIS METHOD AND
`APPARATUS IN A MOBILE STATION
`
`BACKGROUND OF THE INVENTION
`
`The present invention relates to a method for generating
`frequencies in a direct conversion transceiver of a radio
`communication system operating in two different frequency
`bands, wherein a first frequency band comprises a first
`transmission frequency band and a first receiving frequency
`band, and a second frequency band comprises a second
`transmission frequency band and a second receiving fre-
`quency band. The invention also relates to a direct conver-
`sion transceiver of a radio communication system operating
`in two different frequency bands. In addition, the invention
`relates to the use of the method and the direct conversion
`transceiver in a mobile station.
`It is a known method to use a reference oscillator and one
`
`or more frequency synthesizers to generate local oscillator
`frequencies for the transmitter and receiver. For practical
`reasons, the frequency of the reference oscillator is consid-
`erably lower than the local oscillator frequencies. As is
`generally known and as shown in FIG. 2, the frequency
`synthesizer consists of a phase locked loop (PLL), by which
`the output frequency of the VCO is locked at the reference
`frequency. In this loop,
`the reference frequency and the
`frequency of the voltage controlled oscillator (VCO) are
`taken as divided to a phase comparator, the filtered output
`voltage of which is the control voltage of the voltage
`controlled oscillator VCO. The control voltage controls the
`oscillator in a manner such that its frequency is locked at the
`frequency coming from the reference frequency branch to
`the phase comparator.
`In a direct conversion receiver, the local oscillator fre-
`quency is generated at
`the received channel frequency,
`whereby the received baseband signal is obtained directly as
`the difference between the received radio frequency signal
`and the local oscillator frequency. Similarly,
`in a direct
`conversion transmitter the local oscillator frequency is tuned
`at the transmission channel, whereby in addition to the local
`oscillator frequency, the modulating signal is also directed to
`the mixer. The mixing result includes a modulated signal at
`the transmission frequency, which is directed through the
`normal filter and amplifier stages to the antenna. A direct
`conversion transceiver is of a simple construction; particu-
`larly the number of radio frequency blocks is smaller than in
`the ordinary transceivers which include intermediate fre-
`quency stages.
`the local oscillator
`In a direct conversion transceiver,
`frequency is at
`the actual transmission or reception fre-
`quency. If full duplex operation is not required in the system,
`it is possible to implement the transceiver using only one
`frequency synthesizer. However, a problem is,
`that
`the
`tuning range of the frequency synthesizer can become very
`wide, because it must cover both the transmission and
`reception channel frequencies. Furthermore, the frequency
`synthesizer must be capable of removing the duplex offset
`frequency when switching from reception to transmission or
`vice versa (Duplex Separation). Implementing a dual band
`direct conversion transceiver, such as a GSM/PCN mobile
`
`2
`station, with the solutions known at present practically
`always requires the use of at least two frequency synthesiz-
`ers. In some cases as much as four separate frequency
`synthesizers are needed, that is, separate frequency synthe-
`sizers for both the receiver and transmitter and for both
`
`frequency bands, because the tuning range required may
`become too wide when only one frequency synthesizer for
`each frequency band is used.
`In the dual band transceiver shown in FIG. 1, separate
`frequency synthesizers for each frequency band are used in
`the radio frequency part.
`In a mobile station, such as
`GSM/PCN, this means that the frequency synthesizer used
`in the GSM mode of operation operates in the frequency
`band 880 to 960 MHz, whereas the frequency synthesizer
`used in the PCN mode of operation operates in the frequency
`band 1710 to 1880 MHz.
`
`Since the PCN frequency band is about twice the GSM
`frequency band, it might be possible to implement a solution
`with only one frequency synthesizer if the PCN frequencies
`were generated by multiplying the frequency generated by
`the frequency synthesizer by two. In this case, the tuning
`range of the frequency synthesizer would be 855 to 940
`MHz in the PCN mode of operation. Thus the tuning range
`of the frequency synthesizer should be 855 to 960 MHz,
`which is still relatively high, approximately 11.6%. The
`frequency offset that the frequency synthesizer should be
`capable of 45 MHz (Duplex Separation) is also relatively
`high.
`One more possible implementation of the prior art solu-
`tions is to mix the frequency of the frequency synthesizer
`either on the transmission or reception side with a fixed
`frequency, 45 MHz in a GSM/PCN receiver, in order to
`remove the frequency offset between transmission and
`reception. Thus the tuning range of the frequency synthe-
`sizer can be decreased. However, in order to generate a fixed
`frequency, the oscillator must have a very stable frequency
`and it must be capable of being locked at an exact reference.
`This oscillator can be implemented by means of frequency
`synthesis, for example. The practical implementations thus
`resort to the use of two frequency synthesizers.
`In practice, due to the requirements concerning speed,
`noise etc., the maximum tuning range with the frequency
`synthesizer is approx. 10% of the nominal frequency. The
`speed and noise requirements are opposed, that is, the faster
`change of frequency is desired, the higher is the noise of the
`frequency synthesizer, and, respectively, the less noise is
`desired, the slower is the change of frequency. The speed
`requirement, that is, the time allowed to move from one
`frequency of the frequency synthesizer to another varies
`between radio systems, being 600 to 800 microseconds in
`GSM and PCN, for example.
`SUMMARY OF THE INVENTION
`
`An object of the present invention is to attain a method
`and apparatus by which it is possible to implement a dual
`band direct conversion transceivers in which only one
`frequency synthesizer is used to generate the required trans-
`mission and reception channel frequencies.
`An accurate reference oscillator is needed in the mobile
`
`station in any case: as the locking reference of the frequency
`synthesis and as the timing signal of the baseband frequency
`part. In this invention,
`it has been found that the same
`reference oscillator can be used to decrease the tuning range
`and frequency offset of the frequency synthesis by using it
`directly or multiplied as an offset oscillator for the mixer.
`Thus the reference oscillator operates as an accurate fre-
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`5,983,081
`
`3
`quency reference for the frequency synthesis, as an accurate
`constant frequency source for the mixer and as a timing
`signal for the baseband part.
`The invention is based on the idea that one frequency
`synthesizer and one reference oscillator, which generates an
`essentially constant frequency, are used to generate the
`required frequencies. Thus the transmission and reception
`channel frequencies are generated from the frequency mix-
`ing results of the frequency synthesizer and the reference
`oscillator. In accordance with a first embodiment of the
`invention the reception channel frequencies of the first
`frequency band are generated with a frequency synthesizer,
`the transmission channel frequencies of the first frequency
`band are generated by mixing an essentially constant mixing
`frequency with the frequency generated by the frequency
`synthesizer, and the reception channel frequencies of the
`second frequency band are generated by multiplying the
`frequency generated with a frequency synthesizer by a
`constant coefficient, and the transmission channel frequen-
`cies of the second frequency band are generated by mixing
`an essentially constant mixing frequency with the frequency
`generated by the frequency synthesizer, and by multiplying
`the frequency generated as the mixing result by a constant
`coefficient.
`
`According to a second embodiment of the invention, the
`transmission channel frequencies of the first frequency band
`are generated by the frequency synthesizer, the reception
`channel frequencies of the first frequency band are generated
`by mixing an essentially constant mixing frequency with the
`frequency generated by the frequency synthesizer, the trans-
`mission channel frequencies of the second frequency band
`are generated by multiplying the frequency generated with
`the frequency synthesizer by a constant coefficient, and the
`reception channel frequencies of the second frequency band
`are generated by mixing an essentially constant mixing
`frequency with the frequency generated by the frequency
`synthesizer, and by multiplying the frequency thus generated
`as the mixing result by a constant coefficient.
`According to a third embodiment of the invention, two
`mixing results are generated from the frequency generated
`by the frequency synthesizer and the frequency generated by
`the reference oscillator, whereby the reception channel fre-
`quencies of the first frequency band are generated from the
`first mixing result, the transmission channel frequencies of
`the first frequency band are generated from the second
`mixing result,
`the reception channel frequencies of the
`second frequency band are generated by multiplying the first
`mixing result by a constant coefficient, and the transmission
`channel frequencies of the second frequency band are gen-
`erated by multiplying the second mixing result by a constant
`coefficient.
`
`The method according to the invention is characterized in
`what is said in the characterizing part of the appended claim
`1.
`
`The apparatus according to the invention is characterized
`in what is said in the characterizing part of the appended
`claim 9.
`
`By the present invention it is possible to implement a
`direct conversion transceiver operating in two different
`frequency bands by using only one frequency synthesizer.
`The tuning range of the frequency synthesizer can also be
`made relatively small compared to the nominal frequency,
`whereby a high-quality frequency synthesizer of the prior art
`technology can be used as the frequency synthesizer in the
`invention.
`
`BRIEF DESCRIPTION OF THE INVENTION
`
`In the following, the invention will be described in more
`detail with reference to the accompanying drawings in
`which
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`FIG. 1 shows modulator and demodulator blocks of a
`
`prior art dual band direct conversion transceiver as a sim-
`plified block diagram,
`FIG. 2 shows a radio frequency part of a direct conversion
`transceiver according to a preferred embodiment of the
`invention as a simplified block diagram,
`FIG. 3a shows a radio frequency part of a dual band direct
`conversion transceiver according to a preferred embodiment
`of the invention as a simplified block diagram,
`FIG. 3b shows an embodiment of the frequency synthe-
`sizer as a simplified block diagram,
`FIG. 4 shows the construction of a mobile station as a
`
`simplified block diagram,
`FIG. 5a shows another preferred embodiment of the
`invention for generating the local oscillator frequencies, and
`FIG. 5b shows a third preferred embodiment of the
`invention for generating the local oscillator frequencies.
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`Some frequency values selected to generate the frequen-
`cies used in the GSM and PCN systems are mentioned in
`connection with the description by way of example. The
`transmission and reception frequency bands used in these
`systems are as follows, and the following frequencies are
`selected as the internal frequencies of the mobile stations
`used in the systems in order to apply the first embodiment of
`the invention:
`GSM
`
`Reception
`reference oscillator frequency LO4 (constant): 52 MHz
`reception frequency band: 925 to 960 MHz
`frequency synthesizer frequency LO3: 925 to 960 MHz
`receiver local oscillator frequency LO1: 925 to 960 MHz
`Transmission
`
`reference oscillator frequency LO4 (constant): 52 MHz
`transmission frequency band: 880 to 915 MHz
`frequency synthesizer frequency LO3: 932 to 967 MHz
`transmitter local oscillator frequency LO2
`(=LO3—LO4): 880 to 915 MHz
`PCN
`
`Reception
`reference oscillator frequency LO4 (constant): 52 MHz
`reception frequency band: 1805 to 1880 MHz
`frequency synthesizer frequency LO3: 902.5 to 940 MHz
`receiver local oscillator frequency LO1: 1805 to 188 MHz
`Transmission
`
`reference oscillator frequency LO4 (constant): 52 MHz
`transmission frequency band: 1710 to 1785 MHz
`frequency synthesizer frequency LO3: 907 to 944.5 MHz
`transmitter local oscillator frequency LO2
`(=2~(LO3—LO4)): 1710 to 1785 MHz
`From the above it can be seen that the frequency synthe-
`sizer frequencies LO3 of reception and transmission in both
`systems are essentially the same.
`FIG. 2 shows a block diagram of the radio frequency parts
`of a mobile station operating in two different frequency
`bands in accordance with the invention. Blocks 2 to 5 depict
`the radio frequency part of the receiver, block 12 depicts a
`frequency synthesizer and blocks 7 to 11 depict a transmitter.
`Block 14 depicts a reference oscillator, which generates an
`essentially constant frequency.
`In the embodiment of FIG. 2, the frequency generated by
`the reference oscillator 14 is 52 MHz. This can be imple-
`mented by a 13 MHz crystal oscillator, for example, the
`frequency of which is multiplied by four (not shown). Block
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`5,983,081
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`5
`12 depicts a frequency synthesiZer, which is selected as
`operating in the 900 MHZ band in both systems. To
`advantage, the frequency synthesiZer 12 comprises a voltage
`controlled oscillator VCO and a phase-locked loop PLL. In
`addition, a reference oscillator 14, such as a crystal
`oscillator, is needed. The frequency of the reference oscil-
`lator must be as precise as the general frequency precision
`of the system requires. Generally, the reference oscillator is
`locked at the base station frequency
`This precise
`frequency can then be used to lock the local oscillator
`frequencies by means of phase-locked loops (PLL). In a
`digital mobile station, the local oscillator frequency is deter-
`mined by the channel spacing and the bit rate. In GSM, for
`example, the channel spacing is 200 kHZ and the bit rate
`270.833 kbits/s=270 5/6 kbits/s. The oscillator frequency
`must have an integer ratio to each of these frequencies: the
`phase-locked loop needs a frequency of the channel spacing
`as its phase comparison frequency, and the logic part of the
`phone needs a frequency in proportion to the bit rate as its
`timing signal. The smallest common frequency is thus 13
`MHZ.
`
`The fourth harmonic of this frequency is 52 MHZ, which
`differs only by 7 MHZ from the duplex separation. Thus by
`using the 52 MHZ frequency, the 45 MHZ frequency hop
`required by the duplex separation can be considerably
`decreased, and the tuning range of the voltage controlled
`oscillator VCO can also be decreased accordingly. In the
`PCN system, the duplex separation is 95 MHZ, and there a
`suitable frequency for decreasing the frequency hop of the
`voltage controlled oscillator VCO is 104 MHZ=8><13 MHZ.
`In this invention, however, the generation of PCN frequen-
`cies is based on multiplication by two, and so the reference
`oscillator frequency can be 52 MHZ in both systems. The
`implementation of a voltage controlled oscillator VCO and
`a phase-locked loop PLL is known to a person skilled in the
`art, and therefore they are not described in more detail here.
`The received signal comes from the antenna ANT to the
`transmission/reception switch 1. At the reception stage the
`transmission/reception switch is in a position in which the
`received signal is directed to the receiver blocks. From the
`transmission/reception switch 1, the received signal is taken
`in the reception frequency FRX through a first passband
`filter 2, a pre-amplifier 3 and a second passband filter 4 to a
`demodulator 5. In the demodulator 5,
`the reception fre-
`quency signal FRX is mixed with the local oscillator fre-
`quency LO1 of the receiver, which has the reception channel
`frequency, whereby the mixing result is a baseband signal
`corresponding to the original signal. The demodulator 5 is
`most advantageously an I/Q demodulator, whereby the
`demodulator 5 generates both the I and Q demodulated
`signals. The demodulated signals are directed through the
`first low pass filter 6. From the output of the first low pass
`filter 6 the signals I-RX, Q-RX are directed to the other
`stages (not shown) of the receiver for further processing.
`Similarly,
`in the transmitter the outgoing signal I-TX,
`Q-TX coming to the modulator is taken from the modulator
`7, preferably an I/Q modulator, to a buffer amplifier 8. In
`addition to the outgoing baseband signal, the local oscillator
`frequency LO2 of the transmitter, which has the transmis-
`sion channel frequency, is brought to the modulator 7 as the
`carrier signal of the modulator. In the buffer amplifier 8 the
`transmission frequency signal FTX is amplified and taken
`through a third passband filter 9 to a power amplifier 10. The
`amplified transmission frequency signal is taken from the
`power amplifier 10 through the transmission frequency low
`pass filter 11 to the transmission/reception switch 1. At the
`transmission stage, the transmission/reception switch 1 is in
`
`6
`the transmission position, whereby the transmission fre-
`quency signal coming to the switch 1 is led to the antenna
`ANT. If a requirement of the system, such as that of spurious
`transmission, so demands, the low pass filter 11 must be
`replaced by a passband filter. The passband filter can be
`combined with the first passband filter 2 on the reception
`side,
`in which case the filter is actually a duplex filter,
`whereby the transmission/reception switch 1 is not needed.
`Intermediate frequency stages are not used at all in a
`direct conversion receiver, whereby the local oscillator
`frequency LO1 of the receiver is set at the received channel
`frequency. Similarly in a direct conversion transmitter, the
`local oscillator frequency LO2 of the transmitter is set at the
`transmitted channel frequency FTX. In the embodiment
`shown in FIG. 2, frequency LO3 generated by the frequency
`synthesiZer 12 is used as the local oscillator frequency LO1
`of the receiver. The local oscillator frequency LO2 of the
`transmitter is generated so that frequency LO4 of the refer-
`ence oscillator 14 is mixed with the frequency of the
`frequency synthesiZer 12 in the mixer 13.
`In the GSM system, the difference between the transmis-
`sion and reception frequency bands is 45 MHZ, whereby the
`difference between the local oscillator frequency LO1 of the
`receiver and the local oscillator frequency LO2 of the
`transmitter is 45 MHZ. Correspondingly, in the PCN system
`the difference between the transmission and reception fre-
`quency bands is 95 MHZ, whereby the difference between
`the local oscillator frequency LO1 of the receiver and the
`local oscillator frequency LO2 of the transmitter is 95 MHZ.
`FIG. 3a shows a block diagram of the radio frequency
`parts of a dual band direct conversion transceiver according
`to the invention. For each frequency band the receiver has a
`separate front end, which comprises a first passband filter 2,
`15, a pre-amplifier 3, 16 and a second passband filter 4, 17.
`Correspondingly in the transmission side there is a buffer
`amplifier 8, 18, a third passband filter 9, 19, a power
`amplifier 10, 20 and a second low pass filter for each
`frequency band.
`In order to use different parts when operating in different
`frequency bands, two-way switches 22, 23 are applied in the
`transmission/reception switch 1 and the antenna-side line of
`the demodulator 5. In the transmission side, the two-way
`switches for selecting the frequency band are in the
`transmission/reception switch 1 and in the modulator-side
`line of the buffer amplifier 8. When operating in two
`different frequency bands, the two first passband filters 2, 15,
`the two pre-amplifiers 3, 16 and the two second passband
`filters 4, 17 are used in signal reception. Correspondingly,
`two buffer amplifiers 8, 18, two third passband filters 9, 19,
`two power amplifiers 10, 20 and two second low pass filters
`11, 21 are used in signal transmission. By means of the
`two-way switches 22, 23, 24, 25, one block of each two
`similar blocks can always be selected for use. In order to
`generate two different local oscillator frequencies LO1 of
`the receiver, two-way switches 26, 27 and a first frequency
`multiplier 28 are applied between the frequency synthesiZer
`12 and the demodulator 5. Thus either the frequency gen-
`erated by the frequency synthesiZer 12 or that frequency
`multiplied by N can be selected as the local oscillator
`frequency LO1 of the receiver to be directed to the demodu-
`lator 5. On the transmission side, two-way switches 29, 30
`and a second frequency multiplier 31 are applied between
`the mixer 13 and the modulator 7 for generating two
`different local oscillator frequencies LO2 of the transmitter.
`The frequency generated by the frequency synthesiZer 12
`is also directed to the mixer 13,
`in which a frequency
`generated by the reference oscillator 14 is mixed with that
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`7
`frequency. The mixing result of the mixer 13 is an interme-
`diate mixed frequency LO5. By means of two-way switches
`29 and 30, either the intermediate mixed frequency LC)5 or
`LO5 multiplied by the constant N can be selected as the local
`oscillator frequency LO2 of the receiver to be directed to the
`modulator 7. At the frequencies presented here, the antenna
`ANT is a combined antenna of multiple frequency bands.
`For the rest, the radio frequency parts are similar for both
`transmission and reception frequency bands.
`The two-way switches 22-27, 29, 30 can be implemented
`by PIN diodes or GaAsFET transistors, for example. The
`two-way switches are preferably controlled by a two-level
`signal SYSSEL, whereby at the first level of the control
`signal the two-way switches are in a position in which the
`radio frequency circuits of the first frequency band are used,
`and at the second level of the control signal the two-way
`switches are in a position in which the radio frequency
`circuits of the second frequency band are used. The values
`corresponding to the first and second level of the control
`signal SYSSEL depend, among other things, on the imple-
`mentation of the two-way switches. Instead of the two-way
`switches 22-27, 29, 30, some other known method can be
`applied for changing the route of the radio frequency signal.
`The two-way switches 23, 25 can be replaced by matching
`means, which are known as such, and when the first fre-
`quency band is used,
`the radio frequency circuits of the
`second frequency band have a high impedance for the
`signals of the first frequency band. Correspondingly, when
`the second frequency band is used,
`the radio frequency
`circuits of the first frequency band have a high impedance
`for the signals of the second frequency band. Thus the radio
`frequency circuits of different frequency bands do not inter-
`fere with the operation of one another.
`In the embodiment according to FIG. 2, the selection of
`the local oscillator frequency LO1 has been implemented by
`means of one frequency multiplier 28, the coefficient N of
`which can be set by the control signal SYSSEL. Thus on the
`first level of the control signal SYSSEL, the coefficient N is
`one, for example, and on the second level of the control
`signal SYSSEL, the coefficient N is two. In the embodiment
`of FIG. 2, the second mixing frequency LO2 of the trans-
`mission side has been generated by the second frequency
`multiplier 31, in which the coefficient N can be changed
`accordingly by means of the control signal SYSSEL. In this
`embodiment, separate two-way switches 26 97, 29, 30 are
`not needed.
`
`The signal SYSSEL, which controls the two-way
`switches 22-27, 29, 30, is preferably generated in the control
`part 32 of the mobile station (FIG. 4), which preferably
`includes a microprocessor. The control part 32 generates a
`signal according to the system change command given by
`the user from the keypad 33. The system selection can be
`menu-based, for example, in which case the desired system
`is selected from a menu shown on the display 34 by pressing
`a key. Thus the control part 32 generates a control signal
`SYSSEL, which corresponds to the system selected. The
`system change command can also be given by a mobile
`network system, whereby the mobile station receives data
`sent by another system. The received data can include a
`system change command, on the basis of which the control
`part performs the system change. A control program has
`been stored in the memory 35 of the control part 32,
`preferably EPROM or EEPROM, which control program
`monitors the received data, and when it detects a system
`change command in the data it gives the control part a
`command to set
`the control signal SYSSEL in a state
`determined by the selection command.
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`8
`The control part 32 generates a TUNE signal, by which a
`dividing numeral is set for the divider 38 of the frequency
`synthesizer 12 which numeral corresponds to the selected
`channel frequency. Then the divider 38 of the frequency
`synthesizer forms a phase comparison frequency to the
`phase comparator 39 from the frequency of the voltage
`controlled oscillator VCO. In the GSM system, the channel
`spacing is 200 kHz, whereby the phase comparison fre-
`quency is 200 kHz. Because of the duplication, the phase
`comparison frequency is 100 kHz in the PCN system. This
`can be avoided by placing a frequency multiplier in the
`phase comparison loop of the frequency synthesizer 12. This
`alternative is shown in FIG. 3b, in which only the blocks
`necessary for illustrating the implementation of the fre-
`quency synthesizer 12 are shown.
`The circuit functions in a manner such that the frequency
`generated by the reference oscillator 14 is directed to the
`reference divider X of the frequency synthesizer 12, and the
`divider X forms the first phase comparison frequency to the
`phase comparator 39. The frequency of the voltage con-
`trolled oscillator VCO is directed to the divider 38 of the
`
`frequency synthesizer either directly or multiplied by the
`constant N. This selection is preferably made by switches
`26, 27. The divider 38 forms a second phase comparison
`frequency for the phase comparator 39 from that frequency.
`The control part 32 generates a TUNE signal, by which a
`dividing number is set for the divider 38 of the frequency
`synthesizer 12, according to the selected channel frequency.
`Thus in a direct conversion transceiver of a GSM/PCN
`system, the channel spacing 200 kHz can be used as the
`phase comparison frequency in each system, whereby side-
`band problems can be avoided. The division relation of the
`reference divider X need not be changed, either, when
`changing between systems. The frequency of the voltage
`controlled oscillator VCO is directed to the demodulator 5
`
`and the mixer 13 either directly or multiplied by the constant
`N. The frequency generated by the reference oscillator 14 is
`directed to the mixing frequency of the mixer 13 either
`directly or multiplied by the constant N. This selection is
`preferably made by switches 29, 30.
`FIG. 3a shows a dual band direct conversion transceiver
`
`according to the invention, in which the first frequency band
`comprises the frequency band of the GSM system, and the
`second frequency band comprises the frequency band of the
`PCN system. Thus the passband width of the first passband
`filter 2 and the second passband filter 4 of the first frequency
`band is approx. 925 to 960 MHz. The passband width of the
`first passband filter 15 and the second passband filter 17 of
`the second frequency band is approx. 1805 to 1880 MHz.
`Correspondingly in the transmitter the passband width of the
`third passband filter 9 of the first frequency band is approx.
`880 to 915 MHz and the passband width of the third
`passband filter 18 of the second frequency band is approx.
`1710 to 1785 MHz. The frequency band of the second low
`pass filter 11 of the first frequency band comprises frequen-
`cies under 915 MHz, and the frequency band of the second
`low pass filter 21 of the second frequency band comprises
`frequencies under 1785 MHz. In the embodiment of FIG. 3a,
`the frequency synthesizer 12 has been set to operate in the
`reception frequency band of the first frequency band, that is,
`in the reception frequency band 925 to 960 MHz of the GSM
`system. When a signal of the GSM system is received, the
`frequency synthesizer 12 is set at the reception channel, that
`is, at some of the reception frequencies of the GSM system
`between 925 and 960 MHz. The frequency of the transmis-
`sion channel corresponding to the reception channel is 45
`MHz lower, and thus the local oscillator frequency LO2 of
`
`12
`
`12
`
`

`
`5,983,081
`
`9
`the transmitter must be 45 MHZ lower than the local
`
`oscillator frequency LO1 of the receiver. The frequency of
`the reference oscillator 14 is advantageously generated as a
`multiple of 13 MHZ, as is set out in the description of the
`embodiment shown in FIG. 2, and thus 52 MHZ is selected
`as the reference oscillator frequency in the embodiment
`shown in FIG. 3a. At the transmission stage, the frequency
`of the frequency synthesiZer 12 is then raised by 7 MHZ
`regardless of the channel in which the operation takes place
`and the mixing result is an intermed