Abstract:
This invention provides a wireless transmitter circuit for mobile communication apparatus and this circuit can be configured with fewer components and is suitable for downsizing. A single PLL synthesizer serves as both RF frequency band PLL and IF frequency band PLL among three oscillators for TX, RX and IF frequency bands, which have been required in conventional mobile communication apparatus. The number of necessary oscillators occupying a large area within a chip is reduced and the number of components is decreased. Specifically, circuitry is arranged to generate local oscillation signals for RF and IF frequency bands by frequency dividing the output of a VCO of the RF frequency band PLL.

Description:
CLAIM OF PRIORITY  
       [0001]     The present application claims priority from Japanese application JP 2004-184779 filed on Jun. 23, 2004, the content of which is hereby incorporated by reference into this application.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to a transmitter circuit for mobile communication apparatus and, in particular, to a wireless transmitter circuit and a transceiver using that circuit in a transmitter section suitable for large-scale integration.  
       BACKGROUND OF THE INVENTION  
       [0003]     With explosive growth of mobile communication apparatus, there is a strong demand for smaller and less costly wireless apparatus. Consequently, application of a transceiver integrated circuit (IC) with enhanced integration is hoped for. One example of the transceiver IC with improved integration by prior art is a direct-conversion transceiver IC introduced by Hitachi, Ltd. in 2001 (see non-patent document 1). This uses an offset PLL transmitter circuit and, in the transmitter, there are an RF frequency band phase-locked loop (PLL), an intermediate frequency (IF) band PLL, and a transmission oscillator; altogether three oscillation circuits and PLL circuits including oscillators.  
         [0004]     A mobile phone configuration that uses a modulation type fractional divider PLL frequency synthesizer in which a digital dither circuit is provided between a digital input and a delta-sigma (ΔΣ) modulator to supply a digital output which discretely changes from the digital input with its time-average corresponding to the digital input to the delta-sigma (ΔΣ) modulator is known (e.g., see patent document 1). Since the “delta-sigma modulator” is generally called a “sigma-delta (ΣΔ) modulator,” the term “sigma-delta (ΣΔ) modulator” will be used hereinafter.  
         [0000]     [Patent document 1] Japanese Patent Laid-Open No. 2002-152044  
         [0000]     [Non-patent document 1] S. Tanaka et al., “GSM/DCS1800 Dual Band Direct-Conversion Transceiver IC,” Proceedings of the 27th European Solid-State Circuits Conference pp. 492-495, 2001  
       SUMMARY OF THE INVENTION  
       [0005]     The former prior art example requires altogether three oscillators for TX (transmitting frequency), RF (radio frequency), and IF frequency. When these oscillators are integrated into an IC chip, because of low noise requirement for each oscillator, an inductor occupying a large area on the IC is required per oscillator and the IC size is difficult to be reduced.  
         [0006]     In the latter prior art example, for example, a local oscillator circuit configuration to generate both IF and RF local oscillation signals using one PLL is disclosed, but no consideration is taken for spurious generated by the arrangement for IF and RF. The document (patent document 1) on this prior art example also refers to a sigma-delta ΣΔ synthesizer preferable for use in this local oscillator and describes the need of a dither circuit to solve a spurious problem proper to the ΣΔ synthesizer. However, it provides no description of a concrete configuration to avoid the spurious problem and enable more downsizing.  
         [0007]     Then, an object of the present invention is to provide a transmitter circuit that avoids the spurious problem, is suitable for downsizing, can be configured with fewer components, and allows for compact integration.  
         [0008]     Another object of the invention is to provide a transceiver using the above transmitter circuit.  
         [0009]     An example of typical means of the present invention will be presented below. A transmitter circuit according to the present invention comprises a first frequency-generating circuit, a first frequency-converting circuit, a second frequency-converting circuit to which output of the first frequency-converting circuit is input, and a first divider, characterized in that output of the first frequency-generating circuit is supplied via the first divider to the first frequency-converting circuit as a local oscillation signal for the first frequency-converting circuit, output of the first frequency-generating circuit is supplied directly or via a second divider to the second frequency-converting circuit as a local oscillation signal for the second frequency-converting circuit, and a frequency division factor of the first divider is set to a variable number. As for the above components, referring to  FIG. 1 , the first frequency-generating circuit corresponds to a synthesizer SS, the first frequency-converting circuit corresponds to a quadrature modulator  8 , the second frequency-converting circuit corresponds to an offset PLL, the first divider corresponds to dividers  30 ,  31 , and the second divider corresponds to dividers  27 ,  29 . 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a diagram showing a transceiver circuit configuration of a first embodiment of the present invention.  
         [0011]      FIG. 2  is a graph to explain control of a frequency division ratio in the first embodiment of the invention.  
         [0012]      FIG. 3  is a diagram showing an example of a control section configuration involved in the first embodiment of the invention.  
         [0013]      FIG. 4  is a flowchart illustrating the operation of a control part in the first embodiment of the invention.  
         [0014]      FIG. 5  is a diagram showing an example of a configuration of the control part according to a second embodiment of the invention.  
         [0015]      FIG. 6  is a diagram showing a part of RFIC receiving control data from a baseband in the second embodiment of the invention.  
         [0016]      FIG. 7  is a diagram showing an example of a configuration of a voltage controlled oscillator that is used in a third embodiment of the invention.  
         [0017]      FIG. 8  is a diagram showing a circuit configuration for controlling the voltage controlled oscillator shown in  FIG. 7   
         [0018]      FIG. 9  is a diagram showing an example of a configuration of a first-order ΣΔ modulator that is used in the third embodiment of the invention.  
         [0019]      FIG. 10  is a diagram showing an example of a configuration of a third-order ΣΔ modulator that is used in the third embodiment of the invention.  
         [0020]      FIG. 11  is a timing chart of signals to explain the precision of counting signal cycles of the oscillator in the third embodiment of the invention.  
         [0021]      FIG. 12  is a table to demonstrate the effect of calibration when the third embodiment of the invention is applied to major GSM channels.  
         [0022]      FIG. 13  is a diagram showing a third-order ΣΔ modulator with a pseudo random number generator, which is a fourth embodiment of the invention.  
         [0023]      FIG. 14  is a diagram showing an example of a configuration of the pseudo random number generator that is used in the fourth embodiment of the invention.  
         [0024]      FIG. 15  is a diagram showing another example of a configuration of the pseudo random number generator that is used in the fourth embodiment of the invention.  
         [0025]      FIG. 16  is another configuration of a third-order ΣΔ modulator with a pseudo random number generator, which is the fourth embodiment of the invention.  
         [0026]      FIG. 17  is a graph of characteristic lines to demonstrate the result of the fourth embodiment of the invention.  
         [0027]      FIG. 18A  is a circuit diagram wherein a loop filter is ideally grounded, according to a fifth embodiment of the invention.  
         [0028]      FIG. 18B  is a circuit diagram wherein the loop filter is connected to a common ground on the package.  
         [0029]      FIG. 18C  is a circuit diagram wherein a lag-lead filter and a low-pass filter are connected to separate grounds on the package.  
         [0030]      FIG. 18D  is a circuit diagram wherein the whole loop filter is mounted out of the package.  
         [0031]      FIG. 19  is a graph to demonstrate the effect of the fifth embodiment.  
         [0032]      FIG. 20  is a diagram showing a sixth embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]     Embodiments of the present invention will now be described in detail with reference to the accompanying drawings.  
       Embodiment 1  
       [0034]     A first embodiment of the present invention will be described with  FIGS. 1 through 4 .  FIG. 1  shows an RFIC configuration in which a transceiver circuit using a wireless transmitter circuit of the present invention is integrated. This RFIC  34  is designed to support four frequency bands: GSM850, GSM900, DCS1800, and PCS1900.  
         [0035]     GSM850 uses a transmitting band of 824-849 MHz and a receiving band of 869-894 MHz. GSM900 uses a transmitting band of 880-915 MHz and a receiving band of 925-960 MHz. DCS1800 uses a transmitting band of 1710-1785 MHz and a receiving band of 1805-1880 MHz. PCS1900 uses a transmitting band of 1850-1910 MHz and a receiving band of 1930-1990 MHz.  
         [0036]     A received signal is input via a band pass filter (BPF)  1  to a low noise amplifier (LNA)  2  on the IC which is a filter circuit to remove interference introduced from the antenna ANT. The signal amplified by the LNA is frequency converted into two I and Q baseband signals by mixers  3 . The baseband signals respectively pass through PGA/LPF circuits  5  and are output as I and Q output signals from the RFIC  34 . The PGA/LPF circuit  5  consists of a programmable gain amplifier (PGA) and a low-pass filter (LPF) for removing interference from the frequency converted signal.  
         [0037]     A local signal which is input to the mixers  3  is generated by frequency-dividing an output signal of a voltage controlled oscillator (VCO)  21 . In the case of GSM850 and GSM900, the mixers  3  are driven by the VCO output signal with a frequency divided by 4, that is, the frequency is divided by a divide-by-2 divider  32  selected by a switch  33  and further divided by a divide-by-2 divider  4 . In the case of DCS1800 and PCS1900, the mixers  3  are driven by the VCO output signal with a frequency divided by 2, that is, the frequency is divided by only the divide-by-2 divider  4  and the divide-by-2 divider  32  is deselected by the switch  33 . By thus using the dividers, local oscillation signals for I and Q with a phase shift of exactly 90 degrees can be generated. The VCO  21  operates under stable frequency control by a synthesizer SS comprised of a variable divider (DIV)  22 , a phase discriminator (PD)  23 , a charge pump circuit (CP)  24 , and a loop filter  25 . A frequency division factor of the variable divider  22  is set by a control circuit (CONT)  26  and the VCO frequency can be controlled to a desired oscillation frequency.  
         [0038]     A transmitter circuit modulates transmit signals by GMSK (Gaussian filtered Minimum Shift Keying) modulation and input signals are input through two I and Q paths. The input signals go through attenuators  6  for signal level matching with a quadrature modulator  8  at the following stage and low-pass filters  7  to reduce out-of-band noise and are applied to the quadrature modulator  8 . An IF signal that is output from the quadrature modulator passes through a limiter circuit  10  and drives a phase discriminator  11 . Output of the phase discriminator passes through a loop filter  12  and controls a control voltage of a transmitting VCO  13 . To the output side of the VCO  13 , a divide-by-4 divider  14  and a divide-by-2 divider  15  are attached.  
         [0039]     Transmit signals in GSM850 and GSM900 bands are output by the divide-by-4 divider  14  and transmit signals in DCS1800 and PCS1900 bands are output by the divide-by-2 divider  15 . A transmit signal is frequency converted into an IF signal band by a mixer  16 , an error from the input IF signal is detected by the phase discriminator  11 , and the signal is fed back to the VCO  13 . In consequence, a GMSK modulated signal with a predetermined center frequency can be generated at the output side of each of the dividers  14 ,  15 . Output of the divide-by-4 divider  14  is amplified by a power amplifier  17  (for GSM850 and 900) and output of the divide-by-2 divider  15  is amplified by a power amplifier  18  (for DCS1800 and PCSl900). After harmonics are removed from the amplified transmit signals by low-pass filters (LPFs)  19 ,  20 , respectively, the transmit signals are output via a switch S/W  21  to the antenna.  
         [0040]     This type of transmitter circuit is known as an offset PLL circuit (O-PLL) and this circuit generally requires an IF local oscillation signal to drive the quadrature modulator and an RF local oscillation signal to drive the mixer in the PLL feedback loop. As noted above, in the prior art, these signals are generated by two separate synthesizers. However, in the present embodiment, these signals are generated by one synthesizer, so that the IC will be downsized.  
         [0041]     The VCO  21  that is used to generate local oscillation signals in the receiver circuit is also used to generate RF and IF signals for transmission. RF local oscillation signals are generated by frequency-dividing the output of the VCO  21 . Specifically, these signals for GSM850 and 900 are generated by dividing the VCO output frequency by four, using two divide-by-2 dividers  27 ,  29  selected by a switch  28 . These signals for DCS1800 and 1900 are generated by dividing that frequency by two, using only one divide-by-2 divider  27  selected by the switch  28 .  
         [0042]     On the other hand, IF local oscillation signals are generated by dividing the VCO  21  output frequency with a variable divider (IFDIV)  30 . In general, the duty ratio of the variable divider output is not 50%. Therefore, by dividing the output by four, using two divide-by-2 dividers  31 ,  9 , local oscillation signals for I and Q with a phase shift of exactly 90 degrees are generated.  
         [0043]     The reason why the variable divider  30  is used to generate IF local oscillation signals in this embodiment will be described with  FIG. 2 . In the present embodiment, transmission frequency is denoted by fTx, VCO  21  oscillation frequency fVCO, RF local oscillation signal frequency fRF, IF local oscillation signal frequency fIF, and reference frequency fREF. 
 
 fTX=fRF−fIF   (1) 
 
 fRF=fVCO /(2* m )  (2) 
 
 fI=fVCO /(4* N )  (3) 
        where, m=1 for DCS and PCS and m=2 for GSM850 and 900, and N is a variable frequency division factor.        
 
         [0045]     From the above equations (1) to (3), the following equations (4) and (5) are derived. 
 
 fRF=fIF* 2 N/m   (4) 
 
 fTX=fIF*( 2 N/m− 1)  (5) 
 
         [0046]     Equations (4) and (5) indicate that, even if transmission frequency fTX changes, there are always an integral ratio between transmission frequency fTX and IF frequency fIF and an integral ratio between RF local oscillation frequency fRF and IF frequency fIF. Because harmonic spurious due to IF signals corresponds to a transmission frequency band in which a large signal is generated, a spurious problem is not presented.  
         [0047]     On the other hand, harmonics of reference frequency fREF is constant without regard to frequency setting. Some transmission frequency setting may cause spurious to occur near the transmission center frequency.  FIG. 2  shows this phenomenon in the case of GSM900 as an example. The abscissa represents transmission frequency fTX and the ordinate represents RF local oscillation frequency fRF. If reference frequency fREF=26 MHz, the frequencies of harmonics equaling the reference frequency multiplied by a factor of 37, 38, and 39, respectively, and the frequencies of RF signals generated with variable frequency division factor N=10 and N=11, respectively, are plotted.  
         [0048]     Assuming that N is fixed to 11, the RF signal line (N≦11) meets the line of a harmonic having 26 MHz multiplied by a factor of 38 at or near 898 MHz. Therefore, transmission spurious is liable to occur in the vicinity of this frequency. To avoid this, by changing N from 11 to 10 at transmission frequency fTX=895 MHz and from 10 to 11 at fTX=910 MHz, the RF local oscillation frequency fRF is separated from the harmonic frequency of reference frequency fREF. Roughly, this separation is set for 3-5 MHz here, as an offset PLL operates in a band of 1 MHz or above.  
         [0049]      FIG. 3  is a block circuit diagram showing detailed configuration of the control circuit  26  that realizes changing the variable frequency division factor N explained with  FIG. 2 . In  FIG. 3 , the transceiver components corresponding to those shown in  FIG. 1  are assigned the same reference numbers. However, a divider  432  is a simplified representation in a block of a circuit consisting of the divide-by-2 dividers  4 ,  32 , and the switch  33  for switching between division-by-2 and division-by-4. This circuit is used to generate local oscillation signals to be input to the mixers  3  from the output of the VCO  21 , according to the frequency band in use which may be GSM, DCS, etc., as described for  FIG. 1 . A divider  279  is a simplified representation in a block of a circuit consisting of the divide-by-2 dividers  27 ,  29 , and the switch  28  for switching between division-by-2 and division-by-4. This circuit is used to generate RF local oscillation signals from the output of the VCO  21 . Dividers  432  and  279  in  FIG. 5  and  FIG. 20 , which will be described later, are the same representations.  
         [0050]     The frequency division factor of the various divider  22  included in the synthesizer SS is set by using a time-changing signal generated by a sigma-delta (ΣΔ) modulator  43 . In a frequency division ratio setting register (CON_REG)  42 , data for setting a frequency pitch for integral multiplication of reference frequency fREF and data for setting an input to the sigma-delta (ΣΔ) modulator  43  are stored. The former data is used to directly control the variable divider  22  via an adder  44 . The latter data is converted into a time-changing signal with a low bit length by the sigma-delta (ΣΔ) modulator  43  and this signal is supplied to control the variable divider  22  via the adder  44 . This type of PLL circuit including the variable divider whose frequency division factor is changed over time is called a fractional PLL.  
         [0051]     To change the value of variable frequency division factor N as required, a frequency setting register F_REG, an initial frequency setting register FS_REG, first and second switching frequency registers FSW 1  and FSW 2 , first and second switching value setting registers DDIV 1  and DDIV 2 , an initial IF division ratio setting register INI_DIV, and an IF division ratio setting register DIV_REG are used. Operation for this setting change is performed by the following procedure which is illustrated in  FIG. 4 .  
         [0052]     First, in step S 1 , data is set in the initial frequency setting register FS_REG.  
         [0053]     In step S 2 , data is set in the initial IF division ratio setting register INI_DIV.  
         [0054]     In step S 3 , the data set in the initial frequency setting register FS_REG is compared with the data in the first switching frequency register FSW 1 . If the result of the comparison is true, step S 6  is executed; if the result is false, step S 4  is executed.  
         [0055]     In step S 4 , the data in the initial frequency setting register FS_REG is stored into the frequency setting register F_REG.  
         [0056]     Then, the procedure proceeds to step S 5  where the data in the initial IF division ratio setting register INI_DIV is stored into the IF division ratio setting register DIV_REG and the operation terminates.  
         [0057]     If the procedure proceeds to step S 6 , the data set in the initial frequency setting register FS_REG is compared with the data in the second switching frequency register FSW 2 . If the result of the comparison is true, step S 9  is executed; if the result is false, step S 7  is executed.  
         [0058]     In step S 7 , a calculation is executed and the result is stored into the frequency setting register F_REG.  
         [0059]     Next, in step S 8 , the sum of the data in the initial IF division ratio setting register INI_DIV and the data in the first switching value setting register DDIV 1  is stored into the IF division ratio setting register DIV_REG and the operation terminates.  
         [0060]     In step S 9 , a calculation is executed and the result is stored into the frequency setting register F_REG.  
         [0061]     In step S 10 , the sum of the data in the initial IF division ratio setting register INI_DIV and the data in the second switching value setting register DDIV 2  is stored into the IF division ratio setting register DIV_REG and the operation terminates.  
         [0062]     The operation described above is an example and the essence of the present invention resides in variable setting the frequency division factor of the divider for generating IF local oscillation signals.  
       Embodiment 2  
       [0063]     A second embodiment of the present invention will be described with  FIGS. 5 and 6 . This embodiment is an example where the operation illustrated in  FIG. 4 , which is performed within the control circuit  26  in the first embodiment, is performed by a baseband LSI (BBLSI)  59 .  
         [0064]      FIG. 5  is a diagram corresponds to  FIG. 3  for the first embodiment and the same components assigned the same reference numbers and their explanation is not repeated. In this embodiment, unlike the configuration shown in  FIG. 3 , the IF division ratio setting register DIV_REG is provided within the control circuit  26 . Instead, the initial frequency setting register FS_REG, first and second switching frequency registers FSW 1 , FSW 2 , first and second switching value setting registers DDIV 1  and DDIV 2 , and initial IF division ratio setting register INI_DIV are removed from the control circuit  26 .  
         [0065]      FIG. 6  is a diagram showing transfer of control data between the RFIC  34  and the BBLSI  59 . For storing data that is transferred between the RFIC  34  and the BBLSI  59 , a main data register MREG and specific data registers PREG  61  are provided in the RFIC  34 . In the main data register MREG, clock signal CLK, data DT, and enable signal EN transferred from the BBLSI  59  are stored.  
         [0066]     The BBLSI  59  executes a calculation with the contents of the frequency setting register F_REG and the IF division ration setting register DIV_REG and transfers resulting control data to the RFIC  34 . The transferred data DT is once held on the main data register MREG and, when an EN signal is issued from the BBLSI  59 , the data DT is stored into specific data registers PREG  61 . The frequency setting register F_REG and the IF division ratio setting register DIV_REG included in the control circuit  26 , shown in  FIG. 5 , are specific data registers  61 .  
         [0067]     The control data DT, which is transferred from the BBLSI  59  by serial transmission, comprises an address field AAD in which the number of a specific data register is stored and a field DAT in which data to be written is stored. In this embodiment, switching the frequency division factor of the divider for generating IF local oscillation signals can be performed by a program and, thus, flexibility of algorithm is enhanced.  
       Embodiment 3  
       [0068]     A third embodiment of the invention relates to a VCO calibration method that is suitable for an instance where the frequency division factor of the variable divider  22  included in the synthesizer SS is controlled, using sigma-delta (ΣΔ) modulators. This embodiment will be described with  FIGS. 7 through 12 .  
         [0069]     In this embodiment, a VCO that is configured as shown in  FIG. 7  is used. This VCO is a positive feedback type oscillator using PMOSFETs  77   a ,  77   b  which are controlled by a current source  76  from a power source  75 . A control voltage is applied to a terminal  84  to control variable bias capacitors  78   a ,  78   b . Oscillation frequency deviation which may occur due to variations in the PMOSFETs  77   a ,  77   b , capacitors  78   a ,  78   b ,  79   a ,  79   b , the amount of bias current, and inductors  82   a ,  82   b  is controlled by bias application from the k number of terminals  81 - 1  to  81 - k , wherein the bias application is controlled by turning on/off switches  80 , each connected between a pair of capacitors  79   a ,  79   b . Terminals  83   a ,  83   b  are differential output terminals. Suffices a, b attached to the reference numbers of the elements indicate a pair of elements which ideally have identical characteristics.  
         [0070]     This control is executed by a circuit which is shown in  FIG. 8 . The output of a fixed bias generator  64 , when it is connected by a switch  65 , is applied through a loop filter  25  to a bias control terminal of the VCO  21 . This opens the phase locked loop so that calibration can be performed. In order that the output of the variable divider  22  used in the synthesizer is consistent with the reference signal fREF (26 MHz), appropriate values are set for a sigma-delta (ΣΔ) input data register CON_REGF for first-order (1st) and third-order (3rd) sigma-delta (ΣΔ) modulators  70 ,  71  and a reference frequency multiplication setting register CON_REGI included in a division ratio setting register  42 . The reference frequency multiplication setting register CON_REGI is a register in which a frequency division factor by which the reference frequency fREF is integrally multiplied must be set; for example, a frequency division factor corresponding to an integral multiple of fREF=26 MHz is stored. On the other hand, the sigma-delta (ΣΔ) input data register CON_REGF is a register in which a frequency pitch other than the above integral multiple must be set; for example, a pitch of 100 kHz is set as its value. This value is output via the first-order ΣΔ modulator  70  or third-order ΣΔ modulator  71  and added to the set value of the reference frequency multiplication setting register CON_REGI by an adder  73 , and the result of the addition sets the frequency division factor of the variable divider  22 . The first-order ΣΔ modulator  70  or third-order ΣΔ modulator  71  is selected by switch action of a switch  72 .  
         [0071]     To put it graphically, VCO calibration is performed through the paths denoted by dotted lines. First, the loop filter  25  is connected to the fixed bias generator  64  by the switch  66  and the PLL loop is disconnected. After the loop is opened, the output of the variable divider  22  and the reference frequency fREF are, respectively, divided into 400 kHz signals, for example, by divide-by-65 dividers Div65A, DiV65B. A speed comparator FSDET counts the cycles of the 400 kHz signals of both, namely, the divider output and fREF, and observes which is finished first.  
         [0072]     In order that difference between the cycle counts of both becomes zero, that is, the capacitors  78   a ,  78   b  included in the VCO shown in  FIG. 7  will have capacitances adapted to variations under the set bias condition, the values of the sigma-delta (ΣΔ) input data register CON_REGF and the reference frequency multiplication setting register CON_REGI are set. By a VCO band switching circuit (VCO Band)  69 , the switches  80  in the VCO shown in  FIG. 7  are switched so that the capacitances in accordance with the set values of these registers will be obtained, and a set control voltage, namely, a calibrated voltage is applied to the control terminals of the VCO  21 . Then, by switch action of the switch  65 , the loop filter  25  is connected to the charge pump  24 ; that is, the loop is closed, the calibration terminates, and stable VCO oscillation action under the PLL control continues. In this case, the use of the sigma-delta (ΣΔ) modulators gives rise to an error as will be described below.  
         [0073]      FIG. 9  shows a configuration of the first-order ΣΔ modulator and  FIG. 10  shows a configuration of the third-order ΣΔ modulator. The first-order ΣΔ modulator  70  is comprised of a delay element (D), 1-bit quantizer (O), amplifier (G), and subtractor  93 .  
         [0074]     The thus configured first-order ΣΔ modulator  70  operates as follows. A fixed input, 10-bit digital signal input from an input terminal  85  is input to the delay element  90 . This output result of the delay element  90  and the output of the delay element  90  which is the previous input to the quantizer are quantized to one bit by the 1-bit quantizer  91 . The results are amplified by the amplifier  92  and a difference therebetween is obtained by the subtractor  93 . The difference is added to a signal input to the input terminal by the adder  89  and the result of the addition is input to the delay element again. The output result of the delay element is quantized by the quantizer in the same way as above, thereby one-bit signals are generated, and these signals are then amplified and fed back. By repeating a series of these operations, a sequence of data (time-changing signals) as an average of values input at the input terminal  85  is output from a terminal  97 .  
         [0075]     The third-order ΣΔ modulator employs three first-order ΣΔ modulators  86 ,  87 ,  88  configured as above and differential elements  94 ,  95 . It is configured as follows. The first-order ΣΔ modulators  86 ,  87 ,  88  are connected in series. The output of the third-stage ΣΔ modulator  88 , after passing through a differential element  95 , and the output of the second-stage ΣΔ modulator  87 , after passing though one delay element, are added. The result of the addition, after passing through a differential element  94 , is further added to the output of the first-stage ΣΔ modulator  86 , after passing through two delay elements.  
         [0076]     The third-order ΣΔ modulator  71  configured as above operates as follows. Essentially, the ΣΔ modulators of all stages operate in the same manner as described for the above first-order ΣΔ modulator. However, the first-stage modulator takes fixed input and operates in the same way as depicted in  FIG. 9 , but the second and third ΣΔ modulators  87 ,  88  take non-fixed input signals. An error produced by the first-stage ΣΔ modulator  86  is input to the second-stage ΣΔ modulator  87  and an error produced by the second-stage one is input to the third-stage one. Since a ΣΔ modulator works as an integrator intrinsically, the output of the third-stage ΣΔ modulator is reverted through the differential element  95  and added to the output of the second-stage one and this addition output is further reverted through the differential element  94  and added to the output of the first-stage one. As a result, a 4-bit sequence of data is output from a terminal  96 . For adjustment of timing of addition, two delay elements are attached to the first-stage ΣΔ modulator  86  and one delay element is attached to the output side of the second-stage ΣΔ modulator  87 .  
         [0077]     For the first-order ΣΔ modulator, its output involves an error within ±0.5 (1 bit) for one operation, whereas the output of the third-order ΣΔ modulator involves an error within ±7 (4 bit). It is understood that a lower-order modulator is desirable. Therefore, in the configuration of  FIG. 8 , by switch action of the switch  72 , the third-order ΣΔ modulator  71  shown in  FIG. 10  is selected when the synthesizer operation is performed and the first-order ΣΔ modulator  70  is selected when calibration is performed.  
         [0078]      FIG. 11  shows a timing chart of signals to explain an error involved when the VCO is conditioned to generate local oscillation signals with 890.2 MHz and the first-order ΣΔ modulator is used.  FIG. 11  shows the pulses of reference frequency fREF of 26 MHz, oscillation frequency of 890.2 MHz that the VCO  21  generates, and output DIVOUT of the variable divider  22  during a period of 2.5 μs. In  FIG. 11 , cycles are numbered and boxed. 65 cycles of pulses of fREF (hereinafter, cycles are referred to as “clocks”) occur during 2.5 μs, corresponding to 2225.5 clocks of VCO. At the frequency of the output DVOUT of the divide-by-65 divider (Div65B) when the first-order ΣΔ modulator  70  is used, 2225.5±0.5 clocks occur for the corresponding period, indicating an error of ±0.5 clock from the VCO oscillation frequency.  
         [0079]      FIG. 12  shows a table listing calculation results for other major frequencies. In the table shown in  FIG. 12 , CH is a GSM channel number, f is applied frequency [MHz], Ni is the value of the reference frequency multiplication setting register CON_REGI, Nf is the value of the sigma-delta (ΣΔ) modulator input data register CON_REGF, THEO is a theoretical value [clocks], ΣΔ (min) is a minimum value of the first-order ΣΔ modulator, ΣΔ (max) is a maximum value of the first-order ΣΔ modulator, TMERR (max) is a maximum timing error [clock], and TMERRO (min) is a minimum timing error [clock]. From this table, an error within 0.5 clock is verified for all channels.  
       Embodiment 4  
       [0080]     A fourth embodiment relates to a modulator configuration in which a pseudo random number is added to reduce fixed spurious of the ΣΔ modulator. This embodiment will be described with  FIGS. 13 through 17 .  FIG. 13  shows a third-order ΣΔ modulator that is used in this embodiment. Components corresponding to those of the third-order ΣΔ modulator shown in  FIG. 10  are assigned the same reference numbers and their explanation is not repeated. A difference from the modulator configuration shown in  FIG. 10  is that an adder  98  is attached to the input of the third-order ΣΔ modulator and output of a pseudo random number generator  100  is applied via a differential element  99  to cut DC components.  
         [0081]     It is important here is to prevent a frequency error by eliminating DC components.  FIG. 14  shows an example of a configuration of the pseudo random number generator  100 . This pseudo random number generator, which is called PN15, is comprised of 15 delay elements  101  and three exclusive OR elements  102 . As shown in  FIG. 15 , by AND  103  operation of the outputs of two delay elements, an error probability of 1 is controlled and noise power can be reduced by, for example, a factor of 4.  
         [0082]     As shown in  FIG. 16 , by applying the output of the pseudo random number generator  100  to an adder  104  or an adder  105  (denoted by a dotted circle) that is attached to the input of the second-stage ΣΔ modulator, the differential element  99  in  FIG. 13  can be dispensed with.  FIG. 17  shows a spectral analyzer display, a graph of characteristic lines as results of simulation of GSM channel number 95 by a spectral analyzer, where a reference band width RBW=30 kHz. The ordinate represents spectrum power level SP (dB) and the abscissa represents frequency f (kHz). A characteristic line  106  is the one observed when the modulator configuration of  FIG. 10  where a pseudo random number is not applied is used and spurious appears. On the other hand, as seen in a characteristic line  107  observed when the modulator circuitry of  FIG. 13  where a random pseudo number is applied is used and a characteristic line  108  observed when the modulator circuitry of  FIG. 16  is used, it is apparent that spurious is suppressed.  
       Embodiment 5  
       [0083]     In this embodiment, influence of the package on an internal loop filter of the ΣΔ modulator is discussed.  FIG. 18A  shows a synthesizer comprised of the VCO  21 , phase discriminator  23 , charge pump circuit  24 , and loop filter  25 . The loop filter  25  is comprised of a lag-lead filter  109  consisting of parallel connections of a circuit in which a capacitor C 1  and a resistor R 1  are connected in series and a capacitor C 2 , and a low-pass filter  110  consisting of a resistor R 2  and a capacitor C 3 . In  FIG. 18A , Kv/S denotes a transfer function of the VCO  21  and 1/n denotes a frequency division factor of the variable divider  22 .  
         [0084]     Although an example where the first-order low-pass filter is used is shown in this embodiment, it is needless to say that low-pass filters more than the first-order may be used.  
         [0085]      FIG. 18A  is an example where ideal grounding is accomplished.  FIG. 18B  shows an example where the loop filter is connected to a common ground.  FIG. 18C  shows an example where the lag-lead filter  109  and the low-pass filter  110  are connected to separate grounds; that is, the lag-lead filter  109  is grounded to the ground terminal for the charge pump  24  and the low-pass filter  110  is grounded to the ground terminal for the VCO  21 .  FIG. 18D  shows an example where the whole loop filter is mounted out of the package. In  FIGS. 18B  to  18 D, parasitic inductors Lp in the package are added.  
         [0086]      FIG. 19  shows a graph of gain frequency characteristic curves observed respectively for the grounding schemes of  FIGS. 18A  to  18 D, wherein marks A to D correspond to  FIGS. 18A  to  18 D. In  FIG. 19 , the ordinate represents gain GA (dB) and the abscissa represents frequency f (Hz). From  FIG. 19 , it is apparent that, for the grounding schemes of  FIGS. 18B and 18D , the attenuation effect is restricted, affected by the inductors Lp in the package, but such restriction can be lessened by using separate grounds as shown in  FIG. 18C .  
       Embodiment 6  
       [0087]     While the foregoing embodiments has mostly discussed the offset PLL, a sixth embodiment relates to the application of the invention to a two-step transmitter which is shown in  FIG. 20 . Here, the circuit configuration shown in  FIG. 3  is applied. Components corresponding to those shown in  FIG. 3  are assigned the same reference numbers and their explanation is not repeated. In this embodiment, instead of the offset PLL section O-PLL in  FIG. 3 , a transmission frequency band TX section is comprised of a transmitting mixer  112 , an RF band-pass filter  113 , and an IF band-pass filter  114 , which differs from the circuitry of  FIG. 3 . As is the case for the circuitry of  FIG. 3 , in this embodiment as well, the VCO  21  included in the synthesizer SS used for local oscillation for the receiver is also used to generate RF local signals for transmission and IF local signals to drive the quadrature modulator  8 .  
         [0088]     Output of the quadrature modulator  8  passes through the IF band-pass filter  114  and is input to one input of the mixer  112  and an RF local signal generated by frequency-dividing the output from the VCO  21  under control of the control circuit  26  is input to the other input of the mixer. Output of the mixer  112  passes through the RF band-pass filter  113  and a transmission frequency band signal is generated. As a matter of course, the operation for avoiding transmission spurious, explained with  FIG. 2 , is performed, and, by using the VCO for both receiving and transmission, the IC will be downsized.  
         [0089]     While the preferred embodiments of the present invention have been described, focusing the transceiver on a mobile phone for use in Europe, it will be appreciated that the present invention is not limited to the above embodiments and various design changes may be made without departing from its spirit and scope. For instance, of course, the present invention can be applied to mobile phones of other types such as those for CDMA system and wireless LAN or the like.  
         [0090]     According to the present invention, a single PLL synthesizers serves as both RF frequency band PLL and IF frequency band PLL; therefore, the number of necessary oscillators can be reduced and the wireless transmitter is downsized.