Abstract:
A multimode communications system includes a first communications module, a transmission module, a switch and a second communications module. The first communication module has a frequency modulator for modulating a dividing ratio to adjust an oscillating signal and selectively enabling the oscillating signal to have its frequency change with a variety of contents of a first communications signal by modulating the dividing ratio according to the contents of the first communications signal on a modulating mode or enabling the oscillating signal to have its frequency constant by keeping the dividing ratio unchanged. The switch is to selectively transmit the oscillating signal either to the transmission module when the frequency modulator is operating on the modulating mode or to a receiving end when the frequency modulator is operating on the constant frequency mode.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. provisional Application No. 60/481,737, which was filed on Dec. 4, 2003 and is included herein by reference. 

   BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention relates to an RF transceiver, and more particularly, to a multi-mode &amp; multi-band RF transceiver, like a GSM/EDGE &amp; WCDMA dual-mode &amp; multi-band cellular phone, and related communications method. 
   2. Description of the Prior Art 
   The past decade has shown an explosive growth in wireless communications systems. A variety of communications systems, such as GSM and CDMA, have been introduced to the market of cell phones to realize wireless communications functions. A cell phone comprises a wireless RF transceiver to transmit/receive wireless signals. An RF transceiver usually comprises a phase-locked loop (PLL) as a frequency synthesizer to generate a carrier signal for a local oscillator (LO). 
   In general, a typical multi-mode or multi-band RF transceiver has to comprise more than one LO, and more than one PLL accordingly, to generate more than one carrier signal for a variety of bands or system modes, therefore increasing the system complexity and the product cost due to an enlarged chip size. 
   An RF transceiver comprises a transmitter to emit wireless communications signals. Please refer to  FIG. 1 , which is a function block diagram of a wireless transmitter  10  according to the prior art. The basic function of the transmitter  10  is to modulate, or to decode baseband information, such as voice, video, data or other information, onto a high frequency sine wave carrier that can be radiated by a transmit antenna. The reason for this is that signals at higher frequencies can be radiated more effectively, and use the RF spectrum more efficiently, than the direct radiation of the baseband signals. The transmitter  10  comprises a first local oscillator (LO)  12  for generating a first LO signal, an I/Q modulator  14  for modulating an I/Q baseband signal with the first LO signal into an intermediate frequency (IF) signal of a frequency usually ranging from 10 to 100 MHz, a first bandpass filter  16  for passing frequency components within a narrow passband while rejecting frequency components like noises outside the passband, a second LO  18  for generating a second LO signal, a mixer  20  to up-convert the IF signal output from the first bandpass filter  16  into a sum and a difference of the IF signal and the second LO signal by mixing the IF signal with the second LO signal, a second bandpass filter  22  connected to the mixer  20  for passing the sum of the IF signal and the second LO signal only, a power amplifier  24  for increasing the power of signals output from the second bandpass filter  22 , and a transmit antenna  26  for converting signals with amplified power from the power amplifier  24  to propagating electromagnetic place waves. 
   The first LO  12  and the second LO  18  both are made up of a PLL having a voltage-controlled oscillator installed. A feedback control circuit of the PLL enables the voltage-controlled oscillator to precisely track the phase of a stable reference oscillator. The two-staged transmitter  10 , which is applied to WCDMA communications system, has advantages of reduced LO pulling, lower LO feedthrough, and milder cross-talk between I/Q channels. 
   An RF transceiver comprises not only a transmitter, but also a receiver. Please refer to  FIG. 2 , which is a function block diagram of a superheterodyne receiver  30  according to the prior art. The receiver  30  comprises an antenna  32 , a third bandpass filter  34 , a low noise amplifier  36  for amplifying possibly very weak received signals received by the antenna  32  while minimizing noise power that is added to the received signals, a third LO  38  for generating a third LO, a second mixer  40  to down-convert signals transmitted from the low noise amplifier  36  into an IF signal, a fourth bandpass filter  42  connected to the second mixer  40 , a fourth LO  44  for generating a fourth LO, and a demodulator  46  connected to the fourth bandpass filter  42  and to the fourth LO  44  for recovering an I/Q baseband signal from signals filtered by the fourth bandpass filter  42 . 
   According to  FIG. 1  and  FIG. 2 , a transceiver for a cellular phone comprises four elaborate LOs. 
   Because of the very competitive nature of the cellular phone market, there is a strong demand to reduce the parts count, size, weight, and cost of the transmitter  10 , and of the receiver  30  as well. Direct conversion transmitters, which are applied to GSM communications system, are therefore of significant interest, because the first LO  12 , the first bandpass filter  16  and the second bandpass filter  22  of the transmitter  10  are eliminated here with this type of transmitter topology. 
   The above drawback can be overcome if a multi-mode or multi-band RF transceiver can share a single frequency synthesizer capable of generating a variety of carrier signals without impacting the quality of transmitting and receiving signals dramatically. Such an RF transceiver has a simple structure and a low cost. Please refer to  FIG. 3 , which is a function block diagram of a direct conversion transmitter  50  according to the prior art. A pair of I/Q signals enter a fifth bandpass filter  52  and a sixth bandpass filter  54  respectively and mix with an LO signal generated by a fifth LO (PLL)  56  for orthogonalization. The orthogonalized I/Q signals enter a power amplifier  58  for power amplification and are transmitted by a transmit antenna  60 . 
   Despite having an advantage of comprising a minimum of components, the transmitter  50  may still suffer from a problem of LO injection pulling and must requires an additional topology for isolation. Additionally, the PLL, the mixer and the adder of the transmitter  50  have to be designed elaborately to have very low phase noise, therefore increasing the complexity to designing IC circuit and semiconductor manufacturing process. 
   SUMMARY OF INVENTION 
   It is therefore a primary objective of the claimed invention to provide a multi-mode &amp; multi-band RF transceiver of a minimum of components and easy to be fabricated and related wireless communications method. 
   According to the claimed invention, the multimode communications system includes an oscillator for generating an oscillating signal by determining a control signal, the oscillating signal having a frequency corresponding to the control signal, a first frequency divider electrically connected to the oscillator for generating a compare signal by determining the oscillating signal and a dividing ratio, the compare signal having a frequency equal to a product of the frequency of the oscillating signal and the dividing ratio, and a frequency modulator electrically connected to the first frequency divider for modulating the dividing ratio, the frequency modulator capable of operating either on a modulating mode or on a constant frequency mode. When operating on the modulating mode, the frequency modulator enables the oscillating signal to have its frequency change with a variety of contents of a first communications signal by modulating the dividing ratio according to the contents of the first communications signal. When operating on the constant frequency mode, the frequency modulator enables the oscillating signal to have its frequency constant by keeping the dividing ratio unchanged. The communications system further comprises a frequency phase detector electrically connected to the first frequency divider for adjusting the control signal according to the frequency of the compare signal, a transmission module for transmitting communications signals output from the communications system, a switch capable of transmitting the oscillating signal either to the transmission module when the frequency modulator is operating on the modulating mode or to a receiving end when the frequency modulator is operating on the constant frequency mode, and a second communications module. The second communications system comprises a first mixer electrically connected to the receiving end for mixing a second communications signal with a receiving signal received by the receiving end and generating a third communications signal carried over the receiving signal. 
   It is an advantage of the claimed invention that a multimode communications system can be applied to a variety of communications systems with a minimum of components. These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a function block diagram of a wireless transmitter according to the prior art. 
       FIG. 2  is a function block diagram of a receiver according to the prior art. 
       FIG. 3  is a function block diagram of a direct conversion transmitter according to the prior art. 
       FIG. 4  is a function block diagram of an RF transmitter of the preferred embodiment according to the present invention. 
       FIG. 5  is a function block diagram of an RF transceiver of a second embodiment according to the present invention. 
       FIG. 6  is a function block diagram of an RF transmitter of a third embodiment according to the present invention. 
       FIG. 7  is a function block diagram of an RF transmitter of a fourth embodiment according to the present invention. 
   

   DETAILED DESCRIPTION 
   Please refer to  FIG. 4 , which is a function block diagram of a multi-mode &amp; multi-band RF transmitter  70  of a preferred embodiment according to the present invention. The RF transmitter  70  can be applied to a GSM, EDGE, CDMA, WCDMA, CDMA2000, and WLAN communications system etc. The transmitter  70  comprises a fractional-N frequency synthesizer  72 , a first switch  74  capable of selectively connecting a transmitting end  76  electrically connected to the frequency synthesizer  72  to a first, a second, or a third receiving end  78 ,  80  or  82  by determining signals transmitted from the frequency synthesizer  72 , a GSM-900 transmission module  84  electrically connected to the first receiving end  78 , a GSM-1800 transmission module  86  electrically connected to the second receiving end  80 , and a WCDMA (WLAN 802.11b or WLAN 802.11g) module  88  electrically connected to the third end  82 . 
   The frequency synthesizer  72  is capable of selectively generating a carrier wave (CW) (local oscillator wave) having a constant frequency f vco  for up-conversion of a first baseband signal T x1  in a first mode, or an RF signal whose frequency is varied by the dividing ratio of the fractional-N divider  94  controlled by the sigma-delta modulator  90  according to a second baseband signal T x2  on a second mode. The frequency synthesizer  72  comprises a voltage-controlled oscillator (VCO)  92  for generating an oscillating signal according to a control signal, the oscillating signal having a frequency corresponding to the control signal, a first fractional-N frequency divider  94  electrically connected to the VCO  92  for generating a compare signal according to the oscillating signal and to a dividing ratio, the compare signal having a frequency equal to a product of the frequency of the oscillating signal and the dividing ratio, a frequency phase detector  96  electrically connected between the first frequency divider  94  and the VCO  92  for adjusting the control signal according to the frequency of the compare signal, and the sigma-delta modulator  90 , a frequency modulator, electrically connected to the first frequency divider  94  for modulating Tx 2  data into the dividing ratio. 
   The fractional-N frequency divider  94  can be implemented with an add/accumulator and selectively divides an input signal by either N or N+1 according to overflows output from the add/accumulator. Since fraction-N frequency dividers are well known in the art and therefore will not be described for further details. 
   The GSM-900 transmission module  84  comprises a GSM-900 antenna, a low pass filter, a power amplifier (PA), and a PA driver. The GSM-1800 transmission module  86  has a structure similar to that of the GSM-900 transmission module  84 . 
   In  FIG. 4 , the WCDMA module  88  has a heterodyne structure, comprising a second frequency divider  98  for dividing signals received by the third receiving end  82 , an I/Q modulator  100  for modulating Tx 1  data (WCDMA/EDGE), a first mixer  102  for mixing the modulated signals from the I/Q modulator  100  with the signals received by the third receiving end  82 , and an antenna  104  for emitting the mixed signals. 
   When the RF transmitter  70  is operating on a GSM mode, the frequency synthesizer  72  is controlled to generate an RF signal according to the Tx 2  data input to the sigma delta modulator  90  and the first switch  74  is controlled to electrically connect the transmitting end  76  either to the first receiving end  78  or to the second receiving end  80 , and in last the GSM-900 transmission module  84  or the GSM-1800 transmission module  86  emits the RF signal. 
   Alternatively, when the RF transmitter  70  is operating on a WCDMA mode, the frequency synthesizer  72  is controlled to generate the CW (the local oscillator wave) as the diving ratio of the fractional-N divider is a constant, and the first switch  74  is controlled to electrically connect the transmitting end  76  to the third receiving end  82 . The second frequency divider  98  of the WCDMA module  88  divides the CW and generates a divided signal having a second frequency 
               f   vco       N   2       .         
The modulator  100  modulates the Tx 1  data with the divided signal and generates a modulated signal, an IF signal. The first mixer  102  mixes the modulated signal with the CW having the first frequency f vco  and generates two mixed RF signals having two kinds of frequencies,
 
             f   vco     ±         f   vco       N   2       .           
Finally, the antenna  104  emits one of the two mixed RF signals.
 
   The related communications method according to the present invention comprises the following steps: (a) generating a RF signal by the fractional-N frequency synthesizer  72  whose dividing ratio is controlled according to the Tx 2  data and emitting the RF signal when on the GSM mode; (b) generating a CW tone having a constant frequency since the dividing ratio of the fractional-N frequency synthesizer  72  is kept as a constant when on the WCDMA mode; (c) mixing the divided signal generated by the second frequency divider  98  with the modulated signal generated by the I/Q modulator  100  according to the Tx 1  data and directly up-converting the Tx 1  signal into the mixed signal having the frequencies of 
               f   vco     ±       f   vco       N   2         ;         
and (d) emitting the mixed signal with the antenna  104 .
 
   A transceiver comprises both a transmitter and a receiver. Please refer to  FIG. 5 , which is an RF transceiver  400  of a second embodiment according to the present invention. The RF transceiver  400  can be operating on the GSM mode to transmit/receive data asynchronously or operating on the WCDMA mode to transmit/receive data synchronously. 
   In addition to the frequency synthesizer  72  and the WCDMA module  88 , the transceiver  400  further comprises an RF receiving module  402 . When the transceiver  400  is controlled to operate on the WCDMA mode, a second switch  404  connects the frequency synthesizer  72  to the WCDMA module  88  and to the RF receiving module  402  concurrently, and the CW tone having the first frequency f vco  from the frequency synthesizer  72  can be used by the WCDMA module  88  and by the RF receiving module  402 . Please note that the transceiver  400  comprises only one LO. 
   The RF receiving module  402  comprises a third frequency divider  406  for dividing a CW (an local oscillator wave) and for generating a divided CW, a second mixer  408  for mixing the CW tone with the divided CW tone and for generating a mixed signal, a third mixer  410  for down-converting a wireless signal received by the antenna  104 , and a demodulator  412  for demodulating the down-converted signal into an I/Q signal. 
   The RF receiving module  402  may also serve as a GSM receiver. On a GSM receiving mode, the second switch  404  connects the frequency synthesizer  72  with the RF receiving module  402  that provides a local oscillator wave since its dividing ratio is kept as a constant. The RF receiving module  402  therefore down-converts an incoming RF signal according to the local oscillator wave. 
   The second frequency divider  98  of the WCDMA module  88  of the transmitter  70  shown in  FIG. 4  can be alternatively electrically connected to the frequency synthesizer  72  directly rather than to the third receiving end  82  of the first switch  74 , as shown in  FIG. 6 , which is a function block diagram of an RF transmitter  120  of a third embodiment according to the present invention. The transmitter  120  has a structure similar to that of the transmitter  70  shown in  FIG. 4  except that the second frequency divider  98  is electrically connected to the frequency synthesizer  72  directly rather than to the third receiving end  82  of the first switch  74 , as does the third frequency divider  110  of the communications system  70 . 
   Please refer to  FIG. 7 , which is a function block diagram of an RF transmitter  140  of a fourth embodiment according to the present invention and has a homodyne structure. The transmitter  140  also has a structure similar to that of the transmitter  70  except that a WCDMA module  142  of the transmitter  140  comprises a mixer mixing the CW from the frequency synthesizer  72  with another CW to generate a further CW utilized by the I/O modulator  100 . 
   In contrast to the prior art, the present invention can provide a multi-mode &amp; multi-band RF transceiver and related wireless communications method capable of operating on dual modes (GSM/EDGE &amp; WCDMA/WLAN 802.11b &amp; 11g) as well as on multiple bands (GSM 900 &amp; 1,800) with a fractional-N frequency synthesizer consisting of only one LO, therefore reducing the system complexity and the cost to manufacture an integrated circuit. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.