Abstract:
A rejection converter is disclosed for use in a transmitter for operating in at least either of a first mode for transmitting signals within a first frequency range, and a second mode for transmitting signals within a second frequency range. The rejection converter includes an input unit for receiving an input signal in at least either of the first or second frequency ranges. The rejection converter also includes a rejection unit for rejecting at least one spurious harmonic signal associated with the first frequency range that falls within the second frequency range. The rejection converter permits signals in the second frequency range to be passed when the output signal is within the second frequency range.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to the field of transmitters for radio frequency communication systems, and particularly relates to transmitters including constant envelope modulation systems. 
     As wireless communication systems have become increasingly popular, a demand has developed for less expensive yet spectrally clean radio frequency (RF) transmitters having constant envelope modulation systems. High quality RF transmitters typically include relatively expensive components. For example, certain bandpass filters, such as surface acoustic wave (SAW) filters provide excellent performance yet are relatively expensive. Many applications further require transmitters that exhibit low power consumption. It is also desirable that transmitters be suitable for use with any of a plurality of standards for modulation, e.g., global system for mobile communication (GSM) or digital cellular system (DCS). 
     Constant envelope modulation systems including translation loop modulators are known to provide circuits having relatively less expensive filtering requirements. Translation loop modulators generally include a feedback loop in communication with the output oscillator that is coupled to a transmission antenna. The feedback loop permits the circuit itself to provide bandpass filtering since the output signal may be locked to a given center frequency. 
     A conventional translation loop modulation system is shown in FIG.  1 . The system  10  includes quadrature modulation circuitry  12 , phase comparitor circuitry  14 , a voltage controlled oscillator (VCO) 16  coupled to an output antenna (not shown) a feedback coupler  17 , and a feedback path  18 . Input signals representative of the information to be modulated and transmitted may be applied to the I and Q channels of the quadrature modulator. The input signals may be modulated to adjust the phase or angle of a reference signal. This phase information is converted to a voltage signal by the phase comparitor circuitry  14 , and the voltage signal is then converted to a frequency signal by the VCO  16 . The feedback path  18  provides a phase locked loop to lock the VCO  16  to a given center frequency. 
     It is conventionally known that transmitter circuits should be designed to reduce the possibility of spurious signals (e.g., harmonics as well as foreign signals) being introduced into the system. In certain situations, the origin of some spurious signals may be extremely difficult to discern, particularly if they appear only sporatically, and may be nearly impossible to simulate. To address this problem, it is conventionally believed that transmitter circuits of the type shown in FIG. 1 should be designed to be flexible so that they may be adjusted to remove any noise. 
     For example, in certain situations, a circuit may be most easily corrected by adjusting either the voltage controlled oscillator  20  in the phase comparitor circuitry, or the voltage controlled oscillator  22  in the feedback path. Employing two separate oscillators facilitates adjustment for reducing noise since either may be adjusted independent of the other. Moreover, the frequencies may be chosen so as to not be harmonically related, which minimizes the chance of harmonic spurious signals being produced by the oscillators. 
     Unfortunately, however, some oscillators are rather expensive. For example, certain oscillator circuits that are formed of synthesizers produce very stable output signals, but are relatively expensive. It is also desirable that the use of relatively expensive filters be avoided. 
     There is a need, therefore, for inexpensive yet efficient constant envelope modulation systems for use with dual mode transmitters. There is further a need for a translation loop modulator that is spectrally efficient yet economical to produce. 
     SUMMARY OF THE INVENTION 
     The invention provides a rejection converter for use in a transmitter, such as a translation loop modulator, for operating in at least either of a first mode for transmitting signals within a fast frequency range, and a second mode for transmitting signals within a second frequency range. The converter includes an input unit for receiving an input signal in at least either of the first or second frequency ranges. The converter also includes a rejection unit for rejecting at least one spurious harmonic signal associated with the first frequency range that falls within the second frequency range. The converter permits signals in the second frequency range to be passed when the output signal is within the second frequency range. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following detailed description may be further understood with reference to the accompanying drawings in which: 
     FIG. 1 shows a functional block diagram of a conventional translation loop modulator; 
     FIG. 2 shows a functional block diagram of a translation loop modulator including an image rejection downconverter of an embodiment of the invention; and 
     FIG. 3 shows a functional block diagram of the image rejection downconverter shown in FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It has been discovered that a translation loop modulator may include a reference oscillator that provides a local oscillator signal to both the phase comparator circuitry and to the feedback path. As shown in FIG. 2, a system  30  of an embodiment of the invention includes quadrature mixer circuitry including two mixers  32  and  34 , a phase shift device  36 , a summing device  38  and a bandpass filter  40 . One input signal to the first mixer  32  is the I channel (or In Phase channel) input modulation signal, and the other is a feedback signal  56  without a phase shift. The output of the mixer  32  is coupled to the summing device  38 . One input signal to the second mixer  34  is the Q channel (or quadrature channel) input modulation signal, and the other is a phase shifted feedback signal that is produced by the phase shift device  36 . In other embodiments, various combinations of phase shifting may be employed to achieve quadrature modulation of the input signals. The output of the mixer  34  is combined with the output of the mixer  32  at the summing device  38  to produce a combined signal. This combined signal is filtered by bandpass filter  40  to produce a quadrature modulation signal. 
     The phase comparitor circuitry of the embodiment shown in FIG. 2 includes an m frequency divider  42 , a phase comparitor device  44 , an n frequency divider  46 , and a loop filter  48 . The quadrature modulation signal is input to the m frequency divider  42 . The phase comparitor device receives one input from the output of the m frequency divider  42 , and the other input from the n frequency divider  46 . The output of the phase comparitor device  44  is coupled to a low pass filter  48 , the output of which is coupled to an output VCO  50 . The VCO  50  produces the transmitter output signal  52 , and is coupled to a power amplifier (not shown) as well as an antenna (not shown). 
     Downconverter circuitry  54  is provided together with low pass filter  60  and bandpass filter  62  in the feedback path to translate the frequency of the output signal  52  RF OUT  to an intermediate frequency RF IF  (the frequency of the feedback signal  56 ). The downconverter circuitry  54  is provided a reference signal  58  at a local oscillator frequency RF LO , and this reference signal is provided to the n frequency divider  46  of the phase comparator circuitry. One VCO only may, therefore, provide an oscillator signal to both the phase comparator circuitry and to the downconverter mixer in the feedback circuitry. This is achieved through careful selection of components and frequency plan. 
     Each mixer will produce signals having frequencies at the sum as well as at the difference between the frequencies of the two input signals. In particular, the product of two sine functions sin(α)×sin(β) equals ½cos (α−β)−½cos(α+β). The two frequencies produced at the output, therefore, would be F 1 +F 2  and F 1 −F 2 . One of the two signal frequencies may then be filtered out. The quadrature modulation signal is then coupled to phase comparator circuitry. 
     The circuit provides that the frequency of the transmitter output signal (RF OUT ) may be related to the frequency of the local oscillator signal (RF LO ) in either of two ways, either RF LO /n=(RF LO −RF OUT )/m, or RF LO /n=(RF LO +RF OUT )/m. The first relationship provides that RF LO =RF OUT ×n/(n−m) and the second relationship provides that RF LO =RF OUT ×n/(n+m). The values of m and n may be chosen such that the transmitter output signal may be at 900 MHz for GSM, and may be at 1800 MHz for DCS. This may be achieved by recognizing that R OUT =RF LO +RF IF  for DCS and RF OUT =RF LO −RF IF  for GSM where RF IF  is the frequency of the intermediate frequency signal, which is the feedback signal to the quadrature modulator. 
     During operation, the output of the phase comparator  44  provides a do voltage responsive to the phase difference between two input signals of the same frequency. For example, the input signals to the phase comparator  44  may each be 225 MHz in frequency. If m=2 and n=6, then the signal input to the m frequency divider  42  must be 450 MHz in frequency, and the signal input to the n frequency divider  46  must be 1350 MHz. For GSM, the output signal produced by the transmit oscillator will be 900 MHz in frequency. This signal is output to the transmitter antenna (not shown). For these values of m and n, therefore, RF LO ={fraction (3/2)} RF OUT  for GSM, RF LO =¾ RF OUT  for DCS. 
     By controlling I and Q, the phase (or angle) of the 450 MHz signal that is input to the m divider  42  may be precisely controlled. For example, if zero volts is applied on the Q input and one volt is applied to the I input, then the signal provided to the divider circuitry would be a 450 MHz signal at zero degrees. If zero volts is applied on the Q input and negative one volt on the I input, then the quadrature output signal would be a 450 MHz signal at 180 degrees. If one volt is applied on the Q input and zero volts on I input, then the output signal would be a 450 MHz signal at 90 degrees. If negative one volt is applied on the Q input and zero volts is applied to the I input, then the output signal would be a 450 MHz signal at −90 degrees. By adjusting the I and Q inputs, the angle of the 450 MHz signal may be fully adjusted. 
     The quadrature modulator therefore provides the modulation for the RF output signal. The output of the phase comparator produces a signal at the frequency of the sum of the inputs, as well as a signal at a frequency of the difference between the inputs. The signal at the sum frequency (450 MHz) is filtered out at the filter  48 , and the do signal (zero MHz.) is input to the voltage controlled oscillator, which in turn, produces the output signal for the antenna. The filter  48  also filters any other noise that may develop in the system. The output of divider  46  is not modulated, whereas the output of divider  42  is modulated. The output of device  44  and filter  48  is a DC voltage including modulation information. 
     With proper selection of the downconverter oscillator, the filters  40  and  48 , and the values of the frequency dividers  42  and  46 , a translation loop modulator circuit may be provided using one oscillator that is coupled to the phase comparator circuitry and the feedback circuitry. In other embodiments, the values of m and n may be different, e.g, m=2 and n=6. In this case as well, however, RF LO ={fraction (3/2)} RF OUT  for GSM, and RF LO =¾ RF OUT  for DCS. 
     Higher order intermodulation products may be tow pass filtered, but the image frequency must be filtered or rejected. Since RF IF =(m/n) RF LO , the frequency of the output signal RF OUT  may be described as RF OUT =RF LO  (1+m/n) for DCS and RF OUT =RF LO  (1−m/n) for GSM. The translation, therefore, of the frequency of the output signal RF OUT  to the intermediate frequency RF IF  may be described as RF IF =|K×RF OUT −J×RF LO |, where K and J are integers. In a distortion-less system, K=J=1. In a system exhibiting some distortion, J and K represent the harmonic orders of both RF LO  and RF OUT  respectively, which may also create products at RF IF . These harmonics may be generated by the transmit VCO  50 , the reference oscillator, or by non-linearities in the downconverter circuitry. Since J represents the harmonic value of RF LO , and K represents the harmonic value of RF OUT , the values of J and K may be determined from the relationship J=K+(K±1)(m/n) for DCS and J=K−(K±1)(m/n) for GSM, again, where J, K, m and n are integers. 
     For example, in a GSM system where m=2 and n=6, K=2 (the second harmonic of the transmit VCO) and J=1 (the fundamental of the reference oscillator). For a dual mode radio, the second harmonic of the GSM transmit output signal falls into the DCS transmit band, and cannot be simply filtered. For this choice of m and n, RF OUT =2RF IF  since RF OUT =RF LO −RF IF ={fraction (6/2)}RF IF −RF IF . The second harmonic of the GSM frequency is the image frequency of the desired response. An image rejection mixer may be used to accommodate both GSM and DCS modes. 
     The circuit may include two separate VCOs with transmit filters to provide the necessary filtering, but such a circuit adds costs and complexity to the circuit, and further may add distortion. A switched filter may also provide the required filtering, but such filters also add cost and complexity to the circuit, and also may add distortion. 
     As shown in FIG. 3, the downconverter circuitry  54  of the embodiment shown in FIG. 2 includes a VCO  64  that is coupled to the n frequency divider  46  (shown in FIG. 2) and a quadrature divider  66 . The VCO  64  produces the local oscillator signal  58 . The quadrature divider  66  is coupled to two mixers  68  and  70 , each of which is also coupled to the low pass, filter  60  in communication with the output transmit signal. The quadrature divider  66  produces two signals, one having a phase shift of 90 degrees and the other with zero degrees phase shift. The zero degree phase shift signal from the quadrature divider  66  is input to the first mixer  68 , and the 90 degree phase shift signal from the quadrature divider  66  is coupled to the second mixer  70  via a selective inverter  69 . The output of the low pass filter  60  is also input to each of the mixers  68  and  70 . The selective inverter  69  selectively inverts an input signal responsive to a selection signal at  71 . In GSM mode the selective inverter is disabled and therefore does not invert the input signal thereof, and in DCS mode the selective inverter is enabled by applying an enable signal at  71 . 
     The output of the first mixer  68  is coupled to a 90 degrees phase shift input of a quadrature combiner  72 , and the output of the second mixer  70  is coupled to a zero degrees phase shift input of the quadrature combiner  72 . The output of the quadrature combiner is coupled to the bandpass filter  62 , which in turn, is coupled to the quadrature modulation circuitry shown in FIG. 2 . 
     The operation of the circuit may be further described with reference to FIG. 3, as well as a discussion of the relationships between the signals. In short, the described circuit provides that signals at the GSM frequency are passed, as are signals at the DCS frequency, but signals at twice the GSM frequency are rejected. 
     For GSM operation, RF OUT =RF LO −RF IF , which may be written as ω G =ω L −ω I . This represents the difference product. The feedback signal (a 1 ) from the low pass filter  60  may be represented as sin (ω L −ω I )t=sin ω G t. The signal (a 2 ) produced by the VCO  64  and passed through the quadrature divider  66  (at zero phase) to the mixer  68  may be represented as: 
     a 2 =sin ω L t, and the phase shifted signal that is coupled to the other mixer is: 
     
       
           a   2 ′=sin(ω L   t +π/2)=cos ω L   t.   
       
     
     The signal (a 3 ) produced by the mixer  68  is given by: 
     
       
           a   3 =sin(ω L −ω I ) t ×sin ω L   t =½cos (ω L −ω I −ω L ) t− ½cos(ω L −ω I +ω L )  t;    
       
     
     
       
         or  
       
     
     
       
           a   3 =½cos(−ω I ) t −½cos(2ω L −ω I ) t    
       
     
     The second term is ultimately filtered by filter  62 , so it can be ignored here. Since cos (−α)=cos α, a 3  may be expressed as: 
       a   3 =½cos ω I   t   
     The signal (a 4 ) that is produced by the mixer  70  is provided by: 
     
       
           a   4   =a   1   a   2 ′=sin(ω L −ω I ) t× sin(ω L   t +π/2)= 
       
     
     
       
         sin(ω L −ω I ) t× cos ω L   t=   
       
     
     
       
         ½sin(2ω L −ω I ) t ×½sin(−ω I ) t=   
       
     
     
       
         ½sin(2ω L −ω I ) t ×½sin(−ω I ) t   
       
     
     The first term is rejected by filter  62 , so it can be ignored. Since sin (−α)=−sin α, then a 4 may be expressed as a 4=− ½sin ω I t. 
     The output signal a 0  may be expressed as a 3 ′+a 4 , where 
     
       
           a   3 ′½cos(ω I   ′t +π/2)=−½sin ω I   t.   
       
     
     So, a 0 =−½sin ω I   t +(−½sin ω I   t )=−sin ω I   t.   
     The undesired GSM harmonic is filtered as follows. The sum product ω L +ω I =2ω G . 
     In this case, 
     
       
           a   I =sin(ω L +ω I ) t= sin 2ω G   t   
       
     
     
       
           a   2 =sin ω L   t ,  
       
     
     
       
         and 
       
     
     
       
           a   3   =a   1   a   2 =sin(ω L +ω I ) t× sin ω L   t=   
       
     
      ½cos(ω L −ω I −ω L ) t −½cos(ω L +ω I +ω L ) t=   
     
       
         ½cos ω I   t −½cos(2ω L +ω I ) t   
       
     
     Again, the second term is filtered by the filter  62 , so a 3 =½cos ω I t. The phase shifted term (a 3 ′) is expressed as, 
     
       
           a   3 ′=½cos(ω I   t+π/ 2)=−½sin ω I   t   
       
     
     Since a 4 =a 1 a 2 ′, 
     
       
           a   4 =sin(ω L +ω I ) t× sin(ω L   t +π/2)= 
       
     
     
       
         sin(ω L +ω I ) t× cos ω L   t=   
       
     
     
       
         ½sin(ω L +ω I +ω I ) t +½sin(ω L +ω I −ω L ) t=   
       
     
     
       
         ½sin(2ω L +ω I ) t +½sin ω I   t   
       
     
     Again, the first term is rejected by filter  62 , so it can be ignored. The signal (a 4 ) may therefore be expressed as a 4 =½sin ω I  t. 
     In this case, the output signal a 0  may be expressed as a 3 ′+a 4 , where 
     
       
           a   3 ′=½cos(ω I   t+π/ 2)=−½sin ω I   t.   
       
     
     So, a 0 =−½sin ω I t+½sin ω I  t=0. 
     For DCS, the signal (a 2 ) must be inverted prior to being input to the mixer  70 . The signal a 2  is inverted by the selective inverter  69  that inverts an input signal responsive to a selector signal at  7   i . In GSM mode, the selector signal does not direct the selective inverter  69  to invert the input signal. In DCS mode, the selector signal is enabled causing the selective inverter  69  to invert its input signal (a 4 ) and produce an inverted signal (−a 2 ) which is input to the mixer  70  as shown in FIG.  3 . 
     Solving for the signal (a 1 ) entering mixers  68  and  70  from the filter  60 , a 1 =sin (ω L +ω I ) t=sin ω D t. The signal produced by the VCO  64  (a 2 ) is given by 
     
       
           a   2 =sin ω L , 
       
     
     
       
         and  
       
     
     
       
           a   2 ′=sin(ω L   t +π/2)=cos ω L   t.   
       
     
     The signal produced by the mixer  68  (a 3 ) is given by a 3 =a 1 a 2 , or 
     
       
         a 3 −sin(ω L +ω I ) t× sin ω L   t=   
       
     
     
       
         ½cos(ω L +ω I −ω L ) t −½cos(ω L +ω I +ω L ) t=   
       
     
     
       
         ½cos ω I   t −½cos(2ω L +ω I ) t   
       
     
     Again, the second term is filtered, so a 3 =½cos ω I t. The signal a 3 ′ produced internally by the quadrature divider  66  responsive to a 3  is provided by: 
       a   3 ′=½cos(ω I   t +π/2)=−½sin ω I   t.   
     The signal a 4  produced by the mixer  70  is given by a 4 =a 1 ×(−a 2 ′), or 
     
       
           a   4 =sin(ω L +ω I ) t×−sin(ω   L   t +π/2)= 
       
     
     
       
         −sin(ω L +ω I ) t× cos ω L   t=   
       
     
     
       
         −½sin(ω L +ω I +ω L ) t −½sin(ω L +ω I −ω L ) t=   
       
     
     
       
         ½sin(2ω L +ω I ) t −½sin ω I   t.   
       
     
     Since the first term is filtered, a 4 =−½sin ω I t 
     Solving, therefore, for the feedback signal (a 0  ) for DCS provides that: 
     
       
           a   0   =a 3 ′+a 4, 
       
     
     
       
         or 
       
     
     
       
           a   0 =−½cos ω I   t +½cos ω I   t,    
       
     
     
       
         or 
       
     
     
       
           a   0 =−sin ω I   t.   
       
     
     The product, therefore, resulting from twice the signal for GSM is rejected while the fundamental GSM signal (x 1 ) is passed. By inverting the signal a 2 ′, DCS mode is accommodated with a minimum amount of additional circuitry. 
     Those skilled in the art will appreciate that numerous modifications and variations may be made to the above disclosed embodiments without departing from the spirit and scope of the invention.