Abstract:
A single integrated chip has first and second voltage controlled oscillators and first and second buffers. The first voltage controlled oscillator has a first output defined by a first frequency band, and the second voltage controlled oscillator has a second output defined by a second frequency band. The first and second buffers selectively couple the first and second outputs to a common output. The first buffer is coupled between the first frequency controlled oscillator and the common output, and the second buffer is coupled between the second frequency controlled oscillator and the common output. The first buffer has a high output impedance when the second buffer couples the second output to the common output, and the second buffer has a high output impedance when the first buffer couples the first output to the common output.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The present invention relates to a voltage controlled oscillator capable of producing an output in a selected one of multiple frequency bands.  
         BACKGROUND OF THE INVENTION  
         [0002]    It is desirable to increase the level of integration of an integrated circuit, such as an integrated RF circuit designed for wireless communications, because an increase in the level of integration generally lowers the cost of an integrated circuit. One way to increase the level of integration of an integrated circuit is to design the integrated circuit as a chip that can be used in different regions of the frequency spectrum. The same integrated circuit can then be used in multiple applications.  
           [0003]    The present invention is directed to an integrated circuit that has the capability of providing an output in different frequency bands.  
         SUMMARY OF THE INVENTION  
         [0004]    In accordance with one aspect of the present invention, a single integrated chip comprises a first voltage controlled oscillator, a second voltage controlled oscillator, and first and second buffers. The first voltage controlled oscillator has a first output defined by a first frequency band. The second voltage controlled oscillator has a second output defined by a second frequency band, and the second frequency band is different from the first frequency band. The first and second buffers selectively couple the first and second outputs to a common output. The first buffer is coupled between the first frequency controlled oscillator and the common output, and the second buffer is coupled between the second frequency controlled oscillator and the common output. The first buffer has a high output impedance when the second buffer couples the second output to the common output, and the second buffer has a high output impedance when the first buffer couples the first output to the common output.  
           [0005]    In accordance with another aspect of the present invention, a single integrated chip comprises a first voltage controlled oscillator, a second voltage controlled oscillator, an output terminal, and a selection apparatus. The first voltage controlled oscillator has a first output defined by a first frequency band. The second voltage controlled oscillator has a second output defined by a second frequency band, and the second frequency band is different from the first frequency band. The selection apparatus selectively couples the first and second outputs to the output terminal.  
           [0006]    In accordance with still another aspect of the present invention, an integrated circuit comprises a first voltage controlled oscillator, a second voltage controlled oscillator, first and second buffers, a first transmission gate, and a second transmission gate. The first voltage controlled oscillator has a first output defined by a first frequency band. The second voltage controlled oscillator has a second output defined by a second frequency band, and the second frequency band is different from the first frequency band. The first and second buffers selectively couple the first and second outputs to a common output. The first buffer is coupled between the first frequency controlled oscillator and the common output, and the second buffer is coupled between the second frequency controlled oscillator and the common output. The first transmission gate is coupled so as to selectively control the first buffer. The second transmission gate is coupled so as to selectively control the second buffer.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    These and other features and advantages will become more apparent from a detailed consideration of the invention when taken in conjunction with the drawings in which:  
         [0008]    [0008]FIG. 1 illustrates a chip having a multi-band voltage controlled oscillator; and,  
         [0009]    [0009]FIG. 2 illustrates features of the chip shown in FIG. 1 in additional detail. 
     
    
     DETAILED DESCRIPTION  
       [0010]    As shown in FIG. 1, a chip  10  includes a reference voltage generator  12  that generates an output voltage reference so that the output voltage reference is independent of temperature and supply voltage variations. The voltage reference supplied by the reference voltage generator  12  is provided to an oscillator regulator  14  and to a buffer regulator  16 . The oscillator regulator  14  and the buffer regulator  16  function as stable voltage/current sources for a multi-band voltage control oscillator stage  18  and an output buffer stage  20 , respectively.  
         [0011]    The multi-band voltage control oscillator stage  18  includes a first voltage controlled oscillator  22  coupled to the output of the oscillator regulator  14  by way of a first oscillator transmission gate  24  and a second voltage controlled oscillator  26  coupled to the output of the oscillator regulator  14  by way of a second oscillator transmission gate  28 . The first oscillator transmission gate  24  is responsive to a first frequency select input  30  to turn the first voltage controlled oscillator  22  on and off, and the second oscillator transmission gate  28  is responsive to a second frequency select input  32  to turn the second voltage controlled oscillator  26  on and off. The first and second oscillator transmission gates  24  and  28  may be electronic switches and generally function so that, while one of the first and second voltage controlled oscillators  22  and  26  is on, the other of the first and second voltage controlled oscillators  22  and  26  is off.  
         [0012]    The output buffer stage  20  includes a first output buffer  34  coupled between the output of the first voltage controlled oscillator  22  and output terminals  36 , and a second output buffer  38  coupled between the output of the second voltage controlled oscillator  26  and the output terminals  36 . A first output buffer transmission gate  40  is coupled between a control terminal of the first output buffer  34  and the output of the buffer regulator  16 , and a second output buffer transmission gate  42  is coupled between a control terminal of the second output buffer  38  and the output of the buffer regulator  16 .  
         [0013]    The first output buffer transmission gate  40  is responsive to the first frequency select input  30  to turn the first output buffer  34  on and off, and the second output buffer transmission gate  42  is responsive to the second frequency select input  32  to turn the second output buffer  38  on and off. The first and second output buffer transmission gates  40  and  42  may also be electronic switches and generally function so that, while one of the first and second output buffers  34  and  38  is on, the other of the first and second output buffers  34  and  38  is off.  
         [0014]    The first and second voltage controlled oscillators  22  and  26  produce output signals that vary through correspondingly different frequency bands. Hence, if the first voltage controlled oscillator  22  is selected by the first frequency select input  30 , the first voltage controlled oscillator  22  produces a first output signal whose frequency may be varied from frequency f 1  to frequency f 2  within the frequency band f 1 -f 2 . On the other hand, if the second voltage controlled oscillator  26  is selected by the second frequency select input  32 , the second voltage controlled oscillator  26  produces a second output signal whose frequency may be varied from frequency f 3  to frequency f 4  within the frequency band f 3 -f 4 . The frequency band f 3 -f 4  is different than the frequency band f 1 -f 2  at least in the sense that the frequency bands f 1 -f 2  and f 3 -f 4  are not identical. Any known voltage controlled oscillators may be used for the first and second voltage controlled oscillators  22  and  26  as long as the components of such known voltage controlled oscillators are chosen so as to produce the two different frequency bands f 1 -f 2  and f 3 -f 4 .  
         [0015]    The frequency of the output signals provided by the first voltage controlled oscillator  22  may be varied by the use of one or more control lines  23 . Similarly, the frequency of the output signals provided by the second voltage controlled oscillator  26  may be varied by the use of one or more control lines  27 . Accordingly, this arrangement provides a continuous frequency selection capability within the frequency bands f 1 -f 2  and f 3 -f 4 .  
         [0016]    When the first voltage controlled oscillator  22  is selected by the first frequency select input  30 , the first output buffer  34  is also selected by the first frequency select input  30  so that the output signal from the first voltage controlled oscillator  22  is provided to the output terminals  36 . Similarly, when the second voltage controlled oscillator  26  is selected by the second frequency select input  32 , the second output buffer  38  is also selected by the second frequency select input  32  so that the output signal from the second voltage controlled oscillator  26  is provided to the output terminals  36 .  
         [0017]    The first and second output buffers  34  and  38  provide isolation between the corresponding first and second voltage controlled oscillators  22  and  26  and downstream circuitry coupled to the output terminals  36 . In order for the first and second output buffers  34  and  38  to provide such isolation, the first output buffer  34  preferably presents a high impedances to the downstream circuitry coupled to the output terminals  36  when the first output buffer  34  is off. Similarly, the second output buffer  38  preferably presents a high impedances to the downstream circuitry coupled to the output terminals  36  when the second output buffer  38  is off.  
         [0018]    The output buffer design of FIG. 2 achieves these high impedance off states. Although the output buffer design of FIG. 2 is assumed to be the design of the first output buffer  34 , it should be understood that the output buffer design of FIG. 2 could also be used for the design of the second output buffer  38 .  
         [0019]    As shown in FIG. 2, the first output buffer  34  includes a bias circuit  50  coupled between supply lines  52  and  54 . The supply line  52  may be supplied with positive potential from a positive potential supply terminal  56  through a pFET  58  under control of a frequency select enable input  60  coupled to the gate of the pFET  58 . The supply line  54  may be held at a reference potential such as ground.  
         [0020]    The bias circuit  50  has a pFET  62  whose source terminal is coupled to the supply line  52  through a resistor  64  and whose drain terminal is coupled to one side of the first output buffer transmission gate  40  through series coupled resistors  66 ,  68 , and  70 . The gate of the pFET  62  is coupled to the common junction of the resistors  66  and  68 . Similarly, the bias circuit  50  has an nFET  72  whose source terminal is coupled to the supply line  54  through a resistor  74  and whose drain terminal is coupled to the other side of the first output buffer transmission gate  40  through series coupled resistors  76 ,  78 , and  80 . The gate of the nFET  72  is coupled to the common junction of the resistors  76  and  78 .  
         [0021]    The first output buffer transmission gate  40  is sourced from the supply lines  52  and  54  and is controlled by the frequency select enable input  60  as well as a frequency select enable-not input  82 . The frequency select enable input  60  and the frequency select enable-not input  82  form the first frequency select input  30  of FIG. 1.  
         [0022]    The junction between the drain terminal of the pFET  62  and the resistor  66  is coupled to the gate of a pFET  84  through a resistor  86 , and to the gate of a pFET  88  through a resistor  90 . Similarly, the junction between the drain terminal of the nFET  72  and the resistor  76  is coupled to the gate of an nFET  92  through a resistor  94 , and to the gate of an nFET  96  through a resistor  98 . The source terminals of the pFET  84  and the nFET  92  are coupled to the supply lines  52  and  54 , and the source terminals of the pFET  88  and the nFET  96  are coupled to the supply lines  52  and  54 .  
         [0023]    One output of the first voltage controlled oscillator  22  is coupled to the gate of the pFET  84  through a capacitor  100 . This one output of the first voltage controlled oscillator  22  is also coupled directly to the gate of the nFET  92 . The other output of the first voltage controlled oscillator  22  is coupled to the gate of the pFET  88  through a capacitor  102 . This other output of the first voltage controlled oscillator  22  is also directly to the gate of the nFET  96 .  
         [0024]    A pFET  104  has a source terminal coupled to the positive potential supply terminal  56  and a drain terminal coupled to the gate of the pFET  84  through the resistor  86  and to the gate of the pFET  88  through the resistor  90 . The gate of the pFET  104  is coupled to the frequency select enable input  60 . Also, an nFET  106  has a source terminal coupled to the supply line  54  and a drain terminal coupled to the gate of the nFET  92  through the resistor  94  and to the gate of the nFET  96  through the resistor  98 . The gate of the nFET  106  is coupled to the frequency select enable-not input  82 .  
         [0025]    The first output buffer  34  has an output line  108  taken from the junction of the drain terminals of the pFET  84  and the nFET  92 , and the first output buffer  34  also has an output line  110  taken from the junction of the drain terminals of the pFET  88  and the nFET  96 . As shown in FIG. 1, the output lines  108  and  110  are coupled to the output terminals  36 .  
         [0026]    The bias circuit  50 , the pFETs  84  and  88 , and the nFETs  92  and  96  achieve a high impedance state when the first output buffer  34  is off. Accordingly, when the first output buffer  34  is off, the gates of the nFETs  92  and  96  are pulled to ground, and the gates of the pFETs  84  and  88  are pulled to a high voltage. Consequently, the pFETs  84  and  88  and the nFETs  92  and  96  are in a condition where their gate to source voltages are zero so that these FETs conduct no current. Because these FETs conduct no current, their output impedance is high so that the first output buffer  34  does not load the second output buffer  38  which is active (on) when the first output buffer  34  is inactive (off).  
         [0027]    Certain modifications of the present invention have been described above. Other modifications will occur to those practicing in the art of the present invention. For example, the chip  10  is shown with only two voltage controlled oscillators. However, the chip may be provided with a greater number of voltage controlled oscillators in order to operate over additional frequency bands.  
         [0028]    Moreover, because the first oscillator transmission gate  24  and the first output buffer transmission gate  40  operate in unison, the first oscillator transmission gate  24  and the first output buffer transmission gate  40  may be replaced by a single transmission gate coupled both to the first voltage controlled oscillator  22  and to the first output buffer  34 . Similarly, because the second oscillator transmission gate  28  and the second output buffer transmission gate  42  operate in unison, the second oscillator transmission gate  28  and the second output buffer transmission gate  42  may be replaced by a single transmission gate coupled both to the second voltage controlled oscillator  26  and to the second output buffer  38 .  
         [0029]    Accordingly, the description of the present invention is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which are within the scope of the appended claims is reserved.