Abstract:
A voltage controlled oscillator (VCO), suitable for use in a frequency shift keying (FSK) system. The VCO device comprises a switching varactor unit, having a first terminal and a second terminal, wherein the switching varactor unit produces a capacitance, according to a frequency-selection voltage. A VCO core has a first output terminal, a second output terminal complementary to the first output terminal, and an input terminal. Wherein, the switching varactor unit is coupled in parallel with the VCO core at the first output terminal and the second output terminal to produce a capacitance effect with respect to the capacitance, so as to adjust a frequency constant √{square root over (LC)} of the VCO core.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The present invention relates to a frequency shift keying (FSK) modulator. More particularly, the present invention relates to a voltage controlled oscillator (VCO) suitable for use in a FSK system. 
   2. Description of Related Art 
   An oscillator is an electric or electronic circuit that produces an output signal that oscillates at a principal oscillation frequency and, in some instances, contains harmonic frequency signals (i.e., signals having frequencies that are multiples of the principal oscillation frequency). Some oscillators have a principal oscillation frequency that may be varied by the application of a suitable control voltage. Oscillators of this type, called voltage-controlled oscillators or VCO&#39;s, are used in many communication and other signal-processing applications. Because suppression or reduction of higher-order harmonics of the principal oscillation frequency and other noise in the output signal of a VCO is often desirable. 
   In a VCO for switching oscillation frequency bands by supplying a band switching signal to an LC resonator circuit that is connected in a phase locked loop having a phase comparator and a loop filter and sets oscillation frequencies, the LC resonator circuit includes at least one inductor and one capacitor, the connection and non-connection of a part of which are switched in response to a band switching signal and a capacitor element. Wherein, a voltage variable capacitor, or called varactor, element whose capacitance value is adjusted in response to the control voltage from the loop filter is connected in series to impedance elements acting as a composite element that exhibits a capacitance value in oscillation frequency ranges. With this arrangement, when an oscillation signal is selectively output in a high or low frequency band by switching the LC resonator circuit, the voltage controlled oscillator can increase the difference between the rates of change in the high and low frequency bands. 
   In communication system, the VCO is implemented in a FSK modulator.  FIG. 1  is a block diagram, schematically illustrating the conventional FSK modulator. In  FIG. 1 , a conventional FSK modulator includes a VCO  20 , a frequency divider  22 , a low pass filter (LF)  24 , and a frequency mixer  26 . The VCO  20  further includes an oscillator for producing a f osc  ∝1/√{square root over (LC)}, where C contains all related capacitance at VCO output. The frequency divider  22  is used for dividing the oscillation output. The mixer  26  is used for synthesizing frequency of the divided output with that of a reference wave signal. The LF  24  is used for removing high-frequency components of this output. The input data (DATA) is, for example, a no-return-zero (NRZ). In addition, it is assumed that a data row of the input data is temporarily at a 0 or 1 in voltage signal level. 
   In wireless communication, the digital binary data is carried by a central frequency f 0  for transmission.  FIG. 2  is a drawing, schematically the typical frequency spectrum of FSK. In order to represent the digital value 0 and digital value 1, a positive frequency shift +fm and a negative frequency shift −fm are used. In the configuration of  FIG. 1 , the frequency of the reference signal which is an output of a reference signal generator which is not shown is synthesized with that of a divided output of the frequency divider  22  by means of the mixer  26  so as to output a desired frequency f 0 . The VCO  20  detects an output of the mixer  26  and converts it to a voltage so as to synthesize it with a voltage level (0 [V] or 1 [V]) of a data row. For example, an output frequency of the VCO  20  becomes a frequency f 1  (f 1 =f 0 −fm 1 ) when a voltage level of a data row is at level 0 [V] and it becomes a frequency f 4  (f 2 =f 0 +fm 4 ) when it is at level 1 [V], so that there is a difference of fm 1 +fm 4  between these frequencies. As described above, the FSK modulation is achieved by shifting a frequency from the intermediate frequency f 0  by ±fm. Since the actual frequency being used is not a fixed value, and can be varied. Therefore, various frequency shifts are necessary to be set, in order to have the optimal frequency to transmit data without errors. 
   Conventionally, the frequency fi can be selected by setting different voltage Vi. The desired frequency spacing between digital level 0 [V] and 1 [V] can be obtained by selecting a pair of frequency [fi, fj] with the desired frequency spacing. 
   However, in the conventional FSK modulator, the frequency spacing is fixed. There is no way to further adjust the frequency spacing to get more precise frequency spacing. If the frequency is more precise, the data error is less. Therefore, it is an issue about how to get more precise frequency spacing to represent the binary data. 
   SUMMARY OF THE INVENTION 
   The invention provides a VCO circuit, of which the characteristic frequency of VCO core can be shifted by coupling in parallel with a switching varactor unit. The switching varactor unit changes the frequency constant √{square root over (LC)} of the VCO core, so as to shift the operation frequency and the frequency spacing. 
   The invention provides a FSK modulator, which includes switching varactor unit coupled with a VCO core in parallel, so that the operation frequency and the frequency spacing can be adjusted to the optimal condition. As a result, the errors occurring in data transmission can be effectively reduced. 
   As embodied and broadly described herein, the invention provides a voltage controlled oscillator (VCO), suitable for use in a frequency shift keying (FSK) system. The VCO device comprises a switching varactor unit, having a first terminal and a second terminal, wherein the switching varactor unit produces a capacitance, according to a frequency-selection voltage. A VCO core has a first output terminal, a second output terminal complementary to the first output terminal, and an input terminal. Wherein, the switching varactor unit is coupled in parallel with the VCO core at the first output terminal and the second output terminal to produce a capacitance effect with respect to the capacitance, so as to adjust a frequency constant √{square root over (LC)} of the VCO core. 
   In the foregoing VCO device, the switching varactor unit comprises a switching diode unit for receiving a mode selection signal with at least one bit data, wherein the switching diode unit includes:
         a plurality of diode pairs coupled in parallel, wherein the diode pairs can be switched on with respect to a quantity of the mode selection signal, so as to produce the capacitance, wherein the diode pair has one common terminal coupled to the frequency-selection voltage and another terminals coupled to the first terminal and the second terminal, respectively.       

   The present invention further provides a frequency shift keying (FSK) system, comprising: a frequency selection unit, for receiving an input signal and a mode selection signal, and exporting a frequency-selection voltage according to the mode selection signal. A switching varactor unit has a first terminal and a second terminal, wherein the switching varactor unit produces a capacitance, according to a frequency-selection voltage. A VCO core has a first output terminal, a second output terminal complementary to the first output terminal, and an input terminal. Wherein, the switching varactor unit is coupled in parallel with the VCO core at the first output terminal and the second output terminal to produce a capacitance effect with respect to the capacitance, so as to adjust a frequency constant √{square root over (LC)} of the VCO core. A first VCO buffer is coupled to the first output terminal of the VCO core and exporting a desired frequency. A second VCO buffer is coupled to the second output terminal of the VCO core. A phase locked loop unit is coupled between an output of the second VCO buffer and the input terminal of the VCO core to form a feedback loop and produce the desired frequency. 
   The invention also provides a method to adjust an operation frequency of a voltage controlled oscillator (VCO) core. The method comprises providing a voltage controlled oscillator (VCO) core, having an output terminal, a feedback output terminal complementary to the output terminal, and an input terminal. The feedback output terminal and the input terminal form a feedback loop through a phase locked loop unit. A switching varactor unit is provided for producing a capacitance with respect to a mode selection signal. The switching varactor unit is coupled with the VCO core in parallel, so as to shift a frequency constant √{square root over (LC)} of the VCO core. 
   It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
       FIG. 1  is a circuit block diagram, schematically illustrating configuration of a conventional FSK modulator. 
       FIG. 2  is a drawing, showing the typical frequency spectrum of FSK. 
       FIG. 3  is a circuit block diagram, schematically illustrating configuration of a FSK system, according to one preferred embodiment of this invention. 
       FIG. 4  is a circuit diagram, schematically illustrating a circuit architecture of a switching varactor unit used in the FSK system, according to one preferred embodiment of this invention. 
       FIG. 5  is a process diagram, showing a testing procedure for the FSK modulator, according to one preferred embodiment of this invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention is directed to a VCO device, of which the characteristic frequency of VCO core can be shifted by coupling in parallel with a switching varactor unit. The switching varactor unit changes the frequency constant √{square root over (LC)} of the VCO core, so as to shift the operation frequency and the frequency spacing. In addition, the VCO device of the invention can be implemented into a FSK modulator, so that the operation frequency and the frequency spacing can be adjusted to the optimal condition. As a result, the errors occurring in data transmission can be effectively reduced. An example is provided for describing the features of the present invention as follows. 
     FIG. 3  is a circuit block diagram, schematically illustrating configuration of a FSK system, according to one preferred embodiment of this invention. In  FIG. 3 , the FSK system or FSK modulator includes a frequency selection unit  100 , a VCO unit  200 , buffers  38   a ,  38   b , and a PLL unit  40 . 
   The frequency selection unit  100  is used to receive an input signal of data or enabling and mode selection signals Mod_ 2 , Mod_ 3  and export a frequency-selection voltage at a node X, according to the mode selection signals. The frequency selection unit  100  includes, for example, a control circuit  30 , and selector switch  32 , and a constant voltage generator  33 . The control circuit  30  receives input signal DATA or ENABLE and mode selection signals Mod_ 2 , Mod_ 3 , and then export a control signal to the selector switch  32 . The DATA is either 0 or 1, and the ENABLE signal deter-mines whether or not the modulation signal is transmitted. The selector switch  32  also receives a voltage signal from the constant voltage generator  33 . The constant voltage generator  33 , for example, can generate five voltage levels in a relation, such as V 1 &gt;V 2 &gt;V 0 &gt;V 3 &gt;V 4 . As described in  FIG. 2 , the frequency spacing is selected according to the voltage. For example, (V 1  &amp; V 2 ) represent the “0” level, and (V 3  &amp; V 4 ) represent the “1” level. V 0  is un-modulated voltage before modulation. The desired frequency spacing can be obtained by properly adjusting the voltage value. The frequency spacing can be combination of fmi, such as [fm 1 , fm 4 ], [fm 1 , fm 3 ], [fm 2 , fm 4 ] or [fm 2 , fm 3 ]. The frequency selection unit  100  then exports a signal of frequency-selection voltage at the node X. 
   The VCO unit  200 , coupled to the node X, further includes a switching varactor unit  34  and a VCO core  36 . The switching varactor unit  34  can, for example, be a switching diode unit  34  having at least a diode  34   a  and at least a second diode  34   b , which form as a diode pair. The switching diode unit  34  will be described in detail later. The positive ends of the diodes are commonly coupled to the node X and output ends of the diode are a first terminal and a second terminal. The switching varactor unit  34  produces a capacitance, according to the frequency-selection voltage. 
   The VCO core  36 , having a first output terminal VCO_ 1 , a second output terminal VCO_ 2  complementary to the first output terminal VCO_ 1 , and an input terminal. It should be noted that the switching varactor unit  34  is coupled in parallel with the VCO core  36  at the first output terminal VCO_ 1  and the second output terminal VCO_ 1  to produce a capacitance effect with respect to the capacitance produced from the switching diode unit  34 , so as to adjust a frequency constant √{square root over (LC)} of the VCO core  36 . 
   In the invention, since the VCO core  36  is coupled in parallel with the switching varactor unit  34 , the frequency constant √{square root over (LC)} can be further adjusted by adjusting the capacitance C, so as to have the frequency spacing in more precise value. This can effectively reduce the data errors. The design of VCO unit  200  with the switching varactor unit  34  in the invention is obviously novel to the prior art. 
   In order to have the practical application of the FSK modulator, the output terminal VCO_ 2  of the VCO unit  200  is coupled back to the input terminal through the VCO buffer  38   a  and the PLL  40 . The feedback loop can lock the desired frequency. The output terminal VCO_ 1  is complementary to the output terminal VCO_ 2  and then output the desired frequency signal. Usually, another VCO buffer  38   b  is coupled to the output terminal VCO_ 1  of the VCO core  36 , and then a power amplifier (PA) is coupled to the VCO buffer  38   b  to amplify the frequency signal. 
   For the FSK system of the invention, since the frequency spacing in the VCO unit  200  can be further shifted by varying the capacitance of the switching varactor unit  34 , the data error can be effectively reduced during transmission. The invention can improve the communication quality. 
   In the foregoing switching varactor unit  34 , it can be designed as the circuit architecture shown in  FIG. 4 . In  FIG. 4 , the switching varactor is designed by using bipolar junction diode or metal-oxide semiconductor (MOS) diode. The switching diode unit include a decoder  46  and a diode pair part. Each diode pair has a diode  34   a , and a diode  34   b  coupled in reverse direction. In other words, the positive ends are commonly coupled to the node X and the other ends of the diodes are coupled to, the switching device  42  and  44 , respectively. The output end of the switching device  42  is the output terminal VCO_ 1 , and the output end of the switching device  44  is the output terminal VCO_ 2 . The diode pairs are coupled in parallel. 
   The decoder  46  receives a digital signal with at least one bit. In the example, two bits of Mod_ 1  and Mod_ 0  are used for descriptions. The decoder  46  has the output channels b 0 -b 3 , which have the number equal to 2n, where n is the input bit number. In the example, n=2. The output channels are respectively coupled to the switching devices  42  and  44  for each diode pairs. The decoder then decodes the quantity of the digital signal and applying the enabling signal to the switching devices, accordingly. For example, when the data  00  is received, then the b 0  channel is at enable state, and then the switching devices  42 ,  44  at the first channel are turned on. Then, the capacitance contributed form the diodes is enabled and added to the VCO core  36  (see  FIG. 3 ). Likewise, if the binary digital data  10  is received, then channels b 0  and b 2  are turned on. The capacitance from the two channels are added together to have another quantity of capacitance. 
   Since the diode is the varactor diode, the capacitance is also depending on the frequency-selection voltage at the node X. By this design, the switching diode unit  34  is coupled with the VCO core  36  in parallel, so as to provide the desired capacitance and properly shift the frequency spacing in better precision. 
   From the operation point of view, the invention can further adjust the frequency spacing by coupling the switching varactor unit  34  with the VCO core  36  in parallel, so as to shift a frequency constant √{square root over (LC)} of the VCO core. 
   In order to obtain the desired frequency spacing for the FSK modulator, a procedure can be performed, as shown in  FIG. 5 . In  FIG. 5 , a micro control unit (MCU)  50 , a status memory  52 , a control block  54 , a FSK modulator, and a tester  58  are coupled in cascade manner as a testing loop. The MCU  50  send the signal of ENABLE or DATA to the FSK modulator  56 . The selection signals of Mod_ 0 -Mod_ 3  are first chosen. Then the tester  58  tests the data transmission error. The selection signals of Mod_ 0 -Mod_ 3  are then stored in the status memory  52  and written to the control block  54  when each time of power. The test procedure is repeated by changing the selection signals of Mod_ 0 -Mod_ 3  until the test result satisfies the requirement. The procedure in  FIG. 5  is a routing work to estimate the frequency spacing. 
   In conclusions, the invention has introduce the VCO unit, which include the VCO core  36  and the switching varactor unit  34 , coupled in parallel. In this design, the frequency constant √{square root over (LC)} of the VCO core can be more precisely adjusted. As a result, the error in data transmission can be effectively reduced. 
   It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention covers modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.