Abstract:
Improved voltage controlled oscillator (VCO) circuits are disclosed. A symmetrical voltage controlled oscillator (VCO) system according to the embodiments of the present invention comprises a frequency tuning circuit containing one or more varactors for receiving a predetermined tuning signal and a frequency tuning bias signal for altering capacitances of the varactors, a modulation circuit coupled in parallel with the frequency tuning circuit containing one or more varactors for modulating one or more outputs, and a core circuit coupled in a parallel with the tuning circuit and the modulation circuit for providing an oscillation mechanism, wherein the core circuit has an inductance module coupled in a parallel fashion with the frequency tuning circuit and the modulation circuit, wherein circuit elements of the VCO system are symmetrically arranged for increasing oscillation efficiency thereof and the varactors are tuned to deliver the output at an output frequency.

Description:
CROSS REFERENCE  
       [0001]     This application claims the benefits of U.S. Provisional Patent Application Ser. No. 60/599,260, filed on Aug. 4, 2004, and entitled “HIGHLY LINEAR SIGNAL MODULATION VOLTAGE-CONTROLLED OSCILLATOR”. This application further relates to co-pending applications entitled “HIGHLY-LINEAR SIGNAL-MODULATED VOLTAGE CONTROLLED OSCILLATOR” filed on Jan. 31, 2005, under Attorney Docket No. VIT04-172, and “SYMMETRICAL LINEAR VOLTAGE CONTROLLED OSCILLATOR”, filed on Jan. 31, 2005, under Attorney Docket No. VIT04-171. 
     
    
     BACKGROUND  
       [0002]     The present invention relates generally to semiconductor voltage controlled oscillator (VCO) devices, and more particularly to improved integrated designs of inductance-capacitance tank VCO devices.  
         [0003]     The popularity of mobile telephones has placed exceptional attention to wireless architectures and circuit techniques. In addition, the reduction in scaling of complementary metal-oxide semiconductor (CMOS) technologies in recent years has resulted in significant improvements in the radio frequency (RF) performance of MOS devices. As an example of the CMOS RF technology improvements, single-chip transceiver designs have already been demonstrated using low-cost CMOS technology. RF CMOS integrated circuit (IC) technology has advanced to the point of commercial deployment.  
         [0004]     One of the key elements of the wireless communications transceivers is voltage controlled oscillators (VCOs). They are part of the frequency synthesizer that generates the local oscillator (LO) signal for both up-conversion and down-conversion of the baseband signal. For monolithic integration into CMOS devices, inductance-capacitance (LC) tank oscillators are preferred over other oscillators due to its better relative phase noise performance and its low power consumption. Despite continuous improvements in VCOs, however, VCO design still remains both a bottleneck and the main challenge for RF transceiver design. These challenges include reducing phase noise, power consumption, and optimizing frequency tuning range. In LC tank VCOs, phase noise and power consumption depend primarily on the quality factor (Q) of the tank and the non-linearities of varactors, which are specially-designed P-N junction diodes, whose capacitance change significantly in the reverse bias mode. There are numerous varactor types: PN-junction, standard mode p/nMOS, or accumulation mode p/nMOS varactors. The frequency tuning range is determined by the capacitance tuning range of the varactor and the parasitic characteristics of the VCO. Therefore, the main task is to optimize the performance of the inductors and varactors. The control voltage applied to the VCO changes the capacitance value of the varactor, which determines the oscillation frequency of the VCO. The inductance, L, and the parallel capacitance, C, determine the oscillation frequency, f, of the VCO by the following equation: 
 
 f= 1/2π( LC ) 1/2  
 
         [0005]     Varactors are used to cover a certain frequency band. The active devices of the VCO overcome the losses in the tank. To reduce the phase noise of the VCO, the passive elements of the tank need to have large quality (Q) factors, since the quality factors of the tank quadratically influence the phase noise of the VCO. At frequencies suitable for mobile communications, the quality factors of integrated inductors are usually much lower than the quality factors of conventional diodes or MOS varactors. In these applications, the inductors determine the worst-case phase noise and whether or not the VCO specifications can be met.  
         [0006]     The performance of integrated inductors is strongly influenced by losses through undesired currents in the substrate, or by the serial resistance of the inductor windings. In digital CMOS technologies, the thickness of the metal layers is much smaller than in bipolar and bi-CMOS technologies, thus leading to much higher serial resistances. Further the substrates are highly doped, thus leading to large substrate losses. Digital CMOS technologies allow the integration of both digital and analog functions on the same chip without exponentially increasing the cost of digital CMOS technology fabrication.  
         [0007]     Moreover, conventional VCOs require a large die size, have low linearity, and have no signal modulation capability. The parasitic effects of the physical layout increase the variability of the set-on oscillator frequency. As such, oscillator frequency cannot be reliably predicted.  
         [0008]     Therefore, desirable in the art of VCO designs are improved VCO designs with a smaller footprint, higher linearity, improved set-on oscillator frequency stability and signal modulation capability incorporated thereto.  
       SUMMARY  
       [0009]     In view of the foregoing, this invention provides two VCO circuit topologies to improve VCO circuit performance.  
         [0010]     Improved voltage controlled oscillator (VCO) circuits are disclosed. A symmetrical voltage controlled oscillator (VCO) system according to the embodiments of the present invention comprises a frequency tuning circuit containing one or more varactors for receiving a predetermined tuning signal and a frequency tuning bias signal for altering capacitances of the varactors, a modulation circuit coupled in parallel with the frequency tuning circuit containing one or more varactors for modulating one or more outputs, and a core circuit coupled in a substantially parallel fashion with the frequency tuning circuit and the modulation circuit for providing an oscillation mechanism, wherein the core circuit has an inductance module coupled in a substantially parallel fashion with the frequency tuning circuit and the modulation circuit, wherein circuit elements of the VCO system are symmetrically arranged for reducing phase noise and increasing oscillation efficiency thereof.  
         [0011]     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  presents a conventional LC tank VCO circuit.  
         [0013]      FIG. 2  presents a block diagram of a high-linearity, signal-modulated symmetrical LC tank VCO circuit in accordance with one embodiment of the present invention.  
         [0014]      FIG. 3  presents a separated, symmetrical LC tank VCO circuit in accordance with another embodiment of the present invention.  
         [0015]      FIGS. 4A and 4B  present another separated, symmetrical LC tank VCO circuit in accordance with another embodiment of the present invention.  
         [0016]      FIG. 5  presents an integrated, symmetrical LC tank VCO circuit in accordance with another embodiment of the present invention. 
     
    
     DESCRIPTION  
       [0017]     The following will provide a detailed description of improved VCO circuits according to various embodiments of the present invention. Various embodiments illustrate how various capacitance and inductance devices are adjusted so that the collective capacitance and inductance of the VCO are tuned for delivering an output at a selected frequency or over a frequency band if the output is modulated.  
         [0018]      FIG. 1  presents a conventional LC tank VCO circuit  100 . The circuit  100  comprises two varactors  102 , two inductors  104 , two NMOS cross-coupled MOSFET structures  106 , and a constant current power source  108 . The NMOS cross-coupled MOSFET structures  106  provide the necessary negative resistance to cancel the loss of the resonator. According to the Barkhaussen rule, oscillations occur when the loop gain is larger than one and when the image portion of the impedance is zero. The VCO oscillation frequency is determined by the equation: 
   f= 1/2π( LC ) 1/2    
 where L is the total inductance of the two inductors  104 , and C is the network capacitance comprising the capacitance of the two varactors  102  and a circuit parasitic capacitance. 
 
         [0019]     Since this design does not utilize a symmetrical topology, the parasitic capacitances could be quite large and indeterminable. Thus, the VCO output frequency can not be predicted with any accuracy with a large parasitic capacitance of the circuit  100 . It is noted that the circuit  100  does not have a built-in modulation capability, and therefore requires an external modulation circuit. The circuit  100  also has low linearity, thereby producing additional flicker noise in the output. Due to the asymmetrical topology of this design, even-mode harmonics are not suppressed. Because of the above factors, the loaded quality factor of the total LC tank circuit cannot be predicted reliably and accurately.  
         [0020]      FIG. 2  presents a block diagram of a high-linearity, signal-modulated symmetrical LC tank VCO circuit  200  in accordance with one embodiment of the present invention. The circuit  200  comprises a carrier frequency tuning circuit  202 , a carrier modulation circuit  204 , a core circuit  206 , and VCO outputs such as OUTPUT_P and OUTPUT_N. Since the VCO outputs are at a particular output frequency, they can also be referred to as carrier outputs. A frequency tuning bias signal, VTUNE_BIAS, may provide predetermined voltage such as a fixed voltage to the VCO to tune the circuit  200 . Depending upon the VCO circuit design, the VCO may contain just one or multiple frequency bands. The fixed voltage level is dependent upon the type of varactors in a high-linearity signal-modulated varactor circuit, such as PN-junction, standard mode P/NMOS, or accumulation mode P/NMOS. This circuit will be discussed in detail in  FIG. 3 .  
         [0021]     The VCO circuit  200  is further optionally coupled with an external control circuit  210  such as an analog baseband (ABB) circuit, which provides a modulation control signal VTUNE_MODULATION, and further coupled to a phase lock loop (PLL) frequency synthesizer, or simply, PLL module  212 . If necessary, the VTUNE_BIAS signal can be generated by the external control circuit  210 . The PLL module  212 , external to the VCO, utilizes a tuning signal VTUNE (e.g., a phase-locked feedback signal) for locking the phase of the output when the output frequency is locked. It is understood that VTUNE and VTUNE_BIAS together control the varactors of the frequency tuning circuit and the VTUNE_BIAS does not have to be locked to a fixed voltage as these two signals work jointly to alter the total capacitance of the varactors. The VCO output is sent to the PLL module and is sampled by the PLL to maintain the output frequency and phase stability. The PLL module  212  provides precise VCO output frequency control as well as phase control by varying the voltage VTUNE, applied to the carrier frequency varactors in the carrier frequency tuning circuit  202 , that changes the varactor&#39;s capacitance. As such, by changing the capacitance and inductance or LC tank characteristics, the output oscillation frequency will be changed. The VTUNE_MODULATION signal may vary its voltage, when applied to the modulation varactors in the carrier modulation circuit  204 , that changes the varactor&#39;s capacitance, and hence the modulation of the output frequency. Any modulation type may be utilized to modulate the VCO output frequency such as AM (amplitude modulation), FM (frequency modulation), FSK (frequency shift keying), etc. It is noted that without the modulation, the output from the VCO is on a particular output frequency, while with the modulation, the output is presented as a waveform carried by a modulated carrier frequency. The magnitude of the output in either case is largely determined by a quality factor Q of the circuit.  
         [0022]      FIG. 3  presents a separated, symmetrical LC tank VCO circuit  300  in accordance with the first embodiment of the present invention. The circuit  300  includes a highly-linear, signal-modulated varactor circuit  302 , a core circuit including a power source  304 , an inductor  306 , a pair of PMOS/NMOS cross-coupled transistor structures  308  and  310 , and two VCO outputs OUTPUT_P and OUTPUT_N. It is understood that the two VCO outputs are complementary to each other. From manufacturing perspective, the circuit  300  can be fabricated on a semiconductor substrate such as a P-type substrate using standard CMOS fabrication processes.  
         [0023]     The circuit  300  receives its power from the power source  304  such as VCC or another current source, and is connected to an electrical ground  312  or VSS. The circuit  300  is fabricated on the CMOS substrate, thereby resulting in a smaller footprint and hence a lower fabrication cost structure than in conventional VCO designs. The topology of the circuit  300  has excellent symmetry in that the circuit designs of the cross-connected transistor structures  308  and  310 , the varactor circuit  302 , and the inductor  306  are symmetrical designs. This symmetrical VCO design reduces the even-mode VCO harmonics, and significantly reduces the flicker noise in the VCO output compared to conventional VCO designs. It is understood that the inductance module  306  provides a predetermined inductance to the circuit  300  and connects to both outputs on two sides thereof. In this embodiment, it is assumed that this inductor does not have an alterable inductance so that the frequency tuning is largely carried out through the tuning of the varactors. However, it is understood that the inductor can be made in such a manner so that the inductance is a controllable variable.  
         [0024]     In this embodiment, the varactor circuit  302  is a “separated” design in that a modulation circuit  313  and a frequency tuning circuit  331  are two relatively independent circuits. The separation of the modulation circuit  313  and the frequency tuning circuit  331  allows for the implementation of either just one or both of the circuits to meet a particular design specification, thereby requiring a smaller footprint as the modulation can be an optional feature.  
         [0025]     The modulation circuit  313  of the varactor circuit  302  includes capacitors  314  and  316  connected in series respectively with varactors  318  and  320 , thereby increasing circuit linearity. In this embodiment, the varactors  318  and  320  are PMOS or NMOS varactors. The circuit  313  provides a high linearity varactor circuit capable of utilizing any modulation type. The capacitors  314  and  316  are also in series with resistors  322  and  324  respectively. The resistor  322  supplies a determinable or relatively fixed voltage at a node  326 , which connects the capacitor  314  and the varactor  318  on both sides, while the resistor  324  supplies a determinable or relatively fixed voltage at a node  328 , which connects the capacitor  316  and the varactor  320  on both sides. There are two low pass filters within this structure for eliminating external noises: the resistor  322  and the capacitor  314 , and the resistor  324  and the capacitor  316 . The circuit  300  is tied to the device ground at a node  330 , which is a mid point between the two resistors. This node, if not connected to ground, it can still be viewed as a virtual AC ground as the circuit is set up as a differential model. As shown, the varactors  318  and  320  are coupled in a substantially parallel fashion with the resistors  322  and  324 . The circuit  300  output is modulated by applying the modulation signal VTUNE_MODULATION to a mid point between the varactors  318  and  320 . The voltage applied thereto changes the capacitance values of the varactors  318  and  320 . As the capacitance changes, the frequency is also altered. It is understood that the VCO output frequency can be modulated using AM (amplitude modulation), FM (frequency modulation), FSK (frequency shift keying) or other modulation types.  
         [0026]     The frequency tuning circuit  331  of the varactor circuit  302  controls the output frequency and phase. The output frequency is adjusted through the control of two signals VTUNE_BIAS and VTUNE. With the VTUNE signal provided by the PLL module, it is used to close a control loop to maintain the output frequency and phase stability. Capacitors  332  and  334  are respectively coupled in series with varactors  336  and  338 , thereby increasing circuit linearity. The capacitors  332  and  334  are also in series with resistors  340  and  342 , respectively. The resistor  340  and the capacitor  332  combination, similar to the resistor  342  and the capacitor  334 , can be seen as a differential low-pass filter that is used to eliminate external noise. The VTUNE_BIAS signal may be provided by a voltage source external to the circuit  300  and supplies a relatively fixed voltage through the resistors  340  and  342  to nodes  344  and  346 , which connect to the capacitor  332  and the varactor  336 , and to the capacitor  334  and the varactor  338  respectively. It is understood that the voltage level provided by the voltage source is dependent upon the type of varactors in the varactor circuit  302 , such as PN-junction, standard mode p/nMOS, or accumulation mode p/nMOS varactors. This stable frequency tuning bias signal VTUNE_BIAS provides a reference voltage and along with the signal VTUNE help to alter the capacitance of the varactor  338  thereby tuning the output frequency of the circuit  300  to a predetermined frequency. It should be understood that the output frequency is determined collectively by all the capacitors, varactors, and inductors of the circuit  300 , and the use of VTUNE and VTUNE_BIAS is only one way to adjust the frequency. Furthermore, the provided VTUNE_BIAS signal helps to stabilize the output and avoid flicker noise. As it is understood, a proper choice of the electrical characteristics of the varactor circuit may significantly reduce the up-conversion of flicker noise.  
         [0027]     In this embodiment, the core circuit is coupled in parallel to the frequency tuning circuit and the modulation circuit. The core circuit provides the power source and other elements for producing oscillation, from which an output frequency can be generated by selectively tuning through the above illustrated tuning mechanism. As shown, there is at least one PMOS cross-coupled transistor structure  308  having at least a pair of cross-coupled PMOS transistors with sources thereof coupled to a power source. Similarly, there is at least one NMOS cross-coupled transistor structure having at least a pair of cross-coupled NMOS transistors with sources thereof coupled to a second power supply that complements VCC such as an electrical ground GND or VSS of the circuit. The drains of the PMOS and NMOS transistors are coupled to at least one output, either the first or second outputs. The cross-couple arrangement is such that a gate of a PMOS or NMOS transistor is cross-coupled to a drain of another PMOS or NMOS transistor of the corresponding pair. The transistor structures  308  and  310  provide the necessary negative resistance to increase the power source for compensating the losses of the parallel LC resonator tank.  
         [0028]     It is understood that since the output frequency is dependent on the total values of all capacitance and inductance provided by various components in the VCO circuit, the integration of the varactors  332 ,  334 ,  336  and  338  with the inductor  306  to form the VCO LC tank contributes significantly to the performance of the circuit  300 . If the modulation circuit is included, the capacitance devices  314 ,  316 ,  318 , and  320  also contribute to the final performance of the VCO circuit. Mathematically, the relation of the output frequency and the varactors and the inductor  306  can be presented as follows: 
 
 f= 1/(( C 1 +C 2) L ) 1/2  
 
 where C1 is the total capacitance of the varactors  318  and  320 , C2 is the total capacitance of the varactors  336  and  338 , and L is the inductance of the inductor  306 . As it is presented, when C1 and C2 are altered with the L unchanged, the output frequency is largely determined. In addition, the proper symmetrical design of the aforesaid components reduces the inductor and circuit parasitic capacitance, thereby reducing even mode harmonic and flicker noise, and phase noise interference. 
 
         [0029]     One advantage of the varactor circuit  302  is its excellent linearity. The serial alignment of the capacitors  314  and  316  with the varactors  318  and  320 , and that of the capacitors  332  and  334  with the varactors  336  and  338 , increase the linearity of the varactor circuit  302 . In the circuit  300 , the PMOS/NMOS cross-coupled transistor structures  308  and  310 , the inductor  306 , and the circuits  313  and  331 , are all placed symmetrically. This symmetry reduces the even-mode VCO harmonics, and further reduces the flicker noise in the VCO output compared to conventional VCO designs. The symmetrical design of the circuit  300  significantly reduces the parasitic capacitances within the circuit, thereby providing output frequency stability and output frequency set-on accuracy during the design stage. For this embodiment, another advantage of the varactor circuit  302  is the built-in signal modulation and PLL functions. The built-in modulation function eliminates the need for an external signal modulator, which reduces the chip size (35 to 45% size reduction) and reduces fabrication costs. Further, the built-in low-pass filter eliminates the external noise without additional components. Finally, by separating the tuning and modulation circuitries, this embodiment allows for a reduced chip size if the modulation circuit is not required in the design specification.  
         [0030]     This embodiment utilizes a complementary cross-coupled topology with a symmetrical inductor design. Compared with conventional designs that use asymmetrical inductors, this new design can improve output voltage swing and phase noise by 65% and 2.3 dB, respectively, for a given level of power consumption, as shown by some experiments. At the same time, the required chip area can be reduced as much as 36%, compared to conventional inductor designs.  
         [0031]      FIGS. 4A-4B  present two separated, symmetrical LC tank VCO circuit  400  in accordance with another embodiment of the present invention. In  FIG. 4A , the varactor circuit  402  is identical to circuit  302  in the circuit  300 . However, the inductor  306  has been divided into two symmetrical sections  406 A and  406 B. Two output signals OUTPUT_P and OUTPUT_N continue to provide circuit outputs. Also, while the structure  308  is now absent, the NMOS cross-coupled structure  410  remains. The circuit  400  has performance characteristics and advantages similar to those of the circuit  300 . Similarly, in  FIG. 4B , the two inductor modules  406 A and  406 B can be placed close to ground instead of the positive power supply (e.g., VCC), replacing the cross coupled transistor structure  310  of  FIG. 3  with the transistor structure  408  remained in the circuit.  
         [0032]      FIG. 5  presents an integrated, symmetrical LC tank VCO system  500  in accordance with another embodiment of the present invention. In this embodiment, a highly-linear, signal-modulated varactor circuit  502  exhibits an “integrated” design in that circuitries for modulation and frequency tuning are electrically and physically combined. Comparing to the circuit  300 , the VCO system  500  does not require the capacitors  332  and  334 , and the resistors  340  and  342 . By reducing the number of components and integrating functionalities, a smaller footprint is permitted. After the integration of these circuitries, the circuit parasitic effects can be easily compensated for, thus allowing for easy and precise calculation of the overall loaded quality factor of the VCO system. A core circuit including an inductor  506  and two cross-coupled transistor structures  508  and  510  are identical to that shown in the circuit  300 . Similarly, the VCO system  500  is powered by two power supply lines, e.g., a constant power source  504  and an electrical ground  512 . The ground connection point  512  can be viewed as an AC virtual ground. This AC virtual ground at the mid point of the VCO system  500  reduces the series resistance of the LC tank circuit, thus improving the quality factor of the LC tank.  
         [0033]     The combined tuning and modulation part of the varactor circuit  502  controls the output frequency and phase. A capacitor  514  is in series with a varactor  518 , while a capacitor  516  is in series with a varactor  520 , thereby increasing the linearity of the tuning part of the varactor circuit  502 . In the tuning part of the varactor circuit  502 , the varactors  518  and  520  are CMOS varactors, and provide the PLL function. The capacitors  514  and  516  are also in series with resistors  522  and  524 , respectively. The resistor  522  and the capacitor  514 , and similarly, the resistor  524  and the capacitor  516 , can be seen as two low-pass filter that is used to eliminate external noise. A VTUNE_BIAS signal is connected to a voltage source that is external to the VCO system  500  and supplies a fixed voltage at the junction between the resistors  522  and  524 . The fixed voltage level is dependent upon the type of varactors in the varactor circuit  502 . A VTUNE signal is provided to the junction between two varactors  518  and  520 . The VTUNE and VTUNE_BIAS signals together control the adjustment of the varactors  518  and  520 .  
         [0034]     The modulation part of the varactor circuit  502  includes the capacitors  514  and  516 , which are in series with varactors  536  and  538 . A modulation signal VTUNE_MODULATION directs the modulation by applying a voltage at the node between the varactors  536  and  538 . The varactors  536  and  538  are MOS varactors in this example. This modulation part of the varactor circuit  502  provides a high linearity varactor circuit capable of utilizing any modulation type and this circuit&#39;s small VCO gain (K VCO ) allows for easy modulation. It is understood that the VCO output frequency can be modulated using AM (amplitude modulation), FM (frequency modulation), FSK (frequency shift keying) or other modulation types.  
         [0035]     This embodiment integrates the functionalities of a high-linearity signal-modulated varactor circuit. Although the frequency tuning part and the modulation part are integrated, they can still be viewed as in a substantially parallel arrangement. The circuit elements, like the ones in the separated model, are arranged in a substantially symmetrical fashion. This integration allows the circuit parasitic effects to be easily compensated for, thus allowing for easy and precise calculation of the overall loaded quality factor. This embodiment also eliminates some circuit components, thereby reducing the footprint and cost while maintaining the performance advantages.  
         [0036]     The above illustration provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims.  
         [0037]     Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.