Abstract:
In accordance with an embodiment of the disclosure, circuits and methods are provided for using a reconfigurable voltage controlled oscillator to support multi-mode applications. A voltage control oscillator circuit comprises a resonant circuit, a first oscillator circuitry coupled to the resonant circuit, and a second oscillator circuitry coupled to the resonant circuit. The voltage control oscillator circuit further comprises switching circuitry configured to select, based on an operating metric, one of the first oscillator circuitry and the second oscillator circuitry for providing an output voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 61/604,657, filed Feb. 29, 2012, which is hereby incorporated by reference herein in its entirety. 
     
    
     FIELD OF USE 
       [0002]    The present disclosure relates generally to oscillator circuits and methods, and more particularly, to circuits and methods for a reconfigurable voltage controlled oscillator for supporting multi-mode applications. 
       BACKGROUND OF THE DISCLOSURE 
       [0003]    The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the inventors hereof, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
         [0004]    Wireless communication technologies are constantly evolving leading to improvements in new wireless standards for communicating data. For example, multiple wireless standards, such as GSM, W-CDMA, and LTE, have improved one over another in delivering higher data rates and better quality of service. Each of these standards co-exists with one another providing users with wireless coverage throughout the world. However, each of these wireless standards operate at different frequency bands. 
         [0005]    Wireless clients need to support each of these frequency bands in order to support the different wireless standards. Voltage controlled oscillators (VCO) are an important component of a multi-standard wireless client, which are used for handling and generating signals in different frequency bands. In order to support multiple frequency bands, multiple VCOs are typically used. However, minimizing the amount of additional hardware is critical in a wireless client, since as the number of wireless standards increase, adding hardware for each of these standards becomes prohibitive in terms of cost and area. In designing a VCO, design constraints such as, cost, area, power consumption, and phase noise need to be considered. Therefore, it is difficult to design a VCO which meets all the required constraints while also supporting multiple frequency bands. 
       SUMMARY OF THE DISCLOSURE 
       [0006]    In accordance with an embodiment of the disclosure, a circuit is provided for using a reconfigurable voltage controlled oscillator to support multi-mode applications. A voltage control oscillator circuit comprises a resonant circuit, a first oscillator circuitry coupled to the resonant circuit, and a second oscillator circuitry coupled to the resonant circuit. The voltage control oscillator circuit further comprises switching circuitry configured to select, based on an operating metric, one of the first oscillator circuitry and the second oscillator circuitry for providing an output voltage. 
         [0007]    In certain implementations, the operating metric comprises maximum power consumption, maximum current required, and minimum phase noise. 
         [0008]    In certain implementations, the first oscillator circuitry comprises NMOS transistors and the second oscillator circuitry comprises PMOS transistors and NMOS transistors. 
         [0009]    In certain implementations, the first oscillator circuitry and the second oscillator circuitry share a plurality of transistors. 
         [0010]    In certain implementations, the first oscillator circuitry comprises a plurality of NMOS transistors cross coupled together. The second oscillator circuitry comprises a plurality of PMOS transistors cross coupled together and the plurality of NMOS transistors of the first oscillator circuitry cross coupled together. 
         [0011]    In accordance with another embodiment of the disclosure, a method is provided for using a reconfigurable voltage controlled oscillator to support multi-mode applications. Control circuitry is used to determine an operating metric, and switching circuitry is used to select one of a first oscillator circuitry and a second oscillator circuitry to provide an output voltage based on the determined operating metric. The first oscillator circuitry and the second oscillator circuitry are coupled together to a resonant circuit, and an output voltage is provided based on the selected first and second oscillator circuitry. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Further features of the disclosure, its nature and various advantages will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
           [0013]      FIG. 1  shows an example of an n type voltage controlled oscillator according to an embodiment of the present disclosure; 
           [0014]      FIG. 2  shows an example of a p-n type voltage controlled oscillator according to an embodiment of the present disclosure; 
           [0015]      FIG. 3  shows an example of a reconfigurable voltage controlled oscillator according to an embodiment of the present disclosure; 
           [0016]      FIG. 4  shows an example of a selected configuration of a reconfigurable voltage controlled oscillator according to an embodiment of the present disclosure; 
           [0017]      FIG. 5  shows an example of another selected configuration of a reconfigurable voltage controlled oscillator according to an embodiment of the present disclosure; 
           [0018]      FIG. 6  shows a graph of the performance characteristics of a reconfigurable voltage controlled oscillator according to an embodiment of the present disclosure; and 
           [0019]      FIG. 7  shows an illustrative process for selecting a configuration of a reconfigurable voltage controlled oscillator according to an embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    To provide an overall understanding of the disclosure, certain illustrative embodiments will now be described. However, the systems and methods described herein may be adapted and modified as is appropriate for the application being addressed and that the systems and methods described herein may be employed in other suitable applications, and that such other additions and modifications will not depart from the scope hereof. 
         [0021]    Designing a reconfigurable VCO provides a way to support multiple frequency bands while minimizing the cost and area utilized by using multiple oscillators. However, no VCO, which is capable of supporting multiple wireless standards, such as W-CDMA and GSM, has been shown that is also competitive with the power consumption achieved using separate VCO. In fact, the very demanding GSM phase noise specifications require a current up to four times higher, depending on the duplexer selectivity, than that used in the W-CDMA case, which makes designing a VCO with supports multiple frequency bands difficult. 
         [0022]    In the design of oscillators using resonant circuits, such as inductor and capacitor (LC tank) resonant circuits, phase noise normalized to power consumption (i.e., figure of merit, FoM), reaches an optimum at the maximum oscillation amplitude compatible with a supply voltage VDD. This condition complicates the design of a reconfigurable VCO, since there is only one value of bias current yielding the highest FoM, once an LC tank and VDD are chosen. On the other hand, making the LC tank reconfigurable invariably results in a degradation of its Q factor. The Q factor is a metric which characterizes a resonator circuit. The Q factor describes the peak energy stored in a resonator circuit divided by the average energy dissipated in it per cycle at its resonance frequency. Low Q factor circuits are more damped and lossy in terms of energy, which reduces the quality of the resonator, and reduces the FoM of the oscillator. 
         [0023]    The present disclosure describes a reconfigurable VCO which supports multiple frequency bands whose power consumption can be reconfigured while maintaining an almost constant FoM. The benefit is that the same optimized tank may be used, reducing area and cost, while minimizing the degradation of the Q factor of the resonator, and providing a method for switching the VCO topology from different types of oscillators. For example, using switching circuitry to switch between an N type VCO and a p-n type VCO within the same VCO topology. Although, an N type VCO and p-n type VCOs are described, other VCOs which include switching circuitry using similar techniques may be used. 
         [0024]      FIG. 1  shows an example of an N type VCO  100 . N type VCO  100  includes a resonator circuit, also called a tank, which includes inductor  102  and capacitor  106 . Inductor  102  and capacitor  106  may include multiple components which set the inductance and capacitance value of inductor  102  and capacitor  106  respectively. As shown in  FIG. 1 , inductor  102  and capacitor  106  are coupled in parallel, however any configuration of inductor  102  and capacitor  106  may be provided in order to set the desired resonator properties, such as resonance frequency and Q factor. The Q factor of the resonator circuit is directly related to the inductance and capacitance value of inductor  102  and capacitor  106 . 
         [0025]    N type VCO  100  also includes NMOS transistors  112  and  116 , which are cross coupled together. N type VCO  100  uses only NMOS type transistors  112  and  116  in its oscillator circuitry, which is why it is called an N type VCO  100 . NMOS transistors  112  and  116  are coupled at the source to electrical ground  120 . N type VCO  100  also includes a bias current generator  124  which is coupled to the oscillator and to VDD voltage  128 . 
         [0026]    Properties of N type VCO  100  are that it has low phase noise, however it consumes a lot of power. When high phase noise can be tolerated, the benefit of using N type VCO  100  is diminished, since power consumption is high. An alternative VCO to N type VCO  100  is a P-N type VCO, which is shown in  FIG. 2 . 
         [0027]      FIG. 2  shows an example of a P-N type VCO  200 . P-N type VCO  200  includes a resonator circuit, which includes inductor  202  and capacitor  206 . Inductor  202  and capacitor  206  may include multiple components which set the inductance and capacitance value of inductor  202  and capacitor  206  respectively. As shown in  FIG. 2 , inductor  202  and capacitor  206  are coupled in parallel, however any configuration of inductor  202  and capacitor  206  may be provided in order to set the desired resonator properties, such as resonance frequency and Q factor. The Q factor of the resonator circuit is directly related to the inductance and capacitance value of inductor  202  and capacitor  206 . 
         [0028]    P-N type VCO  200  also includes NMOS transistors  212  and  216  and PMOS transistors  240  and  244 , which are cross coupled together. Unlike N type VCO  100  which uses only NMOS type transistors in its oscillator circuitry, P-N type VCO  200  uses both NMOS and PMOS transistors. NMOS transistors  212  and  216  are coupled at the source to electrical ground  220  and PMOS transistors  240  and  244  are coupled at their source to bias current generator  224 . P-N type VCO  100  also includes a current bias generator  224  which is coupled to the oscillator and to VDD voltage  228 . 
         [0029]    P-N type VCO  200  has a lower voltage amplitude oscillation as N type VCO  100 . This results in a higher phase noise in P-N type VCO  200  than N type VCO  100  if the same tank is used. For example, for a given VDD voltage  128  and  228 , inductor value  102  and  202 , and capacitor value  106  and  206 , the maximum amplitude oscillation in P-N type VCO  200  is half compared to N type VCO  100 , resulting in a minimum phase noise 6 dB higher in P-N type VCO  200 . However, P-N type VCO  200  has a double efficiency compared to N type VCO  100  and for the same phase noise draws half of the current, which means it consumes much less power. For the same resonator circuit values and VDD voltage, N type VCO  100  and P-N type VCO  200  have the same maximum FoM. By leveraging both N type  100  and P-N type  200  VCO topologies, a reconfigurable VCO may be realized which has multiple power consumption values, and multiple phase noise properties, with a constant FoM for whichever topology is selected. For example, N type VCO  100  may be used for the low phase noise constraint scenarios and P-N type VCO  200  may be used when a higher phase noise can be accepted. 
         [0030]    Designing a reconfigurable VCO based on an N type VCO, such as VCO  100 , and a P-N type VCO, such as VCO  200 , is not trivial. For example, the PMOS transistors must be completely switched off when the N type VCO is selected or the Q factor of the resonator circuit may become seriously degraded, which would affect the efficiency of the reconfigurable VCO. 
         [0031]      FIG. 3  shows an example of an implementation of a reconfigurable VCO which efficiently combines multiple VCO topologies and provides switching circuitry which prevents degradation of the resonator circuit while providing a constant FoM. 
         [0032]      FIG. 3  shows an example of a reconfigurable VCO  300 . Reconfigurable VCO  300  includes cross coupled NMOS transistors  312  and  316 , a resonant circuit which includes inductor  302  and  306 , and PMOS transistors  340  and  344 . NMOS transistors  312  and  316  have their sources coupled to electrical ground  320 . Reconfigurable VCO  300  generates an oscillating voltage output at  394  and  396 . 
         [0033]    Switching circuitry  360 ,  368 ,  388 ,  386 ,  384 , and  372 , are used to switch between an N type VCO, such as VCO  100 , and a P-N type VCO, such as VCO  200 . Switching circuitry  360 ,  368 ,  388 ,  386 ,  384 , and  372 , may be implemented as transistor switches configured by a control signal generated by control circuitry. The switches may be configured and controlled by the control signals generated by the control circuitry based on an operating metric determined by the control circuitry. An operating metric may include a selected maximum power consumption, maximum current required, and minimum phase noise. Based on the determined operating metric, the control circuitry may generate control signals which configure the switching circuitry in the reconfigurable VCO to select one of the VCO topologies in the reconfigurable VCO. Switching circuitry may also be implemented in any other way which provides selectable electrical isolation between electrical connections. Capacitors  374  and  380 , resistors  376  and  378 , and switching circuitry  372 , which are coupled to PMOS transistors  340  and  344 , provide a tunable RC network which switches off the PMOS transistors  340  and  344  when the N type VCO is selected. Resistors  376  and  378  and capacitors  374  and  380  may be implemented using PMOS or NMOS transistors. PMOS transistors  340  and  344  are cross coupled through the tunable RC network. When switching circuitry  372  is opened or closed, a high pass cut-off frequency is set in the tunable RC network which turns off or turns on PMOS transistors  340  and  344 . The switching circuitry and tunable RC network provides a mechanism to shut off PMOS transistors  340  and  344 , preventing degradation of the resonant circuit. 
         [0034]    Switching circuitry  360  and  386  is closed, and switching circuitry  368 ,  372 ,  384 , and  388  is open, when reconfigurable VCO  300  is configured as a P-N type VCO. Switching circuitry  360  provides a connection to bias current generator  324  which is coupled to VDD voltage  328 . Switching circuitry  386  coupled PMOS transistors  340  and  344  to the resonator circuit, which includes inductor  302  and capacitor  306 . 
         [0035]    Switching circuitry  368 ,  372 ,  384 , and  388  is closed, and switching circuitry  360  and  386  is open, when reconfigurable VCO  300  is configured as a N type VCO. Switching circuitry  360  provides a connection from bias current generator  390 , which is coupled to VDD voltage  392 , to the resonator circuit, which includes inductor  302  and  306 . Switching circuitry  384  and  368  provide connections to VDD voltages  364  and  382 , which along with switching circuitry  372  and the tunable RC network, switch off PMOS transistors  340  and  344 . 
         [0036]      FIG. 4  shows an example of a selected configuration  400  of a reconfigurable  300  in an N type VCO configuration. Switching circuitry  368 ,  474 ,  384 , and  388  are closed and switching circuitry  360  and  386  are opened in  FIG. 4 . Switching circuitry  474  a PMOS switch shown as an implementation of switching circuitry  374 . Switching circuitry  474  has electrical ground coupled to the gate of the PMOS in order to enable the PMOS transistor, effectively closing the switch. 
         [0037]    In the N type VCO configuration of reconfigurable VCO  400 , both sources and gates of PMOS transistors  340  and  344  are biased to VDD and switching circuitry  474  is closed. In this case, the drains and gates of PMOS transistors  340  and  344  are AC coupled through the tunable RC network. This configures the tunable RC network to have a high-pass cut-off frequency which creates sufficient attenuations between the VCO outputs and PMOS transistors  340  and  344 , thereby keeping them switched off. The high-pass cut-off frequency in this configuration may be given by 2/(C*Ron), where Ron is the on-resistance of the PMOS transistors of switching circuitry  474  and C is the capacitance value of the tunable RC network. Ron should be configured to be small enough to provide sufficient attenuation above the oscillating frequency of selected VCO configuration  400 . 
         [0038]      FIG. 5  shows an example of a selected configuration  500  of a reconfigurable  300  in a P-N type VCO configuration. Switching circuitry  368 ,  374 ,  384 , and  388  are opened and switching circuitry  360  and  386  are closed in  FIG. 5 . 
         [0039]    In the P-N type VCO configuration of reconfigurable VCO  500 , switching circuitry  374  is opened and switching circuitry  386  is closed, coupling PMOS transistors  340  and  344  to the resonant circuit. PMOS transistors  340  and  344  are coupled to the resonant circuit through resistors  376  and  378 . The tunable RC network needs to be configured to decrease the high-pass cut-off frequency of the tunable RC network well below the oscillator frequency to ensure proper operation of PMOS transistors without degrading the resonator circuit. The high-pass cut-off frequency in this configuration may be given by 1/(C*Rb), where Rb is the resistance of resistors  376  and  378  and C is the capacitance value of the tunable RC network. Rb should be configured to be large enough to make the high-pass cut-off frequency substantially less than the oscillator frequency of the selected VCO configuration  500 . 
         [0040]      FIG. 6  shows a graph  600  of the performance characteristics of a reconfigurable VCO  300 . Graph  600  shows phase noise versus frequency offset from the oscillator frequency f LO  of reconfigurable VCO  300 . At a 2 MHZ frequency offset  604 , the phase noise  616  for the P-N type configuration  500  and the phase noise  610  for the N type configuration  400  is shown. At the frequency offset of 2 MHz at  604 , the phase noise is −129.3 dBc/Hz and −134.7 dBc/Hz for the P-N type configuration  500  and N type configuration  400  respectively. Since the N type configuration  400  uses four times more current than the P-N Type configuration  500  at maximum efficiency, the FoMs are almost the same between the two configurations. The FoM for the P-N type configuration  500  is 185.6 dBc/Hz and the FoM for the N type configuration  400  is 185 dBc/Hz. This shows that reconfigurable VCO  300  provides an almost constant FoM while allowing for reconfiguration of power consumption, phase noise, and current, while sharing the same resonator circuit. 
         [0041]      FIG. 7  shows an illustrative process  700  for selecting a configuration of a reconfigurable VCO, such as reconfigurable VCO  300 . At  704 , an operating metric is determined using control circuitry. An operating metric may include a selected maximum power consumption, maximum current required, and minimum phase noise. At  708 , based on the determined operating metric, the control circuitry may generate control signals which configure the switching circuitry in the reconfigurable VCO to select one of a plurality of implemented VCO topologies in the reconfigurable VCO. For example, in reconfigurable VCO  300 , based on the determined operating metric, the N type VCO  400  or the P-N type VCO  500  may be selected. 
         [0042]    The selection circuitry may configure the tunable RC network within VCO  300 , which configures the high-pass cut-off frequency of the tunable RC network. This allows the PMOS transistors, such as PMOS transistors  340  and  344  in VCO  300 , to be turned off or on depending on the topology used. At  712 , once the selected VCO configuration is selected, the reconfiguration VCO  300  may provide the oscillator output voltage. 
         [0043]    The above described embodiments of the present disclosure are presented for purposes of illustration and not of limitation, and the present disclosure is limited only by the claims which follow.