Abstract:
Many electronic devices, such as computers and printers, communicate data to each other over wireline or wireless communications links. One component vital to such communications is a voltage-controlled oscillator (VCO)—a circuit that outputs an oscillating signal having an oscillation frequency based on a control voltage. Conventional VCOs adjust frequency based on a single control voltage input, which makes them vulnerable to unintended changes in the control voltage (and power-supply voltages relative to the control voltage.) These voltage changes cause frequency deviations that can make communications between devices less reliable. Accordingly, the present inventors devised a VCO that includes differential frequency control—frequency control based on the difference of two control voltages. The VCO, which uses a differentially tunable impedance, rejects common-mode noise (undesirable voltage variations that affect both control voltages) and thus bolsters reliability of communication circuits, such as phase-lock loops, receivers, transmitters, transceivers, and other devices that use it

Description:
TECHNICAL FIELD 
     The present invention concerns voltage-controlled oscillators, particularly voltage-controlled oscillators used to facilitate communications between electronic devices or circuits. 
     BACKGROUND 
     Today there are a wide variety of computer and telecommunications devices, such as personal computers (PCs), mobile telephones, and personal data assistants (PDAs), that need to share information with each other. Generally, this information is communicated from a sending device to a receiving device. 
     The sending device generally has the data in the initial form of a set of digital words (sets of ones and zeros). In the sending device, a transmitter circuit converts each word into a sequence of electrical pulses, with each pulse timed according to a data clock, and transmits the timed sequence of pulses through a cable, circuit board, or other medium to the receiving device. The receiving device includes a receiver circuit that first determines the timing of the pulses and then identifies each of the pulses in the signal as a one or zero, enabling it to reconstruct the original digital words. 
     A key component in both the transmitter and the receiver is a voltage-controlled oscillator—a circuit that produces a signal that varies back and forth between two voltage levels at a frequency based on an input control voltage. The transmitter uses a VCO to place digital information into a high-frequency carrier signal, and the receiver uses a VCO to separate the digital information from the high-frequency carrier signal. Thus, reliable and precise VCO operation is critical to transmission and reception of digital information. 
     One problem the present inventors identified in conventional VCOs is the use of single-ended control voltages. In other words, conventional VCOs include only one input point for receiving a control voltage. The single-ended control voltage forces VCO oscillation frequencies to deviate randomly from desired frequencies in response to unintended, yet inevitable, variations in the control voltage (or power-supply voltages relative to the control voltage.) For example, for a transmitter VCO intended to produce an oscillation frequency of 2.4 Gigahertz (2.4 billion oscillations per second), control-voltage variations, stemming from inevitable power-supply fluctuations, may cause the frequency to vary from 2.4001 Gigahertz to 2.3992 Gigahertz to 2.4010 Gigahertz. Similarly, a receiver VCO frequency will also vary based on its own local control voltage. Such frequency deviations can make it more difficult to reliably communicate data between electronic devices. 
     SUMARY 
     Accordingly, there is a need for better voltage-controlled oscillators. 
     To address these and other needs, the present inventors devised an exemplary voltage-controlled oscillator (VCO) circuit that includes two frequency-control inputs and a differentially tunable impedance that varies according to the voltage difference F between the frequency-control inputs. The differentially tunable impedance rejects noise (undesirable voltage variations) common to both of the frequency-control inputs and thus provides the VCO with a noise immunity, known as common-mode rejection, that is not available in conventional VCOs with single control inputs. 
     Other aspects of the invention include phase-lock loops, receivers, transmitters, and transceivers that incorporate the exemplary VCO. Additionally, various embodiments of the invention comply with Bluetooth and/or other wireless communications standards. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an exemplary voltage-controlled oscillator  100  incorporating teachings of the invention; 
     FIG. 2 is a block diagram of an exemplary phase-lock loop  200  incorporating voltage-controlled oscillator  100 ; 
     FIG. 3 is a block diagram of an exemplary transceiver  300  incorporating phase-lock loop  200 . 
     FIG. 4 is a block diagram of an exemplary system  400  incorporating transceiver  300 . 
     FIG. 5 is a block diagram of an exemplary programmable logic device  500  incorporating transceiver  300 . 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The following detailed description, which references n and incorporates the above-identified figures, describes and illustrates one or more specific embodiments of the invention. These embodiments, offered not to limit but only to exemplify and teach, are shown and described in sufficient detail to enable those skilled in the art to implement or practice the invention. Thus, where appropriate to avoid obscuring the invention, the description may omit certain information known to those of skill in the art. 
     Exemplary Oscillator with Differential Frequency Control 
     FIG. 1 shows an exemplary voltage-controlled oscillator  100 , which incorporates teachings of the present invention. Oscillator  100  includes supply nodes V 1  and V 2 , a bias-current-control input BCC, differential frequency-control inputs FCP and FCN, a common-mode control input CMC, differential oscillator outputs VCOP and VCON, an adjustable current source  110 , an upper negative-impedance circuit  120 , a differentially tunable tank circuit  130 , a lower negative impedance circuit  140 , and an adjustable current sink  150 . 
     More particularly, adjustable current source  110  includes p-channel field-effect transistors  112 ,  114 , and  116 . Like the other transistors-in this exemplary circuit, each of transistors  112 ,  114 , and  116  includes a control node known as a gate and two non-control nodes: known as a source and a drain. (In FIG. 1, the gates are designated by the three short vertical dashes along with the adjacent vertical line segment, and the source is designated with the letter S; the drain is opposite the source and carries no specific designation.) The sources of transistors  112 ,  114 , and  116  are coupled to supply node V 1 , and the gates of transistors  112 ,  114 , and  116  are coupled together. The gate of transistor  112  is coupled also to its drain and to bias-current-control input BCC. The interconnection of transistors  112  and  116  form a current mirror, which supplies a bias current based on bias-current-control input BCC through the drain of transistor  116  to negative-impedance circuit  120 . 
     Negative-impedance circuit  120  includes matched p-channel field-effect transistors  122  and  124 , which have their sources coupled together and to the drain of transistor  116 . The gate of transistor  122  is coupled to the drain of transistor  124 , and the gate of transistor  124  is coupled similarly to the drain of transistor  122 , defining a cross-coupled differential pair. Also coupled to the drains of transistors  122 , and  124  is differentially tunable tank circuit  130 . 
     Tank circuit  130  includes four matched accumulation-mode varactors, or variable capacitors,  132 ,  134 ,  136 , and  138 ; and a center-tapped inductor  139 . Varactors  132 - 138 —which collectively define a differentially tunable impedance, or more precisely a differentially tunable capacitance—each have respective positive and negative nodes, or terminals. (The figure designates each positive node as the base of the triangle in the pictured circuit symbol.) The positive nodes of varactors  132  and  134  are coupled together and to positive frequency-control input FCP, and the negative nodes of varactors  132  and  134  are coupled respectively to oscillator outputs VCOP and VCON. Varactors  136  and  138  have their negative nodes coupled together and to frequency control input FCN, and their positive nodes coupled respectively to oscillator outputs VCOP and VCON. Center-tapped inductor  139  has end nodes  139 . 1  and  139 . 2  coupled to respective oscillator outputs VCOP and VCON, and a tap node  139 . 3  coupled to common-mode control input CMC. In this exemplary embodiment, tap node  139 . 3  divides inductor  139  into two substantially equal inductances. 
     Lower negative-impedance circuit  140  includes matched n-channel field-effect transistors  142  and  144 . Like transistors  122  and  124  in negative-impedance circuit  120 , transistors  142  and  144  form a cross-coupled differential pair. However, in the exemplary embodiment, transistors  142  and  144  are roughly half the size of transistors  122  and  124  (in terms of channel width-to-length ratio) to ensure similar transconductances. In particular, the gate of transistor  142  is coupled to the source of transistor  144 ; the gate of transistor  144  is coupled to the source of transistor  142 , and the sources of transistors  142  and  144  are coupled together and to the drain of an n-channel field effect transistor  154  within adjustable current sink  150 . 
     In addition to transistor  154 , adjustable current sink  150  includes an n-channel field-effect transistor  152 . Transistors  152  and  154  are coupled together to form a one-to-five current mirror, with transistor  152  mirroring the bias current established in current source  110  to transistor  154 . In terms of specific connections, the gate and drain of transistor  152  are coupled together and to both the drain of transistor  114  (at top of the figure) and the gate of transistor  154 , and the sources of transistors  152  and  154  are coupled together and to supply node V 2 . 
     In general operation ostcillator  100  produces a differential oscillating signal at differential outputs VCOP and VCON, with the frequency of oscillation determined by the differential tuning voltage, that is, the voltage difference between frequency-control inputs FCP and FCN. As tank circuit  130  changes the voltages at outputs VCOP and VCON, transistors  122  and  144  switch on and then off in tandem, and transistors  124  and  142  switch off and then on in tandem, continually sourcing and sinking current to compensate for losses within the tank circuit and thereby sustaining oscillations. Increasing the differential tuning voltage (FCP—FCN) increases the oscillation frequency, and decreasing the tuning voltage decreases it. 
     More particularly, increasing the differential tuning voltage entails increasing the voltage at frequency-control input FCP and decreasing the voltage at frequency-control input FCN. A voltage increase at frequency-control input FCP, which controls the voltage at the positive nodes of varactors  132  and  134 , decreases the average reverse bias voltage on varactors  132  and  134  and thus decreases their combined capacitance. A voltage decrease at frequency-control input FCN reduces the average reverse bias voltage on varactors  136  and  138 , and thus decreases their combined capacitance. As a result of the decreased varactor capacitances, the oscillation frequency, which is inversely proportional to the square root of the inductance capacitance product (LC), increases. 
     Conversely, decreasing the differential tuning voltage entails decreasing the voltage at frequency-control input FCP and increasing the voltage at frequency-control input FCN. Decreasing the FCP voltage increases the average reverse bias voltage on varactors  132  and  134  and thus increases their combined capacitance. Increasing the FCN voltage increases the average reverse bias voltage on varactors  136  and  138  and thus increases their combined capacitance. The increase in total capacitance decreases the oscillation frequency. 
     Notably, this differential tuning mechanism also rejects common-mode voltages, such as power-supply noise, at differential frequency-control inputs FCP and FCN. Positive voltage changes at input FCP increase the capacitance of varactors  132  and  134 , whereas positive voltage changes at input FCN decrease the capacitance of varactors  136  and  138 . Thus, assuming ideal matching of all four varactors, a given positive voltage change at both inputs FCP and FCN has no effect on the total capacitance of the tank circuit and thus has no effect on the oscillation frequency. Similarly, any negative voltage change occurring at both inputs FCP and FCN decreases the capacitance of varactors  132  and  134 , and increases the capacitance of varactors of  136  and  138 . Thus, the exemplary oscillator provides common-mode rejection through matching or counterbalancing any increase in capacitance in one portion of the total capacitance, or more generally, total impedance, with a corresponding decrease in another portion. The inventors contemplate that similar counterbalancing mechanisms could be formed using two inductances and/or combined inductance capacitances. 
     In addition to the frequency-control inputs FCP and FCN, operation of oscillator  100  is also affected by bias-current-control input BCC and common-mode-control inputs CMC. Bias-current-control input BCO determines the magnitude of the current sourced by current source  110  and sunk by current sink  160 , and thus determines the magnitude of the negative impedances presented by negative impedance circuits  120  and  140 . (As known in the art, the bias current governs the transconductance of the transistors in each cross-coupled differential pair, and the magnitude of the negative impedance is approximately half this transconductance.) The negative impedances in circuits  120  and  140  respectively compensates for losses in the tank circuit to sustain oscillation of the signals at differential outputs VCOP and VCON. 
     Common-mode-control input CMC is provided to compensate for inevitable mismatches between current source  110  and current sink  150 . In operation, source  110  and sink  150  allow the oscillator to essentially float between the power-supply voltages, effectively isolating the oscillator from the power supply. In general, common-mode-control should provide a voltage midway between the voltages at the frequency-control inputs FCP and FCN. 
     Some other embodiments incorporating teachings of exemplary oscillator  100  may use other types of transistors, such as bipolar junction transistors, or alter other aspects of the exemplary embodiment. For examples, some embodiments can use other types of current sources, sinks, or negative-impedance circuits. Still other embodiments can place a parallel set of reversed varactors in parallel or in series with a fixed capacitance to constrain the tuning range of the oscillator. Other embodiments could also provide an arrangement of opposing variable inductances coupled to fixed or tunable capacitances. Other embodiments may also implement tapped inductor  139  as two inductors. 
     Exemplary Phase-Locked Loop 
     FIG. 2 shows an exemplary differential phase-lock loop  200  incorporating differential voltage-controlled oscillator  100 . Phase-lock loop  200 , which generally operates in a conventional manner, further includes a phase-frequency detector  210 , a charge pump  220 , a loop filter  230 , a common-mode control circuit  240 , and a frequency divider  250 . Common-mode control circuit  240  provides a voltage midway between the loop filter output voltages. Phase-lock loop  200  includes a differential reference-clock input REFCLK for receiving a reference clock signal. 
     Exemplary Wireless Transceiver 
     FIG. 3 shows an exemplary transceiver  300  that incorporating exemplary amplifier  100  of FIG.  1 . In particular, transceiver  300 , which generally operates according to conventional principles, includes an antenna  310 , a receiver  320 , a transmitter  330 , and a digital transceiver (XCVR) interface  340 . Receiver  320  includes an amplifier  321 , a mixer (or down-converter)  322 , an intermediate-frequency (IF) filter  326 , a demodulator  328 , an analog-to-digital converter  329 . Transmitter  330  comprises a digital-to-analog converter  332 , a modulator  334 , an up-converter  336 , and an output amplifier  338 . Receiver  320  and transmitter  330  share antenna  310  and a voltage-controlled oscillator  324 ; however, other embodiments provide separate antennas and oscillators. The exemplary transceiver complies with one or more versions of the Bluetooth specification and thus receives and transmits signals in the 2.4 Gigahertz band. However other embodiments use different communication bands. 
     Although the exemplary embodiment shows a wireless transceivers, the scope of the invention also includes wireline transceivers. 
     Exemplary Field Programmable Integrated Circuit 
     FIG. 4 shows a block diagram of an exemplary field-programmable integrated circuit  400  including exemplary wireless transceiver  300  (of FIG. 3.) Integrated circuit  400  also includes a field-programmable logic device (FPLD)  420 , such as a field-programmable gate array (FPGA), and an FPLD interface  430 . Although not show for clarity of illustration, various embodiments of logic device  420  includes one or more individually and collectively configurable logic blocks, as well as an on board processor and memory, which facilitate configuration of the device to perform desirable signal and data processing functions. FPLD Interface  430  provides conventional communications and program support capabilities. 
     Notably, the incorporation of wireless transceiver  400  also makes it possible to wirelessly program or reconfiguration the logic device  420  using a compatible wireless device with a conventional capability for programming field-programmable logic devices. 
     Exemplary System 
     FIG. 5 shows an exemplary system  500  including two or more electronic devices that incorporate field-programmable integrated circuit  400  of FIG.  4 . In particular, system  500  includes electronic devices  510  and  520  and a wireless communications link  530 . Devices  510  and  520  include respective processors  512  and  522 , memories  514  and  524 , and integrated programmable logic circuits  516  and  526 . Circuits  516  and  526  incorporate the teachings of exemplary integrated circuit  400  in FIG.  3  and thus provide devices  510  and  520  with capability for communicating over communications link  530  to each other (or to one or more other suitably equipped devices. Communications link  530  carries voice and/or digital data on a 2.4 Gigahertz carrier signal, according to a version of the Bluetooth communications protocol. However, other embodiments may use other communications protocols. 
     Devices  510  and  520  can assume a wide variety of forms. For example, in various embodiments, one or both of the devices are a computer, monitor, mouse, key board, printer, scanner, fax machine, network communications device, personal digital assistant, cordless telephone, headset, mobile telephone, vehicle, appliance, entertainment equipment, and industrial controller. Indeed, virtually any device that currently communicates with another device wirelessly or via a cable or that would be more useful with such communication could incorporate teachings of the present invention. 
     Conclusion 
     In furtherance of the art, the inventors have presented an exemplary voltage-controlled oscillator (VCO) that includes differential frequency-control inputs and a differentially tunable impedance. The differentially tunable impedance rejects noise common to both the frequency-control inputs and thus provides the VCO circuit with a noise immunity not found in conventional, single-ended VCOs. Additionally, the exemplary oscillator is coupled respectively to upper and lower supply nodes via a current source and a current sink which provide power-supply isolation. VCOs having this form of noise immunity and/or power-supply isolation ultimately promise to improve performance of not only phase-locked loops, receivers, transmitters, and transceivers, but also programmable integrated circuits, electronic devices, and systems that incorporate them. 
     The embodiments described above are intended only to illustrate and teach one or more ways of practicing or implementing the present invention, not to restrict its breadth or scope. The actual scope of the invention, which embraces all ways of practicing or implementing the teachings of the invention, is defined only by the following claims and their equivalents.