Abstract:
A noise tolerant voltage controlled oscillator is described. The voltage controlled oscillator include a varactor element as part of an LC tank circuit. The varactor element is biased by a bias signal and a bias-dependent control signal. The bias-dependent control signal tunes the LC tank circuit. Because the control signal is bias-dependent, noise and other deleterious influences do not cause the varactor element to deviate in capacitance. Instead, the bias-dependent control signal is a tuning signal that is centered around the bias signal, which allows the varactor element to provide a constant capacitance in the event of a varying bias signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application claims priority to U.S. Provisional Application No. 60/684195, entitled “Clock Data Recovery Architecture”, filed May 24, 2005, which is hereby incorporated by reference herein in its entirety. 

   FIELD OF THE INVENTION 
   The present invention relates generally to a voltage controlled oscillator, and more particularly, to a voltage controlled oscillator that comprises a variable capacitive element. 
   BACKGROUND 
   Many circuits today use voltage controlled oscillators (VCOs) to generate a reliable periodic signal. For example, phase locked loops (PLLs) typically include a VCO for duty cycle correction, phase/delay compensation, and/or frequency multiplication. Although VCOs come in various configurations (e.g., a ring oscillator, a crystal oscillator, or an LC tank circuit), a VCO generally receives a voltage tuning signal indicative of a desired periodic output signal. The applied voltage signal may be varied to adjust, or tune, the frequency of the output signal. For example, using such a voltage signal, a PLL may tune a VCO until the VCO outputs a desired waveform. If, at any point of operation, the waveform needs to be adjusted or corrected, the PLL will adjust the magnitude of the voltage signal to tune the VCO. 
   Unfortunately, because VCOs are located in proximity to other circuitry, often sharing the same substrate and biasing, VCOs are subjected to noise (e.g., common-mode voltage shifts). Moreover, environmental factors, such as temperature and electromagnetic radiation, may also produce noise. Such noise reduces the performance of a VCO and may increase operational overhead. For example, common-mode voltage noise within a VCO may increase the capacitance of circuit elements that the VCO comprises. When the VCO receives a tuning signal, the increased capacitance may increase the amount of time it takes the VCO to produce the correct waveform. Such a lag in time may likewise reduce the performance of circuits and devices that use the VCO. Therefore, it is desirable to provide a VCO that may be subjected to noise without comprising performance. 
   SUMMARY 
   An apparatus and a method of operation of a noise tolerant VCO are presented. In one example, a VCO includes a varactor element having two input nodes and two output nodes. The output nodes of the varactor element provide an effective capacitance, and are intercoupled with an inductive element. The varactor element, for example, may comprise four varactor diodes, which are arranged to receive an input signal. 
   Together the inductive and varactor elements act as a differential tank circuit (which may be viewed as two tank circuits), producing a desired LC constant of the VCO. To properly bias the varactor element, a bias circuit supplies a bias signal to the output nodes of the varactor element. The bias circuit, for example, may be a bandgap reference circuit. 
   To tune the VCO, the input nodes of the varactor element are coupled to receive a bias-dependent control signal, which is a combination of a tuning signal and the bias signal. In a preferred embodiment, the common mode, or average, of the bias-dependent signal is substantially equal to the bias signal. The bias-dependent control signal adjusts the capacitance value of the varactor element. Accordingly, the LC constant may be altered, producing a periodic output signal at a desired frequency. Advantageously, because the bias-dependent control signal is a combination of the tuning signal and the bias signal, any noise, including a common-mode voltage shift, is mitigated. 
   In one example, the bias-dependent control signal may be produced by a tuning circuit. The tuning circuit may include a differential amplifier, and the tuning and bias-dependent control signals may likewise be differential signals. The differential amplifier, for example, may further mitigate common-mode voltages that may potentially occur across the tuning signal. To increase tuning efficiency, or speed, the differential amplifier of the tuning circuit may also be coupled to the varactor element via a low-impedance driver, such as emitter-follower circuitry. 
   In an additional example, the VCO may include a pair of resistive elements, a pair of capacitive elements, and a negative resistive component, all of which are intercoupled with the output nodes of the varactor element. The resistive elements may be used to distribute the bias signal to the output nodes of the varactor element. For example, the bias signal may be applied to a common node where both of the resistive elements are coupled together. In addition, the bias circuit may be coupled to the common node via a low-impedance driver such as emitter-follower circuitry. 
   The capacitive elements, on the other hand, may be used to promote the coupling of the varactor element to the negative resistive component. In general, the negative resistance component provides energy to the VCO, to sustain the output waveform. In one example, the negative resistance may include a differential pair, which provides a negative transconductance across the output nodes of the varactor element. 
   In an alternative example, a method of operating a VCO is described. In the examplary method, a VCO receives a tuning signal and a bias signal. The VCO combines the tuning and bias signal into a bias-dependent control signal and provides the bias-dependent control signal to the control nodes of a variable capacitance. The variable capacitance may be located within an oscillator circuit, for example. The VCO also uses the bias signal to bias two output nodes of the variable capacitance. Because the bias-dependent control signal includes a bias signal component, the VCO has an increased tolerance to noise. For example, when the bias signal is influenced by common-mode voltage noise, the bias-dependent control signal is likewise influenced. Consequently, the capacitance value of the variable capacitance is prevented from spiking or increasing. 
   In another example, the tuning signal and the bias-dependent control signals may be differential signals. Furthermore, the bias-dependent control signal may be viewed as the tuning signal centered around the bias signal. 
   These as well as other aspects and advantages will become apparent to those of ordinary skill in the art by reading the following detailed description, with reference where appropriate to the accompanying drawings. Further, it is understood that this summary is merely an example and is not intended to limit the scope of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Certain examples are described below in conjunction with the appended drawing figures, wherein like reference numerals refer to like elements in the various figures, and wherein: 
       FIG. 1  is a schematic diagram of a VCO, according to an example; 
       FIG. 2A  is a schematic diagram of a negative resistance component, according to an example; 
       FIG. 2B  is a schematic diagram of a tuning circuit, according to an example; 
       FIG. 3A  is a graph of capacitance vs. a bias signal without bias-dependent control signaling; 
       FIG. 3B  is a graph of capacitance vs. a bias signal with bias-dependent control signaling; and 
       FIG. 4  is a flow diagram depicting a method of operating a VCO. 
   

   DETAILED DESCRIPTION 
   Turning now to the figures,  FIG. 1  is a schematic diagram of VCO  10 . The VCO  10  includes a varactor element  12  that includes input nodes  14 ,  16  and output nodes  18 ,  20 . The output nodes  18 ,  20  receive a bias signal (V BIAS ). The input nodes receive a bias-dependent control signal (V BDC+ , V BDC− ), which is a combination of the bias signal and a tuning signal. In the example of  FIG. 1 , the bias-dependent control signal is a differential signal. In an alternative example, the bias-dependent control signal may be single ended. Generally speaking, however, differential signaling may be beneficial as such signaling mitigates common-mode voltage influences. Moreover, the bias and bias-dependent control signals, although described as voltage signals, may be current signals. 
   In general, the varactor element  12  serves as a variable capacitor. The VCO  10 , however, may incorporate other types of variable capacitors and the varactor element  12  should not be viewed as limiting. The VCO  10  also includes an inductor  20 , which is intercoupled with the varactor element  12 . The inductor  20  and the varactor element  12  together form two tank circuits that establish a frequency of a periodic output signal generated across the inductor  20  (nodes  22 ,  24 ). The varactor element  12  is symmetric about a common node  21  and the inductor  20  is symmetric about an applied voltage Vcc. 
   The capacitance value of the varactor element  12  may be adjusted by applying the bias-dependent control signal at the input nodes  14 ,  16 . The varactor element  12  includes varactor diodes  26 - 29 . The diodes  26 - 29  each comprise a PN junction that is operated under a reverse bias voltage. By increasing the amount of reverse bias across each PN junction, the depletion zone associated with each of the diodes  26 - 29  increases, reducing the capacitance of the diodes  26 - 29 . Conversely, decreasing the reverse bias ultimately increases the capacitance of the diodes  26 - 29 . The bias-dependent control signal directly determines the amount of reverse bias applied to the diodes  26 - 29 , establishing the capacitance of the tank circuits and therefore establishing the frequency of the periodic output signal. 
   To communicate the bias-dependent control signal to the varactor element  12 , the VCO  10  includes bipolar junction transistors (BJTs)  31 - 34  and resistors  35 ,  36 . The BJTs  31 ,  34  are coupled to a tail current source  39 . The BJTs  32 ,  33 , on the other hand, are respectively coupled to tail current sources  38 ,  40 . 
     FIG. 1  shows the BJTs  31 - 32  and the resistor  35  arranged in a common-emitter amplifier and an emitter-follower configuration. Similarly, the BJTs  33 - 34  and the resistor  36  are also in a common-emitter/emitter-follower configuration. The emitter-follower configurations, although not necessary, shift a frequency pole at each of the input nodes  14 ,  16 . For example, if the tail current source  38  has a current of approximately 1 mA, the input resistance is approximately equal to 0.026 Ohm (1 mA/V t ). Overall, minimizing the input resistance increases the tuning speed of the VCO  10 . In some examples, however, other methods of applying the bias-dependent control signal are possible. 
   To communicate the bias signal to the output nodes  18 ,  20 , the VCO  10  includes BJTS  41 ,  42  and resistors  43 - 45 . The BJTs  41 ,  42  are respectively coupled to tail current sources  46 ,  48 . The arrangement of the BJTs  41 ,  42  may also enhance the performance of the VCO  10 , similar to the common-emitter/emitter-followers described above with reference to the BJTs  31 - 34 . 
   The resistors  43 ,  44  are coupled together at the common node  21 . The common node  21  receives the bias signal, and communicates the bias signal to both the output nodes  18 ,  20 . The resistors  43 ,  44 , although not limited to a specific resistance value, should be sufficiently small in order to prevent dampening of the periodic output signal across the nodes  22 ,  24 . In one example, the resistors  43 ,  44  may have resistance values of 1 kohm and provide an adequate coupling of the bias signal to the output nodes  18 ,  20 . 
   The VCO  10  also includes a negative resistance component  50  for promoting the amplification and oscillation of the periodic output signal (i.e., providing energy to the tank circuits). The capacitors  52 ,  54  couple the negative resistance component  50  to the output nodes  18 ,  20 , and provide isolation between the bias in associated with the negative resistance component  50  in the biasing of the varactor element. Also, because the varactor capacitance is much smaller than the capacitors  52 ,  54 , the varactor capacitance dominates the value of the LC tank circuit capacitance. Generally speaking, the negative resistance component  50  is an active circuit which cancels losses in the tank circuits. The negative resistance components  50  should provide a closed loop gain within the VCO  10  which is greater than or equal to unity magnitude. Losses in the VCO  10  may be attributed to parasitic resistances within the diodes  26 - 29 , the resistors  43 ,  44 , the inductor  20 , and any other active component. 
     FIG. 2A  shows a differential amplifier  100 , which may be used as the negative resistance component  50 . The amplifier  100  includes resistors  102 ,  104 , capacitors  106 ,  108 , BJTs  110 ,  112 , and tail current source  114 . The resistors  102 ,  104  are coupled to receive a reference signal and communicate the reference signal to the capacitors  106 ,  108 . Both of the tank circuits provide a differential load to the amplifier  100 , where the common node  21  remains at a potential associated with the bias signal. Operationally, the BJTs  110 ,  112  provide positive feedback to the tank circuits. As described above, the negative resistance of the amplifier should be designed to overcome all of the resistive losses of the tank circuits. 
   As described above, the bias-dependent control signal is a combination of the tuning signal and the bias signal. In a preferred embodiment, the common mode, or average, of the bias-dependent signal is substantially equal to the bias signal.  FIG. 2B  shows a tuning circuit  200  that may be used to generate the bias-dependent control signal. The tuning circuit  200  receives the bias signal and the tuning signal, and produces the bias-dependent control signal. The tuning circuit  200  includes a differential amplifier  202  which is coupled to receive the tuning signal (V TUN+ , V TUN− ). The tuning circuit  200  also includes a pair of differential amplifiers  204 ,  206  for effectively summing the bias signal with the tuning signal. 
   To sum the bias and tuning signals, the differential amplifiers  204 ,  206  are biased by both the bias-dependent control signal and the bias signal. At one end of the differential amplifiers  204 ,  206  are BJTs  208 ,  210  which are respectively coupled to the negative and positive differential ends of the bias-dependent control signal. At the other end of the common mode differential amplifiers  204 ,  206  are BJTs  212 ,  214  which are coupled to receive the bias signal. 
   The outputs of amplifier  202  may be altered by the tuning signal, which is applied to BJTS  220 ,  222 . When the input signal at the BJTs  220 ,  222  is altered, the voltage at the input BJTs  208 ,  210  will likewise be altered and therefore provide feedback to the amplifiers  204 ,  206  via FET  218 . The overall effect is that the common mode feedback amplifiers  204 ,  206  attempt to drive the average of the outputs of differential amplifier  202  to be equal to V CC  minus V BG , which is the value of the bias signal. The feedback between the differential amplifier  202  and the amplifiers  204 ,  206  allows the tuning signal to be centered around the bias signal. Note also that if the bias signal changes (via a common mode noise voltage, for instance), the amplifiers  204 ,  206  provide feedback to the amplifier  202  via field effect transistors (FETs)  216 - 219 , influencing the bias-dependent control signal. It should be understood, however, that the tuning circuit  200  is an example only, and other tuning circuits could be used to combine the tuning signal and the bias signal into the bias-dependent control signal. In particular, the common mode of the tuning signal may be some multiple or some proportional factor of the bias signal. 
   Demonstrating the advantages of the bias-dependent control signal are the  FIGS. 3A and 3B , which are plots showing capacitance across the varactor element  12 .  FIGS. 3A and 3B  show C 1 , the capacitance across the input node to the output nodes all and C 2 , the capacitance across the output nodes to the input node. C EQV  is the series combination of C 1  and C 2 . 
     FIG. 3A  shows a plot of the general behavior of varactor biased according to the techniques of the prior art. The effect of a change in the bias on C 1 , C 2 , and C EQV , if the tuning signal were used to drive the input nodes (in lieu of the bias-dependent control signal) are plotted for a fixed tuning voltage input. As  FIG. 3A  demonstrates, as the bias signal varies, C 1  and C 2  vary, causing C EQV  to vary. Consequently, the periodic output signal of the VCO  10  may be deleteriously impacted by this varying capacitance. Variation in C EQV  may produce jitter and delayed convergence, resulting in unreliable output signals. Moreover, increases in C EQV  may likewise increase the capacitance of the tank circuits, which may reduce the tuning speed of the VCO  10 . Generally speaking, it is difficult to completely remove noise influence from the bias signal. For example, common mode voltage noise may typically cause the bias signal to vary in DC offset during normal operation. 
   Because noise cannot be removed completely from the tuning signal, it is desirable for the tuning voltage to be centered around the bias signal.  FIG. 3B  reflects the behavior of C 1 , C 2 , and C EQV  using the bias-dependent control signal to drive the input nodes  14 ,  16 .  FIG. 3B  shows that when the tuning signal is centered around the bias signal, C EQV  remains substantially constant. In operation, when the bias voltage shifts, the bias-dependent control voltage shifts, preventing an asymmetric capacitance variation to occur across the varactor element (i.e., C 1  remains approximately equal to C 2 ). Furthermore, any change in the bias signal is filtered out, and the tuning signal is allowed to establish the voltage drops across each of the diodes  26 - 29  without the bias signal influence. 
   The bias signal, in general, should be designed to be a stable predictable value. The bias signal may be generated by a variety of bias circuits. For example, a bandgap reference circuit, which uses the intrinsic voltage of a semiconductor bandgap, may generate the bias signal. Typically, bandgap reference circuits provide a predictable output signal that scales with temperature, allowing the VCO to compensate for active currents (through FETs or BJTS) that may increase with temperature. Other bias circuits, however, are possible. 
     FIG. 4  is a block diagram of a method  300 , which generally describes operation of a VCO using the bias-dependent control and the bias signals. At blocks  302 ,  304  a VCO receives a tuning signal and a bias signal. The tuning signal, in one example, may be a differential signal, which communicates current or voltage pulses intended for adjusting the capacitance value of a variable capacitance. The variable capacitance may be located within an oscillator circuit, such as the VCO described above. The bias signal, also described above, may be generated by bias circuit. 
   At block  306  in the method  300 , the tuning and bias signal are combined into a bias-dependent control signal. At block  308 , the bias-dependent control signal is provided to the variable capacitance. For instance, the bias-dependent may be provided to varactor diode inputs. At block  310 , the bias signal is also provided to the variable capacitance. The bias signal and the bias-dependent controls signal are then used to differentially control the variable capacitance, shown at block  312 . In effect, the method  300  may be used to fully differentially control a VCO. Such a differential control prevents the bias signal from causing the capacitance value of the variable capacitance from drifting. 
   A variety of examples have been described above. More generally, those skilled in the art will understand that changes and modifications may be made to these examples without departing from the true scope and spirit of the present invention, which is defined by the claims. Thus, for example, a VCO should not be limited to the capacitance, inductive, or resistance values of any of the described circuit elements and components described above. Any of the above circuit elements and components may be modified to achieve a desired periodic output signal. In one example, the described VCO may be capable of producing a periodic output signal with a frequency of up to about 10 GHz having a tuning range of about 1 GHz. If the maximum voltage drop across the varactor element  12  is about 1.4 V (0.7 V for each diode), the gain of the VCO  10  is about 1 GHz/1.4 V. Other examples operating characteristics are possible. 
   Accordingly, the description of the present invention is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which are within the scope of the appended claims is reserved.