Patent Application
20140167871
The present disclosure relates generally to electronic circuits and, more particularly, to variable capacitors for digitally controlled oscillators.
Digitally controlled oscillators (“DCOs”) are used in a variety of integrated circuit applications. A critical performance parameter of a DCO is the frequency of its oscillating output signal. Some integrated circuit applications may require the oscillator frequency to be adjustable. In some applications, the oscillating frequency of a DCO may be based on the resonant frequency of an inductor-capacitor resonator (“LC-resonator”). In such applications, the oscillating frequency of the DCO may be adjusted by varying the capacitance in the LC-resonator. However, the tuning resolution of the frequency may be limited by the minimum step size of the variable capacitor. Moreover, a single fixed step size for the variable capacitor may result in different frequency tuning step sizes at different DCO frequency ranges.
In accordance with some embodiments of the present disclosure, a variable leaf capacitor may comprise a first alternating current coupling capacitor having a first terminal coupled to a first differential node and a second terminal coupled to a first common-mode node, a second alternating current coupling capacitor having a first terminal coupled to a second differential node and a second terminal coupled to a second common-mode node, and a varactor having a bias terminal, a first common-mode terminal coupled to the first common-mode node, and a second common-mode terminal coupled to the second common-mode node, wherein the capacitance of the varactor is based on the voltage from the first common-mode terminal of the varactor to the bias terminal of the varactor and on the voltage from the second common-mode terminal of the varactor to the bias terminal of the varactor.
The object and advantages of the invention will be realized and attained by means of at least the features, elements, and combinations particularly pointed out in the claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed.
A more complete and thorough understanding of the present embodiments and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein:
Gain stage 110 may be configured to apply a gain to a resonant oscillator signal across the OUTN and OUTP outputs of DCO 100. In some embodiments, gain stage 110 may include p-type metal-oxide semiconductor field-effect transistor (“PMOS”) 112 that may have a gate coupled to OUTP, a drain coupled to OUTN, and a source coupled to a high-side power supply. Gain stage 110 may also include PMOS 111 that may have a gate coupled to OUTN, a drain coupled to OUTP, and a source coupled to a high side power supply. Likewise, gain stage 110 may include n-type metal-oxide semiconductor field-effect transistor (“NMOS”) 114 that may have a gate coupled to OUTP, a drain coupled to OUTN, and a source coupled to ground. Gain stage 110 may also include NMOS 113 that may have a gate coupled to OUTN, a drain coupled to OUTP, and a source coupled to ground. Though
Inductor 108 may have a first terminal coupled to OUTN and a second terminal coupled to OUTP. In some embodiments, inductor 108 may be a differential inductor, and may also include a common mode node (not expressly shown). Similarly, each of capacitor 120, capacitor 130, variable leaf capacitor 140, and DAC-controlled variable leaf capacitor 150 may have a first terminal coupled to OUTN and a second terminal coupled to OUTP. The oscillating frequency of DCO 100 may depend on the resonant frequency of inductor 108 and the total capacitance applied between OUTN and OUTP. For example, the oscillating frequency (“fo”) of DCO 100 may be as described by Equation 1:
where L may be the inductance value of inductor 108 and C may be the total combined capacitance value of capacitor 120, capacitor 130, variable leaf capacitor 140, DAC-controlled variable leaf capacitor 150, and any other parasitic capacitance associated with the OUTN and OUTP nodes.
The capacitance of capacitor 120 may be varied by any suitable means. For example, in some embodiments, capacitor 120 may include an array of individually selectable capacitors that may be switched in or out to contribute to the total capacitance of capacitor 120. Similarly, in some embodiments, the capacitance of capacitor 130 may be varied by any suitable means. For example, in some embodiments, capacitor 130 may include an array of individually selectable capacitors that may be switched in or out to contribute to the total capacitance of capacitor 130.
In some embodiments, capacitor 120 may be configured to provide for coarse tuning of DCO 100 and capacitor 130 may be configured to provide for a fine tuning of DCO 100. For example, capacitor 120 may have a relatively large variation range with relatively large step sizes as compared to capacitor 130. Capacitor 130 may have a relatively small variation range with relatively small step sizes as compared to capacitor 120. In such embodiments, the large variation of capacitor 120 may be utilized to coarsely tune DCO 100 to a desired frequency within a broad range of potential frequencies, and the small step size of capacitor 130 may be utilized to finely tune DCO 100 to the desired frequency. The fine tuning capabilities provided by capacitor 130 alone may, however, be limited by inherent characteristics of capacitor 130 and/or other design limitations. For example, the minimum step size of capacitor 130 may be affected by the number of control settings that are available and the fine-tuning range that may be required in a given application. Further, in some applications, the minimum step size may be limited, for example, by the minimum delta capacitance of the smallest varactor in a given process. Moreover, a given minimum capacitance step size for variable capacitor 130 may result in different frequency step sizes for DCO 100 at different frequency ranges. For example, in some embodiments, inductor 108 may be configured to be 1.0 nH, capacitor 120 may be configured to vary from approximately 1.0 pF to approximately 10.92 pF in thirty-two discrete steps with a step size of 320 fF, and capacitor 130 may be configured to vary from 100 fF to 410 fF in thirty-two discrete steps with a step size of approximately 10 fF. In such embodiments, if the total capacitance between OUTP and OUTN is set to approximately 4 pF, the frequency of DCO 100 may be tuned to approximately 2.52 GHz, and the fine-tuning frequency step resulting from increasing or decreasing the capacitance of capacitor 130 by 10 fF may be approximately 3.14 MHz. However, in such embodiments, if the total capacitance between OUTP and OUTN is set to approximately 8 pF, the frequency of DCO 100 may be tuned to approximately 1.78 GHz, and the fine-tuning frequency step resulting from increasing or decreasing the capacitance of capacitor 130 by 10 fF may be approximately 1.11 MHz.
Accordingly, additional techniques of varying the capacitance across OUTP and OUTN may be desired in order to provide an extra fine tuning resolution and to provide for a controllable frequency step size of DCO 100.
Variable leaf capacitor 140 and DAC-controlled leaf capacitor 150 may be configured to provide an extra fine tuning resolution as well as a controllable frequency step size for DCO 100. As shown in
DAC-controlled variable leaf capacitor 150, described in greater detail below with reference to
Capacitor 211 and capacitor 212 may be configured as alternating current (“AC”) coupling capacitors. For example, in some embodiments, capacitor 211 may have a first terminal coupled to OUTP, and a second terminal coupled to a first common mode input VCMP. Similarly, in some embodiments, capacitor 212 may have a first terminal coupled to OUTN and a second terminal coupled to a second common-mode input VCMN. As described above in reference to
In some embodiments, varactor 220 may include two voltage-dependent capacitors 221 and 222. Voltage-dependent capacitor 221 may have a first terminal coupled to the first common mode terminal of varactor 220 and a second terminal coupled to the bias terminal of varactor 220. The first common-mode terminal of varactor 220 may be coupled to the first common-mode input VCMP. Similarly voltage-dependent capacitor 222 may have a first terminal coupled to the second common-mode terminal of varactor 220 and a second terminal coupled to the bias terminal of varactor 220. The second common-mode terminal of varactor may be coupled to the second common-mode input VCMN.
The bias terminal of varactor 220 may be configured to be driven by a bias voltage. In some embodiments, variable leaf capacitor 140 may include a bias switch 230. In some embodiments, bias switch 230 may include a first input terminal that may be coupled to VBIAS1 and a second input terminal that may be coupled to VBIAS2. Bias switch 230 may also include an output that may be coupled to the bias terminal of varactor 220. Bias switch 230 may be controlled by a digital signal, and may be configured to apply a first bias voltage (e.g., VBIAS1) to varactor 220 in a first state or a second bias voltage (e.g., VBIAS2) to varactor 220 in a second state.
The capacitance of varactor 220 may vary as a function of the voltage from its first and second common-mode terminals to its bias terminal. In turn, the capacitance of variable leaf capacitor 140 may vary as a function of the varying capacitance of varactor 220. In some embodiments, the capacitance of variable leaf capacitor 140, with capacitor 211, varactor 220, and capacitor 212 coupled in series to each other, may be as described by Equation 2:
where C211 may be the capacitance of capacitor 211, C212 may be the capacitance of capacitor 212, and Cvar220 may be the capacitance of varactor 220. As described above, there may be multiple means of varying the voltage from the first and second common-mode terminals to the bias terminal of varactor 220, and accordingly, there may be multiple means of varying the capacitance of varactor 220, and in turn, the total capacitance of variable leaf capacitor 140. For example, in some embodiments, varying bias voltages may be applied to the bias terminal of varactor 220 via bias switch 230. In addition, a variable common-mode voltage may be applied to the first and second common-mode terminals of varactor 220 via the VCMP and VCMN inputs. Accordingly, in some embodiments, the capacitance across DCO 100 outputs OUTP and OUTN, and in turn the frequency of DCO 100, may be varied as a function of bias voltage applied to varactor 220 as well as the common-mode voltage applied to varactor 220.
Though some embodiments of varactor 220 may be modeled as depicted in
In some embodiments, the capacitance of variable leaf capacitor 140 may be varied by varying the capacitance of capacitor 211 and/or the capacitance of capacitor 212. Capacitor 211 and/or capacitor 212 may be varied by any suitable means. For example, in some embodiments, capacitor 211 may include an array of individually selectable capacitors that may be switched in or out to contribute to the total capacitance of capacitor 211. In some embodiments, capacitor 211 may include a varactor that may vary as a function of the voltage across the capacitor 211. Likewise, in some embodiments, capacitor 212 may include an array of individually selectable capacitors that may be switched in or out to contribute to the total capacitance of capacitor 212. In some embodiments, capacitor 212 may include a varactor that may vary as a function of the voltage across the capacitor 212.
For the purposes of the present invention, the term “variable leaf capacitor” may refer generically to any type of variable leaf capacitor, including a DAC-controlled variable leaf capacitor. The term “DAC-controlled variable leaf capacitor” may further indicate that the variable leaf capacitor may be controlled by a DAC.
Capacitor 311 and capacitor 312 may be configured as AC-coupling capacitors. For example, in some embodiments, capacitor 311 may have a first terminal coupled to OUTP and a second terminal coupled to a first common-mode input VCMP. Similarly, in some embodiments, capacitor 312 may have a first terminal coupled to OUTN and a second terminal coupled to a second common-mode input VCMN. As described above in reference to
In some embodiments, varactor 320 may include two voltage-dependent capacitors 321 and 322. Voltage-dependent capacitor 321 may have a first terminal coupled to the first common-mode terminal of varactor 320 and a second terminal coupled to the bias terminal of varactor 320. The first common-mode terminal of varactor 320 may be coupled to the first common-mode input VCMP. Similarly, voltage-dependent capacitor 322 may have a first terminal coupled to the second common-mode terminal of varactor 320 and a second terminal coupled to the bias terminal of varactor 320. The second common-mode terminal of varactor may be coupled to the second common-mode input VCMN.
The bias terminal of varactor 320 may be configured to be driven by a bias voltage. In some embodiments, DAC-controlled variable leaf capacitor 150 may include a DAC input DAC_IN coupled to the bias terminal of varactor 320. Accordingly, in some embodiments, the bias terminal of varactor 320 may be driven by DAC 180.
The capacitance of varactor 320 may vary as a function of the voltage from its first and second common-mode terminals to its bias terminal. In turn, the capacitance of DAC-controlled variable leaf capacitor 150 may vary as a function of the varying capacitance of varactor 320. In some embodiments, the capacitance of DAC-controlled variable leaf capacitor 150, with capacitor 311, varactor 320, and capacitor 312 coupled in series to each other, may be as described by Equation 3:
where C311 may be the capacitance of capacitor 311, C312 may be the capacitance of capacitor 312, and Cvar320 may be the capacitance of varactor 220. As described above, there may be multiple means of varying the voltage from the first and second common-mode terminals to the bias terminal of varactor 320, and accordingly, there may be multiple means of varying the capacitance of varactor 320, and in turn, the total capacitance of DAC-controlled variable leaf capacitor 150. For example, in some embodiments, varying bias voltages may be applied to the bias terminal of varactor 320 via DAC 180. In addition, a variable common-mode voltage may be applied to the first and second common-mode terminals of varactor 320 via the VCMP and VCMN inputs. Accordingly, in some embodiments, the capacitance across DCO 100 outputs OUTP and OUTN, and in turn the frequency of DCO 100, may be varied as a function of bias voltage applied to varactor 320 by DAC 180 as well as the common-mode voltage applied to varactor 320.
Though some embodiments of varactor 320 may be modeled as depicted in
In some embodiments, the capacitance of DAC-controlled variable leaf capacitor 150 may be varied by varying the capacitance of capacitor 311 and/or the capacitance of capacitor 312. Capacitor 311 and/or capacitor 312 may be varied by any suitable means. For example, in some embodiments, capacitor 311 may include an array of individually selectable capacitors that may be switched in or out to contribute to the total capacitance of capacitor 311. In some embodiments, capacitor 311 may include a varactor that may vary as a function of the voltage across the capacitor 311. Likewise, in some embodiments, capacitor 312 may include an array of individually selectable capacitors that may be switched in or out to contribute to the total capacitance of capacitor 312. In some embodiments, capacitor 312 may include a varactor that may vary as a function of the voltage across the capacitor 312.
In some embodiments, voltage bias 160 may provide, for example, a VBIAS1 of 0V and a VBIAS2 of 160 mV. As shown by curve 401, when the common-mode voltage is set to a first common-mode value, variable leaf capacitor 140 may have a first capacitance represented by point 411 at a bias voltage of 0V, and a second capacitance represented by point 412 at a bias voltage of 160 mV. Similarly, as shown by curve 402, when the common-mode voltage is set to a second common-mode value, variable leaf capacitor 140 may have a third capacitance represented by point 421 at a bias voltage of 0V, and a fourth capacitance represented by point 422 at a bias voltage of 160 mV. As shown in
Graph 400 of
In some embodiments, variable leaf capacitor 140 and/or DAC-controlled variable leaf capacitor 150 may be configured to provide DCO 100 with an additional means of tuning DCO 100 as well as a means of controlling the step size of the varying capacitance across OUTP and OUTN such that a substantially consistent frequency tuning step size may be maintained across a wide range of potential frequencies.
For example, in some embodiments, DCO 100 may include inductor 108 that may have an inductance of 1.0 nH, capacitor 120, capacitor 130, DAC-controlled variable leaf capacitor 150, and sixty-four individual instantiations of variable leaf capacitor 140. Each instantiation of variable leaf capacitor 140 may be configured such that, at a first common mode voltage, the difference between its capacitance at a first bias voltage and its capacitance at a second bias voltage may be 0.203 fF. In other words, in reference to
In such embodiments, if the total capacitance across OUTP and OUTN is set to approximately 4 pF and the frequency of DCO 100 is roughly tuned to approximately 2.52 GHz, the common-mode bias 170 may be configured to provide a first common-mode voltage and the resulting 0.203 fF capacitive step size for each of the sixty-four variable leaf capacitors 140 may provide sixty-four frequency tuning settings with a step size of 64 KHz. If DCO 100 is later configured to output a different frequency, the common mode voltage may be adjusted such that the frequency step size may be maintained. For example, if the total capacitance across OUTP and OUTN is adjusted to approximately 8 pF and the frequency of DCO 100 is roughly tuned to approximately 1.78 GHz, common-mode bias 170 may be configured to provide a second common-mode voltage and the resulting 0.575 fF capacitive step size for each of the sixty-four variable leaf capacitors 140 may provide sixty-four frequency tuning settings with a step size of 64 KHz.
In such embodiments, the sixty-four instantiations of variable leaf capacitor 140 that each may have a 64 KHz step size, may provide a total tuning range of approximately 4 MHz. In some embodiments, such a tuning range may be sufficient to allow a Phase-Locked Loop (“PLL”), in which DCO 100 may be incorporated, to stay locked. In some alternative embodiments, however, DCO 100 may include any suitable number of variable leaf capacitors 140, which may be configured to have any suitable capacitive range.
In some embodiments, DAC-controlled variable leaf capacitor 150 may be configured to match variable leaf capacitor 140. For example, in some embodiments, capacitor 311 in DAC-controlled variable leaf capacitor 150 may be configured to match capacitor 211 in variable leaf capacitor 140, and capacitor 312 in DAC-controlled variable leaf capacitor 150 may be configured to match capacitor 212 in variable leaf capacitor 140. Likewise, in some embodiments, varactor 320 in DAC-controlled variable leaf capacitor 150 may be configured to match varactor 220 in variable leaf capacitor 140.
Moreover, in some embodiments, DAC-controlled variable leaf capacitor 150 may be configured to operate in conjunction with variable leaf capacitor 140. For example, DAC 180 may be configured as a 6-bit DAC with an analog output ranging from the first bias voltage provided by voltage bias 160 (e.g., VBIAS1) to the second bias voltage provided by voltage bias 160 (e.g., VBIAS2). Further, DAC-controlled variable leaf capacitor 150 may be configured such that, at a first common mode voltage, the difference between its capacitance at the lowest DAC setting and its capacitance at the highest DAC setting may be 0.203 fF. In other words, in reference to
In such embodiments, if the total capacitance across OUTP and OUTN is set to approximately 4 pF and the frequency of DCO 100 is roughly tuned to approximately 2.52 GHz, the common-mode bias 170 may be configured to provide a first common-mode voltage, and the 64 DAC-controlled settings along the 0.203 fF variable capacitance range may provide 64 settings along a 64 KHz range with a step size of approximately 1 KHz. If DCO 100 is later configured to output a different frequency, the common mode voltage may be adjusted such that an approximately equal frequency step size may be maintained. For example, if the total capacitance across OUTP and OUTN is adjusted to approximately 8 pF and the frequency of DCO 100 is roughly tuned to approximately 1.78 GHz, common-mode bias 170 may be configured to provide a second common-mode voltage, and the 64 DAC controlled settings along the 0.575 fF variable capacitance range may provide 64 settings along a 64 KHz range with a step size of approximately 1 KHz. Accordingly, in some embodiments, DAC-controlled variable leaf capacitor 150 and 64 instantiations of variable leaf capacitor 140 may be operated in conjunction with each other to provide for a extra-fine tuning of DCO 100 across a range of 4 MHz with a minimum step size of 1 KHz at multiple different tuning frequency ranges.
Example embodiments are described above with specific values to provide for example calculations of frequencies for DCO 100. However, the various values described for voltage bias 160, common-mode bias 170, capacitor 120, capacitor 130, variable leaf capacitor 140, and DAC-controlled variable leaf capacitor 150, for example, are merely exemplary. Any other suitable values may be used to achieve a desired tuning resolution as well as a controllable frequency tuning step size across a range of different frequencies for DCO 100. For example, smaller capacitance values may be used for some embodiments of DAC-controlled variable leaf capacitor 150 to achieve a tuning resolution finer than 1 KHz. In such embodiments, smaller capacitance values may be used to achieve, for example, a resolution of 800 Hz or less.
As described above, the capacitance of variable leaf capacitor 140 and DAC-controlled variable leaf capacitor 150 may depend on both the common-mode voltage and the bias voltage. As described above, in some embodiments, the step size for a given variation of the bias voltage may be set by controlling the common-mode voltage, and the capacitance of the device may then be controlled by setting the bias voltage. In some embodiments, the roles of the bias voltage and the common-mode voltage may be reversed. For example, in some embodiments, the step size for a given variation of the common-mode voltage may be set by controlling the bias voltage, and the capacitance of the device may then be controlled by setting the common-mode voltage.
In some embodiments, the matching between variable leaf capacitor 140 and DAC-controlled variable leaf capacitor 150 may provide improved tuning performance of DCO 100. For example, when the AC-coupling capacitors 311 and 312 and the varactor 320 in DAC-controlled variable leaf capacitor 150 match the respective AC-coupling capacitors 211 and 212 and the varactor 220 in instantiations of variable leaf capacitor 140, the common-mode setting of variable leaf capacitor 140 and DAC-controlled variable leaf capacitor 150 can be controlled together via the same common-mode voltage signals. Matching corresponding components and driving those components with the same common-mode voltage signals VCMP and VCMN may allow the physical semiconductor layout of the devices to be located closely to each other, optimizing the physical matching of the respective devices. Accordingly, physical mismatches due to unintended semiconductor processing effects may be minimized, and in turn, any frequency tuning gaps introduced by physically mismatched components may be eliminated or minimized.
Moreover, although some embodiments of variable leaf capacitor 140 and DAC-controlled variable leaf capacitor 150 are described above as being operated in conjunction with each other to perform a single fine-tuning scheme, some embodiments of variable leaf capacitor 140 and some embodiments of DAC-controlled variable leaf capacitor 150 may be controlled and/or operated independently of each other.
For some embodiments of DCO 100, capacitor 120 may be described as providing a “coarse” tune, capacitor 130 may be described as providing a “fine” tune, and DAC-controlled variable leaf capacitor 150 and variable leaf capacitor 140 may be described as providing an “extra fine” tune. However, the terms “coarse,” “fine,” and “extra fine” merely indicate potential relative tuning resolutions as compared to other methods of tuning that may be available in a given embodiment of DCO 100. For example, in some embodiments, DCO 100 may include capacitor 120 and DAC-controlled variable leaf capacitor 150, and in such embodiments, capacitor 120 may be varied such that it provides a “coarse” tune as compared to DAC-controlled variable leaf capacitor 150, and DAC-controlled variable leaf capacitor 150 may be varied such at it provides a “fine” tune as compared to capacitor 120.
At step 502, method 500 may require coarsely tuning DCO 100 to a desired frequency. In some embodiments, DCO 100 may include an LC-resonator formed by an inductor 108 and one or more capacitive elements such as capacitor 120, capacitor 130, variable leaf capacitor 140, and/or DAC-controlled variable leaf capacitor 150. The frequency of the digitally controlled oscillator may depend on the inductance of inductor 108 and the total capacitance of the capacitive elements. The frequency of DCO 100 may be coarsely tuned by adjusting the inductance of inductor 108 and/or by adjusting the capacitance of, for example, capacitor 120 and/or capacitor 130.
At step 504, method 500 may require setting the capacitive step-size of a variable leaf capacitor such as variable leaf capacitor 140 or DAC-controlled variable leaf capacitor 150 to a capacitive step size that corresponds to a desired frequency tuning step size of the digitally controlled oscillator 100 at the desired frequency. In some embodiments, the capacitance of variable leaf capacitor may depend on a bias voltage and a common-mode voltage, and the capacitive step size achieved by a variation of the bias voltage may be dependent on the common-mode voltage.
Accordingly, in some embodiments, setting the capacitive step size of the variable leaf capacitor may comprise setting the common-mode voltage.
At step 506, method 500 may require finely tuning DCO 100 to the desired frequency by setting the capacitance of the variable leaf capacitor. In some embodiments, setting the capacitance of the variable leaf capacitor may comprise setting the bias voltage applied to the variable leaf capacitor.
Although
This disclosure encompasses all changes, substitutions, variations, alterations, and modifications to the example embodiments herein that a person having ordinary skill in the art would comprehend. Similarly, where appropriate, the appended claims encompass all changes, substitutions, variations, alterations, and modifications to the example embodiments herein that a person having ordinary skill in the art would comprehend. Moreover, reference in the appended claims to an apparatus or system or a component of an apparatus or system being adapted to, arranged to, capable of, configured to, enabled to, operable to, or operative to perform a particular function encompasses that apparatus, system, component, whether or not it or that particular function is activated, turned on, or unlocked, as long as that apparatus, system, or component is so adapted, arranged, capable, configured, enabled, operable, or operative.
All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present inventions has been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.
