1. Technical Field
The subject matter described herein relates to communication receivers. In particular, the subject matter described herein relates to receivers having a high dynamic range analog-to-digital converter (ADC) architecture with a continuous-time (CT) multiplying digital to analog converter (MDAC).
2. Description of Related Art
Wideband and narrowband receivers are generally required to have high dynamic range (i.e., ability to handle a wide range of signal strengths) to receive weak signals in an environment with strong blocker signals (blockers). A first problem is that wideband (e.g., multi-channel or multi-carrier) receivers generally comprise multiple narrowband receivers, which leads to multiplication of power and area consumption and a necessity for external passive filters. A second problem is that filters and other blocker cancellation techniques used by receivers are not effective to handle multiple blockers or wideband blockers for multi-channel or multi-carrier signals.
A third problem is that higher performing ADCs (analog-to-digital converters) consume excessive power. The dynamic range of a receiver is often limited by its ADC. ADC dynamic range is limited by thermal noise power (i.e., kT/C) and sampling nonlinearity. The resolution quality of a recovered signal is often specified by an effective number of bits (ENOB), which is the number of bits representing a signal, excluding the number of bits representing noise. In order to achieve a high ENOB (e.g., signal resolution in excess of 12 bits) at high speed, ADCs consume excessive power to overcome increasing noise accompanying increasing speed.
State of the art ADC architectures do not support a direct sampling receiver with high speed (e.g., multi GHz), high dynamic range (e.g., greater than 70 dB), high resolution (e.g., greater than 11 bit ENOB), low noise figure (NF) (e.g., lower than 20 dB) without pre-amplification, with reduced area, power consumption and cost, suitable for a wide variety of applications, e.g., full-band, narrow-band, multi-channel and single-channel, in a wide variety of communication markets, e.g., terrestrial TV, cable, satellite, multi-media over coax alliance (MoCA), uWave, WiFi, WiMax and cellular communications.
Methods, systems, and apparatuses are described for a direct sampling receiver that includes a CT MDAC, substantially as shown in and/or described herein in connection with at least one of the figures, as set forth more completely in the claims.
The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate a plurality of embodiments and, together with the description, further serve to explain the principles involved and to enable a person skilled in the pertinent art(s) to make and use the disclosed technologies. However, embodiments of the disclosed technologies are not limited to the specific implementations disclosed herein. Unless expressly indicated by common numbering, each figure represents a different embodiment where components and steps in each embodiment are intentionally numbered differently.
Embodiments will now be described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears.
Reference will now be made to embodiments that incorporate features of the described and claimed subject matter, examples of which are illustrated in the accompanying drawings. While the technology will be described in conjunction with various embodiments, it will be understood that the embodiments are not intended to limit the present technology. The scope of the subject matter is not limited to the disclosed embodiment(s). On the contrary, the present technology is intended to cover alternatives, modifications, and equivalents, which may be included within the spirit and scope the various embodiments as defined herein, including by the appended claims. In addition, in the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the present technology. However, the present technology may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the embodiments presented.
References in the specification to “embodiment,” “example” or the like indicate that the subject matter described may include a particular feature, structure, characteristic, or step. However, other embodiments do not necessarily include the particular feature, structure, characteristic or step. Moreover, “embodiment,” “example” or the like do not necessarily refer to the same embodiment. Further, when a particular feature, structure, characteristic or step is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not those other embodiments are explicitly described.
Certain terms are used throughout the following description and claims to refer to particular system components and configurations. As one skilled in the art will appreciate, various skilled artisans and companies may refer to a component by different names. The discussion of embodiments is not intended to distinguish between components that differ in name but not function. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” Also, the term “couple” or “couples” is intended to mean either an indirect or direct electrical connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection or through an indirect electrical connection via other devices and connections.
Methods and apparatuses are described for a multi-GHz direct sampling receiver having high dynamic range (e.g., over 70 dB) and low noise figure (e.g., under 20 dB) with a continuous-time (CT) multiplying digital-to-analog converter (MDAC) architecture. Difficulties in a wideband, high speed and strong blocker environment are overcome by a multipath architecture where the input signal is sampled in the quantizer path, but not the signal path. In the signal path, only a filtered residue signal is sampled. Coarse bits generated in the quantizer path and fine bits generated in the input signal path are digitally combined to reconstruct the analog input signal in the digital domain. Filtering, aperture error control and digital equalization remove sources of errors, resulting in high performance. Advantages include toleration of multiple blocker signals, simultaneous reception of multiple weak channels and/or strong and weak channels, operation with simplified gain control and reduced pre-amplification as well as operation without excessive power consumption, external filters and tunable local oscillator (LO).
Methods, systems, and apparatuses will now be described for a direct sampling receiver with CT MDAC architecture having high speed, high dynamic range and low noise figure. Many embodiments of systems, devices and methods may be implemented, each with various configurations and/or steps. While several detailed features and embodiments are discussed below, many more embodiments are possible. In Section II, an exemplary communication system is described. In Section III, an exemplary direct sampling receiver with CT MDAC architecture is described. In Section IV, exemplary spectrums of a direct sampling receiver with CT MDAC Architecture are described. In Section V, an exemplary method of analog to digital conversion by an exemplary CT MDAC architecture is described.
Communication system 100 may be any wired and/or wireless communication system, including, but not limited to, terrestrial TV (television), cable, satellite, multi-media over coax alliance (MoCA), uWave, WiFi, WiMax and cellular communication systems. Communication device 105 may be any wired or wireless communication device, including, but not limited to, a TV or satellite set top box, cellular telephone (e.g., a smart phone), a transceiver, an access point, a router, a modem, and/or other type of communication device. Receiver 110 may be a standalone receiver or a receiver in any communication device or system having a CT MDAC. ADC 115 may be a standalone ADC or an ADC in any receiver, device or system having a CT MDAC.
CT MDAC 120 is illustrated with a simplified block diagram representative of many possible embodiments having one or more CT MDACs. CT MDAC 120 has multiple signal paths, e.g., a quantizer path 150 and a continuous-time (CT) path 155. Each signal path may include one or more electrical conductors (e.g., metal traces or other types of electrical conductors), as well as the respective components described herein and/or further or alternative components. CT MDAC 120 comprises an M-bit coarse ADC 125, an M-bit coarse DAC 135, and a residue calculator 140. CT MDAC based ADC 115 comprises a CT MDAC 120, an N-bit fine ADC 130, and a digital combiner 145. M-bit coarse ADC 125 and N-bit fine ADC 130 have heterogeneous sampling, i.e., they do not sample the same signal and may not have the same sampling rates and phases. M-bit coarse ADC 125 samples an analog input signal VIN 160 in quantizer path 150 while N-bit fine ADC 130 samples an analog residue signal VR, instead of input signal VIN 160, in CT path 155. M-bit coarse ADC 125 generates an M coarse bits Dc while N-bit fine ADC generates an N fine bits Df, with both M coarse bits Dc and N fine bits Df being digital signals. M-bit coarse DAC 135 converts M coarse bits Dc into a coarse analog signal VC. Residue calculator 140 subtracts coarse analog signal VC from analog input signal VIN 160 to generate analog residue signal VR. Digital combiner 145 digitally combines M coarse bits Dc and N fine bits Df to generate digital output DOUT 165, which is a digital representation of analog input signal VIN 160.
The following section describes example embodiments for CT MDAC 120
III. Exemplary Direct Sampling Receiver with CT MDAC Architecture
As one of many examples of receiver topology in which embodiments may be implemented,
Generally, CT MDAC based receiver 200 comprises a multi-path ADC with a quantizer path through M-bit ADC 210 and a continuous time path through N-bit ADC. The paths take heterogeneous samples where the quantizer path samples input signal VIN 160 at M-bit ADC 210 and the CT path (or signal path) samples a residue signal portion of input signal VIN 160, which is determined by residue calculator 235, at N-bit ADC 260. Input signal VIN 160 is not sampled on the CT path. In some embodiments, receiver 200 may comprise one of multiple channels in a multi-channel time-interleaved ADC. An ultra-high speed ADC, e.g., 10 GS/s (giga-samples per second), may be realized by a multi-channel or time-interleaved ADC. It must be understood that the embodiment shown in
More specifically, M-bit ADC 210 provides coarse quantization by converting a portion of analog input signal VIN 160 into M coarse bits Dc. In other words, M-bit ADC 210 may be considered to digitize a portion of input signal VIN 160, or to digitize input signal VIN 160 into a coarse or low resolution form. Coarse bits or M-bits are the M more significant bits (MSBs) that represent analog input signal VIN 160 in the digital domain with low resolution. In some embodiments, the sampling time of M-bit ADC 210 may be adjusted. Coarse quantization bits Dc, which includes a number M of bits in each sample, are provided to digital combiner 270. M-bit DAC 230 converts coarse quantization Dc to analog form to generate coarse analog signal VC.
In parallel with the operation of M-bit ADC 210, and M-bit DAC 230, delay 225 delays input signal VIN 160 to output a delayed version of input signal VIN 160. In the analog domain, residue calculator 235 determines residue input signal VR by subtracting coarse analog signal VC and noise PN from the delayed input signal VIN 160. Amplifier 240 amplifies residue signal VR with a gain of 2K (e.g., when k is 3, amplifier 240 provides a gain of 8). In some embodiments, K may be less than M. There is a greater tolerance for inaccuracy by M-bit ADC 210 so long as K is less than M. When K is less than M, redundancy or overranging is provided to tolerate errors by M-bit ADC 210. In some embodiments, the gain of amplifier 240 may be variable.
Filter 245 receives the amplified residue signal VR from amplifier 240, and filters noise and distortion from the amplified residue signal. Sampler 259 and N-bit ADC 260 receive and digitize the amplified and filtered residue signal, including noise (if present), into N fine bits Df. Fine bits or N-bits are the N less significant bits (LSBs) that represent analog input signal VIN 160 in the digital domain with high resolution. In some embodiments, digital equalizer 365 may correct amplitude and phase frequency in N fine bits Df.
Digital combiner 270 receives and combines M coarse bits Dc and N fine bits Df, and subtracts noise PN (when present), to generate digital output DOUT 165, which is a digital representation of analog input signal VIN 160. For instance, digital combiner 270 may create each digital output value (e.g., a bit string) in a stream of digital output values that represent DOUT 165 by using M coarse bits Dc as the MSBs of the digital sample, and N fine bits Df as the LSBs of the digital sample, to create the digital output value to have a bit length of M+N.
In some embodiments, M may be in the range of four to six, such that M-bit ADC 210 is a 4-bit to 6-bit ADC, providing MSBs or lower resolution. In some embodiments, N may be in the range of eight to ten, such that N-bit ADC 260 is an 8-bit to 10-bit ADC, providing LSBs or higher resolution. In other embodiments, M and N may be any number of bits. The sampling rates of M-bit ADC 210, M-bit DAC 230 and N-bit ADC 260 may be the same or different. For example, in some embodiments, the sampling rate of M-bit ADC 210 and M-bit DAC 230 may be twice the sampling rate of N-bit ADC 260 in order to obtain higher residue signal gain and to simplify delay 225.
In some embodiments, M-bit ADC 210 and/or N-bit ADC 260 may comprise high speed flash ADCs with very small quantizer delays. M-bit DAC 230 may comprise a high speed current or resistor DAC. High speed ADCs and DAC provide very low latency.
In some embodiments, delay 225 may be a fixed analog delay. The delay may vary with process, voltage, and/or temperature (PVT). In other embodiments, delay 225 may be a variable, controllable, delay. Delay 225 may comprise a 1st or 2nd order low-pass or all-pass filter with non-ideal group delay, which, in at least some embodiments, need not be accurate or flat.
In some embodiments, filter 245 may comprise a 1st or second order low-pass, anti-aliasing filter. In some embodiments, amplifier 240 and filter 245 may be combined into one block. In other embodiments, amplifier 240, filter 245 and sampler 259 may be combined into one block Output linearity of amplifier 240 need not exceed the linearity requirement of N-bit ADC 260.
In some embodiments, delay mismatch in the quantizer and CT paths, the frequency responses of amplifier 240 and filter 245, PVT variations and/or other issues may necessitate corrective circuitry. A primary challenge for CT MDAC architecture is the aperture error between the quantizer path and CT path. The aperture error may cause over-ranging in N-bit ADC 260. Delay 225 may reduce the error. A flash ADC with a very low quantizer delay implemented as M-bit ADC 210 may also reduce the error.
As one of many examples of receiver topology in which embodiments may be implemented,
Note that some embodiments may comprise corrective or compensating circuitry while others may not.
A digital phase/delay tracking/control loop may be present to adjust the sampling time of M-bit ADC 210 in order to minimize an input power to N-bit ADC 260 that increases with aperture error. Accordingly, phase rotator 315 and phase tracker 355 may be present. In this embodiment, the digital phase tracking loop comprises phase tracker 355 coupled to N fine bits Df and phase rotator 315 coupled to M-bit ADC 210. Other embodiments may implement no phase tracking loop or a different phase tracking loop. Phase rotator 315 may be provided with a clock input from a phase-locked loop (PLL) (not shown). Phase rotator 315 generates a sampling clock for M-bit ADC 210. Phase tracker 355 may provide a timing recovery signal to phase rotator 315 to cause phase rotator 315 to adjust the sampling clock phase.
A digital gain control loop may be present to track PVT gain variation and adjust the gain of amplifier 240 in the CT path. Accordingly, gain control 350 may be present. Gain control 350 may generate and provide a gain control signal to amplifier 240, which adjusts gain according to the gain control signal. In this embodiment, the gain control loop comprises gain control 350 coupled to N fine bits Df and amplifier 240. Other embodiments may implement no gain control loop or a different gain control loop. In such embodiments, gain control 350 may not be present.
Digital correction of the amplitude and phase responses of M-bit DAC 230, residue calculator 235, amplifier 240 and filter 245 may be provided by digital equalizers and background calibration. For instance, digital equalization may be applied to M coarse bits Dc and N fine bits Df before digital combiner 270. In such an embodiment, digital coarse equalizer EQc 362 and fine equalizer EQf 365 may be present to provide equalization and calibration may be provided by PNGEN 305. In the embodiment of
PNGEN 305 and noise injector 230 may be present in some embodiments. In this embodiment, prior to providing coarse quantization bits Dc to M-bit DAC 230, noise injector 320 may be present to inject noise PN (“pseudo noise” or “pseudo random noise”) generated by PNGEN 305. PNGEN 305 may provide background calibration. In order to calibrate equalizer 365, PNGEN generates noise PN that is injected (by noise injector 320) into M coarse bits Dc before M-bit DAC 230 converts M coarse bits Dc into coarse analog signal VC used by residue calculator 235 to calculate residue signal VR. Noise PN characterizes CT MDAC amplitude and phase frequency responses. PNGEN 305 tracks variations in the amplitude and phase frequency responses due to PVT and due to the phase and gain loops. Noise PN is digitally injected, converted to analog by M-bit DAC 230 and re-digitized by N-bit ADC 260. This permits equalizer 365 to be calibrated by operation of PNGEN 305.
There are numerous advantages to the CT MDAC architectures described herein. Because strong blockers dominate coarse quantization, the strong blockers are removed from residue signal VR in the quantizer path, and residue signal VR may include the desired signal. By removing any strong blockers, increased gain may be applied to residue signal VR to amplify the weak desired signal before sampling by N-bit ADC 260. The increased signal gain improves the signal-to-noise ratio (SNR).
Noise and distortion caused by M-bit ADC 210 do not significantly impact overall performance because M coarse bits Dc, which drive the conversion by M-bit DAC 230, are known during reconstruction, i.e., during digital combination.
Due to low pass filtering of residue signal VR by filter 245 before sampling by sampler 259 and N-bit ADC 260, SNR is not limited by kT/C noise. SNR is limited by K+N bits, and typically may be less than K+N bits. Noise, high-frequency transients and images introduced by M-bit DAC 230, residue calculator 235 and amplifier 240 are reduced by filter 245.
Due to gain provided to residue signal VR by amplifier 240 before sampling by sampler 259 and N-bit ADC 260, the effective number of bits (ENOB) is not limited by sampling nonlinearity.
While M-bit DAC 230 should have low noise, its integral non-linearity (INL) can be calibrated. The gain error, nonlinearity and frequency response of M-bit DAC 230 may be estimated by a PN sequence and compensated during reconstruction performed by equalizer 365.
Furthermore, an inaccuracy of amplifier 240 can be tolerated. The gain error, nonlinearity and frequency response of amplifier 240 may be estimated by a PN sequence, and compensated for during reconstruction performed by equalizer 365. In this manner, AGC (automatic gain control) may be simplified.
A single, robust direct sampling receiver with a CT MDAC ADC tolerates multiple blocker signals, simultaneously receives multiple weak and/or strong signals and tolerates substantial inaccuracy, requiring only N-bit sampling linearity, while providing higher dynamic range, higher bandwidth, lower power, lower area and lower cost than state of the art Multi-GHz high resolution receivers and ADCs. ENOB requirement for ADCs may be reduced. Less pre-amplification is necessary. External passive filters are unnecessary. An ADC having an M-bit ADC and an N-bit ADC is much easier to implement and consumes much less total power than an M+N−1 bit ADC. Multi-GHz ADCs with ENOB greater than 11 bits may be practically implemented. High dynamic range, full band, single-channel and multi-channel communication systems and receivers with NF (noise figure) less than 20 dB may be practically implemented with simplified gain control and reduced pre-amplification without excessive power consumption, external filters and tunable local oscillator (LO). This enables full band communication systems in previously difficult to impossible systems, such as terrestrial TV and cellular communications.
The following section provides some example output spectrums for CT MDAC embodiments for some example analog input signals.
IV. Exemplary Spectrum of Direct Sampling Receiver with CT MDAC Architecture
The following section shows further example CT MDAC operational embodiments.
Embodiments may also be implemented in processes or methods. For example,
Flowchart 600 begins with step 605. In step 605, in a first path an input signal is received and sampled in an M-bit analog-to-digital (ADC) converter to generate a coarse signal in the digital domain. For example, as shown in
At step 610, the coarse signal in the digital domain is converted to a coarse signal in the analog domain in an M-bit digital-to-analog converter (DAC). For example, as shown in
At step 615, in a second path, the input signal is received but not sampled. For example, as shown in
At step 620, the coarse analog signal is subtracted from the input signal in the analog domain to generate a residue signal. For example, as shown in
At step 625, the residue signal is sampled in an N-bit ADC to generate a fine signal. For example, as shown in
At step 630, the coarse and fine signals are digitally combined to generate a digital representation of the input signal. For example, as shown in
One of many detailed embodiments of the method in
Flowchart 700 comprises steps 705 to 765, which may be performed in a continuous operation. However, other embodiments may operate in other ways. Other structural and operational embodiments will be apparent to persons skilled in the relevant art(s) based on the foregoing discussion of embodiments. No order of steps is required unless expressly indicated or inherently required. There is no requirement that a method embodiment implement all of the steps illustrated in
Flowchart 700 begins with step 705. In step 705, in a first path an input signal is received and sampled in an M-bit analog-to-digital (ADC) converter to generate a coarse signal in the digital domain. For example, as shown in
At step 710, the coarse signal in the digital domain is converted to a coarse signal in the analog domain in an M-bit digital-to-analog converter (DAC). For example, as shown in
At step 715, in a second path, the input signal is received but not sampled. For example, as shown in
At step 720, a delay is applied to the input signal. For example, as shown in
At step 725, the coarse analog signal is subtracted from the delayed input signal in the analog domain to generate a residue signal. For example, as shown in
At step 730, the residue signal is amplified to generate an amplified residue signal. For example, as shown in
At step 735, the amplified residue signal is filtered. For example, as shown in
At step 740, the filtered residue signal is sampled in an N-bit ADC to generate a fine signal. For example, as shown in
At step 745, an equalized fine signal is generated from the fine signal to correct amplitude and phase frequency responses. For example, as shown in
At step 750, the coarse and equalized fine signals are digitally combined to generate a digital representation of the input signal. For example, as shown in
At step 755, the equalizer is calibrated by adding a pseudo-noise signal to the coarse signal for digitization by N-bit ADC and removal by digital combination. For example, as shown in
At step 760, a phase from the fine signal is tracked and a sampling rate of the M-bit ADC is controlled. For example, as shown in
At step 765, PVT gain variation is tracked from the fine signal and an amplification of the residue signal is controlled. For example, as shown in
In embodiments where a plurality of ADCs operate according to flowchart 600 or 700 as a plurality of channels, flowchart 600 and/or 700 may further comprise combining digital output DOUT 165 data with digital output from at least one other channel operation of the method. In some embodiments, continuous background calibration may be performed.
A device (i.e., apparatus), as defined herein, is a machine or manufacture as defined by 35 U.S.C. §101. Devices may be digital, analog or a combination thereof. Some devices may be implemented with a semiconductor process or semiconductor technology, including one or more of a Bipolar Junction Transistor (BJT), a heterojunction bipolar transistor (HBT), a metal oxide field effect transistor (MOSFET) device, a metal semiconductor field effect transistor (MESFET) or other transconductor or transistor technology device. Such alternative devices may require alternative configurations other than the configuration illustrated in embodiments presented herein.
Techniques, including methods, described herein may be implemented by hardware (digital and/or analog) or a combination of hardware with software and/or firmware. Techniques described herein may be implemented by one or more components. Embodiments may comprise computer program products comprising logic (e.g., in the form of program code or software as well as firmware) stored on any computer useable medium, which may be integrated in or separate from other components. Such program code, when executed in one or more processors, causes a device to operate as described herein. Devices in which embodiments may be implemented may include storage, such as storage drives, memory devices, and further types of computer-readable storage media. Examples of such computer-readable storage media include, but are not limited to, a hard disk, a removable magnetic disk, a removable optical disk, flash memory cards, digital video disks, random access memories (RAMs), read only memories (ROM), and the like. In greater detail, examples of such computer-readable storage media include, but are not limited to, a hard disk associated with a hard disk drive, a removable magnetic disk, a removable optical disk (e.g., CDROMs, DVDs, etc.), zip disks, tapes, magnetic storage devices, MEMS (micro-electromechanical systems) storage, nanotechnology-based storage devices, as well as other media such as flash memory cards, digital video discs, RAM devices, ROM devices, and the like. Such computer-readable storage media may, for example, store computer program logic, e.g., program modules, comprising computer executable instructions that, when executed, provide and/or maintain one or more aspects of functionality described herein with reference to the figures, as well as any and all components, steps and functions therein and/or further embodiments described herein.
Such computer-readable storage media are distinguished from and non-overlapping with communication media (do not include communication media). Communication media typically embodies computer-readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave. The term “modulated data signal” means a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wireless media such as acoustic, RF, infrared and other wireless media, as well as signals transmitted over wires. Embodiments are also directed to such communication media.
Proper interpretation of subject matter described herein and claimed hereunder is limited to patentable subject matter under 35 U.S.C. §101. Subject matter described in and claimed based on this patent application is not intended to and does not encompass unpatentable subject matter. As described herein and claimed hereunder, a method is a process defined by 35 U.S.C. §101. As described herein and claimed hereunder, each of a circuit, device, apparatus, machine, system, computer, module, media and the like is a machine and/or manufacture defined by 35 U.S.C. §101.
While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Embodiments are not limited to the functional blocks, detailed examples, steps, order or the entirety of subject matter presented in the figures, which is why the figures are referred to as exemplary embodiments. A device, apparatus or machine may comprise any one or more features described herein in any configuration. A method may comprise any process described herein, in any order, using any modality. It will be understood by those skilled in the relevant art(s) that various changes in form and details may be made to such embodiments without departing from the spirit and scope of the subject matter of the present application.
The exemplary appended claims encompass embodiments and features described herein, modifications and variations thereto as well as additional embodiments and features that fall within the true spirit and scope of the disclosed technologies. Thus, the breadth and scope of the disclosed technologies should not be limited by any of the above-described exemplary embodiments or the following claims and their equivalents.
This application claims the benefit of U.S. Provisional Application No. 61/836,469, filed Jun. 18, 2013, the entirety of which is hereby incorporated by reference herein.
Number | Date | Country | |
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61836469 | Jun 2013 | US |