Abstract:
A method and system for implementing a gain control with fine resolution and minimal additional circuitry. The fine digital gain control may be deployed in conjunction with a coarse switched gain at the front end of a sampling receiver. The fine digital gain control mechanism is configured to receive an input signal and moderate gains applied to the received input signal. The output of a low noise amplifier (LNA) is connected to a switched attenuator which provides fine gain stepped gain control. The output of this stage is connected to the switch stage whose output is connected to a charge redistribution successive approximation register digital-to-analog converter (SAR ADC) configured to convert an analog waveform into a digital representation.

Description:
BACKGROUND 
     Embodiment generally relate to electronic circuit designs, and more specifically to improvements in architectural arrangements which enable enhanced performance and/or features for direct sampling tuner, and specifically to direct conversion sampling receivers which include a successive approximation analog-to-digital converter (SAR-ADC) to enhance quality of sampling receivers, where the SAR-ADC incorporates a current redistribution digital-to-analog converter (DAC), with gain control. 
     Direct conversion sampling receivers (DSRs) are a relatively new realization and are highly suited to implementation on an ultra-high speed digital process since the receiver architecture eliminates the requirement for significant analogue circuits such as operational amplifier (op-amp) based continual time filters. DSRs are used in, for example, cable modems, satellite set top boxes, cable set top boxes, and the like. However, in many DSRs in order to compensate for a wide amplitude range of received signals, the input signals are subjected to amplitude adjustment using fine digital gain control (“FDGC”). FDGC allows for the selection and adjustment of gain to be applied to an input signal. Amplitude adjustment or so called gain adjustment of an incoming signal by an FDGC is used to achieve an amplitude level well above the noise and offset thresholds. Without the application of gain adjustment, it may not be feasible to perform further post processing of an incoming signal, such as adaptive equalization and digital conversion. 
     Many techniques are known for implementing fine digital gain control such as switched gm stages, field effect transistor (FET) switched R-2R ladders and the like. All these approaches have major disadvantages such as adding to thermal noise, intermodulation associated with the additional circuits, adding to circuit complexity, and since they are typically preceded by an amplifier stage with a fixed gain or with a small range of coarse gain steps the output amplitude will increase in sympathy with the input and so potentially lead to compression and further intermodulation distortion in the output. 
     Therefore, there is a need in the art for an architectural arrangement which substantially overcomes the aforementioned undesired characteristics. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which: 
         FIG. 1  is an illustration of a direct sampling tuner/receiver (DSR) having charge redistribution SAR-ADC architecture with variable gain control component in accordance to an embodiment; 
         FIG. 2  is an illustration of the DSR of  FIG. 1  during the sample phase in accordance to an embodiment. 
         FIG. 3  is an illustration of the DSR of  FIG. 1  during the conversion phase in accordance to an embodiment; 
         FIG. 4  is an illustration of gain control characteristic with C GAIN  expressed as a ratio to C ADC  in accordance to an embodiment; 
         FIG. 5  is an illustration of gain control characteristic with C Tot  ratio of 4:2:1 in frequency domain in accordance to an embodiment; and 
         FIG. 6  is an illustration of gain control characteristic with C Tot  ratio of 4:2:1 in time domain in accordance to an embodiment; and 
         FIG. 7  is a flow diagram illustrating actions in a method  700  for introducing variable gain control to an analog to digital conversion based on the architecture of  FIG. 1  in accordance to an embodiment. 
     
    
    
     It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate corresponding or analogous elements. 
     DETAILED DESCRIPTION 
     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the preset invention may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the present invention. 
     Although embodiments of the invention are not limited in this regard, discussions utilizing terms such as, for example, “processing,” “computing,” “calculating,” “determining,” “establishing”, “analyzing”, “checking”, or the like, may refer to operation(s) and/or process(es) of a computer, a computing platform, a computing system, or other electronic computing device, that manipulate and/or transform data represented as physical (e.g., electronic) quantities within the computer&#39;s registers and/or memories into other data similarly represented as physical quantities within the computer&#39;s registers and/or memories or other information storage medium that may store instructions to perform operations and/or processes. 
     Although embodiments of the invention are not limited in this regard, the terms “plurality” and “a plurality” as used herein may include, for example, “multiple” or “two or more”. The terms “plurality” or “a plurality” may be used throughout the specification to describe two or more components, devices, elements, units, parameters, or the like. For example, “a plurality of stations” may include two or more stations. The terms “first,” “second,” and the like, herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. 
       FIG. 1  is an illustration of a direct sampling tuner/receiver  100  (DSR) having charge redistribution SAR-ADC architecture with variable gain control component in accordance to an embodiment. The illustrated direct sampling receiver (DSR)  100  is implemented to process received signals  105 , such as signal V in  (t), with charge redistribution SAR-ADC  125 . A front end of the DSR  100  includes a low noise amplifier (LNA). The LNA may be a gm stage  110  outputting a current which is switched to the SAR-ADC. The Gm stage  110  amplifies the received signal  105  (Vin (t)). In one example, the front end of the DSR  100  also includes a sampling switch  120 . The sampling switch  120  is selectively switched in accordance with a sampling clock signal (sample clock  121 ), switched to pass a selected sample of the amplified signal to the SAR-ADC  125 . 
     In one example, the DSR  100  further includes a variable gain control component  180 , which provide a variable load to the Gm stage  110 . Known techniques for implementing gain control include switched gm stages, FET switched R2R ladders and the like. These devices are known for introducing thermal noise, adding circuit complexity, and causing or increasing inter-modulation. Some of these known techniques additionally tend to cause compression and inter-modulation distortion with down stream components which tends to manifest in the output. Gain control component  180  is implemented as a capacitor or an array of capacitors. A gain control capacitor (C Gain ) component can deliver an accurate and predictable gain step that overcomes additive noise and intermodulation associated with traditional techniques. Gain control component  180  comprises capacitor  182 , which may consist of a multiple component array, and switch  185 . Switch  185  is positioned based on the operation of SAR-ADC  125 , i.e., the sample and conversion phases. During the sampling phase, switch  185  like mode switch  160  is placed in sample period position. As shown there is one mode switch  160  per capacitor. During the sample period the switches form a circuit with the output side of sampling switch  120 . During the conversion phase the connection with the sampling switch is broken. 
     SAR ADC  125  can include various subcircuits, including comparator circuit  125 , internal digital-to-analog converter (DAC)  150 , SAR logic  140 , and control logic block  170  with result register. Comparator  130  can compare an input voltage (Vin(*)  106 ) which is the output voltage  152  (Vcomp) of DAC  150  against a reference voltage Vref  131  and can output the result of the comparison to SAR LOGIC block  140 . SAR logic  140  can include a successive approximation register designed to supply an approximate digital code of the input voltage, Vin(*)  106 , to DAC  150 . DAC  150  is shown associated with comparator  130 , capacitor banks or array of capacitors  155 , mode switches  160  each coupled to a capacitor in the array of capacitors, and sampling switch  120  is associated with a low noise amplifier (LNA) and one element of the array of capacitors. An input of comparator  130  is coupled to a reference voltage Vref. The closing or opening of each of the mode switches  160  is controlled by control block  170 . The closing or opening of the sampling switch  120  is controlled by a sampling clock signal to selectively activate. A resulting code of a digital approximation of the sampled input voltage Vin (*)  106  can be outputted at the end of a conversion to an output register  172  at control block  170  or as a separate circuit block. In accordance with various embodiments, SAR-ADC  125  can be implemented as a charge redistribution SAR-ADC. For clarity, power supplies (positive Vdd, negative Vee), as well as ground connections, are assumed to be present, but not shown in the Figure. 
     The closing or opening of each of the mode switches  160  and/or switch  185  is controlled by control block  170 . The closing or opening of sampling switch  120  is controlled by a sampling clock signal to selectively activate the sampling switch. The sample clock may be externally generated, generated by control block  170 , or by a programmed multivibrator in DSR  100 . 
     A control block  170  based on the stored instructions such as values and/or number of iterations that correspond to a predetermined resolution, determines the switching of switch  160 . The opening and closing of the switch is predetermined by the number of samples from sample switch  120  which sets the sampling phase duration, and the number of cycles required to run the logic to switch the charge redistribution digital-to-analog converter output. Switch  160  is controlled to two distinct configurations or phases. These configurations are (a) a sampling mode configuration and (b) a conversion mode configuration. 
     In the first configuration, referred to as the sampling phase/mode, the Gm stage,  110 , through the switch,  120 , charges the array of capacitors  155 , i.e., mode switch  160  couples the capacitors to the output of sampling switch  120 , to integrate the current output sampled by the sampling switches; each capacitor in the array of capacitors would normally be discharged before this period. In addition during the sampling period capacitor  182  in gain control element  180  is also connected to the output of sampling switch  120 . ADC capacitor array and capacitor  182  create a parallel bank of capacitors during the sampling phase. 
     In the second configuration, referred to as the conversion mode/phase, After the requisite number of samples the DAC array of capacitors is then isolated from the input sampling switches and transitioned back to normal charge distribution function within the SAR-ADC wherein the array of capacitors are switched between supply voltage (Vdd) and ground (Vss) which redistributes the stored charge between the elements such that the resultant voltage on the capacitor is V=Q/C trends towards the reference voltage of the comparator  130 . The output of comparator  130  then processed through quantization loop of SAR logic  140  and CDAC  150  until the number of iterations is produced and a predetermined resolution. Since the illustrated architecture is based on charge sampling and redistribution around the SAR-ADC capacitors the performance can be enhanced by redistribution of the capacitors and amplifiers (gm) during the sampling phase. 
     While a single stage is shown in this embodiment the gain control component can be deployed in an interleaved system, in which case the C GAIN  may be preferably reused between multiple placements of the SARADC which may be incorporated so that when a first SARADC  125  is sampling the second is converting, and vice versa. More than 2 SAR ADCs  125  may be deployed in this manner for example if the conversion periods are substantially longer than the sampling; for example consider a sampling to conversion period ratio of 1:2, three segments may be deployed so that when one segment is converting firstly a second segment samples the input for half the conversion period of the first and then a third segment samples for the other half conversion period of the first. In all such cases the capacitor  182  can be reused for the sampling period of each and all segments, since it is only used during the sampling period. 
       FIG. 2  is an illustration of the DSR of  FIG. 1  during the sample phase in accordance to an embodiment. Here, a gain control component  180 , capacitor  182  (C Gain ), can be introduced to scale the dynamic input range of the SAR-ADC  125 . During the sampling phase, the SAR-ADC  125  can be connected to the output of Gm stage  110  via sampling switch  120 . The total charge stored in SAR-ADC  125  and the gain control components after the sampling phase can be defined as: Q tot =(C ADC +C Gain )*Vin; where Vin is the voltage at sampling switch  120  after it was amplified and C ADC +C Gain =C TOT . As shown, from the point of view of the front end of DSR  100  it appears that the load is two capacitors, ADC capacitor array  155  and capacitor  182 , connected in parallel. The theoretical load is represented in the “s” domain by: 1/SC TOT  where C TOT =C ADC +C GAIN . By switching the capacitor  182  in during the sample phase the voltage generated (Vin) will be reduced by the additional capacitance, so providing gain control. The Gain control can be raised or lowered by changing the overall capacitance of gain control component  180 . 
       FIG. 3  is an illustration of the DSR of  FIG. 1  during the conversion phase in accordance to an embodiment. After the requisite number of samples the array of capacitors  155  is then isolated from the input sampling switch  120  and transitioned back to normal charge distribution function within the SAR-ADC wherein the array of capacitors are switched (mode switch  160 ) between supply voltage (Vdd) and ground (Vss) which redistributes the stored charge between the elements such that the resultant voltage on the capacitor is V=Q/C trends towards the reference voltage  131  of comparator  130 . The output of comparator  130  then processed through quantization loop of SAR logic  140  and DAC  150  until the number of iterations is produced and a predetermined resolution. Additionally, during the conversion stage the additional capacitance in gain control component  180  (Capacitance  182 ) must be switched or isolated away from the SAR-ADC capacitor to allow correct operation of the charge redistribution DAC, i.e., ADC capacitor array  155 . Switching the additional capacitance away from the ADC capacitor array  155  will not modify the voltage stored in the ADC capacitor hence the reduced signal amplitude generated during the sampling period will be maintained into the conversion period. Further, the isolated additional capacitor may be discharged (connecting capacitor  182  to ground) to allow correct operation during the next sample/conversion cycle. 
       FIG. 4  is an illustration of gain control characteristic with C GAIN  expressed as a ratio to C ADC  in accordance to an embodiment. The variation in gain with C GAIN  is shown in  FIG. 4 . 
       FIG. 5  is an illustration of gain control characteristic with C Tot  ratio of 4:2:1 in frequency domain in accordance to an embodiment. The gain control characteristic is constant with frequency offset as displayed in  FIG. 5 . This figure shows the gain characteristic for C TOT  a 1:2:4 for a 4 segment (Gm segment  1  . . . Gm segment  4 ) direct sampling receiver. As can be seen the gain offset between simulations is approximately six (6) dB as predicted and furtherly that the relative attenuation is constant with frequency maintaining the sampled filter characteristic. 
       FIG. 6  is an illustration of gain control characteristic with C Tot  ratio of 4:2:1 in time domain in accordance to an embodiment. As can be seen from  FIG. 6 , an additional feature of this architecture is that since the signal is input as a current there is theoretically no change in phase as the gain is adjusted Phase shift can be a particularly problematic effect since digital modulation techniques employ phase information of the carrier as part of the encoding, for example 64 QAM has 64 data locations each with a unique phase and amplitude information, therefore any phase shift associated with a gain change can lead to a corruption in the data location and corruption in the data. 
       FIG. 7  is a flow diagram illustrating actions in a method  700  for introducing variable gain control to an analog to digital conversion based on the architecture of  FIG. 1  in accordance to an embodiment. Method  700  begins with start  710 . Control is then passed to action  715 . Action  715  is based on the positioning of mode switch  160 . If the sampling mode has been selected then control is passed to action  720  for further processing in accordance to a sampling process. Action  720  introduces variable gain control into the process by toggling switch  185  to the sample phase. After action  720  control is passed to action  725 . In action  725  if switch  160  is in the conversion mode then control is passed to action  730  for further processing. If switch  160  is not set to conversion mode then control is passed to action  715  for further processing. When control is passed to action  730  the gain control component is isolated from the array of capacitors at SAR-ADC  125  and the resultant voltage in the capacitors are compared against a reference voltage Vref  131  at comparator  130 . Control is then passed to action  750  where SAR-ADC is performed on the charges (Q) in the array of capacitors. After a predetermined number of iterations control is then passed to action  710  and the process is restarted. 
     The techniques described herein may be embodied in a computer-readable medium for configuring a computing system to execute the method. The computer readable media may include, for example and without limitation, any number of the following: magnetic storage media including disk and tape storage media; optical storage media such as compact disk media (e.g., CDROM, CDR, and the like) and digital video disk storage media; holographic memory; nonvolatile memory storage media including semiconductor-based memory units such as FLASH memory, EEPROM, EPROM, ROM; ferromagnetic digital memories; volatile storage media including registers, buffers or caches, main memory, RAM, and the like; and data transmission media including permanent and intermittent computer networks, point-to-point telecommunication equipment, carrier wave transmission media, the Internet, just to name a few. Other new and various types of computer-readable media may be used to store and/or transmit the software modules discussed herein. Computing systems may be found in many forms including but not limited to mainframes, minicomputers, servers, workstations, personal computers, notepads, personal digital assistants, various wireless devices and embedded systems, just to name a few. A typical computing system includes at least one processing unit, associated memory and a number of input/output (I/O) devices. A computing system processes information according to a program and produces resultant output information via I/O devices. 
     Realizations in accordance with the present invention have been described in the context of particular embodiments. These embodiments are meant to be illustrative and not limiting. Many variations, modifications, additions, and improvements are possible. Accordingly, plural instances may be provided for components described herein as a single instance. Boundaries between various components, operations and data stores are somewhat arbitrary, and particular operations are illustrated in the context of specific illustrative configurations. Other allocations of functionality are envisioned and may fall within the scope of claims that follow. Finally, while certain features of the embodiment have been illustrated and described herein, many modifications, substitutions, changes, and equivalents may occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.