Abstract:
A current-mode direct conversion RF receiver is presented. In one set of embodiments the RF receiver comprises a simple transconductor input stage to create a current-mode modulated signal from a voltage-mode modulated signal. A downconversion mixer may be coupled to the transconductor input stage via a low impedance current cascode stage, and may operate to create a set of current-mode quadrature baseband signals from the current-mode modulated signal. The downconversion mixer may be implemented with a transistor-switching network, which may be driven by a phase locked loop (PLL) with quadrature outputs. The set of current-mode quadrature baseband signals may be converted back to the voltage domain by a transimpedance filter, which may perform channel selection for the receiver. The transimpedance filter may additionally include a low frequency zero to remove DC offsets. The receiver may be implemented using CMOS design technologies and operated with minimal self-mixing effects, minimal DC offset in the baseband signal, and utilizing low voltages.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to the field of wireless electronic equipment design and, more particularly, to radio frequency (RF) receiver design. 
   2. Description of the Related Art 
   A traditional narrow band radio system&#39;s receiver portion is generally used for translating a modulated radio frequency (RF) carrier to a low frequency, or baseband, signal, which may then be demodulated to extract an original modulating signal. A primary function of an RF receiver is best described as a “down conversion”, or “mixing”, of an RF modulated carrier to a baseband signal, and a filtering of the down converted signal to remove any artifacts that may have arisen as part of the mixing process. Due to the image effects that may occur during the mixing process, performing down conversion may be handled in multiple stages where two or more down conversion stages may be applied in series. A traditional super-heterodyne receiver often uses two down conversions stages separated by channel filtering to remove any effects from image frequencies. These channel filters have traditionally been implemented with external components due to the frequency and accuracy requirements of the receiver. Often some additional gain is also used before down converting the input RF signal as the mixing process typically adds noise to the baseband signal. 
   Down conversion mixers have traditionally been implemented with Gilbert mixers (as described in U.S. Pat. No. 4,156,283), which are voltage input, voltage output circuits that require large supply voltages. A large supply voltage limits the ability of RF receivers to operate from portable supply voltages, for example a battery, which limits their ease of use. Additionally, low noise amplifier (LNA) gain stages used at the input of RF receivers generally require large supply voltages in order to maintain proper isolation from their inputs. 
   One RF receiver topology that has generated significant interest and research activity is “direct conversion” receivers. Direct conversion receivers typically mix the modulated input RF signal directly to a baseband signal with a single mixing stage. The advantage of this topology is that the image-reject filters of the super-heterodyne receivers are no longer required, which generally translates into reduced cost. Disadvantages of direct conversion receivers include non-ideal effects like DC offsets in the baseband signal and self-mixing with the local oscillator to the RF input due to finite isolation. In the past, these problems were generally difficult to overcome, which is one main reason why a majority of RF receivers have been designed as a variation on the super-heterodyne architecture. 
   One example of a direct conversion receiver design is found in “A 2 GHz Wide-Band Direct Conversion Receiver for WCDMA Applications”, IEEE JSSC vol. 34, no. 12 December 1999. The receiver described in the above cited publication uses a voltage mode LNA at the front end, a Gilbert cell down-conversion mixer, and a voltage mode Butterworth channel selection filter. Typically, most direct conversion receiver architectures use at least one stage of voltage mode circuitry. Generally, direct conversion receivers to this point have relied on tried and true LNA and mixer topologies that are inherently voltage mode circuits. These architectures typically require significantly higher supply voltages than alternative current mode architectures, especially in case of Complementary Metal-Oxide Semiconductor (CMOS) circuits. 
   SUMMARY OF THE INVENTION 
   In one set of embodiments the invention proposes a system and method for designing and building a current-mode direct conversion RF receiver that operates with minimal self-mixing effects, minimal DC offset in the baseband signal, and utilizes low voltages, by employing a direct conversion receiver with a simple input transconductance stage, or voltage-to-current conversion stage, to create a current-mode RF modulated signal. The input transconductor of the input transconductance stage may be isolated from a down conversion mixer, which may be used to generate quadrature baseband signals corresponding to the input RF signal, by a low impedance current cascode stage to minimize self-mixing effects. The down conversion mixer may be implemented with a transistor-switching network, which may be driven by a phase locked loop (PLL) with quadrature outputs. The receiver may be implemented using CMOS design technologies at low voltages. The final quadrature baseband signals may be converted back to the voltage domain by a transimpedance filter, which may perform channel selection for the receiver. The baseband filter may additionally include a low frequency zero to remove DC offsets. Pursuant to the above, all analog signal processing may take place in the current domain, with the only voltage nodes configured at the input of the input transconductance stage and the baseband filter outputs. 
   In one embodiment, the current-mode direct conversion receiver includes an input transconductance stage that converts the input RF signal into a current mode signal. The input transconductor replaces the traditional voltage-mode low noise amplifier (LNA) of conventional RF receivers. A high impedance output of the input transconductor may be connected to a very low input impedance current cascode stage. The current cascode stage may feed into a down conversion mixer stage, which may generate quadrature current outputs. In one embodiment, the current cascode stage is constructed with common source transistors in a feedback loop and may operate to minimize the input impedance (and voltage perturbations) seen at the current cascode inputs. Overall, the current cascode stage may operate to isolate the RF input from any voltage changes seen due to the down conversion mixer stage, in order to minimize self-mixing with the input signal. 
   Because the modulated RF signal is a current-mode signal, in some embodiments the mixer may be constructed from simple single transistor switches, well suited for CMOS implementations. In one embodiment the mixer may be implemented with just eight NMOS switches, which may be driven with CMOS logic level signals from a phase locked loop (PLL). The quadrature baseband currents may then be fed to a transimpedance filter, which may reconstruct the modulated signal in voltage mode at low frequency. The transimpedance filter eliminates unwanted signals in adjacent channels. In one embodiment, the voltage-mode filter output is fed into a demodulator to extract the wanted transmitted information. 
   Thus, various embodiments of the invention may provide a means for designing a current-mode direct conversion RF receiver that operates with minimal DC offset in the baseband signal, minimizes self-mixing effects, and utilizes low voltages. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing, as well as other objects, features, and advantages of this invention may be more completely understood by reference to the following detailed description when read together with the accompanying drawings in which: 
       FIG. 1  illustrates a current-mode direct conversion receiver implemented in accordance with one set of embodiments of the present invention; 
       FIG. 2  illustrates a circuit diagram of one embodiment of the transconductor and active cascode stage; 
       FIG. 3  illustrates a transistor diagram of one embodiment of the intermediate mixer, intermediate filter and current splitter, and quadrature mixers; and 
       FIG. 4  illustrates a flowchart of one embodiment of a method for processing a modulated RF signal. 
   

   While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. Note, the headings are for organizational purposes only and are not meant to be used to limit or interpret the description or claims. Furthermore, note that the word “may” is used throughout this application in a permissive sense (i.e., having the potential to, being able to), not a mandatory sense (i.e., must).” The term “include”, and derivations thereof, mean “including, but not limited to”. The term “coupled” means “directly or indirectly connected”. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   As used herein, when referencing a pulse of a signal, a “leading edge” of the pulse is a first edge of the pulse, resulting from the value of the signal changing from a default value, and a “trailing edge” is a second edge of the pulse, resulting from the value of the signal returning to the default value. A “data-valid window” for a signal represents a time period during which the signal may be considered a valid digital signal. A first signal is said to be “corresponding” to a second signal if the first signal was generated in response to the second signal. A first signal is said to “propagated based on” a second signal, when the second signal controls the propagation of the first signal. Similarly, a first module is said to “use” a clock signal to transfer data to a second module, when propagation of the data from the first module to the second module is controlled and/or triggered by the clock signal. 
     FIG. 1  illustrates a receiver architecture implemented in accordance with one set of embodiments of the present invention. In the embodiment shown in  FIG. 1 , an antenna  102 , which is operable to receive an input RF signal, is coupled to an input transconductance stage (ITC)  104  via capacitor  124  and resistor  122 . While some antennas may require the presence of capacitor  124  and resistor  122  in order to exhibit the desired characteristics, other antennas may operate as required without either capacitor  124  and/or resistor  122 . ITC  104  functions as a voltage-to-current converter and operates to create a current-mode RF modulated signal. ITC  104  may be coupled to an intermediate mixer  106 , which may receive the current-mode RF modulated signal created by input transconductance stage  104 . The output of intermediate mixer  106  may be provided to an intermediate filter and current splitter (IFC)  108 , which may then be coupled to a down conversion mixer stage comprising quadrature mixers  110  and  112 , respectively, and transimpedance filters (TRF)  114  and  116 , respectively. 
   In direct conversion receiver architectures where the local oscillator mixes with an incoming RF signal and transforms the incoming RF signal directly to baseband, any leakage of the local oscillator into the RF signal ports may potentially drown out the RF signal itself. Furthermore, such leakage may also result in the local oscillator mixing with itself, which may additionally lead to a DC offset at the baseband signal depending on phase differences. Larger frequencies of the incoming RF signal may facilitate leaks of the local oscillator throughout the receiver circuit. 
   In the embodiment shown in  FIG. 1 , first mixer  106  operates to reduce the input frequency of the carrier of the input RF signal, by mixing the current-mode RF modulated signal at nodes  184  and  186  with frequency F 1  down to an intermediate frequency. Frequency F 1  may be provided by a local oscillator. Intermediate mixer  106  substantially alleviates the leakage problem described in the previous paragraph by mixing the current-mode modulated RF signal at nodes  184  and  186  to the lower intermediate frequency F 1 . For filtering out residual, but no longer operational frequencies, IFC  108  may be applied to the output of intermediate mixer  106 . IFC  108  may also operate to split the intermediate frequency signals to provide respective differential input pairs  364  and  366 , and  368  and  370  to quadrature mixers  110  and  112 , respectively. In the embodiment shown in  FIG. 1 , processing through intermediate mixer  106  and IFC  108  also take place in the current domain as opposed to the voltage domain. Those skilled in the art will appreciate that while the embodiment shown in  FIG. 1  includes an intermediate mixer and intermediate filter, alternate embodiments may be realized without an intermediate mixer and/or an intermediate filter, and input transconductance stage  104  may be coupled to quadrature mixers  110  and  112  in a variety of ways. 
   Mixer  110  and mixer  112  together may operate to create quadrature current outputs through mixing the current-mode RF modulated signal with frequency F 2 , which may be provided by a local oscillator. In one embodiment, the quadrature current outputs are provided to TRF  114  and TRF  116 , respectively. TRF  114  and TRF  116  together may operate to reconstruct the modulated signal—which was comprised in the input RF signal—as voltage-mode quadrature signals at low frequency. In one embodiment the receiver may be part of a radio link system using several channels spaced apart in frequency. While one of those channels is being accessed all other channels may need to be filtered out in order to prevent interference. Therefore, TRF  114  and TRF  116  may also operate to attenuate any signal outside the signals corresponding to the input RF signal by removing unwanted signals in adjacent channels comprised in the radio link system. 
   In one set of embodiments, for example when performing phase demodulation, the voltage-mode quadrature signals may each be fed into a respective limiter amplifier, where the output of limiter amplifier  118  may provide as its output the I-channel quadrature signal (I)  130  and limiter amplifier  120  may provide as its output the Q-channel quadrature signal (Q)  132 . I  130  and Q  132  may be provided to a demodulator to extract the desired transmitted information. Limiter amplifiers  118  and  120  may operate to amplify the voltage-mode quadrature signals such that they saturate against a supply voltage Vcc and Ground, thus providing a more ideal input into the demodulator to process the signals in the digital domain. In embodiments where an FSK demodulator is used, limiter amplifiers  118  and  120  may also operate to limit the voltage-mode quadrature signals to eliminate interference from amplitude modulated (AM) sources that may be feeding into the receiver through the front-end. While certain embodiments may preferably include limiter amplifiers, in other embodiments limiter amplifiers  118  and  120  may be omitted, and the voltage-mode quadrature signals generated by TRF  114  and TRF  116  may be provided as I  130  and Q  132 , respectively. 
   One embodiment of ITC  104  is shown in  FIG. 2 . In this embodiment, ITC  104  comprises a basic transconductor stage (TS)  200  coupled to a very low input impedance current cascode stage (CS)  202 . TS  200  may be implemented using differential transistor pair M 1   150  and M 2   152 , with current source  170  providing current to the source-terminal of M 1   150  and to the source-terminal of M 2   152 . Differential input signals Rx−  180  and Rx+  182  may be provided to the gate-terminals of M 1   150  and M 2   152 , respectively. CS  202  may be constructed with common source transistors M 3   154  and M 4   156 , each configured in a feedback loop to minimize the input impedance at the respective drain-terminals of M 1   150  and M 2   152 , which also comprise the inputs to CS  202 . M 3   154  and M 4   156  may be coupled to node  188  and  189 , respectively, in an active cascode configuration as shown. The feedback loop for M 3   154  may be implemented by coupling its source-terminal to the inverting input of amplifier  158 , and providing a source reference voltage Vbc  162  to the non-inverting terminal of amplifier  158 . Similarly, the feedback loop for M 4   156  may be implemented by coupling its source-terminal to the inverting input of amplifier  160 , and providing source reference voltage Vbc  162  to the non-inverting terminal of amplifier  160 . 
   In the embodiment shown in  FIG. 2 , the source-terminals of M 3   154  and M 4   156  act as a virtual ground, effectively lowering the input impedance that would otherwise be observed at TS  200  output terminals  188  and  189 . CS  202  also operates to effectively eliminate band limiting at output terminals  188  and  189 , and reduce signal loss that may be affected by TS  200  due to finite output impedance. In this manner, CS  202  provides effective bandwidth extension and allows for weak inversion operation of input transistor pair M 1   150  and M 2   152 , not normally possible due to bandwidth limitation. In one set of embodiments, coupling CS  202  to TS  200  allows for differential pair M 1   150  and M 2   152  to be used in a high frequency application when biased in weak inversion. Furthermore, by keeping the respective non-inverting inputs of amplifiers  158  and  160  at reference voltage Vbc, outputs of TS  200  may be held substantially constant, thus substantially reducing or eliminating the Miller effect that may arise as a result of the presence of parasitic capacitances. Overall, high impedance output of TS  200  (across output nodes  188  and  189 ) may be connected to very low input impedance CS  202 , which may operate to isolate the RF input from any voltage changes seen due to the down conversion mixer stage (mixers  106 ,  100  and  112 ), in order to minimize self-mixing with the input signal, originally at input nodes  180  and  182 . A fully differential topology, as shown in  FIG. 2 , may lead to substantially reduced voltage perturbations. 
   For the current-mode signal provided at output terminals  184  and  186  by ITC  104 , mixers  106 ,  110  and  112  may be constructed using simple single transistor switches ideal for CMOS implementations. In one embodiment, each one of mixers  106 ,  100  and  112  is implemented using four NMOS switches, which can be driven with CMOS logic level signals from a phase-locked loop (PLL) with quadrature outputs.  FIG. 3  illustrates the transistor circuits implementing mixers  106 ,  110 , and  112  and IFC  108  according to one embodiment of the present invention. Transistor circuit  302  corresponds to mixer  106  with differential current inputs  184  and  186  provided to the source-terminals of transistors  350 ,  352 ,  354 , and  356  comprised in circuit  302  as shown. Differential inputs RFLO−  380  and RFLO+  382  may originate from a local oscillator, and may correspond to frequency input F 1  of mixer  106  in  FIG. 1 . The output from circuit  302  is provided to transistor circuit  304  comprising transistors  356 ,  358 ,  360 , and  362 , which correspond to the filter portion of IFC  108 , with IFC  108  enable signal  254  (from  FIG. 1 ) indicated as shown. The output of the filter portion of IFC  108  is then input to transistors  364 ,  366 ,  368  and  370 , also comprised in circuit  304 , which together operate as a current splitter to provide four current-output signals, two current-output signals each respectively to transistor circuit  306  corresponding to I mixer  110  (from  FIG. 1 ), and to transistor circuit  308  corresponding to Q mixer  112 . Differential inputs ILO−  385  and ILO+  384  of mixer  306  may originate from a local oscillator, and may correspond to frequency input F 2  of mixer  110  in  FIG. 1 . Similarly, differential inputs QLO−  387  and QLO+  386  of mixer  308  may also originate from a local oscillator, and may correspond to frequency input F 2  of mixer  110  in  FIG. 1 . Outputs  310  and  312  represent current-mode differential outputs of the I channel while outputs  314  and  316  represent current-mode differential outputs of the Q channel. In one embodiment, outputs  310  and  312  are provided to TRF  114 , and outputs  314 , and  316  are provided to TRF  116 , as illustrated in  FIG. 1 , to reconstruct the modulated signal in voltage-mode at low frequency. 
   TRF  114  and TRF  116  may be designed as demonstrated in U.S. application Ser. No. 10/341,158 titled “Baseband Filter For Receivers” filed on Jan. 13, 2003, invented by Troy L. Stockstad and Klaas Wortel, and which is hereby incorporated by reference as though fully and completely set forth herein. 
     FIG. 4  illustrates a flowchart of one embodiment of a method for processing a modulated RF signal. The modulated RF signal may be received by an antenna, which may generate a voltage-mode signal from the modulated RF signal and provide the voltage-mode signal to a receiver ( 220 ). The receiver may convert the voltage-mode signal to a current-mode signal ( 222 ) and isolate the current-mode signal from voltage changes external to the current-mode signal, such as voltage changes appearing due to a downconversion mixer that may be used in the receiver. In one embodiment, an intermediate current-mode baseband signal is generated from the isolated current-mode signal by mixing the isolated current-mode signal to an intermediate frequency ( 226 ). The frequency used in mixing to the intermediate frequency may be provided by a local oscillator. A set of current-mode quadrature baseband signals may be generated from the intermediate frequency current-mode signal through mixing the intermediate frequency current-mode signal to baseband ( 228 ), where a local oscillator may also provide the frequency used to mix to baseband. The set of current-mode quadrature baseband signals may then be converted to a set of low frequency voltage-mode modulated signals ( 230 ). In one embodiment, a baseband filter performs the conversion. The set of low frequency voltage-mode modulated signals may be amplified ( 232 ) and provided to a demodulator to obtain the desired information ( 234 ). 
   Thus, various embodiments of the systems and methods described above may facilitate design of a current-mode direct conversion RF receiver with minimal self-mixing effects, minimal DC offset in the baseband signal, and low operating voltage levels. 
   Although the embodiments above have been described in considerable detail, other versions are possible. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications. Note the section headings used herein are for organizational purposes only and are not meant to limit the description provided herein or the claims attached hereto.