Abstract:
Disclosed are hybrid heterodyne transmitters and receivers for use in communications systems, or other systems, and the corresponding methods for hybrid heterodyne transmitting and receiving. A heterodyne receiver for converting a continuous time modulated signal to a discrete time digital baseband signal includes a sigma-delta modulator. The sigma-delta modulator is a sigma-delta analog-to-digital converter constructed and arranged to receive a modulated signal at an RF carrier frequency and provide a quantized output at a first intermediate frequency. The heterodyne receiver may also include a digital mixer constructed and arranged to receive a data stream quantized by the sigma-delta analog-to-digital converter and receive a signal at a second mixing frequency. The digital mixer then provides digital signals representative of a baseband signal suitable for digital signal processing.

Description:
[0001]     This application claims priority from U.S. Application Ser. No. 60/602,729 filed on Aug. 19, 2004, which is incorporated by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present inventions are directed to communications systems and more particularly to hybrid heterodyne transmitters or receivers.  
       BACKGROUND OF THE INVENTION  
       [0003]     Modern communication systems use digital transmission since it provides improved efficiency and the ability to detect and correct transmission errors. There are several digital transmission formats such as binary phase shift keying (BPSK), quaternary phase shift keying (QPSK), offset quaternary phase shift keying (OQPSK), m-ary phase shift keying (m-PSK), orthogonal frequency division modulation (OFDM), and quadrature amplitude modulation (QAM). There are different communication systems such as code division multiple access (CDMA) communication systems, or high definition television (HDTV) systems.  
         [0004]     In digital transmission, the digitized data is used to modulate a carrier sinusoid using one of the above-listed formats. The modulated waveform is further processed (e.g. filtered, amplified, and up-converted) and transmitted to a remote station. At the remote station, the transmitted RF signal is received and demodulated by a receiver. A typical receiver includes an antenna that receives the signals and a filter that limits the received signals to the desirable carrier frequency range. The frequency band limited signal received by the antenna is then applied to a low noise amplifier where it is amplified to an amplitude suitable for subsequent processing, as described below.  
         [0005]     Wireless telecommunications systems such as cellular telephone communications systems use several base stations that receive and transmit signals over a particular carrier frequency or channel within an allocated frequency band to communicate with a terminal handset. The terminal handset typically tunes to receive one narrow band channel within the wider frequency band at a time while base stations are typically are required to tune in multiple channels and communicate with multiple terminals at a time.  
         [0006]     In general, communications systems can use several types of RF receivers. A homodyne receiver is perhaps the most basic of RF receivers. The homodyne receiver usually includes a low noise amplifier (LNA) that accepts an RF signal received by an antenna, and amplifies the detected signal. The amplifier provides the amplified signal to an RF filter and to an analog mixer that multiplies the filtered RF signal with an analog mixing signal provided by a frequency generator including a local oscillator (LO). The analog mixer down-converts and recovers the desired baseband signal. (The analog mixing signal may have its frequency tuned for channel selection by a synthesizer.) This homodyne technique is sometimes called a “zero IF” architecture since the RF modulated signal is down-converted directly to zero frequency without an intermediate frequency (IF). In “zero IF” architectures, the LO signal is at the same frequency as the RF receiver signal. The use of the substantially same RF frequency signal (LO) for mixing can have the undesirable effect of the LO signal being radiated out through the antenna. In addition, coupling within the mixer can create a design issue in that the LO appears on the output of the mixer as a large DC offset, potentially jamming the desired signal.  
         [0007]     A super-heterodyne receiver is another type of an RF receiver. A super-heterodyne receiver has several advantages over the zero-IF architecture. A super-heterodyne receiver also includes a low noise amplifier and a filter for filtering the modulated amplified RF signal. The receiver uses an analog RF mixer that receives the modulated RF signal for down-converting. The receiver uses a first frequency generator for providing a first mixing signal (LO 1 ) that is offset from the RF carrier by an intermediate frequency (IF). The analog mixer receives the two offset RF signals and provides the modulated output at the IF frequency to a filter (e.g., a surface acoustic wave filter) having a high Q and a narrow band.  
         [0008]     In super-heterodyning, the difference between the frequency of the modulated signal and the LO1 signal provides advantageous ability to isolate and filter non-idealities from the desired signal. The high Q and narrow band filter provides the filtered IF signal to a second mixer (usually an analog mixer) operating at the IF frequency. The second mixer also receives a second mixing signal (LO 2 ) provided by a second frequency generator. (The second mixer may be replaced by a modulator that also digitizes the analog signal.) The mixer down-converts the IF frequency signal to a baseband signal suitable for processing. Usually, this architecture provides the signal of interest at the frequency RF+/−IF, and an image signal at from RF−/+IF. Therefore, the receiver performs image rejection using a surface acoustic wave (SAW) filter. Alternatively, the receiver may use Weaver mixer architecture to remove the unwanted image. The Weaver mixer architecture separates the modulated signal into an in-phase (I) signal and a quadrature (Q) signal to perform the mixing separately for the I &amp; Q signals. This is done in two stages further separating each signal into two 90° shifted signals for mixing down to a baseband frequency. The baseband signals are combined by first appropriately shifting the phase.  
         [0009]     There is still a need for communications systems and other systems that use hybrid heterodyne transmitters or receivers.  
       SUMMARY OF THE INVENTION  
       [0010]     The present inventions are directed to hybrid heterodyne transmitters or receivers for use in communications systems or other systems. The present inventions are also directed to methods for hybrid heterodyne transmitting or receiving for use in communications systems or other systems.  
         [0011]     According to one aspect, a heterodyne receiver for converting a continuous time modulated signal to a discrete time digital baseband signal includes a sigma-delta modulator and a digital mixer. The sigma-delta modulator is a sigma-delta analog-to-digital converter constructed and arranged to receive a modulated signal at an RF carrier frequency and provide a quantized output at a first intermediate frequency. The digital mixer is constructed and arranged to receive a data stream quantized by the sigma-delta analog-to-digital converter and receive a signal at a second mixing frequency. The digital mixer is constructed to provide digital signals representative of a baseband signal suitable for digital signal processing.  
         [0012]     Preferably, the heterodyne receiver may include an analog frequency generator, including a local oscillator, constructed to provide a mixing signal being less than twice the RF frequency, wherein the sigma-delta analog-to-digital converter includes a sampler such as a mixer for receiving the analog mixing signal and providing an output signal at the first intermediate frequency.  
         [0013]     The sigma-delta analog-to-digital converter is arranged to receive an analog sampling signal from the analog frequency generator. The sigma-delta analog-to-digital converter is arranged to receive an analog sampling signal at a frequency providing over-sampling with respect to the first intermediate frequency. The sigma-delta analog-to-digital converter is arranged to receive, from the analog frequency generator, an analog sampling signal at a frequency providing over-sampling with respect to the first intermediate frequency.  
         [0014]     The heterodyne receiver may further include a digital frequency generator constructed an arranged to provide the signal at a second mixing frequency, the second mixing frequency being comparable to the first intermediate frequency. The heterodyne receiver may further include a digital frequency generator constructed an arranged to provide selectable frequency signals used as the signal at the second mixing frequency, the second mixing frequency being comparable to the first intermediate frequency.  
         [0015]     According to another aspect, a modulated signal receiver includes a means for receiving a plurality of modulated signals, a means for generating a plurality of local oscillator (LO) signals with frequencies less than twice the carrier frequency of the modulated signals, and an over-sampled data converter. The over-sampled data converter is constructed to receive the modulated signals and the LO signals, and includes means for down-converting the modulated signals before quantization. The over-sampled data converter also includes a means for generating a plurality of quantized signals responsive to the down-converted signals, and a means for performing noise-shaping utilizing a plurality of poles in the modulated frequency band and a plurality of poles in the down-converted frequency band.  
         [0016]     According to preferred embodiments, this modulated signal receiver may further include within the over-sampled data converter a means, responsive to the quantized signal, for generating a plurality of feedback signals, and a plurality of summing means responsive to the modulated inputs and the feedback signals for closing the over-sampled converter loop. The feedback means may include a plurality of single bit digital-to-analog converters (DACs). The feedback means may include a multi-bit DAC with Dynamic Element Matching (DEM). The over-sampled converter may be a noise shaping Sigma-Delta converter. The over-sampled converter may include a complex sigma-delta modulator (SDM), responsive to provided in-phase (I) and quadrature (Q) modulated input signals.  
         [0017]     According to yet another aspect, a heterodyne transmitter for converting a discrete time digital baseband signal to a continuous time modulated signal includes a digital mixer and a sigma-delta analog-to-digital converter. The digital mixer is constructed and arranged to receive a digital data stream processed by a digital signal processor and receive a digital signal at a mixing frequency, wherein the digital mixer is constructed to provide digital signals at an intermediate frequency being representative of a baseband signal. The sigma-delta analog-to-digital converter is constructed and arranged to receive digital signals at the intermediate frequency and provide a modulated signal at an RF carrier frequency.  
         [0018]     According to yet another aspect, a method of generating a plurality of quantized signals includes receiving a plurality of modulated signals, and generating a plurality of local oscillator (LO) signals with frequencies less than twice the carrier frequency of the modulated signals. The method also includes receiving the modulated signals and the LO signals, down-converting the modulated signals before quantization; generating a plurality of quantized signals responsive to the down-converted signals, and performing noise-shaping utilizing a plurality of poles in the modulated frequency band and a plurality of poles in the down-converted frequency band. The noise-shaping may be performed during the converting.  
         [0019]     The method may also include generating a plurality of feedback signals responsive to the quantized signal, and summing the feedback signals to close over-sampled loop converting. The input signals may include of a plurality of in-phase (I) and out-of-phase (Q) signals having carriers 90 degrees out of phase with respect to each other. The method may include changing the noise shaping to modify receiver bandwidth and sensitivity.  
         [0020]     Furthermore, this present invention relates to a method and apparatus for converting a continuous time modulated signal to/from a discrete time digital baseband signal. Preferred embodiments utilize a sigma-delta modulator (SDM) replacing a traditional analog mixer by substituting the local oscillator (LO) input to the mixer with the sample clock input to the SDM. This undersamples the modulated signal while it simultaneously over-samples the desired baseband information bandwidth. If the sample clock is chosen such as to create an intermediate frequency (IF) carrier, a second mixing function may be performed on the digital SDM output by a digital multiplier, thereby creating a hybrid analog/digital receiver or a “digital heterodyne” receiver. The receiver may generate quadrature outputs using either a complex SDM or two SDMs with a phase shift network on the RF input, or by utilizing quadrature sample clocks to the two SDMs. The transmitter is designed to complement the receiver as described below.  
         [0021]     The present invention improves system manufacturability and performance by replacing analog/RF components with digital mode devices such as those used in Sigma-Delta data converters. The over-sampling SDM ADC clock is intentionally chosen to be less than twice the carrier frequency, in clear violation of Nyquist sampling criteria, to create an aliased signal at a frequency equivalent to a traditional RF down-converting mixer. Either high side or low side mixing can be realized as long as the sample clock is less than twice the carrier frequency. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]      FIG. 1A  is a block diagram of a digital heterodyne receiver.  
         [0023]      FIG. 1B  is a block diagram of a digital heterodyne transmitter.  
         [0024]      FIG. 2  is a more detailed block diagram of a digital heterodyne receiver shown in  FIG. 1A .  
         [0025]      FIG. 2A  is a detailed block diagram of a sigma-delta modulator used in the digital heterodyne receiver of  FIG. 2 .  
         [0026]      FIG. 2B  is a detailed block diagram of an automatic gain control used in the digital heterodyne receiver of  FIG. 2 .  
         [0027]      FIG. 2C  provides a diagram of a simulation of the digital heterodyne receiver shown in  FIG. 2  using Simulink® software.  
         [0028]      FIGS. 2D-2G  show input, intermediate and recovered signals from the simulation shown in  FIG. 2C .  
         [0029]      FIG. 3  illustrates a digital heterodyne circuit with Weaver image reject.  
         [0030]      FIG. 3A  is a block a Dynamic Element Matching (DEM) used with quadrature SDM, shown in  FIG. 3 .  
         [0031]      FIG. 4  illustrates a complex sigma-delta modulator constructed to provide intermediate frequency in-phase (I) and quadrature (Q) signals for the heterodyne receiver circuit of  FIG. 3 .  
         [0032]      FIG. 5  is a block diagram of a digital heterodyne transmitter. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]      FIG. 1A  illustrates schematically a digital heterodyne receiver  10 . Digital heterodyne receiver  10  includes a low noise RF amplifier  14 , a sigma-delta modulator analog-to-digital converter (SDM ADC)  16 , digital decimators  20  and  26 , and a digital multiplexer  22 . Sigma-delta modulator  16  receives the modulated RF signal and provides a digitized output at the RF frequency (for example, several protocols such as Bluetooth and IEEE 802.11b utilize the 2.4 GHz ISM band where f=2.4 GHz), which enables the replacement of analog RF components with digital components.  
         [0034]     The transmitted signal is detected by an antenna  12  and the analog RF signal is provided to LN amplifier  14 , which amplifies the RF signal and provides it to SDM ADC  16  for modulation, down-conversion and digitalization. SDM ADC  16  receives a sample clock signal from a frequency generator circuit  18  providing the sample clock at a frequency offset from the carrier frequency by a selected intermediate frequency (IF). For the ISM band example, a 2.376 GHz clock would generate a digital IF of 2.4 GHz-2.376 GHz or 24 MHz. The mixing frequency is preferably fixed to enable a low noise construction of the frequency generator. The output of SDM ADC  16  is a digitized signal at the intermediate frequency that could be compared to a mixed-signal equivalent of a super-heterodyne receiver. Typically, the IF is chosen to be between 0 Hz and the signal information bandwidth. Device noise such as 1/f and shot noise as well as DC LO leakage often affect the IF selection and may limit the low side of the range to about 1 MHz. The digitized signal is provided to digital decimator  20 , which removes quantization noise and provides anti-aliasing, as described in more detail below. This signal is provided to digital multiplexer  22 , which also receives a mixing signal (i.e., a digital sine wave signal) from a digital frequency generator  24 , which serves as the second LO2 of the heterodyne receiver. This digital sine wave signal is generated, for example, by using lookup tables, a numerically controlled oscillator (NCO), or by Cordic techniques. Digital multiplexer  22  provides a down-converted output to digital decimator  26 . Digital decimator  26  provides the digital signal to a digital signal processor (DSP).  
         [0035]     At the RF frequency, the sample clock signal  19  over-samples the baseband signal and the over-sampling SDM ADC clock is selected to be at less than twice the carrier frequency, in clear violation of Nyquist sampling criteria, to create an aliased signal at a frequency equivalent to a traditional RF down-converting mixer. The high side or low side mixing can be realized as long as the sample clock is less than twice the carrier frequency. In digital multiplexer  22 , the digital IF output of SDM data converter  16  is mixed (multiplied) with the digital representation of digital oscillator  24  to affect channel selection, and to create a digital representation of the baseband signal. Since digital oscillator  24  provides a programmable frequency output signal, the frequency of the sample clock  18  may be fixed.  
         [0036]     The use of the fixed frequency simplifies the design of frequency generator circuit  18  and potentially reduces phase noise. The low phase noise in frequency generator circuit with low phase noise is important since SDM performance is often limited by the sample clock jitter. This may improve frequency isolation in frequency shift keying (FSK) as used in communication methods like binary frequency shift keying (BFSK), quadrature phase frequency shift keying (QPFSK), and continuous phase frequency shift keying (FSK).  
         [0037]     Another embodiment of digital heterodyne receiver  10  includes a polyphase filter receiving an amplified RF signal from LNA  14  and generating RF in-phase (I) and RF quadrature (Q) signals delivered to two SDMs. This arrangement provides for a quadrature SDM and has the added benefit that both the digital I and Q SDM outputs are available on the same clock edge, thus simplifying the digital timing re-synchronization at the digital mixer operating at the frequency of LO2. For improved image rejection and/or improved dynamic range, the I and Q outputs from the SDMs can be combined into a single complex SDM. Most complex implementations use a band-pass SDM as described for example by Jantzi in “Quadrature Bandpass SDM for Digital Radio,” published in IEEE JSSC Vol. 32: No. 12 in December 1997, which is incorporated by reference.  
         [0038]      FIG. 1B  illustrates schematically a digital heterodyne transmitter  10 A. Digital heterodyne transmitter  10 A includes digital interpolators  21 A and  27 A, digital multiplexer  22 A, a sigma-delta modulator digital-to-analog converter (SDM DAC)  16 A, and an RF power amplifier  15 . For example, digital interpolator  27 A receives a digitalized voice signal from a DSP and provides the output to digital multiplexer  22 A, which also receives input from a digital oscillator  24 A. As described above, digital oscillator  24 A provides a programmable frequency output signal, wherein the frequency of the sample clock is selected for a particular communication channel. Digital interpolator  21 A receives the output of digital multiplexer  22 A, which is a digitized signal at an intermediate carrier frequency. (Depending on the specific embodiment, the digital radio frequency signal may include digitized in-phase (I) and quadrature (Q) signals initially generated by the DSP). SDM DAC  16 A receives the interpolated signal from interpolator  21 A signal and a clock signal from oscillator circuit  18 A. Oscillator circuit  18 A is preferably designed to generate fixed frequency low phase noise signal.  
         [0039]     Digital interpolator  21 A and  27 A are designed to up sample the low clock rate input signal such as to remove aliased versions of the desired signal inherent in the original input signal. This new higher clock signal still has aliases, but they are at a larger offset from the input signal and are therefore more easily filtered later by reconstruction filters.  
         [0040]     SDM DAC  16 A provides an analog RF signal that may be filtered to remove the noise moved outside the communication band by the sigma delta modulation process. A reconstruction filter  15 A, for example an LC tank or a surface acoustic wave filter, filters the RF analog signal and provides it to an RF power amplifier  14 A. The amplified RF signal is transmitted by antenna  12 A.  
         [0041]      FIG. 2  is a block diagram of a multi-standard, multi-band digital heterodyne receiver  200 . Digital heterodyne receiver  200  includes a sigma-delta modulator  300 , digital decimation filters  220 , a demodulator  270 , and other elements described below. Sigma-delta modulator  300  is shown in detail in  FIG. 2A . Digital heterodyne receiver  200  is designed to oversample the information bandwidth and undersample the RF signal.  
         [0042]     Referring still to  FIG. 2 , digital heterodyne receiver  200  receives modulated signal  207  provided to automatic gain controller (AGC)  210 , which provides at its output the product of the input signal  207  and first stage output  218  from automatic gain controller (AGC)  216 , as described below. The output of AGC  210  is fed to an optional limiter  212  (i.e., supply rails  212 ), where the output amplitude is clipped or clamped up to supply rail voltage levels and is provided to sigma-delta modulator  300 . Automatic gain controller  216  adjusts the gain control signal  218  such that the RF signal  298  has the amplitude that matches the desired A/D range and thus limiter  212  do not clip the signal. Otherwise, limiter  212  protects SDM  300 .  
         [0043]     The output signal from SDM  300  (signal  299 ) is provided into the input of half band decimation filters (HBF)  213 ,  214  and  215 , which are connected in series, resulting in frequency division by 8=2 3 , and producing the IF signal at 297 MSPS. The order of each HBF with length N may be decided for each filter. In general, the order will increase as the sample rate is reduced due to the smaller transition band for the filter. Other decimation filters such as sinc or comb may be used. The values of transfer functions and coefficient gains are selected to meet specific application requirements.  
         [0044]     Referring to  FIG. 2A , third order continuous time sigma-delta modulator  300  quantizes the input signal into digital signal outputs and output levels. The output enables the separation of three signal channels, C 1 , C 2  and C 3 , using overflow level logic control  320 . Sigma-delta modulator  300  includes three integrators,  310 ,  311  and  312 , each having a selected transfer function designed to successively provide higher frequency noise shaping and combine the produced Noise Transfer Function (NTF) to meet the application requirements, where the subscript j=1, 2, 3 for each of the three filters  310 ,  311  and  312 , and where the coefficients aj are different for each filter. In the current embodiment, IFb=14, a1=2.3457, a2=2.0000, a3=1.1447, gm=0.000272, dcg=140, where Ifb is the effective number of bits in the digital IF signal, ai is the resultant gain of the three integrator stages (gm/C), and dcg is the practical circuit limitation on the DC gain of each gm stage.  
         [0045]     Sigma-delta modulator  300  also includes a zero order sample-and-hold element  340  for enabling the quantization. The sample rate is chosen similarly to a LO1 in a traditional radio receiver to be offset from the RF frequency by a relatively small IF frequency. The resultant sample frequency of the sample-and-hold should be less than twice the input RF frequency and therefore less than the Nyquist frequency. The sample rate also defines the SDM over sample ratio (OSR=Fs/(2*BW)) with respect to the desired baseband signal bandwidth.  
         [0046]     The quantized output of sample-and-hold 340 is further conditioned through amplifier  341 , bounded in amplitude by upper/lower bounds block  342 . The output level is rounded to the nearest integer level through round  343 . The quantized output levels occupy levels labeled [3, 2, 1, 0, − 1 , − 2 , − 3 ] which are equally spaced over the expected signal dynamic range (e.g., 1V peak-to-peak), i.e., equal intervals of ⅓ rd  of a volt. The output from  341  is additionally provided to overflow logic controller  320 . Depending on the overflow level, the outputs Clmp 2  and Clmp 3  set the switch conditions on switches  326  and  327 . Gain amplifiers  328  and  329  provide gains of −1, thereby eliminating the signal from those channels when switches  326  and  327  are set to connect to the outputs of unity-gain inverting amplifiers  328  and  329 , respectively. This occurs when the output level is zero from amplifier  314 .  
         [0047]     The present SDM eliminates issue of stability present with prior art Sigma-Delta Modulators over second order. The present system monitors the digital output of the SDM and if the output stays max positive or max negative for three cycles in a row, an overflow is declared. The overflow logic states are determined from the input level in logic control  320 . If an overflow is detected, the internal integrators are reset by shorting the output back to the input that puts the modulator back into a valid state.  
         [0048]     The overflow logic signal from the overflow logic  320  additionally enables the overflow condition to be used to control the gain in automatic gain control  216 . Normally, the AGC uses a digital absolute value to rectify the signal followed by a very low frequency filter to extract the average signal level and adjust the output gain signal to obtain the target DC value. The loop frequency response is kept slow, or disabled as not to effect the desired receiver signal. When the overflow bit is high, the gain is decreased at a more aggressive rate, to reduce the overflow and bring the signal back into range of the SDM ADC.  
         [0049]     Referring still to  FIG. 2 , digital heterodyne receiver  200  uses amplifier  250  for amplifying the signal amplitude that is significantly reduced after passing through sigma-delta modulator  300  and decimation filters  213 ,  214 ,  215 . In amplifier  250 , the signal has IFb bits, and therefore a dynamic range of 2ˆ(IFb−1).  
         [0050]      FIG. 2B  is a detailed block diagram of an automatic gain control AGC  216  (also shown in  FIG. 2 ). Automatic Gain Control  216  ensures that the level of the final digital output signal is within digital data range after decimation through decimators HBF 1  through HBF 3  (shown in  FIG. 2 ). To achieve this, the output of the SDM  300  is provided to overflow logic  320  ( FIG. 2A ) in SDM  300  ( FIG. 2A ), which detects the digital overflow. The digital overflow signal  370  ( FIG. 2B ) is fed to the AGC, which provides a signal to remove or add pulses in the multiplier  510 .  
         [0051]     The core of AGC  216  is the adder  530 . The overflow signal  370  is added to the signal from a gain amplifier  541 , or subtracted from the signal from a gain amplifier  540 , as determined from the switch signal at input  520 A. Additionally, the overflow signal is modified in multiplier  510  according to the output of multiplier  510 , and also the one-interval delayed signal from a unit delay  532 . Unit delay  532  of the added signal provides a means of preventing under-compensation or overcompensation of the overflow by preventing repeat applications of the same overflow signal. The output of the multipliers AGC 2  and AGC 4  ( 535 ) is controlled from the output of summer  530  and the inputs from signal generators. A limiter  531  clips the output levels to the specified limits.  
         [0052]     The switch signal  520 A is derived from the first stage decimator  213  (HBF 1  shown in  FIG. 2 ). The absolute magnitude of the signed integer input from  213  is taken in Abc  501 . The series of amplifiers and synchronization blocks  502 ,  503  and  504  represents a unity gain amplifier and makes up for former delays: this effectively advances the signal  230  (RF 1  displayed in the figure below). Amplifier  502  provides a gain to the input of  503  to enhance signal level, while  504  returns the signal to the appropriate output level for the magnitude signal  230  and input to the multiplexer Shown in  FIG. 2B . The signal  230  is indicated in connection with RF 1 . The sign of the signal is recovered through blocks  506  through  509 . A DC level is set in  506 , followed by a delay. The adder  508  actually subtracts the DC level, and hence, if the output from  504  is below the level set in  506 , the output of  509  is negative, while if the signal from  504  is greater than or equal to  506 , the signal output from  509  is positive. As indicated earlier, the output of  509  controls whether the summer adds or subtracts digital input through switch  520 , by providing the controlling signal at the port  520 A. As shown in  FIG. 2 , the delayed output from AGC  216  is provided as input to multiplier  210  to adjust the digital gain.  
         [0053]     Referring again to  FIG. 2 , after decimation in decimator  200  (using decimation filters  213 ,  214 ,  215 ), the digital signal from amplifier  250  is rounded down to the nearest integer level in round  251  provided to demodulator  270 . Demodulator  270  includes a local fixed oscillator signal  271  providing an IF mixing frequency that is mixed with the IF conditioned signal in mixer  272 . The product output signal from mixer  272  is rounded (unit  273 ) and passed through a half band filter  272  to remove the higher sideband for the super-heterodyne function. It is then additionally conditioned in  275  to provide an oversampled signal to FIR filter  276  (finite input response filter  276 ). The low pass FIR filter  276  removes the up-shifted modulated noise.  
         [0054]     Unit delay  277  delays its input by the specified sample period, and this is equivalent to the z−1 discrete-time operator. Delay  277  accepts input from FIR filter  276  and generates an output, which can be either both scalar or both vector. For the vector input, all elements of the vector are delayed by the same sample period. The output is provided to a digital demodulator  279 , which may be as simple as detecting the sign of the signal. Sign block  279  provides the sign of the input so that the output is 1 when the input is greater than zero, the output is 0 when the input is equal to zero, and the output is −1 when the input is less than zero. Demodulated output signal  280  is provided at the base band.  
         [0055]     In general, transmitter  10 A may include various types of Digital to Analog converters and modulators designed to generate the transmitted RF communication signal. Preferably, transmitter  10  includes a quadrature sigma-delta modulator digital-to-analog converter for simultaneously processing the in-phase (I) and quadrature (Q) signals. Alternately, a mixed-signal super-heterodyne transmitter can include two SDMs and four digital mixers to effect a digital version of the traditional Weaver image reject mixer architecture. The Weaver architecture receives an RF modulated input to be effectively sampled in quadrature. This sampling is performed, for example, using two oscillator signals (LO 1   1  and LO 1   Q ) shifter 90 degrees out of phase, often generated using an oscillator circuit running four times faster than the LO1 signal.  
         [0056]      FIG. 2C  is a diagram of a simulation of the digital heterodyne receiver shown in  FIG. 2  using Simulink® software. The input signals are signals  202 ,  203 ,  204 ,  205 , and  206 . An oscilloscope  282  displays the comparison of the original unmodulated signal and the recovered signal (i.e., signal received from sign  279  after processing).  
         [0057]      FIG. 2D  shows the OFDM signal  202  (i.e., signal from the antenna), the 299 signal from ADC  300  and the overflow signal from ADC  300 . The overflow signal from ADC  300  is encoded as a seven level pulse signal. That is,  FIG. 2D  displays the three signals labeled as  232  in  FIG. 2C .  FIG. 2E  displays the gain signal from  233  (in analog form, there being a D/A converter not shown), the magnitude signal from AGC  216 , and the signal from limiter  212 , all labeled as  231  in  FIG. 2C .  
         [0058]      FIG. 2F  shows the three signals provided to  291 . That is,  FIG. 2F  compares the OFDM signal (channel  1 ), the output signal from SDM  300  (channel  2 ) and the decimated signal from decimator  220  (channel  3 ).  FIG. 2G  shows the three signals provided to  282  shown in  FIG. 2C . That is,  FIG. 2G  compares the demodulated recovered signal (channel  1 ), the unmodulated signal  204  (channel  2 ) and the IF signal (channel  3 ).  
         [0059]      FIG. 3  illustrates schematically a heterodyne receiver  40  including a quadrature SDM  46  used to down-convert a plurality of I and Q signals using a complex noise shaping loop. Receiver  40  receives the modulated RF signal that is amplified by a low noise amplifier (LNA)  42  and provided to an optional band pass filter  44  (for example, a surface acoustic wave filter, SAW) to attenuate unwanted out of band signals. A quadrature SDM  46  receives the filtered modulated RF signal and provides quantized I and Q signals, at an intermediate frequency, having a high sample rate of 2.376 GHz and a low bit depth of 6 levels (2.5 bits). The linearity of the feedback DAC within the SDM ADC typically limits system linearity so correction methods can be used.  
         [0060]      FIG. 3A  illustrates one embodiment of a Dynamic Element Matching (DEM)  47  used with quadrature SDM  46 . According to one exemplary embodiment, DEM  47  includes a set of five multiplexers (MUX) each receiving a select signal and reference signals REF  1 , REF  2 , REF  3 , REF  4 , and REF  5 . DEM  47  also includes a set of five comparators (CMP), each receiving input signal  45 , a selected reference signal and a clock signal. DEM  47  also includes a set of five 1 bit DACs connected to a summing circuit providing the analog feedback signal. A second summing circuit receives the output from the five CMPs and provides its digital output signal  51  for decimation.  
         [0061]     Dynamic Element Matching (DEM) is required in most multi-level quantized SDMs (in any application, not just transceivers) to achieve the full performance potential of the SDM ADC as determined by the designed NTF.  
         [0062]     Furthermore, the performance of SDM  46  depends on the feedback from the used DAC to exactly match the feed forward digital path. Thus, the DAC linearity essentially limits the ADC performance, where non-linearities in the DAC are largely caused by mismatch of components, but the novel design successfully deals with these limitations.  
         [0063]     As shown in  FIG. 3A , the novel continuous time (CT) modulator system shuffles the reference inputs to the five CMPs that generate the digital thermometer code. This places the DEM in the forward signal path and allows errors in the DEM to be noise shaped by an NTF similar to the input signal  45 . Since most CT modulators use clocked comparators, the references can be shuffled during the reset phase of the comparator. No additional propagation delay is added to the output of the comparators and therefore the group delay and jitter in the feedback path is minimized. This novel concept is applicable also to switch capacitor modulators or even Flash ADCs.  
         [0064]     Specifically, DEM  47  ( FIG. 3A ) has one 1 bit DAC dedicated to one comparator. The comparator outputs are summed in the digital space to produce the encoded multi-bit forward output signal  51 . The DACs are current summed to produce the multi-level feedback signal. The digital select signal to the analog multiplexers can be changed while the comparator is in a hold or reset state, giving the reference time to settle before the next sample clock edge is received by the comparator. To minimize any harmonic tones related to the shuffle rate being generated in the signal band, the rate of shuffling can be spread using a method such as a PN generator, noise shaping, or a data weighted averaging technique in the digital circuitry which generates the select signal. (We note that other less preferred embodiments may also be use digital logic in the feedback path to shuffle the DAC elements and therefore spread (average) the additional quantization noise caused by the non-linear DAC step size. This shuffling may be done dynamically and not as a separate calibration step.)  
         [0065]     Referring still to  FIGS. 3 and 3 A, the output  51  from DEM  47  is provided to decimation filters  50  and  70 , separately. Decimation filters  50  and  70  suppress quantizer noise and decimate the high sample rate, low bit depth I and Q signals. (Alternatively, receiver  40  may use a single complex decimator.) Receiver  40  also includes a Weaver style image reject mixer  52 , which includes four digital mixers  54 ,  56 ,  72 , and  74  receiving IF signals from two numerically controlled oscillators  60  and  62  having bit depths similar to the bit depth of the decimator output. Numerically controlled oscillators  60  and  62  are programmed to an IF frequency such as to tune a plurality of channels.  
         [0066]     Digital decimation filters  50  and  70  may include a digital low-pass or bandpass filter and a digital decimator. Digital decimation filters  50  and  70  receive digitized amplitude density-modulated signals at an intermediate frequency, wherein the signals are separated by 90°. Each digital decimation filter produces a decimated or down-sampled digital output signal delivered to image reject mixer  52 .  
         [0067]     Referring still to  FIG. 3 , decimation filter  50  receives the in-phase IF signal and provides the decimated signal to mixers  54  and  56 , which also receive digital sine wave signal from digital oscillator circuits  60  and  62 , respectively. Furthermore, decimation filter  70  receives the quadrature IF signal and provides the decimated signal to mixers  72  and  74  that also receive digital sine wave signal from digital oscillator circuits  60  and  62 , respectively. Oscillator circuits  60  and  62  provide numerically generated sine waves of a selected frequency separated by 90°. The outputs of multipliers  54 ,  56  and  72  and  74  are summed and differenced to create I and Q signals with the image rejected. These tuned and image rejected signals are then potentially processed by additional decimation before being output to a digital baseband processor for demodulation.  
         [0068]      FIG. 4  illustrates a complex SDM  46  providing intermediate frequency in-phase (I) and quadrature (Q) signals used in heterodyne receiver circuit  40  shown in  FIG. 3 . The modulated data in the form of a continuous time signal is amplified by a current output Gm amplifier  82  and presented to an error summing node  84 . (Low noise amplifier  42  ( FIG. 3 ) may potentially be realized as Gm amplifier  82 ). Complex SDM  46  includes a bandpass filter  86  providing an in-phase (I) analog signal to analog mixer  92  coupled to integrators  102  and  112  connected to an analog-to-digital converter  116 . Bandpass filter  86  also provides a quadrature (Q) analog signal to analog mixer  94  coupled to integrators  104  and  114  connected to an analog-to-digital converter  118 . A clock generator  96  provides a quantization signal at a sample frequency to both quantizers  116  and  118 , as well as the mixing frequency to analog mixers  92  and  94 .  
         [0069]     Summing node  84  effectively creates an error signal by combining the output current of Gm amplifier  82  and the output current of the SDM feedback digital-to-analog converters  120  and  130  (feed-back loops  122  and  132 ) within the bandpass tank  86  created by the L and C elements. The mixer inside the SDM down-converts this error signal, not the input signal, and as such, a traditional analog IF signal is never generated. This error signal closes the SDM loop and LC filter  86  is an integral part of the noise shaping transfer function. The LC tank  86  creates a complex pole that is typically located within the modulated signal band, much like a bandpass SDM. Multiple GmLC stages may be cascaded for higher order loops. In operation, the circuit of  FIG. 4  includes a network of elements including feedback loops  124 ,  126 ,  134  and  136 , which serve as a phase shift network to form a complex filter. The feedback loops  124 ,  126 ,  134 , and  136  provide for linearity of the circuit.  
         [0070]     Sigma-delta modulator  46  converts the analog RF signal into a low-resolution but very high-speed digital signal for both I and Q. The analog input signal modulates the density of states of the high speed signal. Such signals include, for example, pulse density and pulse duration modulated signals (PDM), pulse code modulation (PCM), pulse position modulation (PPM) for single-bit digital output streams, or amplitude density modulated signals (ADM) for multiple-bit digital output streams. Digital decimation filters  50  and  70  ( FIG. 3 ) reconstruct the analog input signal in digital form. Output reduces the sampling frequency from that of the sampling clock frequency provided by clock generator  96 , to a lower rate generally near the Nyquist sampling rate, with respect to the input signal bandwidth. Decimation filters  50  and  70  suppress quantizer noise outside the bandwidth of complex SDM  46 . Furthermore, the filtered output signal is decimated to a lower sample rate, by selection of every N-th sample. The PDM or ADM signals contain many states over a time period corresponding to an analog input cycle at a frequency near the upper edge of the modulator or signal passband, which means that the sample rate is much higher than the highest frequency of the analog input signal. The sigma-delta processing modulates the density of quantizer states and thus makes better use of the multiple PDM or ADM samples available from SDM  46  for each output sample produced by the decimation filter. The better use is accomplished by shaping the quantization noise spectrum so that it is reduced within the same frequency band as the input signal.  
         [0071]     The digitized I and Q outputs of SDM  46  are separately provided to the respective digital-to-analog converters DACs  120  and  130  via connections  117  and  119 , respectively. DACs  120  and  130  provide analog replicas of the output signals. In summing the circuit  84 , the analog RF input signal is applied to the non-inverting input port, and the analog replica of the PDM output signal is applied to the inverting output via the feedback loops. Thus, summing circuit  84  takes the difference between the actual sampled input signal and the analog replica and produces a difference or error signal representing the deviation of the PDM output from the actual sampled analog input signal.  
         [0072]     Integrators  102  and  112  (and  104 ,  114 ) receive the error signal that is summed with the previously accumulated signal fed to a second non-inverting input port. The previously accumulated signal is updated by addition of the current error signal to produce a new, updated accumulated value. The new, updated, accumulated value is applied to delay elements, which delay the accumulated sum, and makes it available after the delay on output signal.  
         [0073]     The I and Q down-converting may be formed such as a sample and hold amplifier, transmission gate or balanced mixer that is enclosed within the same noise shaping loop, which helps to linearize the down-converter means.  
         [0074]     Sigma-delta converter  46  includes a complex noise shaping filter, which is responsive to both the I and Q down-converted signals. This noise shaping filter adds additional noise shaping before quantization. The filtered signals are quantized to a plurality of digital output levels, which are both fed forward to the digital processor and fed back through a plurality of DACs to effectively close the SDM loop.  
         [0075]     Digital-to-analog converters (DACs)  120  and  130  are connected to the GmC stages potentially utilize DEM for higher linearity. In addition to DEM, the DACs connected to the GmLC stages either utilize return-to-zero (RTZ) or are effectively modulated at the SDM sample clock rate such that they feed back signal in the modulated frequency band. In addition, the tuned LC tank of the bandpass GmLC stage(s) may be tuned for different RF bands while either the Gm or C of the lowpass GmC stages may be tuned for the desired signal bandwidth.  
         [0076]     For high-speed performance, the preferred DEM method utilizes the time during which the clocked comparator is reset to shuffle which comparator is looking at which reference using analog multiplexers. Each comparator is connected to a dedicated single bit DAC for analog feedback to the SDM and the digital outputs are effectively summed for feeding forward to the decimator. To prevent beat tones due to the DEM switch rate, either the mux selection order can be randomized or the frequency of rotating the selects can be randomized using the serial output of a Pseudorandom Number (PN) generator as a rotate/hold signal. Not all stages of the SDM are required to have the same number of bits in the DAC feedback. There are potential linearity improvements if the very first stage is only one bit while there are potential stability and dynamic range improvements for multi-bit DACs.  
         [0077]     There are several possible modifications of the diagram shown in  FIG. 4 . For example, the capacitor network may be located after the mixers to provide the input to quantizers  116  and  118 . Furthermore, the analog mixer may include a diode sample and hold circuit.  
         [0078]      FIG. 5  is a block diagram of a digital heterodyne transmitter  400 . Digital heterodyne transmitter  400  received two digital baseband signals  402  and  404  from, for example, a modulator (not shown). Signals  402  and  404  are provided to the respective summing circuits  406  and  408 , also receiving an IF signal from a numerically controlled oscillator (NCO)  410 . Sigma-delta modulator  420  receives a digital in-phase IF signal  419  and an RF clock signal. Sigma-delta modulator  420  provides an in-phase RF analog signal  424  to a summing circuit  430 . Sigma-delta modulator  422  receives a digital out-of-phase IF signal  417  and an RF clock signal. Sigma-delta modulator  422  provides an out-of-phase RF analog signal  426  to a summing circuit  430 . Circuit  440  is used for carrier tuning.  
         [0079]     In addition, it will be understood by those skilled in the relevant art that control and electronic elements and functional elements of the invention and various structures may vary in many ways from the described above. Numerous other embodiments, and modifications thereof, are contemplated as falling within the scope of the present invention as defined by appended claims and equivalents thereto.  
         [0080]     Additional embodiments are within the following claims: