Abstract:
A wideband receiver system comprises a mixer module, a wideband analog-to-digital converter (ADC) module, and digital circuitry. The mixer module is configured to downconvert a plurality of frequencies that comprises a plurality of desired television channels and a plurality of undesired television channels. The wideband ADC module is configured to digitize the swatch of frequencies comprising the plurality of desired television channels and the plurality of undesired television channels. The digital circuitry is configured to select the desired plurality of television channels from the digitized plurality of frequencies, and output the selected plurality of television channels to a demodulator as a digital datastream.

Description:
PRIORITY CLAIM 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 14/614,543 filed on Feb. 5, 2015 (now U.S. Pat. No. 9,210,362), which is a continuation of U.S. patent application 13/962,871 filed on Aug. 8, 2013 (now U.S. Pat. No. 9,100,622), which is a continuation of U.S. patent application Ser. 12/762,900 filed on Apr. 19, 2010 (now U.S. Pat. No. 8,526,898), which claims the benefit of priority to U.S. provisional application 61/170,526 filed Apr. 17, 2009, now expired. Each of the above referenced documents is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    This invention relates to wideband receiver systems and methods having a wideband receiver that is capable of receiving multiple radio frequency channels located in a broad radio frequency spectrum. In particular, the invention relates to wideband receiver systems that are capable of receiving multiple desired television channels that extend over multiple non-contiguous portions of the broad frequency spectrum and grouping them into a contiguous, or substantially-contiguous, frequency spectrum. 
         [0003]    Receivers used to down-convert and selectively filter TV channels are referred to as tuners, and tuners designed to concurrently receive several TV channels are referred to as wideband tuners. Existing tuners for these applications down-convert a swath of channels to an intermediate frequency, which are then sent to a demodulator. Because the swath of channels is not contiguous, this swath includes the desired channels as well as undesired channels. The demodulator employs a high-speed data converter to capture this swath of desired and undesired channels in the digital domain and subsequently filters out the desired channels. 
         [0004]    In general, television channels broadcasted over the air or over cable networks are distributed across a broad frequency spectrum. That is, the channel frequencies may not be adjacent to each other. In certain applications such as DVR and picture-in-picture, the receiver system may have to concurrently receive several desired channels that may or may not be contiguous. The wideband receiver requirement poses a trade-off to the system to limit either the dynamic range of the wideband tuner or reduce the bandwidth covered by the tuner so that fewer channels may be received and processed by the demodulator. 
         [0005]      FIG. 1  shows a conventional wideband tuner  100 . Tuner  100  may be a direct conversion tuner and includes a low noise amplifier LNA 1  having an input terminal coupled to a radio frequency (RF) input signal  102  and an output terminal coupled to a mixer M 1 . The RF signal may include one or more television channels receiving from a cable network via an RF connector or wirelessly via an antenna. The RF input signal may include the VHF and UHF television channels in a terrestrial television broadcasting system or the CATV channels in cable networks. In order to receive all broadcasted channels present in the RF input signal, LNA 1  must necessarily have a wide tuning range, high linearity, and low noise. Mixer M 1  is coupled to a synthesizer S 1  that can generate an oscillator frequency located around the center of the RF signal. Mixer M 1  frequency down-converts the received RF input signal to a more convenient intermediate frequency (IF) band. Tuner  100  includes an amplifier V 1  having a programmable gain for amplifying the IF signal, which is then band-pass filtered by a filter F 1  before outputting to a demodulator. 
         [0006]    In general, the RF signal includes multiple desired channels that are located in non-contiguous portions of a radio frequency spectrum. As shown in  FIG. 1 , the swath of channels  110  occupies a bandwidth BW 1   120  at an RF center frequency f rfc    130 . Synthesizer S 1  may be tuned to a frequency around the center frequency f rfc    130  for mixing channels  110  to an intermediate frequency f ifc    160 , the frequency down-mixed channels  140  are amplified by amplifier V 1  and then filtered by F 1  to produce a swath of channels  170  centered around frequency f ifc    160 . In an exemplary application shown in  FIG. 1 , bandwidth BW 1  contains  10  channels. In the case where channels are TV channels that are spaced at either 6 MHz or 8 MHz in most parts of the world, bandwidth BW 1   120  would span from 60-80 MHz, i.e., the down-converted bandwidth at the intermediate frequency would require a bandwidth equal to at least BW 1 , or at least 80 MHz when such architecture is used. It is noted that in other applications where the desired RF channels are located in the low band such as channels numbers 2 to 6 (VHF in the terrestrial TV broadcast or CATV) and in the high band such as channels numbers  14  to  83  of the UHF TV broadcast or channel numbers  63 - 158  of the CATV&#39;s ultra band, the bandwidth BW 1  can be 800 MHz or higher. This wide bandwidth of 800 MHz would require a very expensive digital processing circuitry such as very high-speed analog to digital conversion and high-speed processor in the demodulator. 
         [0007]    It is desirable to have wideband receiver systems that can increase the dynamic range without requiring expensive data conversion, filtering and channel selection at the demodulator. 
       BRIEF SUMMARY 
       [0008]    An embodiment of the present invention includes a wideband receiver system that is configured to concurrently receive multiple radio frequency (RF) channels including a number of desired channels that are located in non-contiguous portions of a frequency spectrum and group the desired channels in a contiguous or substantially-contiguous frequency band at an intermediate frequency spectrum, where the term “substantially-contiguous” includes spacing the desired channels close to each other (e.g. as a fraction of the total system bandwidth, or relative to a channel bandwidth) but with a spacing that can be variable to accommodate the needs of overall system. The term “contiguous” heretofore encompasses “substantially-contiguous.” The term “spacing” is referred to as the frequency difference between adjacent channels. The system includes a wideband receiver having a complex mixer module for down-shifting the multiple RF channels and transforming them to an in-phase signal and a quadrature signal in the baseband or low intermediate frequency (IF) band. The system further includes a wideband analog-to-digital converter module that digitizes the in-phase and quadrature signals. The digital in-phase and quadrature signals are provided to a digital frontend module that contains a bank of complex mixers that frequency-shift the number of desired channels to a baseband where the desired channels are individually filtered. 
         [0009]    The digital frontend module may also include a decimator module that decimates the desired RF channels by a factor M before demodulating them to a digital data stream. 
         [0010]    In certain embodiments of the present invention, the wideband receiver system additionally includes an up-converter module having multiple complex up-mixers, each of the complex up-mixers is configured to frequency up-shift each one of the desired RF channels to a sub-portion of an IF spectrum, wherein all sub-portions of the desired channels are adjacent to each another and form a contiguous frequency band in the IF spectrum. The act of frequency shifting the desired channels to the IF spectrum allows the wideband receiver system to directly interface with commercially available demodulators. Allowing the spacing of the desired channels in the contiguous spectrum to be variable allows a system to optimize placement of these desired channels for the purposes of avoiding sensitive portions of the spectrum which may either be vulnerable to spurious signals and interference; or which may generate interference directly or as a harmonic product, to other systems. 
         [0011]    In another embodiment of the present invention, a multi-tuner receiver system having two or more tuners is provided to receive multiple desired RF channels that extend over several non-contiguous sub-portions of a broad frequency spectrum and group them into a contiguous frequency spectrum. The multi-tuner system includes at least a first tuner that processes a first sub-portion of the broad frequency spectrum into a first in-phase signal and a first quadrature signal and a second tuner that processes a second sub-portion of the broad frequency spectrum into a second in-phase signal and a second quadrature signal. The multi-tuner receiver system further includes a first analog-to-digital converter module that digitizes the first in-phase and quadrature signals and a second analog-to-digital converter module that digitizes the second in-phase and quadrature signals. In addition, the multi-tuner system includes a first digital frontend module having a first number of complex mixers corresponding to a first number of the desired RF channels located in the first sub-portion of the broad frequency spectrum and a second digital frontend module having a second number of complex mixers corresponding to a second number of the desired RF channels located in the second sub-portion of the broad frequency spectrum. The first digital frontend module frequency shifts the first number of the desired RF channels to a first plurality of baseband signals and the second digital frontend module frequency shifts the second number of the desired RF channels to a second plurality of baseband signals. 
         [0012]    The multi-tuner system further includes a first up-converter module having a plurality of N complex mixers, wherein N is an integer value equal to the number of desired channel. The first up-converter module frequency up-shifts the first plurality of the baseband signals to a first portion of an intermediate frequency. In addition, the multi-tuner system includes a second up-converter module that frequency up-shifts the second plurality of the baseband signals to a second portion of an intermediate frequency. The first and the second portions of the intermediate frequency are non-overlapping and located adjacent to each other to form a contiguous intermediate frequency (IF) band. The multi-tuner system further includes a digital-to-analog converter that converts the contiguous IF band to an analog waveform signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a schematic block diagram of a conventional wideband tuner; 
           [0014]      FIG. 2  is a schematic block diagram of a wideband receiver system according to an embodiment of the present invention; 
           [0015]      FIG. 3  is a simplified circuit diagram of a complex down-mixer according to an embodiment of the present invention; 
           [0016]      FIG. 4  is a simplified schematic block diagram of a wideband receiver system according to another embodiment of the present invention; 
           [0017]      FIG. 5  is a simplified circuit diagram of a complex up-mixer according to an embodiment of the present invention; 
           [0018]      FIG. 6  is a simplified schematic block diagram of a wideband multi-tuner receiver system according to an embodiment of the present invention; 
           [0019]      FIG. 7  is a block diagram illustrating an exemplary digital front end according to an embodiment of the present invention in more detail; 
           [0020]      FIG. 8  is a block diagram illustrating an exemplary tiled up-converter module according to an embodiment of the present invention in more detail; 
           [0021]      FIG. 9  is a simplified block diagram of a wideband multi-tuner receiver system  900  according to an embodiment of the present invention; and 
           [0022]      FIG. 10  is a simplified block diagram of a wideband multi-tuner receiver system  1000  according to another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]      FIG. 2  is a schematic block diagram of a wideband receiver system  200  according to an embodiment of the present invention. Wideband receiver system  200  includes a radio front end  210  and a digital front end  230 . Radio front end  210  may be a single very wide-band tuner receiver that captures the desired swath of channels located in non-contiguous portions of the spectrum having a frequency bandwidth BW 1   120 . In this example, the number of available channels in BW 1   120  is assumed to be  10  with each channel occupying an 8 MHz bandwidth for a total of 80 MHz. Radio front end  210  is shown as including a low noise amplifier LNA  202  having an input terminal configured to receive an RF input signal  102 . In the example shown, RF signal  102  includes four desired RF channels having the respective carrier frequency f rf1 , f rf2 , f rf3 , and f rf4  that are located in non-contiguous portions of the wide frequency spectrum BW 1 . It is understood, however, that spectrum BW 1   120  may have any other number of desired frequencies that are not contiguous. LNA  202  has a very low noise figure and very high linearity and a wide tuning range (i.e., very high IIP 2  and IIP 3  intercept points) to maximize a signal-to-noise-and distortion ratio (SNDR) at the amplifier output. LNA  202  may have a programmable gain to amplify RF signal  102  to adequate voltage levels for mixers M 1   211  and M 2   221 . 
         [0024]    Mixers M 1   211  and M 2   221  may be conventional mixers formed using, for example, differential Gilbert cells. Each of the mixers  211  and  221  multiplies (mixes) an amplified RF signal  203  with a respective first oscillator frequency signal  205  and a second oscillator frequency signal  207  to generate an in-phase signal  212  and a quadrature signal  222  that have a phase shift of 90° degree between them. Mixers  211  and  221  are identical so that the amplitude of the in-phase signal  212  and quadrature signal  222  are the same. The first and second oscillator frequencies  205  and  207  are identical and have a 90° degree phase shift generated through a 90° degree phase shifter P 1   206 . Synthesizer S 1  may be a single local oscillator operable to generate the oscillator frequency  205  for converting the receive RF signal  102  to a zero-IF or low-IF band. Synthesizer S 1  can be a coarse (large step) phase locked loop. Synthesizer S 1  can also be programmable to cover the wideband frequency of the analog and digital terrestrial broadcast and/or the cable television system. The RF signal  102  may have relatively uniform signal strength in a cable network. However, its signal strength may extend in several orders of magnitude in a terrestrial broadcast system, thus, LNA  202  and/or mixers M 1   211 , M 2   221  are required to have a relatively high dynamic range to handle the large variations in the signal strength. 
         [0025]    In-phase signal  212  and quadrature signal  222  are further amplified and filtered by respective amplifiers V 1   213 , V 2   223  and filters F 1   215 , F 2   225  to generate a filtered in-phase signal  216  and a filtered quadrature signal  226 . Filters F 1   215  and F 2   225  may be passive or active low-pass filters to filter out any unwanted frequency components of the signals  214  and  224  before digitizing them for further processing in digital front end  230 . It is understood that the in-phase path  216  and the quadrature path  226  must have the same amplitude spectrum and maintain a fixed phase relationship, i.e., amplifiers V 1   213 , V 2   223  and filters F 1   215 , F 2   225  must be substantially identical. Because the two paths  216  and  226  are in quadrature, the spectral components from both positive and negative frequencies can be overlaid so that the bandwidth (cutoff frequency) of filters F 1   215  and F 2   225  can be one half of the BW 1  bandwidth  120 . 
         [0026]    Analog-to-digital converters ADC 1   218  and ADC 2   228  are high-speed (i.e., high sampling rate) converters to maximize the dynamic range. In an exemplary application, radio front end  210  operates as a nominal zero-IF down-mixer so that signals  216  and  226  have a nominal bandwidth  290  equal to one half of the RF signal bandwidth BW 1  thanks to the complex down-mixer architecture. In other embodiment, radio front end  210  operates as a low-IF down-mixer so that the nominal bandwidth  290  of signals  216  and  226  is greater than one half of the bandwidth BW 1 . In practice, the sampling rate of ADC 1   218  and ADC 2   228  is chosen to be higher than the Nyquist sampling requirement, i.e., the filtered analog quadrature signals  216  and  226  may be over-sampled in order to reduce or avoid aliasing of undesired signals into the digitized I and Q signals. 
         [0027]    ADC 1   218  generates a digital signal I  232  that is a digital representation of the analog filtered signal  216 ; ADC 2   228  generates a digital signal Q  242  that is a digital representation of the analog filtered signal  226 . Digital signals I  232  and Q  242  are then applied to a bank of N complex mixers  250 , wherein N is an integer value corresponding to the number of desired RF channels located in the non-contiguous portions of the frequency spectrum BW 1 . It is understood that the number N can be any integer value. In one embodiment, N can be equal to the number of all available channels that exist in the licensed frequency spectrum to provide system flexibility. In other embodiments, N can be equal to the number of all receivable channels within a geographic area. In yet another embodiment, N can be an integer value less than the number of receivable channels with the geographic area to reduce system costs. In the exemplary embodiment shown in  FIG. 2 , the number of desired channels is 4. That is, each of the 4 complex mixers  250  mixes in-phase and quadrature signals  232  and  242  with an associated frequency to generate a corresponding baseband, which is then individually filtered, decimated and provided to an associated demodulator. 
         [0028]    Each of the N complex mixers  250  receives the digital signals I  232  and Q  242  from ADCs  218  and  228  to extract a different one of the desired channels and frequency-shifts the extracted signals to the baseband frequency. Each of the frequency shifted desired channels  252  is filtered by an associated filter module (identified as  260   a  to  260   n ). In an embodiment, each of the filtered signals  260   a  to  260   n  may be sent directly to an associated demodulator (identified as  270   a  to  270   n ) for extracting the original information transmitted in the associated desired channel. In another embodiment, each of the filtered signals  262   a  to  262   n  is further decimated before providing to a demodulator. A path of digital front end  230  is described in more detail below. 
         [0029]      FIG. 3  is a simplified circuit diagram of one of the signal paths  272   a  to  272   n  of digital front end  230  shown in  FIG. 2  according to an embodiment of the present invention. In an embodiment, digital signal I  232  may be further filtered by a filter  311  to obtain a filtered signal  312 . Similarly, digital signal Q  242  may be further filtered by a filter  321  to obtain a filtered signal  322 . Thus, digital signals  312  and  322  only contain low-frequency components with undesired high-frequency components being eliminated by respective filters  311  and  321 . It is noted that filtered signals  312  and  322  are interposed between the respective ADCs  218 ,  228  and the bank of N complex mixers  250 . 
         [0030]    Mixer  300 , which represents one of the N complex mixers  250 , includes four multipliers  313 ,  315 ,  323 , and  325 . Multipliers  313  and  315  multiply the filtered signal  312  with respective cos(ω ci t) and sin(ω ci t) signals and generate respective products  314  and  316 . Similarly, multipliers  323  and  325  multiply the filtered Q signal  322  with respective cos(ω ci t) and sin(ω ci t) signals and generate respective products  324  and  326 . An adder  317  sums the products  314  and  326  to generate a frequency-shifted signal I  318 . An adder  327  sums the products  324  and  316  to generate a frequency-shifted signal Q  328 . Basically, complex mixer  300  causes a frequency shift of the filtered components  312  and  322  to respective baseband signals  318  and  328  in the digital domain according to the operation: 
         [0000]        Y ( t )= X ( t )*  e   −jω     c     t   (1)
 
         [0000]    or taken the Fourier transform, we obtain: 
         [0000]        Y (ω)= X (ω−ω c )  (2)
 
         [0031]    Multipliers  313 ,  315 ,  323 , and  325  are identical digital multipliers. In an embodiment, a numerically controlled oscillator with quadrature output generates the cos(ω ci t) and sin(ω ci t) signals. Numerically controlled oscillators (NCO) can be implemented using a phase accumulator and a look-up table. NCOs are known to those of skill in the art and will not be described herein. The frequency ω ci  is so chosen that each one of the desired channels embedded in the digital signals I  232  and Q  242  will be downshifted to the baseband. In the given example shown in  FIG. 2 , the bank of N complex mixers will have four complex mixers, each one of the N (i.e., four) complex mixers is coupled to an individual NCO having a distinct frequency ω ci  so that when mixing the filtered digital I and Q signals  312  and  322  with that frequency, each one of the complex mixers will generate the signals I ( 318 ) and Q ( 328 ) of a corresponding one of the desired channels at the baseband. 
         [0032]    In an embodiment, baseband signals  318  and  328  are further individually filtered by respective filters  330  and  340  that are identified as one of the filters  260   a - n  in  FIG. 2 . Filters  330  and  340  may be band-pass or low-pass filters having a narrow bandwidth equal to the bandwidth of a desired channel. In certain embodiments, filters  330  and  340  can be analog passive or active low-pass or complex band-pass filters such as polyphase filters. In another embodiment, filters  330  and  340  can be digital low-pass filters, such as finite impulse response (FIR) filters to eliminate high frequency components that may be aliased back to the baseband signals Ii ( 332 ) and Qi ( 342 ) when decimated by subsequent decimator  350 . 
         [0033]    The reduced sampling rate of the N desired baseband channels will be sent as a serial or parallel digital data stream to a demodulator using a serial or parallel data interface according to commonly known methods, as shown in  FIG. 2 . This approach provides several advantages over conventional tuner architectures. First, it eliminates the need of expensive data conversion, filtering and channel selection on the demodulator side. Second, it removes undesired channels from the signal path at an early stage, thus relieves the large dynamic range requirement in the demodulator. 
         [0034]      FIG. 4  shows a simplified schematic block diagram of a wideband receiver system  400  according to another embodiment of the present invention. Wideband receiver system  400  includes a radio front end  410 , a digital front end  430 , a tiled up-conversion module  450 , and a summing digital-to-analog converter module DAC  470 . Radio front end  410  includes a low noise amplifier LNA 1  that receives an RF input signal  102  and provides an amplified RF signal  403  to mixers M 1   411  and M 2   421 . Mixer M 1   411  is coupled to an oscillator frequency  405  of a synthesizer S 1  whereas mixer M 2   421  is coupled with the oscillator frequency  405  via a phase shifter P 1   406  that generates a 90° degree phase-shift to the oscillator frequency  405 . Mixers M 1   411  and M 2   421  generate respective in-phase signal  412  and quadrature signal  422  that are further amplified by respective amplifiers V 1   413  and V 2   423 . The amplified in-phase and quadrature signals  414 ,  424  are then filtered by filters F 1   415  and F 2   425  to eliminate undesired frequency components that would be aliased back to the in-phase and quadrature signals when digitally sampled by subsequent analog-to-digital converters ADC 1   418  and ADC 2   428 . Digital signals I  422  and Q  442  at the input of digital front end  430  are digital representations of the filtered analog in-phase and quadrature signals  416 ,  426  before the ADCs. Digital front end  430  include a bank of N complex mixers  432  comprising  432   a  to  432   n  identical mixers, where N is an integer value corresponding to the number of the desired channels located in non-contiguous portions of the frequency spectrum. Each of the N complex mixers  432   a  to  432   n  frequency down-converts signals I  432  and Q  442  to an associated baseband. Each of the frequency down-converted I and Q signals are coupled to respective low-pass, band-pass, or decimating filters  434 . In this regard, the radio front end  410  and the digital front end  430  are similar to respective radio front end  210  and digital front end  230  of  FIG. 2  that have been described in detail above. 
         [0035]    In an alternative embodiment of the present invention, the N filtered and decimated channels  438   a  to  438   n  (where indices a to n correspond to the associated number of desired channels) are not provided to a demodulator for demodulation. Instead, the N filtered and decimated channels  438   a  to  438   n  are further frequency up-converted to an intermediate frequency (IF) spectrum. In order to achieve that, the N filtered and decimated channels are coupled to a tiled up-conversion module  450  that includes a bank of N complex up-mixers, where N is an integer value correspond to the number of desired received channels. The N complex up-mixers include identical digital mixers  452   a  to  452   n  that will be described further in detail below with reference to  FIG. 5 . The N up-shifted channels are then filtered by a subsequent bank of channel filters  454  that, in an embodiment, comprises N individual finite impulse response (FIR) filters. The N filtered channels are then digitally combined and converted to the analog domain by a summing digital-to-analog converter module DAC  470 . The N up-shifted channels are adjacent to each other and form a contiguous or substantially-contiguous set of channels  475  in the IF spectrum centered around f if  as illustrated in  FIG. 4 . In an embodiment, the spectra of the mixed products are spaced in such a way so as to avoid overlap with known frequency bands containing potential or actual interferers. In another embodiment, the spectra of the mixed products are spaced in such a way so as to avoid overlap with frequency bands that might introduce interference to other systems. In general, because the bandwidth BW 2  is substantially lower than BW 1 , the IF frequency f if  can be set proportionally lower, e.g., typically about 16 MHz to accommodate the spectrum of BW 2  of up to 32 MHz (corresponding to the total bandwidth of the four desired channels, each having a bandwidth of 8 MHz in this example). 
         [0036]    The up-conversion approach of  FIG. 4  provides several advantages over conventional tuner architectures. First, it allows the demodulator to operate the data converter at a lower data rate and with lower resolution (fewer bits) due to the fact that the contiguous channels have a narrower bandwidth. Second, the up-conversion approach provides full compatibility with existing demodulators that require an analog IF signal. Third, it removes undesired channels from the signal path at an early stage, thus relieves the requirement of a high dynamic range requirement of the demodulator&#39;s analog-to-digital converter and the demodulator itself. 
         [0037]      FIG. 5  shows a simplified exemplary circuit diagram of a complex up-mixer  500  according to an embodiment of the present invention. Up-mixer  500  is one of the N complex up-mixers  452   a  to  452   n  in tiled up-conversion module  450  shown in  FIG. 4 . Up-mixer  500  includes filter  510  and  520  configured to eliminate unwanted frequency components present in respective input signals I  501  and Q  502 . Filtered signals  512  and  522  are provided to up-mixers UMI  515  and UMQ  525  that multiply the filtered signals  512  and  522  with respective cos(ω u t) and sin(ω u t). The products  516  and  526  are summed in an adder  530  to generate an IF signal  532  according to the following equation: 
         [0000]        IF ( t )= I ( t )*cos(ω u   t )+ Q ( t )*sin(ω u   t )  (3)
 
         [0038]    Up-mixers UMI  515  and UMQ  525  are identical digital multipliers that multiply the respective filtered signal  512  and  522  with a cosine function  505  and a sine function  506  that can be generated from a NCO using a digital phase accumulator and a look-up table. 
         [0039]    As described above, TV channels are grouped into multiple frequency bands in North America. For example, channels  2  through  6  are grouped in VHF-low band (aka band I in Europe), channels  7  through  13  in VHF-high band (band III), and channels  14  through  69  in UHF band (bands IV and V). In order to receive such a wide frequency spectrum, the low noise amplifier and mixer must have very low noise, wide tuning range and high linearity as described above in the wideband receiver systems  200  and  400 . However, a wideband receiver having a single tuner with high sensitivity may have a high power consumption. For certain applications, it may be advantageous to use multiple tuners that are optimized for a given frequency band, such as a dedicated tuner for the low VHF band, another dedicated tuner for the high VHF band and the UHF band, and yet other dedicated tuners for receiving the digital video broadcasting (DVB) via satellite (DVB-S), via cable (DVB-C), or terrestrial digital video broadcasting (DVB-T). The multi-tuner approach may also be advantageously applied to cable networks that carry TV programs on an 88 MHz to 860 MHz according to the Data Over Cable Service Interface Specification (DOCSIS) protocol. 
         [0040]      FIG. 6  shows a simplified schematic block diagram of a wideband multi-tuner receiver system  600  according to an embodiment of the present invention. In an embodiment, multi-tuner system  600  includes low noise amplifier A 1   602  for receiving an RF input signal  601 . Amplifier A 1   602  is coupled to at least a tuner 1   610  and a tuner 2   720 . In another embodiment, multi-tuner system  600  may not include amplifier  602  so that RF input signal  601  can be received directly at each tuner  610  and  720 . 
         [0041]    Tuner 1   610  includes an amplifying filter AF 1   613  that filters and amplifies a first portion BWtuner 1   604  of a broad frequency spectrum  608  that contains a first plurality of RF channels  606  including desired channels  607  having respective channel frequencies f rf1  and f rf2 . The first portion of the broad frequency spectrum BWtuner 1   604  is then frequency down-converted to a low-IF or zero-IF in-phase signal I 1   612  and a quadrature signal Q 1   622  through respective mixer M 1   611  and M 2   621 . Signals I 1   612  and Q 1   622  are further amplified and low-pass filtered before applying to respective analog-to-digital converters ADC 1   618  and ADC 2   628  that convert analog signals Ia 1   616  and Qa 1   626  to respective digital in-phase signal Id 1   631  and digital quadrature signal Qd 1   641 . Because tuner 1   610  only covers a portion BWtuner 1   604  of the entire frequency spectrum  608  having fewer channels, the ADC 1   618  and ADC 2   628  can be slower-speed analog-to-digital converters with a large number of bits, i.e., large dynamic range. 
         [0042]    Digital signals Id 1   631  and Qd 1   641  are then provided to a digital front end DFE  630  that includes a first bank of N complex mixers  632  and channel and decimating filters  634 . The first bank of N complex filters  632  has N identical complex mixers, where N is an integer value equal to the number of desired channels located in the first portion BWtuner 1   604  of the broad frequency spectrum  608 . In an embodiment, each one of the first bank of N complex mixers includes four digital mixers that multiply digital stream Id 1   631  and Qd 1   641  with respective digitized cosine function and sine function to generate the sum and difference frequency components, as shown in  FIG. 3 . The digitized cosine and sine frequency, i.e., the mixer frequency is so chosen so that when mixing signals Id 1   631  and Qd 1   641  will move them to a baseband or a low-IF band. In an embodiment, channel and decimating filters have similar structures as filters  330  and  340  and demodulator  350  as shown in  FIG. 3 . That is, channel and decimating filters include digital low-pass filters  330  and  340  that eliminates unwanted high frequency components of the baseband signals I and Q prior to applying them to a decimator  350  ( FIG. 3 ) that reduces the sample frequency without any loss of information since Id 1   631  and Qd 1   641  are sampled at a much higher frequency by the respective ADC 1   618  and ADC 2   628 . 
         [0043]    The decimated desired channels are then provided to an up-converter module  650  that includes a bank of N up-mixers. The bank of N up-mixers includes N identical up-mixers whose structure is shown in  FIG. 5 . In an embodiment, N is an integer value equal to the number of desired channels present in BWtuner 1   604 . Each one of the up-mixer frequency-shifts the baseband signals I and Q of each one of the desired channels to an appropriate portion of the intermediate frequency band according to Equation (3). In other words, the bank of N up-mixers is “frequency multiplexing” the desired channels onto a first portion  682  of an IF band  686 . 
         [0044]    Similarly, tuner 2   720  includes an amplifying filter AF 2   713  that is configured to receive a second portion BWtuner 2   704  of the broad frequency spectrum  608 . The second portion  704  contains a second plurality of RF channels  706  including a second number of desired channels. In the exemplary illustration of  FIG. 6 , the second portion  704  has a frequency bandwidth of BWtuner 2  that contains desired channels  707  having respective channel frequencies f rf3  and f rf4 . Tuner 2   720  includes elements such as mixers M 3   711 , M 4   721 , amplifiers V 3   714 , V 4   724 , filters F 3   715 , F 4   725  and analog-to-digital converters ADC 3   731  and ADC 4   741  that are substantially the same as the like-named elements of the signal path of tuner 1   610 . Thus, redundant description is omitted herein. 
         [0045]    Digital in-phase signal Id 2   731  and digital quadrature signal Qd 2   741  are then provided to digital front end  740 . Digital front end  740  includes a bank of L complex filters, where L is an integer value equal to the number of desired channels in the second portion BWtuner 2   704  of the broad frequency spectrum  608 . Each one of the bank of L complex filters is a digital complex mixer configured to transform the signals Id 2   731  and Qd 2   741  to baseband signals that are further filtered by individual digital low-pass filters such as FIR filters before decimated by a subsequent decimator. The elements of digital front end  740  are substantially similar to those described in digital front end  630 . Thus, redundant description is omitted herein. 
         [0046]    The decimated baseband I and Q channels are further provided to a subsequent up-conversion module  760  that performs a function substantially similar to that of the up-conversion module  650  already described above. The outputs of up-conversion module  650  and  760  can be tiled to generate a contiguous set of IF frequencies  682 ,  684  centered at f if    686 . In an embodiment, the outputs of up-conversion module  650  and  760  are digitally summed and converted to an analog signal by summing DAC  670 . In another embodiment, the up-conversion modules  650  and  760  and the digital summing function  672  can be performed using an inverse discrete Fourier transform or an inverse Fast Fourier transform operation. 
         [0047]    The multi-tuner architecture provides the flexibility that multiple commercially available tuners can be used without the need of designing a wideband tuner. For example, a tuner designed for a terrestrial broadcast digital TV can be used together with a tuner dedicated to receiving a cable signal and/or a tuner for receiving a satellite broadcast signal. The multi-tuner receiver system provides an additional advantage that other tuners can be added quickly to the system to accommodate any future applications. Additionally, the multi-tuner architecture allows the use of slower speed (i.e., lower cost) analog-to-digital converters with a larger number of bits for achieving large dynamic range. 
         [0048]      FIG. 7  shows a block diagram of an exemplary digital front end of the invention in more detail. In an embodiment, in-phase signal Id 2   731  and quadrature signal Qd 2   741  at the output of respective ADC converters  718  and  728  are provided to each of the L complex mixers  732  comprising mixers  732 A to  732 L. A more detailed description of each of L complex mixers is shown in  FIG. 3 . Mixer  732 A multiplies Id 2   731  and Qd 2   741  with a cosine signal and a sine signal that are generated from an NCO 1  and produces an I- 732 A signal and a Q- 732 A signal that are further individually filtered by an FIR filter before decimating. The bank of L complex mixers corresponds to the block  732  in  FIG. 6 ; and the set of FIR filters and decimator corresponds to the block  734  in  FIG. 6 . Each decimated pair of I- 732   i /M in the baseband, where the index “i” is from A to L, is further provided to a subsequent up-mixer for frequency-shifting to an intermediate frequency as shown in  FIG. 8 . 
         [0049]      FIG. 8  shows an exemplary embodiment of a bank of L complex up-mixers according to the present invention. Each decimated pair of complex signals I- 732   i /M and Q- 732   i /M is provided to an associated complex up-mixer, whose frequency is so chosen that when mixing with the pair of complex signals I- 732   i /M and Q- 732   i /M will generate an associated channel at a predetermined sub-portion of the intermediate frequency band  686  ( FIG. 6 ). A more detailed schematic block of one of the L up-mixers is described above together with  FIG. 5 . 
         [0050]      FIG. 9  is a simplified block diagram of a wideband multi-tuner receiver system  900  according to an embodiment of the present invention. In an embodiment, system  900  includes a crossbar switch  910  having at least an input terminal  912  configured to receive signals from an analog-to-digital converter (ADC)  912  and an input terminal  922  configured to receive signals from an ADC  922 . Crossbar switch  910  also includes an output terminal  924  that is coupled to a digital front end  930 . In an embodiment, input terminals  912  and  922  of crossbar switch  910  have P inputs, where P is an integer value that is equal to the total number of desired channels received by tuner 1   610  and tuner 2   720 . Output terminal  924  of crossbar switch  910  have Q outputs, where Q is an integer value that is equal to the total number of desired channels received by tuner 1   610  and tuner 2   720 . 
         [0051]    In an embodiment, digital front end  930  may include a bank of R complex mixers that frequency shifts the received channels to a baseband. Digital front end  930  may combine digital front end  630  and  740  shown in  FIG. 6 . Similarly, a tiled up-conversion module  950  may include up-converter modules  650  and  760  of  FIG. 6 . 
         [0052]    System  900  further includes a summing DAC that operates similarly as summing DAC  470  and  670  that have been described in detail in relation with respective  FIG. 4  and  FIG. 6  above. Thus, redundant description is omitted herein. 
         [0053]      FIG. 10  is a simplified block diagram of a wideband multi-tuner receiver system  1000  according to another embodiment of the present invention. System  1000  includes at least tuner 1   610  coupled with digital front end  630  through an analog-to-digital converter  620  and tuner 2   720  coupled with digital front end  740  through an analog-to-digital converter  730 . System  1000  further includes a crossbar switch  1010  that is interposed between digital front ends  630 ,  740  and up-conversion modules  650 ,  760 . Crossbar switch  1010  includes an input terminal  1012  having S inputs coupled with DFE  630  and an input terminal  1022  having T inputs coupled with DFE  740 . In an embodiment, S is an integer value equal to the number of desired channels processed in DFE  630  and T is an integer value equal to the number of desired channels processed in DFE  740 . Crossbar switch  1010  further includes an output terminal  1024  having U outputs coupled with up-converter module  650  and an output terminal  1024  having V outputs coupled with up-converter module  760 . In an embodiment, the total number of the outputs U and V is equal to the sum of the inputs S and T. Thus, crossbar switch  1010  allows the routing of any channel from either DFE  630  or DFE  740  to up-converters  650  or  760 . It is understood that system  1000  is not as flexible as system  900  because DFE  630  and DFE  740  are already pre-assigned to respective tuner 1  ( 610 ) and tuner 2  ( 720 ). However, this pre-assigned arrangement allows a simpler implementation of crossbar switch  1010  that operates at lower speeds. 
         [0054]    While several embodiments in accordance with the present invention have been described, it is to be understood that the above description is intended to be illustrative and not restrictive. Many embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined not with reference to the above description, but instead should be determined with reference to the appended claims along with their full scope of equivalents.