Patent Publication Number: US-2007105514-A1

Title: Low Noise, High Linearity TV Tuner Architecture with Switched Fixed-Gain LNA

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application claims the benefit of U.S. Provisional Application No. 60/597,066, filed Nov. 8. 2005. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to electronics, and more specifically, to a TV tuner.  
      2. Description of the Prior Art  
      TV tuners are used in a variety of consumer and commercial systems such as TVs, VCRs, and more sophisticated devices that include cable modems and cable set-top-boxes. A TV tuner is responsible for receiving all available channels, selecting the desired channel and filtering out the others.  
      World Intellectual Property Organization publication WO 02/093732, which is included herein by reference, describes a state-of-the art integrated TV tuner. This type of TV tuner uses double conversion, and thus includes two polyphase filters and two mixers.  
      Another example of an integrated TV tuner is one that has been developed by PHILLIPS. The TDA8275 silicon IC tuner provides a low intermediate frequency (IF) signal to a subsequent decoder device, but relies on proprietary architecture to do so. That is, commonly available decoders or demodulators cannot be used with the TDA8275.  
      A wideband receiver, such as terrestrial digital TV (DVB-T), requires high linearity and low noise. It is also necessary to have a wide continuous gain range to maximize a signal-to-noise-and-distortion-ratio (SNDR) at tuner output. Typically a low-noise amplifier (LNA) with a wide tuning range is employed to simultaneously satisfy three requirements: high linearity, low noise, and wide tuning range. However, performance is compromised by trading off sensitivity and largest signal that can be handled, and often at the expense of high power consumption.  
     SUMMARY OF THE INVENTION  
      According to the invention, a TV tuner comprises a plurality of amplifiers comprising inputs for coupling to an antenna; a plurality of filters comprising inputs coupled to outputs of the plurality of amplifiers; a first multiplexer comprising inputs coupled to outputs of the plurality of filters, the first multiplexer for selecting an output of one of the filters; a mixer comprising an input coupled to an output of the first multiplexer; and an output amplifier having an input coupled to an output of the mixer and an output for providing output of the TV tuner.  
      These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram of a TV tuner according to the present invention.  
       FIG. 2  is a block diagram of another TV tuner according to the present invention. 
    
    
     DETAILED DESCRIPTION  
      The present invention uses a bank of fixed gain low-noise amplifiers (LNAs), followed by a wide continuous gain range intermediate frequency (IF) amplifier. By using more than one fixed gain LNA, noise figure and linearity can be better traded off. For example, a maximum gain LNA can be optimized for minimum noise figure while a minimum gain LNA can be optimized for high linearity. A wide tuning range IF amplifier ensures the overall transfer function has continuous gain.  
      Please refer to  FIG. 1 .  FIG. 1  shows a TV tuner  100  according to the present invention. The TV tuner  100  comprises a plurality of amplifiers  120 - 122  having inputs coupled to an external antenna  110 , a multiplexer  130  (second multiplexer) having inputs coupled to outputs of the amplifiers  120 - 122 , a plurality of filters  140 - 142  having inputs coupled to an output of the multiplexer  130 , another multiplexer  150  (first multiplexer) having inputs coupled to outputs of the plurality of filters  140 - 142 , a mixer  160  having an input coupled to an output of first multiplexer  150 , and an output amplifier  180  having an input coupled to an output of the mixer  160  and an output for providing output of the TV tuner  100 . A phase-locked loop (PLL)  170  is also provided and connected to the mixer  160 . The output of the TV tuner  100  can be connected to an external surface acoustic wave (SAW) filter  190 , for example.  
      The amplifiers  120 - 122  can be fixed-gain low-noise amplifiers. There can be two or more amplifiers  120 - 122 . Each amplifier can have a different fixed gain spanning a required “discrete” gain range. A higher gain amplifier can be optimized for lower noise figure while a lower gain amplifier can optimized for higher linearity. The overall system noise figure is limited by the maximum gain amplifier while the overall system linearity is limited by the minimum gain amplifier.  
      At the multiplexer  130 , an output of one of the amplifiers  120 - 122  is selected by a digital signal Sel_LNA, and the selected output is fed to the bank of filters  140 - 142 . The multiplexer  130  can be any known digital switching device.  
      The filters  140 - 142  can be band-pass filters. There can be one or more band-pass filters  140 - 142  in the bank. More band-pass filters  140 - 142  reduce signal bandwidth for the subsequent stages and relieve their linearity requirements. However, this is done at the expense of circuit complexity, area, and power consumption. Lastly, the filters  140 - 142  can be composed of on-chip and off-chip components. For example, transistors can be put on chip, while passive components, such as resistances and capacitances, can be put off chip. This can allow for better Q values.  
      At the multiplexer  150 , an output of one of the filters  140 - 142  is selected by a digital signal Sel_BPF, and the selected output is fed to the mixer  160 . The digital signal Sel_BPF can be the same as the digital signal Sel_LNA or can be different, i.e., independent. The multiplexer  150  can be any known digital switching device.  
      The mixer  160  receives the output of the selected band-pass filter  140 - 142 . The mixer  160  can be a single mixer, in the case of direct conversion, or more than one mixer for more frequency conversion. The mixer  160  converts the band-pass filter output to a final intermediate frequency with a local oscillator (LO) signal from a frequency synthesizer, which in this case is the PLL  170 . The mixer  160  can be a regular mixer, an image rejection mixer (I-Q mixer), or a harmonic rejection mixer. In addition, the mixer  160  can be configured as a harmonic rejection mixer when certain filters  140 - 142  are selected, and can be configured as an image rejection mixer when other filters  140 - 142  are selected. Generally, lower frequency range filters would require a harmonic rejection mixer, while higher range filters would require an image rejection mixer. In regard to this, the mixer  160  can be set up to receive the selection signal Sel_BPF.  
      The output amplifier  180 , or IF amplifier, receives the output of the mixer  160 . The output amplifier  180  drives the external SAW filter  190 . The output amplifier  180  has a wide continuous gain range, and a gain controlled by an input signal G. The gain range can be chosen to cover the maximum gain step in the bank of fixed-gain low-noise amplifiers  120 - 122 . The continuous gain range facilitates a continuous time control scheme which can be done on-chip. Alternatively, the TV tuner  100  gain can be controlled by a connected demodulation chip.  
      In operation, the signals of the TV tuner  100  are as follows. The antenna  110  feeds a received signal  10  to the bank of fixed-gain low-noise amplifiers  120 - 122 . Each amplifier  120 - 122  outputs a corresponding signal I 1 , I 2  to the multiplexer  130 . The output of the multiplexer  130 , selected according to the selection signal Sel_LNA, is filtered by the bank of band-pass filters  140 - 142  to form respective signals I 3 , I 4 , of which one is selected by the multiplexer  150 , by way of the selection signal Sel_BPF, to be input to the mixer  160  and further to be output as a signal I 5 . At the same time, the mixer  160  also receives a LO signal I 6  from the PLL  170 . Lastly, the signal I 5  is amplified by the output IF amplifier  180  before finally being filtered by the external SAW filter  190  to generate a signal I 8 .  
      In one embodiment of the TV tuner  100 , a total quantity of amplifiers  120 - 122  and a total quantity of filters  140 - 142  is identical. The amplifiers  120 - 122  correspond to the filters  140 - 142  in a one-to-one manner. Therefore, the digital signal Sel_BPF is identical to the digital signal Sel_LNA.  
      In another embodiment, the digital signals Sel_BPF and Sel_LNA are independent from each other. In some applications or circumstances, it may be desirable to be able to arbitrarily match amplifiers  120 - 122  and filters  140 - 142 .  
      It should also be mentioned, that unselected amplifiers and filters, that is, amplifiers and filters not selected by the digital signals Sel_BPF and Sel_LNA, can be temporality shut down to save power.  
      In practical application, there may be four filters  140 - 142 . Each of these four filters may have a different frequency range, such as 50-150 MHz, 150-300 MHz, 300-600 MHz, and 600+ MHz.  
      Please refer to  FIG. 2 .  FIG. 2  illustrates another TV tuner  200  according to the present invention. The TV tuner  200  comprises a plurality of amplifiers  220 - 226  having inputs coupled to an external antenna  210  and a plurality of filters  230 - 236 , each having an input coupled to an output of a different amplifier  220 - 226  (one-to-one connection). The TV tuner  200  further comprises a multiplexer  240  (first multiplexer) having inputs coupled to outputs of the filters  230 - 236 , a mixer  250  having an input coupled to an output of the multiplexer  240 , a PLL  260  coupled to the mixer  250 , and an output amplifier  270  having an input coupled to an output of the mixer  250  and an output for providing output of the TV tuner  200 . The output of the TV tuner  200  can be connected to an external SAW filter  280 , for example.  
      The elements of the TV tuner  200  are substantially the same as the like-named elements of the TV tuner  100 , and redundant description is omitted. However, one key difference is that one selection signal Sel is required since only one multiplexer is used. In this regard, the TV tuner  200  is less flexible than the TV tuner  100 , while having the advantage of requiring fewer components. In general, the other above-described provisions for the TV tuner  100  also apply to the TV tuner  200 .  
      Lastly, it should be noted that the specific quantities of amplifiers and filters shown in  FIG. 1  and  FIG. 2  are merely examples. More or fewer can be used depending on specific requirements.  
      Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.