Abstract:
A method for frequency conversion in a receiver. A signal having a radio frequency and carrying information on a selected channel is received and converted from the radio frequency to a first variable intermediate frequency determined by the selected channel. The signal is further converted from the first variable intermediate frequency to a second variable intermediate frequency determined by the selected channel. The signal is further converted from the second variable intermediate frequency to a constant baseband frequency.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part application of U.S. Utility application Ser. No. 10/762,455, filed Jan. 23, 2004, and hereby incorporated herein in its entirety by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates to frequency conversion and particularly to double or triple conversion of an RF signal in a TV tuner.  
         [0004]     2. Description of the Prior Art  
         [0005]     Broadband tuners are used in a variety of consumer and commercial systems such as TVs, VCRs and other devices that include cable modems and cable set-top-boxes. More than 300 million broadband tuners are produced every year.  
         [0006]     Increased services offered through broadcast TV and cable operators have resulted in a rapidly evolving and convergent market. Incorporation of DVD, VCR, Personal Video Recording, and Internet functionality into TV sets, set-top-boxes, and personal computers is a major goal.  
         [0007]     Serving as the RF front-end of broadband signals, the tuner receives available channels, selecting the desired channel and filtering out the others. The tuners, operating at frequencies from 40 to 900 MHz have different performance requirements than traditional TV tuners. Smaller form factors, low power consumption, high reliability and ease of manufacture are important concerns of such tuner applications.  
         [0008]     In a TV tuner, frequency conversion architecture is essential to a tuner design.  
         [0009]     In U.S. Pat. No. 5,737,035, Robert Rudolf Rotzoll et al. disclose a highly integrated double conversion television tuner on a single microcircuit, as shown in  FIG. 1 . The RF signal enters a TV tuner  100  from an antenna  402  (or cable, not shown) and passes a RF low-pass filter (RFLPF)  404  to limit the incoming band to below 900 MHz. The filtered RF signal is amplified up to 20 dB by a gain-controlled low-noise transconductance amplifier (LNTA)  406 .  
         [0010]     The output of a first local oscillator (LO 1 )  450 , operating between 1200 and 2100 MHz and determined by channel selection, is mixed in a first mixer (MIX 1 )  408  with the RF signal to generate a first IF video carrier frequency of 1200 MHz. The frequency of 1200 MHz is constant, irrespective of the TV channel selected. This approach leads to minimum distortion due to mixer images and harmonic mixing. The first IF is crudely filtered by the bandwidth limitation of the first mixer  408  to minimize harmonic effects.  
         [0011]     The first IF signal of 1200 MHz is mixed in a second mixer (MIX 2 )  410 , an image-rejection mixer, with the fixed 1180 MHz reference output of a second local oscillator (LO 2 )  412  to generate the second IF at 20 MHz visual carrier. Because the RF input signal is lower in frequency than the LO referenced, the mixing of the two signals results in a down conversion of the RF input.  
         [0012]     In such a TV tuner, however, the out-of-band channels must be removed by an external RF SAW (surface acoustic wave) filter, which necessitates a highly linear SAW driver consuming considerable power in the tuner chip. Further, the PLL (phase lock loop) circuit generating the oscillation signal for the first mixer operates at a high frequency, which results in spurious output of the first mixer.  
       SUMMARY OF THE INVENTION  
       [0013]     Embodiments of the invention provide a method for frequency conversion in a receiver. A signal having a radio frequency and carrying information on a selected channel is received and converted from the radio frequency to a first variable intermediate frequency determined by the selected channel. The signal is further converted from the first variable intermediate frequency to a second variable intermediate frequency determined by the selected channel. The signal is further converted from the second variable intermediate frequency to a constant baseband frequency.  
         [0014]     Embodiments of the invention further provide a receiver comprising an antenna, first, second, and third local oscillators, and first, second, and third mixers. The antenna receives an RF signal carrying information from a selected channel. The first local oscillator generates a first oscillating signal having a first frequency. The first mixer mixes the RF signal with the first oscillating signal to generate a first intermediate signal. The second local oscillator generates a second oscillating signal having a second frequency. The second mixer mixes the first intermediate signal with the second oscillating signal to generate a second intermediate signal. The third local oscillator generates a third oscillating signal having a third frequency. The third mixer mixes the second intermediate signal with the third oscillating signal to generate a baseband signal. The first, second, and third frequencies are determined by the selected channel.  
         [0015]     Embodiments of the invention provide another receiver comprising an antenna, first, second, and third local oscillators, and first, second, and third mixers. The antenna receives an RF signal carrying information from a selected channel. The first local oscillator generates a first oscillating signal having a first frequency. The first mixer mixes the RF signal with the first oscillating signal to generate a first intermediate signal. The second local oscillator generates a second oscillating signal having a second frequency. The second mixer mixes the first intermediate signal with the second oscillating signal to generate a second intermediate signal. The third local oscillator generates a third oscillating signal having a third frequency. The third mixer mixes the second intermediate signal with the third oscillating signal to generate a baseband signal. Frequencies of the first and second intermediate signals are variable and determined by the selected channel. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, given by way of illustration only and thus not intended to be limitative of the invention.  
         [0017]      FIG. 1  is a diagram of a conventional TV tuner.  
         [0018]      FIG. 2  is a diagram of a TV tuner according to an embodiment of the invention.  
         [0019]      FIG. 3  is a diagram of an oscillator in a TV tuner according to an embodiment of the invention.  
         [0020]      FIG. 4  is a diagram of a TV tuner according to another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]      FIG. 2  is a diagram of a TV tuner according to an embodiment of the invention. The TV tuner (receiver) includes an antenna  21  receiving an RF signal carrying information in all TV channels, a low noise amplifier  22  coupled to the antenna  21  to amplify the RF signal, a first local oscillator  23  generating a first oscillating signal OS 1  having a first frequency FO 1 , a first mixer  24  mixing the amplified RF signal with the first oscillating signal OS 1  to generate an intermediate signal IS, a second local oscillator  25  generating a second oscillating signal OS 2  having a second frequency FO 2 , a second mixer  26  mixing the intermediate signal IS with the second oscillating signal OS 2  to generate a baseband signal BS, and a SAW driver  27  coupled to an output of the second mixer  26  to drive an external SAW filter (not shown). Both frequencies of output oscillating signals OS 1  and OS 2  are controllable, determined by channel selection, as shown in  FIG. 2 .  
         [0022]     The first local oscillator  23  and mixer  24  form a first frequency conversion stage converting the RF signal from the radio frequency to a variable intermediate frequency IF determined by the selected TV channel. The frequency IF exceeds the radio frequency (up-conversion) and is determined to minimize noise and spurious signals coupled from the other channels into the selected channel. The value of the frequency IF is different for each channel. The second local oscillator  25  and mixer  26  form a second frequency conversion stage converting the signal from the frequency IF to a baseband frequency BF (down-conversion) fixed for all channels. The mixers  24  and  26  are image rejection mixers rejecting in-band noise from the image frequency. The out-of-band signals are rejected by the LC tanks (not shown) inside the mixers  24  and  26 .  
         [0023]     As first mixer  24  up converts the selected TV channel, IF (the frequency of intermediate signal IS) is equal to the sum of the frequency of the selected TV channel and frequency FO 1  of the first oscillating signal OS 1  from the first local oscillator  23 . Mixer  26  down converts intermediate signal IS, deducting the second frequency FO 2  of the second oscillating signal OS 2  from frequency IF and resulting in the baseband frequency BF. This frequency correlation, if the frequency of a selected TV channel is defined as TVF, can be expressed as formula (1): 
 
 BF=IF−FO 2=( FO 1+ TVF )− FO 2  (1) 
 
         [0024]     The last equal sign in formula (1) indicates that the frequency of a selected TV channel, which is variable, as selected by a user, is determined by FO 1  and FO 2 . As BF is a constant and IF is variable, determined by the selected TV channel, the first equal sign indicates that FO 2  is also variable, determined by the selected TV channel. In other words, to change a TV channel, both output frequencies, FO 1  and FO 2 , of the two local oscillators are changed, such that IF is also changed and BF remains constant.  
         [0025]      FIG. 3  is a diagram of the oscillator  23 . The oscillator includes a first frequency divider  231  dividing a frequency FR of a reference signal RS by a divisor N, a phase frequency detector  232  having a first input coupled to an output of the first frequency divider  231 , a charge pump  233  having an input coupled to an output of the phase frequency detector  232 , a loop filter  234  having an input coupled to an output of the charge pump  233 , a voltage controlled oscillator  235  having an input coupled to an output of the loop filter  234 , a second frequency divider  236  dividing a frequency of the signal output from the voltage controlled oscillator  235  by a divisor P and outputting the first oscillating signal OS 1 , and a frequency multiplier  237  multiplying the first oscillating signal OS 1  by a multiplicator M and having an output coupled to a second input of the phase frequency detector  232 . The divisors N and P, and the multiplicator M are determined by the selected channel. The frequency FO 1  of the first oscillating signal OS 1  is derived by: 
   FO 1 =FR*M /( P*N )  
         [0026]     The operation shown in  FIG. 3  is also applicable to the second local oscillator  25  of  FIG. 2 , the output frequency of which is variable and controllable, determined by channel selection.  
         [0027]      FIG. 4  is a diagram of a TV tuner according to another embodiment of the invention. The TV tuner applies triple conversion rather than double conversion (shown in  FIG. 2 ) to the RF signal. The TV tuner includes an antenna  41  receiving an RF signal carrying information from all TV channels, a low noise amplifier  42  coupled to the antenna  41  to amplify the RF signal, a first local oscillator  43  generating a first oscillating signal OS 1  having a first frequency FO 1 , a first mixer  44  mixing the amplified RF signal with the first oscillating signal OS 1  to generate a first intermediate signal IS 1 , a second local oscillator  45  generating a second oscillating signal OS 2  having a second frequency FO 2 , a second mixer  46  mixing the first intermediate signal IS 1  with the second oscillating signal OS 2  to generate a second intermediate signal IS 2 , a third local oscillator  47  generating a third oscillating signal OS 3  having a third frequency FO 3 , a third mixer  48  mixing the second intermediate signal IS 2  with the third oscillating signal OS 3  to generate a baseband signal BS, and a SAW driver  49  coupled to an output of the third mixer  48  to drive an external SAW filter (not shown). All frequencies of output oscillating signals OS 1 , OS 2  and OS 3  are variable and controllable, determined by channel selection.  
         [0028]     The first local oscillator  43  and mixer  44  form a first frequency conversion stage converting the RF signal from the radio frequency to a variable intermediate frequency IF 1  determined by the selected TV channel. The frequency IF 1  exceeds the radio frequency (up-conversion) and is determined to minimize noise and spurious signals coupled from the other channels into the selected channel. The value of the frequency IF 1  is different for each channel. The second local oscillator  45  and mixer  46  form a second frequency conversion stage converting the signal from the frequency IF 1  to a second intermediate frequency IF 2  (down-conversion) which is also variable, determined by the selected TV channel. The third local oscillator  47  and mixer  48  form a third frequency conversion stage converting the signal from the frequency IF 2  to a baseband frequency (down-conversion). The mixers  44 ,  46  and  48  are image rejection mixers rejecting in-band noise from the image frequency. The out-of-band signals are rejected by the LC tanks (not shown) inside the mixers  44 ,  46  and  48 .  
         [0029]     Each of the oscillators  43 ,  45  and  47  can be the same as that shown in  FIG. 3 . When a channel is selected, the divisors N and P, and the multiplicator M of each oscillator ( 43 ,  45  and  47 ) are simultaneously determined.  
         [0030]     It should be noted that the triple conversion tuner shown in  FIG. 4  achieves wide-to-narrow band conversion and down-conversion with two mixers  46  and  48 , while the double conversion tuner shown in  FIG. 2  accomplishes the same down-conversion with a single mixer  26 .  
         [0031]     In conclusion, the invention provides a TV tuner with fewer elements, lower power consumption, and high signal-to-noise ratio. In comparison with the TV tuner disclosed in U.S. Pat. No. 5,737,035, the TV tuner of the invention provides the advantage of no RF SAW filter, eliminating the need for a highly linear SAW driver and thus reducing power consumption.  
         [0032]     Variable intermediate frequencies IF 1  and IF 2  provides more flexibility in system design over TV tuners having constant intermediate frequencies IF 1  and IF 2 . Another benefit is the ability to avoid undesired in-band mixing products by adjusting IF 1  and IF 2 . In case the mixing products of internal local oscillators fall into the intermediate frequencies IF 1  and IF 2 , both can be changed to other frequencies to avoid the undesired mixing products.  
         [0033]     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.