Abstract:
A communications system comprises a local oscillator configured to generate a local oscillator output and a signal processing component coupled to the local oscillator. The signal processing component is configured to receive a clock signal and the clock signal is derived from the local oscillator output. A method of demodulating an input signal comprises deriving a conversion signal from a local oscillator output, deriving a clock signal from the local oscillator output, mixing the input signal with the conversion signal to generate an intermediate frequency signal, and processing the intermediate frequency signal using a signal processing component driven by the clock signal. A method of modulating an input signal comprise deriving a conversion signal from a local oscillator output, deriving a clock signal from the local oscillator output, processing the input signal using a signal processing component driven by the clock signal to generate an intermediate frequency signal and mixing the intermediate frequency signal with the conversion signal to generate a modulated signal.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to communication systems. More specifically, a communications system for transmitting or receiving signals is disclosed.  
       BACKGROUND OF THE INVENTION  
       [0002]     As integrated circuit (IC) technology advances, it has become feasible to implement many functions that were traditionally implemented using analog circuits in the digital domain. It is advantageous to implement more functions using digital circuits since it is generally easier to simplify and scale digital circuits than analog circuits. Some circuits such as transceivers used in communication systems often have both analog and digital modules. These types of digital and analog circuits are sometimes referred to as mixed mode or mixed signal circuits. Mixed mode transceivers typically include a reference frequency source such as a temperature compensated crystal oscillator, used to derive both the analog signal used for modulating/demodulating the input signal and to derive the system clock used for driving the digital circuit.  
         [0003]     Although implementing some of the functions in the digital domain simplifies the design, digital noise coupling may lead to performance degradation. For example, the harmonics of the digital clock frequency may appear in the desired signal band and cause interference. It would be useful if the effects of digital noise coupling can be reduced. Existing mixed signal circuit design has additional limitations. For example, in receiver circuits that employ a first stage analog IF demodulation and a second stage digital baseband demodulation, a digital local oscillator (LO) signal is generated at the IF frequency to demodulate the signal to baseband. In existing mixed mode transceivers, the choices of intermediate frequencies are often constrained due to limitations in the digital LO signals that may be conveniently generated. In these transceiver circuits, the IF signal is typically only adjustable among frequencies that correspond to available digital LO frequencies. The limited selection of intermediate frequencies may be undesirable. It may be useful sometimes to select among many available intermediate frequencies to improve image rejection or avoid interference from a digital clock driving digital circuitry on the chip or other source, or to otherwise avoid feed through of noise. It would be desirable to have a way of generating a variable intermediate frequency with small frequency increments.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]     Various embodiments of the invention are disclosed in the following detailed description and the accompanying drawings.  
         [0005]      FIG. 1  is a block diagram illustrating a mixed mode receiver circuit.  
         [0006]      FIG. 2A  is a block diagram illustrating a receiver embodiment.  
         [0007]      FIG. 2B  is a block diagram illustrating a transmitter embodiment.  
         [0008]      FIG. 3  is a block diagram illustrating another receiver example according to some embodiments.  
     
    
     DETAILED DESCRIPTION  
       [0009]     The invention can be implemented in numerous ways, including as a process, an apparatus, a system, a composition of matter, a computer readable medium such as a computer readable storage medium or a computer network wherein program instructions are sent over optical or electronic communication links. In this specification, these implementations, or any other form that the invention may take, may be referred to as techniques. In general, the order of the steps of disclosed processes may be altered within the scope of the invention.  
         [0010]     A detailed description of one or more embodiments of the invention is provided below along with accompanying figures that illustrate the principles of the invention. The invention is described in connection with such embodiments, but the invention is not limited to any embodiment. The scope of the invention is limited only by the claims and the invention encompasses numerous alternatives, modifications and equivalents. Numerous specific details are set forth in the following description in order to provide a thorough understanding of the invention. These details are provided for the purpose of example and the invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in detail so that the invention is not unnecessarily obscured.  
         [0011]     A communications system is disclosed. In some embodiments, a local oscillator is configured to generate an output that is used to derive a clock signal of a signal processing component. The local oscillator output may also be used to derive a conversion signal used for modulation or demodulation. In some embodiments, the clock signal and the conversion signals change in step. In some embodiments, the local oscillator uses a fractional N phase locked loop to provide an IF signal that can be fine tuned.  
         [0012]      FIG. 1  is a block diagram illustrating a mixed mode receiver circuit. In this example, wireless receiver  100  includes an antenna  102  for receiving the transmitted signal and sending the signal to be amplified by a low noise amplifier (LNA)  104 . The amplified signal from low noise amplifier  104  is down converted by analog mixer  106  by mixing with a conversion signal f C . To supply f C , the output of local oscillator  120  f VCO  is divided by an integer N via divider  122 . Local oscillator  120  includes a phase locked loop (PLL) capable of generating signals at different frequencies. The input reference signal of local oscillator  120 , f ref , is generated by a TCXO or any other appropriate source.  
         [0013]     The down converted IF signal is sent to a filter  108  and the filtered output is sent to digital module  111 . The filtered output is converted to a digital signal by ADC  110 . A digital mixer  112  combines the output of ADC  110  and a digital LO signal generated as a sine wave f sin  to produce a down converted signal, which is then converted to a baseband (zero-IF) analog signal by digital to analog converter (DAC)  114 . The digital sine wave is generated by a sin/cos coefficient table  116  that is clocked by a digital clock signal  119 . The sin/cos coefficient table shown in this example is stored in read only memory (ROM). Since the cost of implementing the ROM table is proportional to the number of entries in the table, it is desirable to keep the table small. To generate digital clock signal  119 , reference frequency f ref  is divided by an integer P via divider  118 .  
         [0014]     The frequency of the digital sine wave can be expressed as the following:  
                 f   sin     =       f   ref     LP       ,           (   1   )             
 
 where f sin  is the frequency of the digital sine wave, f ref  is the frequency of the system clock, L is the number of digital samples per period of the digital sine wave, and P is the clock division ratio for the digital module. For example, assuming that f ref  is the same as the standard reference clock frequency of global system for mobile communications (GSM), which is 13 MHz; also assuming that there are 32 digital samples per period and P is set to 4, then  
               f   sin     =         13   ⁢           ⁢   MHz       4   ×   32       =     101.5625   ⁢           ⁢     kHz   .                 (   2   )             
 
         [0015]     To obtain a baseband signal centered at DC, the IF frequency of the analog component (f IF ) should be equal to the frequency of the digital module (f sin ). To vary the IF frequency and still provide zero IF output, P or L or both will change. Since it is desirable to keep the number of entries in the coefficient table small, changing L is impractical as the change will require additional entries in the coefficient table for each possible IF frequency. The value of L is fixed so that only one set of entries is required. Since the value of P is relatively small, any change in P will result in significant changes in f sin  and f IF . For example, in equation 2 shown above, changing P from 4 to 3 leads to a 30% change in f IF . Since the selection of frequencies for f IF  is limited, it may not be possible to vary f IF  in a manner desired to improve image rejection or avoid noise feed through.  
         [0016]      FIG. 2A  is a block diagram illustrating a receiver embodiment that allows a variable intermediate frequency signal to be generated. In this example, receiver  200  derives both the analog signal for down conversion and the digital clock for the digital module from the local oscillator output. Receiver  200  includes an antenna  202  that receives the transmitted signal. The output of antenna  202  is amplified by an LNA  204 , and then down converted to an intermediate frequency signal f IF  by mixing with an analog conversion signal f C  via mixer  206 . The IF signal is filtered by filter  208 , and the filtered signal is sent to a signal processing component  210  to be further processed and down converted to baseband. The signal processing component may include a digital module. The reference frequency f ref  is sent to local oscillator  220 , whose output is divided by N via divider  222  to produce analog conversion signal f C  and divided by M via  218  to produce digital clock f D . As will be shown in more details below, deriving both the analog IF signal and the digital clock from the local oscillator output increases the number of possible choices for f C  and helps reduce digital noise coupling.  
         [0017]     The technique of deriving a digital system clock from the local oscillator is also applicable to transmitters.  FIG. 2B  is a block diagram illustrating a transmitter embodiment in which the digital clock is derived from the local oscillator output. In this example, transmitter  250  includes a signal processing component  252 , which processes an input for transmission. The analog input is sent to ADC  254  to be converted to digital. DSP  256  processes the digital signal and performs functions such as digital modulation, filtering, etc. The output of DSP  256  is sent to DAC  258  to be converted back to analog and then filtered by a filter  260 . The output of filter  260  is an intermediate frequency signal. Mixer  262  modulates the IF signal with a conversion signal f C  to generate a modulated signal, which is sent to a power amplifier  264 . The output of power amplifier  264  is transmitted via antenna  272 . Reference frequency f ref  is sent to local oscillator  268 , which is configured to provide an output signal that is divided by N via divider  270  to supply the digital system clock f D . The same output signal of the oscillator is divided by M via divider  266  to supply the conversion signal f C .  
         [0018]      FIG. 3  is a block diagram illustrating another receiver example according to some embodiments. In this example, the transmitted signal is received by antenna  302  of receiver  300  and then sent to LNA  304 . Analog mixer  306  down converts the amplified signal from LNA  304  by mixing it with a conversion signal f C , which is generated by dividing the output of local oscillator  320  by N via divider  322 . In some embodiments, the local oscillator includes a fractional N phase locked loop (PLL) that is capable of synthesizing a range of output signals at relatively small frequency increments. Thus, it is possible to fine tune the frequency of f C  and vary the frequency of f IF  with fine granularity to improve the receiver&#39;s image rejection ratio and noise characteristics, as well as to achieve better frequency planning.  
         [0019]     The down converted IF signal is sent to a filter  308 , which sends its output to down converter  310 . In this example, down converter  310  performs down conversion in the digital domain. The filtered output is converted to a digital signal by ADC  312 . A digital mixer  314  down converts the output of ADC  312  to baseband by mixing it with a digital sine wave f sin . The baseband digital signal is then converted to analog by digital to analog converter (DAC)  316 . To generate f sin , the output of local oscillator  320  is divided by M via divider  324 . A look-up table  318  generates samples of a sine wave using this clock.  
         [0020]     In this example, both f sin  and f C  are derived from the output of local oscillator  320 . The relationship between various signals may be expressed as the following: 
 
 f   IF   =f   in   −f   C   (3), 
 
 where f in  is the input signal frequency; and  
                 f   sin     =       Nf   C     ML       ,           (   4   )             
 
 where L is the number of digital samples per period of f sin . 
 
         [0021]     Setting f IF =f in  and solving for f C  results in  
               f   C     =         f   in       1   +     N   ML         .             (   5   )             
 
         [0022]     Substituting (5) into (3) results in  
               f   IF     =       f   in     ⁢         N   ML       1   +     N   ML         .               (   6   )             
 
         [0023]     The frequency of f IF  may be controlled by changing the value of divider M. In many communications applications, the frequency difference between f C  and f sin  is large, thus M can be chosen to be a relatively large value such that a small change in M leads to a small change in f IF . For example, assuming that f in =935 MHz, N=2, L=32 and M=584, the resulting f IF  is equal to 100 kHz. Incrementing or decrementing M by 1 results in less than 0.2% change in the frequency of f IF , allowing the frequency to be tuned on a fine scale. In some embodiments, f sin  may be indirectly derived from the output of local oscillator  320 . For example, the input to divider  324  may be f C  rather than the local oscillator output.  
         [0024]     In some embodiments, the frequency of f IF  is tuned before the transceiver begins its operations. For example, the f IF  frequency of a transceiver used in a cellular phone may be calibrated at the factory based on test measurements. In some embodiments, the f IF  frequency is adjusted during the transceiver&#39;s operation. For example, when a cell phone is switched on, if improved image rejection is deemed necessary or if it is determined that there is excess noise feed through due to signal harmonics, the f IF  of a cell phone transmitter may be tuned to improve the image rejection ratio or noise characteristics or both.  
         [0025]     In the examples shown, the frequencies of the conversion signal and the digital signal track each other. In other words, when the local oscillator output changes, both f C  and f D  change proportionally. This also prevents harmonics of digital clock from falling into the desired signal band of the input. The harmonics of digital noise are at integer multiples of digital clock frequency f D . Since f C  and f D  track each other, there is a relatively stable and predictable relation between f C  and digital noise harmonics. Therefore, it is possible to choose an IF frequency, f IF , that keeps harmonics of digital noise away from f in , which can be expressed as f C ±f IF . The following example shows how to choose a proper f IF  in the system shown in  FIG. 2A , according to some embodiments. Two harmonics of f D  closest to f C  satisfy the following relation: 
 
 nf   D   ≦f   C &lt;( n+ 1) f   D   (7) 
 
 In other words, for some integer number n, the n-th harmonic is the closest harmonic of f D  below or at f C  and the (n+1)-th harmonic is above f C . Dividing this equation by f D  gives: 
 
 n≦f   C   /f   D   &lt;n+ 1  (8) 
 
 This equation can also be written as: 
 
 n =floor[ f   C   /f   D ]  (9) 
 
 From  FIG. 2A :  
               f   D     =       f   LO     M             (   10   )                 f   C     =       f   LO     N             (   11   )             
 
 f   C   /f   D   =M/N=n+r/N  (0≦r&lt;N)  (12) 
 
 Where n is the integer part of the division of f C  by f D  as in Equation (9) and r/N is the fractional part. Applying (7), (9), and (12), the distance between the n-th harmonic and f C , Δ L , is: 
 
Δ L   =f   C   −nf   D   (13) 
 
Δ L   =M/Nf   D   −nf   D   (14) 
 
Δ L =( n+r/N ) f   D   −nf   D   (15) 
 
Δ L   =r/Nf   D   (16) 
 
 Similarly, the distance between (n+1)-th harmonic and f C , Δ H , is: 
 
Δ H =( n+ 1) f   D   −f   C   (17) 
 
Δ H =( n+ 1) f   D   −M/Nf   D   (18) 
 
Δ H =( n+ 1) f   D −( n+r/N ) f   D   (19) 
 
Δ H =(1− r/N ) f   D   (20) 
 
 Since r is limited to values in the range 0-N−1, the possible values of Δ L  and Δ H  are 0, f D /N, 2f D /N, . . . , (N−1)f D /N. This means that the closest two harmonics of f D  are located at f C , f C ±f D /N, fc±2f D /N, etc. With a properly chosen IF frequency, the desired channel at f in  does not substantially coincide with the harmonic locations, thus interference from the digital noise is avoided. For example, f in  may be kept between possible harmonic locations, f C  and f C +f D /N: 
 
 f   C   &lt;f   in   =f   C   +f   IF   &lt;f   C   +f   D   /N   (21) 
 
0 &lt;f   IF   &lt;f   D   /N   (22) 
 
 The digital clock frequency f D  varies as f LO , and M changes. However, this does not necessarily affect system performance because f IF  can be chosen to keep the desired channel away from possible harmonics even with varying f D . For example, if f D  varies from 10 MHz to 20 Mhz (100% variation), and N is 2, choosing f IF  to be less than 5 MHz would guarantee that the harmonics of f D  do not substantially coincide with the desired channel to cause interference. 
 
         [0026]     A technique for generating a variable intermediate frequency signal in a communications system and eliminating noise coupling been disclosed. Although the examples shown above discuss in detail the operations of transceivers used in GSM systems, the technique is also applicable for other standards and frequency ranges.  
         [0027]     Although the foregoing embodiments have been described in some detail for purposes of clarity of understanding, the invention is not limited to the details provided. There are many alternative ways of implementing the invention. The disclosed embodiments are illustrative and not restrictive.