Abstract:
Systems and methods for provided for reducing interference caused by leakage of signals generated by a spread spectrum phase lock loop (SS PLL). Output of SS PLL may be processed to reduce interference. For example, a sinusoidal spreading signal may be used to spread the output of the SS PLL. A notch filter tracks the frequency of the output of the SS PLL to steer the notch in the filter to the instantaneous frequency output from the SS PLL, thus allowing the notch filter to be placed in the path of signals that have unwanted leakage from the SS PLL.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority benefit of U.S. Provisional Patent Application Ser. No. 61/982,827, filed Apr. 22, 2014 and entitled “Method and Apparatus for Spreading Energy of a Phase Lock Loop”, which is herein incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The disclosed method and apparatus relate to phase lock loops and more particular to a method and apparatus for cancelling interference caused by leakage from a spread spectrum phase lock loop. 
       BACKGROUND 
       [0003]    Phase lock loops are commonly used in many devices today. Phase lock loops generate a signal having a desired frequency. Accordingly, receivers require phase lock loops to generate signals on which information is modulated. Furthermore, circuits that have timing requirements require clocks that are generated by phase lock loops. 
         [0004]    One problem that arises when phase lock loops are used is that spurious signals are generated by the phase lock loop and coupled to other nearby circuits. Such spurious signals interfere with the operation of the other circuits. One example of a system in which spurious signals are induced in circuits that lie relatively close to a phase lock loop is a transceiver having a compactly integrated circuit. In such cases, it is common for phase lock loops to generate energy that is induced into the transmit and receive path of the transceiver. In particular, the receive path of a transceiver is vulnerable to interference from energy that leaks from phase lock loops to the receive path. 
         [0005]    One way to deal with the interference from phase lock loops is to spread the energy generated in the output of the phase lock loop over a relatively wide frequency spectrum. By spreading the energy across a broadband frequency spectrum, the amount of energy that is present at narrower frequency bands, such as the frequency at which the transceiver receives signals transmitted by other transceivers, is reduced. In order to evenly distribute the frequency of the phase lock loop over a broad spectrum of frequencies, a sawtooth or triangular spreading signal is used as a spreading signal. That is, a signal that ramps at a constant rate can be used to spread the signal output from the phase lock loop evenly over a relatively broad frequency spectrum. 
         [0006]      FIG. 1  is an illustration of a spread spectrum phase lock loop  100 . A frequency source  102  provides a reference for the operation of the phase lock loop  100 . The frequency source  102  is typically a crystal oscillator or other such stable frequency source. The frequency source  102  is coupled to a phase detector  104 . The phase detector  104  has an output that represents the difference in phase between the output of a divider  106  and the frequency source  102 . The output of the phase detector  104  is a control voltage that is coupled to a low pass filter  108 . The low pass filter  108  is designed to provide the phase lock loop with a sufficiently fast response time that the loop can converge, but not so fast that the loop will overshoot and go into oscillation. The output of the low pass filter  308  is coupled to a voltage controlled oscillator (VCO)  110 . The output of the low pass filter  108  attempts to steer the output frequency of the VCO  110  to a frequency that will cause the error signal from the phase detector  104  to be zero. Accordingly, the VCO  110  outputs a frequency that is N times the frequency source, where N is the value by which the divider  106  divides the output of the VCO  110  before providing a signal to the phase detector  104 . The output of the VCO  100  is coupled to the output port of the phase lock loop  100 . In addition, the output of the divider is fed back to the phase detector  104  to allow the phase detector  104  to produce the control voltage to the VCO  110 . 
         [0007]    In a spread spectrum phase lock loop such as that shown in  FIG. 1 , the divider  106  is programmable with a variable value that is input from a triangle wave generator  112 . The triangle wave generator  112  loads the divider with a value N that increases in even steps up from a minimum value to a maximum value and then back down again in equal steps. Accordingly, the output of the phase lock loop  100  will start at a frequency that is N times the frequency of the source  102  and increases in frequency with increases in the value of N until it hits the maximum frequency. The frequency output from the phase lock loop  100  then decreases from the maximum to the minimum in equal frequency steps. By making the steps equal the frequency moves smoothly across the frequency spectrum spreading the energy equally over the frequency spectrum. 
         [0008]    In some systems that are particularly sensitive to the need to reduce the interference from internal phase lock loops, simply spreading the energy generated by the phase lock loop is not sufficient to mitigate the interference caused by the phase lock loop. In these cases, it is desirable to also provide a filter that can further reduce the power of the interfering spurious signals generated as a byproduct of the phase lock loop. Since the frequency generated by the phase lock loop is spread over a relatively broad frequency spectrum, it is not possible to simply put the interference laden received signal through a filter. Such a filter would impede the passage of the desired received signals as well as the interfering signals generated by the phase lock loop. 
         [0009]    Therefore, there is a need for a method and apparatus that can generate frequencies in a way that will not interfere with the operation of circuits near the phase lock loop. 
       SUMMARY 
       [0010]    Various embodiments of the disclosed method and apparatus for reducing interference generated by leakage of a nearby phase lock loop are presented. In one embodiment, a signal is generated that allows a tunable notch filter to track the output of the phase lock loop creating the interference. Some of these embodiments are directed toward systems and methods for spreading the output of a phase lock loop with a sinusoidal spreading signal. The use of a sinusoidal spreading signal allows the output frequency of the phase lock loop to be easily tracked, despite the characteristics of a low pass filter within a phase lock loop. Because the frequency of the signal at the output of the phase lock loop can be easily tracked, a tunable notch filter can be used to reduce interference in sensitive circuits, such as the receive chain within a transceiver. The tunable notch filter is provided with the frequency of the sinusoidal signal used to spread the output of the phase lock loop. The tunable notch filter can then track the output frequency of the phase lock loop, reducing the interference generated by the signals output from the phase lock loop. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The disclosed method and apparatus, in accordance with one or more various embodiments, is described with reference to the following figures. The drawings are provided for purposes of illustration only and merely depict examples of some embodiments of the disclosed method and apparatus. These drawings are provided to facilitate the reader&#39;s understanding of the disclosed method and apparatus. They should not be considered to limit the breadth, scope, or applicability of the claimed invention. It should be noted that for clarity and ease of illustration these drawings are not necessarily made to scale. 
           [0012]      FIG. 1  is an illustration of a phase lock loop. 
           [0013]      FIG. 2  is a simplified block diagram of a transceiver in accordance with one embodiment of the disclosed method and apparatus. 
           [0014]      FIG. 3  is a simplified block diagram of a phase lock loop in accordance with one embodiment of the disclosed method and apparatus. 
           [0015]      FIG. 4  illustrates the use of a tunable notch filter that is tuned to remove spurious signals generated by the phase lock loop and coupled onto another circuit, such as the receive chain of a transceiver. 
       
    
    
       [0016]    The figures are not intended to be exhaustive or to limit the claimed invention to the precise form disclosed. It should be understood that the disclosed method and apparatus can be practiced with modification and alteration, and that the invention should be limited only by the claims and the equivalents thereof. 
       DETAILED DESCRIPTION 
       [0017]      FIG. 2  is a simplified block diagram of a transceiver  200  in accordance with one embodiment of the disclosed method and apparatus. In one embodiment of the disclosed method and apparatus, the transceiver is used in a set top box. In one such embodiment, the set top box is capable of communicating in accordance with the well-known Multi-media over Coax Alliance (MoCA) standard for broadband communications over coaxial cable. The transceiver has a MoCA transmit chain that includes a digital core  202 , a undesired signal mitigation digital signal processor (DSP)  206 , a digital to analog converter (DAC)  207 , a filter  209  and a power amplifier  211 . A transmit/receive switch  213  allows selection between the transmit chain and a MoCA front end section  215 . Signals are received in a diplexer  217 . The signals are coupled from the diplexer  217  to a MoCA front end section  215  comprising an INA  221 , filter  223  and ADC  25 . The output from the front end section  215  is coupled to a undesired signals mitigation digital signal processor (DSP)  206 . The diplexer  217  is also coupled to a satellite receive front end  219 . The DSP  206  reduces the interference of spurious signals that are coupled by leakage from internal phase lock loops onto the received signals. The output from the DSP  206  is provided to a digital core  202  that communicates with external devices over one or more protocols or network interfaces, such as High-Definition Multimedia Interface (HDMI), Universal Serial Bus (USB), Ethernet, Double Data Rate (DDR), Flash, Reduced Gigabit Media Independent Interface (RGMII), etc. Those skilled in the art will understand that the digital core is essentially a conventional component of a transceiver. 
         [0018]    A spread spectrum phase lock loop (SS PLL)  210  generates a spread spectrum signal  212 . In one embodiment of the disclosed method and apparatus, the spread spectrum signal  212  is used for several purposes throughout the transceiver  200 . In one embodiment, the signal  212  is coupled to circuits within the digital core  202 . Use of such SS PLLs are well known. When such SS PLLs  210  are used in a compactly integrated circuit, the spread spectrum PLL&#39;s tone can easily leak into other parts of the system, such as circuits in the receive path. Such leakage can interfere with these other circuits. The SS PLL  210  provides frequency spreading information on a frequency spreading signal  214  to the DSP  206 . Accordingly, the DSP  206  can use the information regarding the frequency spreading to tune a notch filter  402  see  FIG. 4 ) to track, and thus reject, the tone leaked from the SS PLL  210 . 
         [0019]      FIG. 3  is a simplified block diagram of a phase lock loop  210  in accordance with embodiment of the disclosed method and apparatus. The phase lock loop  210  operates similarly to the phase lock loop  100  described above. However, a sinusoidal wave generator  312  provides the waveform that is used to spread the frequency of the phase lock loop  210  across the frequency spectrum. The output of the sinusoidal wave generator  312  is coupled to a program input to a programmable divider  306 . Accordingly, the signal applied to the signal input of the divider  306  from the output of a VCO  310  will be divided by an amount that varies in response to the sinusoidal wave input to the divider  306  by the generator  312 . By spreading the output of the phase lock loop using a sinusoidal wave generator  312 , the frequency variations of the phase lock loop  210  can be tracked. That is, the effect of the low pass filter  308  on the sinusoid that is generated by a phase detector  304  will be predictable, since the low pass filter  308  has a relatively predictable response to a sinusoid. The output of the VCO  310  is: 
         [0000]      e j2πf(t)   Equation 1:
 
         [0020]    The ideal spreading function uniformly distributes the frequency over the frequency band of interest: 
         [0000]        f ( t )=[( t  mod  W )− W/ 2]+ f   c ;  Equation 2:
 
         [0000]    where f c  is the center frequency of the spreading bandwidth. 
         [0021]    The precise impulse response of the low pass filter  308  h(t) to the signal f(t) of Equation 1 is difficult to estimate. The actual output frequency of the VCO  310  after spreading with the spreading function f(t) is: 
         [0000]      e j2πh(t)*f(t) ;  Equation 3:
 
         [0000]    where * is the convolution operation. 
         [0022]    Small ripples or group delay in the filter response can create an undesirable discrepancy between f(t) and h(t)*f(t). By spreading the output of the phase lock loop  210  with a sinusoidal function, the output of the phase lock loop  210  is more predictable. For example, if: 
         [0000]        f ( t )= A  cos( w×t )+ f   c ;  Equation 4:
 
         [0000]    then h(t)*f(t) can be modeled as: 
         [0000]        h ( t )* f ( t )= A ′ Cos( w×t +Θ)+ f   c   Equation 5:
 
         [0023]    Therefore, an estimate of the amplitude and phase can be made to allow a tunable notch filter to track the signal output from the phase lock loop  210  and thus attenuate any energy coupled from the phase lock loop  210  to other circuits. 
         [0024]    In one embodiment of the disclosed method and apparatus, the SS PLL  210  is implemented by the DSP  206 . However, in an alternative embodiment, the SS PLL  210  is implemented in discrete functional components within the transceiver  200 . It will be clear to those skilled in the art that any combination of the DSP  206 , a second DSP and hardware used to implement the components of the SS PPL are possible and within the scope of the disclosed method and apparatus. 
         [0025]      FIG. 4  illustrates the use of a tunable notch filter  402  that is tuned to remove spurious signals generated by the phase lock loop  210  and coupled onto another circuit, such as the receive chain of a transceiver  200 . In one embodiment of the disclosed method and apparatus, the tunable notch filter  402  is implemented within the DSP  206 . As noted above with respect to the SS PPL  210 , the notch filter  402  can be implemented alternatively using a second DSP (not shown), or discrete hardware (not shown) within the transceiver  200 . 
         [0026]    The phase lock loop  210  provides frequency information  404  to a control input of the tunable notch filter  402 . The frequency information provides the tunable notch filter  402  with the frequency of the sinusoidal wave that is used to spread the output of the phase lock loop  210 . The frequency information is used to steer the frequency of the notch to the frequency of the signal output by the phase lock loop  210 . Since the sinusoidal wave used to spread the frequency output of the phase lock loop is relatively undistorted by the low pass filter  308  (See  FIG. 3 ) of the phase lock loop  210 , the tunable notch filter  402  can accurately track the frequency output from the phase lock loop  210  based on the frequency information  404 . Accordingly, the notch in the filter will track the frequency of the spurious signal to be removed. The fact that the notch filter  402  tracks the signal to be cancelled means that the notch filter  402  can be made narrow. This reduces the amount of desirable in-band energy that is attenuated by the notch filter  402 , while at the same time maximizing the amount of interfering energy that is attenuated. 
         [0027]    The phase of the signal output from the phase lock loop  210  must be synchronized with the tuning of the notch filter  402 . This is accomplished by observing the output of the filter to detect when the spurious signal to be cancelled is at the same frequency as the notch in the notch filter  402 . Once the notch in the notch filter  402  and the spurious signal are at the same frequency, the notch filter  402  will track at the frequency indicated by the information signal from the phase lock loop  210 . In accordance with one embodiment, the DSP  206  can analyze the output from the notch filter  402  to detect whether the spurious signal generated from leakage of the SS PLL  210  is present. If present, the synchronization of the notch filter  402  can be adjusted by an incremental amount. This process can be repeated iteratively until the proper alignment between the notch filter  402  and the SS PLL  210  is achieved as determined by a reduction in the amount of interference at the SS PLL frequency. 
         [0028]    By using a sinusoidal spreading signal to spread the SS PLL  210 , the notch filter  402  can accurately track the output of the SS PLL  210  and thus cancel any leakage that is coupled from the SS PLL  210  to other circuits, in accordance with the disclosed method and apparatus, such accurate tracking only requires the frequency used by the SS PLL  210  to spread the signal be provided to the notch filter  402 . In one embodiment of the disclosed method and apparatus, a clock  406  is used to control the tracking of the notch filter  402 . The output of the clock  406  is synchronized with the same source as the clock used by the sinusoidal wave generator  312  to generate the spreading signal used to spread the SS PLL output. Therefore, the clock  406  used to control the tracking of the notch filter  402  will be phase coherent with the sinusoidal spreading signal used to spread the SS PLL output. Nonetheless, some adjustment to align the filter notch with the interfering signal may be necessary. In one embodiment, the sinusoidal signal output from the sinusoidal wave generator  312  is a series of digital values that each represent the amplitude of a sinusoidal waveform. 
         [0029]    Although the disclosed method and apparatus is described above in terms of various examples of embodiments and implementations, it should be understood that the particular features, aspects and functionality described in one or more of the individual embodiments are not limited in their applicability to the particular embodiment with which they are described. Thus, the breadth and scope of the claimed invention should not be limited by any of the examples provided in describing the above disclosed embodiments. 
         [0030]    Terms and phrases used in this document, and variations thereof, unless otherwise expressly stated, should be construed as open ended as opposed to limiting. As examples of the foregoing: the term “including” should be read as meaning “including, without limitation” or the like; the term “example” is used to provide examples of instances of the item in discussion, not an exhaustive or limiting list thereof; the terms “a” or “an” should be read as meaning “at least one,” “one or more” or the like; and adjectives such as “conventional,” “traditional,” “normal,” “standard,” “known” and terms of similar meaning should not be construed as limiting the item described to a given time period or to an item available as of a given time, but instead should be read to encompass conventional, traditional, normal, or standard technologies that may be available or known now or at any time in the future. Likewise, where this document refers to technologies that would be apparent or known to one of ordinary skill in the art, such technologies encompass those apparent or known to the skilled artisan now or at any time in the future. 
         [0031]    A group of items linked with the conjunction “and” should not be read as requiring that each and every one of those items be present in the grouping, but rather should be read as “and/or” unless expressly stated otherwise. Similarly, a group of items linked with the conjunction “or” should not be read as requiring mutual exclusivity among that group, but rather should also be read as “and/or” unless expressly stated otherwise. Furthermore, although items, elements or components of the disclosed method and apparatus may be described or claimed in the singular, the plural is contemplated to be within the scope thereof unless limitation to the singular is explicitly stated. 
         [0032]    The presence of broadening words and phrases such as “one or more,” “at least,” “but not limited to” or other like phrases in some instances shall not be read to mean that the narrower case is intended or required in instances where such broadening phrases may be absent. The use of the term “module” does not imply that the components or functionality described or claimed as part of the module are all configured in a common package. Indeed, any or all of the various components of a module, whether control logic or other components, can be combined in a single package or separately maintained and can further be distributed in multiple groupings or packages or across multiple locations. 
         [0033]    Additionally, the various embodiments set forth herein are described with the aid of block diagrams, flow charts and other illustrations. As will become apparent to one of ordinary skill in the art after reading this document, the illustrated embodiments and their various alternatives can be implemented without confinement to the illustrated examples. For example, block diagrams and their accompanying description should not be construed as mandating a particular architecture or configuration.