Patent Publication Number: US-6711694-B1

Title: Apparatus and method for generating a modulated clock signal including harmonics that exhibit a known sideband configuration

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention generally relates to digital circuits and, in particular, to an apparatus and method for generating a modulated clock signal including harmonics that exhibit a known sideband configuration. 
     2. Description of Related Art 
     From the early days of electronic components, size has been an important factor in component design. With the ever increasing integration of electronics into everyday life, component size has become a critical aspect of the design of electronic devices. Although the public is demanding more compact electronics, it is not willing to give up features that add size to those electronics. In fact, the demand for more compact electronic devices is only rivaled by the demand for more features in those devices. Electronic component designers are thus forced to miniaturize electronics while they integrate more features and more components into those electronics. 
     In no field is miniaturization and integration more important than in the field of circuit design. It is beneficial for a variety of reasons to be able to miniaturize circuits and to place more components in those circuits. Such miniaturization and integration results in, for example, a reduction in manufacturing costs, a reduction in overall device size, and the provision of more features in the same size device. For many electrical devices, especially wireless mobile terminals, it is equally beneficial to integrate digital and analog components. However, the integration of analog and digital components, when combined with ever increasing miniaturization, leads to electromagnetic compatibility (EMC) problems. These EMC problems need to be solved so that highly-integrated systems of digital and analog circuitry can work reliably. For example, in wireless mobile terminals, high-frequency analog signals are handled near digital circuitry which can easily disrupt the high-frequency analog signals and cause degraded receiver performance or spectrum mask violations of transmitted signals. 
     Presently, EMC issues arising from the integration of digital circuitry and analog circuitry are addressed in many different ways—none of which are wholly satisfactory. Particularly unsatisfactory is the existing digital technology associated with the clock signal which can cause the aforementioned problems if coupled into sensitive analog circuitry. For instance, the harmonic components of the clock signal can fall on a pass band of a radio-frequency (RF) signal and cause interference if that RF signal is coupled to sensitive analog circuitry. Therefore, it is important to control the shape or configuration of the harmonic components of the clock signal. 
     One traditional way of modifying the configuration of the harmonic components of the digital clock signal is by phase-modulating the digital clock signal. Reference is made to U.S. Pat. Nos. 5,426,392 and 5,731,728 for descriptions of prior art methods associated with phase modulation of clock signals. These traditional phase-modulation methods have drawbacks. In some methods, analog elements and noise sources are used for phase modulation; this practice makes the sideband levels difficult to accurately predict and also complicates testing. In other methods, the digital hardware used to generate the phase-modulated signal is clocked at a very high frequency, which causes high power dissipation in CMOS (Complementary Metal-Oxide Semiconductor) implementations. All the methods referred to are difficult to apply to partial designs, since the phase relationship between signals in different parts would not be constant. Therefore, there is a need for an apparatus and a method capable of generating a phase-modulated clock signal cheaply and predictably, which would also be possible to use on partial designs. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention is an apparatus and method that helps address the electromagnetic compatibility issues arising from the integration of digital circuitry and analog circuitry within a wireless mobile terminal (for example). Basically, the apparatus includes a clock generator capable of generating an unmodulated clock signal and a modulator capable of generating a modulated clock signal having selected clock pulses omitted from the unmodulated clock signal. The modulated clock signal includes a fundamental frequency and a plurality of harmonic frequencies that exhibit a known sideband configuration. The known sideband configuration can be selected such that there would be less harmonic content in a band where co-located analog circuitry could be adversely affected by electro-magnetic interference from the modulated clock signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the method and apparatus of the present invention may, be had by reference to the following detailed description when taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 illustrates exemplary signal diagrams of unmodulated clock signals and a modulated clock signal; 
     FIG. 2 is a graph illustrating the differences between the harmonic content of an unmodulated clock signal and the harmonic content of a modulated clock signal of the same effective frequency as the unmodulated clock signal; 
     FIG. 3 is a flowchart illustrating the steps of a preferred method of the present invention; 
     FIG. 4 is a graph illustrating exemplary sideband configurations of two test modulated clock signals as determined by step  304  of the method shown in FIG. 3; 
     FIG. 5 is a simplified diagram illustrating a wireless mobile terminal incorporating an apparatus of the present invention; 
     FIG. 6 is a block diagram illustrating in greater detail a first embodiment of the apparatus shown in FIG. 5; and 
     FIG. 7 is a block diagram illustrating in greater detail a second embodiment of the apparatus shown in FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     To better present and describe the present invention a detailed description of the preferred method  300  and exemplary apparatus  510  of the present invention is deferred pending a brief discussion about unmodulated clock signals, modulated clock signals and the differences between the harmonic contents of the unmodulated clock signal and the modulated clock signal. 
     Referring to FIG. 1, there are illustrated exemplary signal diagrams of an unmodulated clock signal  110  and a modulated clock signal  120 . The modulated (dithered) clock signal  120  can be described as a clock signal which has one or more of every M clock pulses removed (where M is a constant number, after which the process repeats). In contrast, the unmodulated (undithered) clock signal  110  can be described as a clock signal which has no clock pulses removed. An example, of an unmodulated clock signal  110  and a modulated clock signal  120  where the fifth and tenth clock pulses have been removed are shown in FIG.  1 . 
     The modulated clock signal  120  may be mathematically derived by adding a time-varying phase φ(t) to a time base of a unmodulated clock signal  130  having the same effective frequency (i.e., the same number of pulses per unit of time) as the modulated clock signal  120 . The modulated clock signal  120  with an average angular frequency ω may then be described by the expression v(ωt+φ(t)), where the function v(ωt+φ(t)) is periodic and the long-term average value of the phase function φ(t) is zero. The phase function φ(t) can be chosen such that the modulated clock signal  120  can have one or more clock pulses periodically absent when compared to the unmodulated clock signal  110 . In addition, the term “jittering clock” is usually understood to mean a modulated clock signal having a phase function φ(t) that has a smaller magnitude and varies more slowly than the phase function φ(t) of the modulated clock signal  120 . 
     It should be understood that digital circuitry driven by a modulated clock signal has a lower computational throughput than if it were driven by an unmodulated clock signal. This is true since each omitted clock pulse in the modulated clock signal represents a lost opportunity for computation. Thus, the amount of throughput reduction depends directly on the percentage of clock pulses removed from the unmodulated clock signal. Also, digital circuitry using the modulated clock signal can be equivalent throughput-wise to a digital processing system using an unmodulated clock signal of the same effective frequency as the modulated clock signal. 
     Referring to FIG. 2, there is an exemplary graph illustrating the differences between the harmonic content “a” of an unmodulated clock signal and the harmonic content “b” of a modulated clock signal. An unmodulated clock signal can be approximated as a symmetric square wave having finite rise and fall times (see FIG.  1 ). As such, the unmodulated clock signal has main harmonics that fall at odd factors of a fundamental frequency f c , i.e., at (2n+1) f c , n=1, 2, . . . (only one harmonic “a” is shown). In contrast, the modulated clock signal has main harmonics that include sideband harmonics that surround each main harmonic (only one main harmonic and four sideband harmonics “b” are shown) As shown, the signal power in the unmodulated case is concentrated at a main harmonic frequency, and the signal power in the modulated case is “smeared out” across a main harmonic and sideband harmonics. 
     Thus, replacing an unmodulated clock signal with a modulated clock signal of the same effective frequency can have three effects on the harmonics of the modulated clock signal: (1) the main harmonic is moved to a higher frequency; (2) the level of the main harmonic is reduced corresponding to the percentage of clock pulses omitted; and (3) the power removed from the main harmonic is distributed over several sideband harmonics that appear around the main harmonic. 
     Referring to FIGS. 3-7, there are disclosed the preferred method  300  and two embodiments of the exemplary apparatus  510  both of which generate a modulated clock signal including harmonics that exhibit a known sideband configuration in accordance with the present invention Basically, the present invention enables a designer to select a desirable sideband configuration by knowing what the different magnitudes (e.g., signal power) of the main harmonics and corresponding sideband harmonics would be prior to generating a modulated clock signal that exhibits a similar sideband configuration. 
     Referring to FIG. 3, there is a flowchart illustrating the basic steps of the preferred method  300  of the present invention. Beginning at step  302 , at least one of M clock pulses is selected to be removed from a test clock signal to generate a test modulated clock signal. The test clock signal and the test modulated clock signal are not to be confused with the clock signal and the modulated clock signal that are generated by the present invention at steps  308  and  310  (described below). 
     At step  304 , a sideband configuration or power distribution of the test modulated clock signal may be computed by Fourier transforming (for example) the test modulated clock signal. Then at step  306 , the selecting step  302  and the computing step  304  are repeated until a test modulated clock signal is found that exhibits a desirable sideband configuration. To find a test modulated clock signal that exhibits a desirable sideband configuration, one has to determine how many of the clock pulses need be removed and also determine the particular location in a period of the test modulated clock signal at which each clock pulse needs to be removed. Steps  302 ,  304  and  306  may be carried out once and for all during design of the apparatus  510 ; they may be carried out entirely by computing circuitry in the apparatus  510 ; or the work may be distributed across the design phase and the operational phase. Two examples of such computed sideband configurations are shown in FIG.  4 . 
     Referring to FIG. 4, there is a graph illustrating two sideband configurations of two test modulated clock signals each of which has had two clock pulses removed out of a period of 32 clock pulses. The graph illustrates the sideband configuration around the fifth harmonic  402  of each test modulated signal which is computed by performing a Fourier transform on the respective test modulated signal (other sidebands around other odd harmonics are similar). The precise distribution of power in the sidebands also depends on which of the 32 clock pulses are omitted. For instance, the sideband configuration of the first test modulated clock signal (shown as case “a”) where the 1 st  and 4 th  clock pulses of every 32 clock pulses were omitted is different than the sideband configuration of the second test modulated clock signal (shown as case “b”) where the 1 st  and the 17 th  clock pulses of every 32 clock pulses were omitted. Such Fourier transform computations can be used to select a test modulated clock signal that exhibits a desirable sideband configuration. 
     It should be understood that the amount of sideband signal power corresponds to the percentage of clock pulses removed which also corresponds to the percentage of throughput reduction. For instance, a throughput reduction of 10% corresponds directly to a reduction in the power level of the main harmonic by 10%, and to an overall increase in the power of the harmonic&#39;s side bands by 10%. Again, the precise distribution of the power in the harmonic&#39;s sidebands depends on which 10% of the clock pulses are removed. 
     Referring to step  308  of FIG. 3, a clock signal is generated within a digital processing system of a device (e.g., wireless mobile terminal  500  of FIG.  5 ). How this clock signal is generated is well known to those skilled in the art. 
     At step  310 , at least one of M clock pulses is selectively removed from the clock signal to generate the modulated clock signal that can be used within the device. The modulated clock signal is generated to mirror the test modulated clock signal in that the same number of clock pulses at the same locations are omitted. Thus, the modulated clock signal includes a fundamental frequency and a plurality of harmonic frequencies that exhibit a desirable sideband configuration wherein there is little (if any) harmonic content in a band where co-located analog circuitry (see analog circuitry  520  of FIG. 5) could be adversely affected by electromagnetic interference from the modulated clock signal. 
     Referring to FIG. 5, there is illustrated a simplified diagram of a wireless mobile terminal  500  incorporating an apparatus  510  of the present invention. Although two embodiments of the apparatus  510  will be described in detail with reference to the wireless mobile terminal  500 , it will be readily understood by those skilled in the art that the present invention can be used in any type of circuitry that includes analog circuitry and digital circuitry. Accordingly, the apparatus  510  and its two embodiments described should not be construed in a limited manner. 
     The wireless mobile terminal  500  incorporates both digital circuitry  520  and analog circuitry  530 . The apparatus  510  is incorporated within the digital circuitry  520  and operates to reduce the electromagnetic interference on the nearby analog circuitry  530 . Basically, the apparatus  510  includes a clock generator capable of generating an unmodulated clock signal and a modulator capable of generating a modulated clock signal having selected M clock pulses omitted from the unmodulated clock signal. The modulated clock signal includes a fundamental frequency and a plurality of harmonic frequencies that exhibit a known sideband configuration. The known sideband configuration can be selected such that there would be less harmonic content in a band where co-located analog circuitry  530  could be adversely affected by electro-magnetic interference from the modulated clock signal. 
     Referring to FIG. 6, there is illustrated a first embodiment of the apparatus  510  which includes a clock generator  605  and a modulator  610 . The modulator  610  configured as a counter-based state machine is capable of generating a modulated clock signal  615  by gating away at least one of every M clock pulses from an unmodulated clock signal  620  that was generated by the clock generator  605 . 
     More specifically, the modulator  610  includes a counter  625  (e.g., wrap-around binary counter) capable of receiving the unmodulated clock signal  620  from the clock generator  605  and indexing the clock pulses of the unmodulated clock signal  620  to generate an output that increments by 1 after every clock cycle and rolls over at period M. For instance, if period M=4, then the generated output would be a sequence of values represented as 0 1 2 3 0 1 2 3 . . . , or in binary, 00 01 10 11 00 01 10 11 . . . . 
     The modulator  610  also includes at least one comparator  630  each of which is capable of receiving the output from the counter  625 . Each comparator  630  operates to compare the output (e.g., sequence of values 00 01 10 11 00 01 10 11 . . . ) of the counter  625  to a constant value C 1 , C 2  . . . CN. The constant values C 1 , C 2  . . . CN are generally multi-bit constants, which in the simplest case can be implemented by hard-wiring the bits of one input to the comparator  630  to logic ‘1’ and ‘0’, as appropriate (see examples below described with respect to FIG.  4 ). In the alternative, it would also be possible to dynamically tailor the sideband configuration by modifying the constant values C 1 , C 2  . . . CN as needed. Each comparator  630  operates to output a high value (binary ‘1’) when there is a match or a low value (binary ‘0’) when there is no match between the particular output of the counter  625  and the particular constant value C 1 , C 2  . . . CN. 
     The modulator  610  further includes a logic device (shown as an NOR gate  637  and an AND gate  639 ) capable of receiving the output from each comparator  630  and removing a clock pulse from the clock signal whenever a match occurs within any comparator. As shown, the NOR gate  637  has an output that goes low for one cycle whenever any comparator  630  gives a match, so the corresponding clock pulse is gated away from the unmodulated clock signal  620  by the AND gate  639  which then outputs the modulated clock signal  615 . 
     Assuming a designer wanted to configure a modulator  610  that generates a modulated clock signal that exhibits a sideband configuration similar to the sideband configuration shown in cases “a” and “b” of FIG. 4, then the modulator  610  would include a counter  625  with a period M=32, and there would be two comparators  630  because two pulses are to be removed for each M cycles in each case “a” and “b”. In case “a”, the modulated clock signal would have the 1 st  and 4 th  clock pulses of every 32 clock pulses omitted thus the two constant values would have binary values C 1 =1 and C 2 =4. And, in case “b”, the modulated clock signal would have the 1 st  and the 17 th  clock pulses of every 32 clock pulses omitted then the two constant values would have binary values C 1 =1 and C 2 =17. 
     The thick wires in FIG. 6 denote buses of width &gt;1; the bus width can be given by 2 log M. Therefore, the width of the inputs of each comparator is 2 log M. Furthermore in case “b”, it should be understood that the modulator  610  could be simplified because the bit pattern representing C 1 =1 is 00001 and the bit pattern representing C 2 =17 is 10001. Thus, a single comparator  630  is sufficient when it is configured to have an output go high when four LSBs (least significant bits) of the counter&#39;s output have a bit 0001 pattern. Then the NOR gate  637  now has but one input and therefore reduces to an inverter. 
     Referring to FIG. 7, there is illustrated a second embodiment of the apparatus  510  which includes a clock generator  705  and a modulator  710 . The modulator  710  configured as a pseudo-noise generator is capable of generating a modulated clock signal  715  by gating away at least one of every M clock pulses from an unmodulated clock signal  720  that was generated by the clock generator  705 . The modulator  710  includes a shift register (e.g., multiple flip-flops  727 ) with linear feedback and a logic device  730  both of which are designed to generate the modulated clock signal  715 . In particular, the shift register combines the outputs of a predetermined number of flip-flops  727  through a linear logic network  729  and feeds the result back to the shift register input. The flip-flop outputs are also combined in a second logic network  730  which outputs the modulated clock signal  715 . It is possible in principle to generate any modulated clock signal of length 2 K−1  (K is the number of flip-flops  727 ) even though some modulated clock signals require a more sophisticated logic device  730  than others. In the very simplest case, the logic device  730  reduces to a straight connection between one of its inputs and its output. 
     The shift register (e.g., multiple flip-flops  727 ) and the linear logic network  729  in the pseudo-noise generator serves the same purpose as the counter  625  in the finite state machine shown in FIG.  6 : to enumerate uniquely the clock pulses within a period M. Likewise, the logic device  730  in the pseudo-noise generator serves the same purpose as the comparators  630 , the NOR gate  637  and the AND gate  639  in the finite state machine shown in FIG.  6 : to select some of the uniquely-identified clock pulses for deletion. As long as the clock pulses of the modulated clock signal are uniquely identified, it is always possible to construct a logic network which can select any subset of the clock pulses to omit in order to generate the modulated clock signal. 
     Both the pseudo-noise generator and the counter-based state machine can be used to repeatedly select certain clock pulses to be omitted from a sequence of clock pulses having a given length, M. However, the pseudo-noise generator would typically be used when M equals some power of two less one, and the counter-based state machine would be used when M equals some power of two. Another type of state machine would be needed if there was a value of M which is neither a power of two or a power of two less one. Thus, there are many ways one skilled in the art could design these and other state machines for any desired modulated clock signal, given the principles of the present invention. Again, the pseudo-noise generator and the counter-based state machine described above are exemplary for purposes of this discussion. 
     It should be understood that when the digital processing system is used with throughput requirements lower than the highest frequency it was designed to work at, then phase modulation of the undithered clock signal can give a system designer an additional degree of freedom for frequency planning. And, instead of plainly clocking the digital processing system at a reduced frequency, the system designer may elect to use a modulated clock signal which yields the same throughput but has its harmonics placed differently which can be beneficial for the overall system performance. 
     From the foregoing, it can be readily appreciated by those skilled in the art that the present invention provides an apparatus and method capable of generating a modulated clock signal having a desirable sideband configuration that helps address the EMC issues arising from the integration of digital circuitry and analog circuitry. 
     Although two embodiments of the apparatus and a method of the present invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.