Abstract:
A spread spectrum frequency modulated oscillator circuit usable as a clock comprises a reference component such as a resistor, a voltage controlled oscillator and a first circuit coupled to the reference component and voltage controlled oscillator and configured to supply a first control signal to the oscillator to cause the oscillator to oscillate at a frequency corresponding to a value of the reference component. A second circuit configured to supply a random signal to the oscillator causes the frequency of the oscillator to dither. To cause the oscillator to exhibit random frequency modulation that is fast enough to reduce EMI but not too fast for controlled devices such as switching regulators to track, the oscillator includes a third circuit configured to control (1) a rate of change of the oscillator frequency such that the rate of change is a fixed percentage of the oscillator frequency, and (2) an amount of frequency change in the oscillator frequency such that the amount of frequency change is a fixed percentage of the oscillator frequency.

Description:
TECHNICAL FIELD  
       [0001]     This disclosure relates generally to oscillator clock sources, and more particularly to improving electromagnetic interference by frequency modulating a clock using spread spectrum frequency modulation.  
       BACKGROUND DESCRIPTION  
       [0002]     Electronic devices, such as switching regulators often generate electromagnetic interference which can be detrimental to the operation of electronic circuits in which such devices are employed. Switching regulators translate all input voltage at one level to an output voltage at another. Energy from an input voltage source is stored in an inductor or capacitor and then transferred, or switched, to the output. Energy transfer is repeated at a rate determined by the clock source of the regulator. The switching action generates interference, the energy of which is concentrated at frequencies which are harmonics of the clock source.  
         [0003]     The output of a switching regulator is a DC level signal plus interference created by clock-controlled switching. Different switching regulators require different clock frequencies. Consequently, the interference from a switching regulator differs from design to design. Techniques to reduce interference should be effective for clocks of both slow and fast clocking frequencies. If the clock frequency of a regulator is dynamically varied, it is possible to spread the interference energy over a range of frequencies, reducing the energy at any one frequency. This technique is known as spread spectrum frequency modulation.  
         [0004]     Methods to implement spread spectrum frequency modulation have included modulation with periodic waveforms such as triangular or sinusoidal waveforms, and random modulation. However, each of these techniques has drawbacks.  
         [0005]     The shape of a modulating waveform will affect the shape of the interference spectrum.  FIG. 1  illustrates the output of a typical switching regulator with periodic sinusoidal frequency modulation of the clock source. The upper trace  102  is the modulating signal. The lower trace  104  is the output of the regulator. The lower trace  104  appears as an amplitude variation of the switching induced interference. Even though the interference energy near the clocking frequency has been “spread,” a low frequency large amplitude component has been added to the spectrum. This “amplitude modulation” could easily corrupt the operation of electronic circuits, a result which nullifies the benefits of the sinusoidal modulation.  
         [0006]     Random modulation provides an improvement over the use of a periodic waveform. If the clock source of a switching regulator is randomly varying, using, for example, a technique termed “frequency hopping,” then the resulting amplitude modulation of the interference will be random. Random amplitude variations are indistinguishable from noise. However, the difficulty with random modulation lies in the generation of an optimal random signal. A random signal should be provided that produces reduction of EMI at both fast and slow hopping rates within the ability of practical regulators to track them. Lowpass filtering is employed for this purpose, without which the output will tend to exhibit ripple. Too much filtering, however, will negate the benefit of random modulation.  
         [0007]     There is a need for a product which generates a wide range of clock frequencies to cover many switching regulator applications. At any clock frequency, the product should exhibit random frequency modulation which is fast enough for good EMI reduction, yet not too fast for switching regulators to track.  
       SUMMARY  
       [0008]     A spread spectrum frequency modulated oscillator circuit comprises a reference component, a controlled oscillator and a first circuit coupled to the reference component and controlled oscillator and configured to supply a first control signal to the oscillator to cause the oscillator to oscillate at a frequency corresponding to a value of the reference component. Included are a second circuit configured to supply a random signal to the oscillator to cause the frequency of the oscillator to dither, and a third circuit configured to control (1) a rate of change of the oscillator frequency such that the rate of change is a fixed percentage of the oscillator frequency, and (2) an amount of frequency change in the oscillator frequency such that the amount of frequency change is a fixed percentage of the oscillator frequency.  
         [0009]     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     In order that the manner in which the above-recited and other advantages of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:  
         [0011]      FIG. 1  depicts a graph of a typical switching regulator output using periodic frequency modulation of the clock source.  
         [0012]      FIG. 2  depicts a circuit diagram of an oscillator circuit, in accordance with one embodiment of the invention.  
         [0013]      FIG. 3  depicts several waveforms and their corresponding amplitude probability densities.  
         [0014]      FIG. 4  depicts an oscillator circuit diagram, in accordance with one embodiment of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0015]     A spread spectrum frequency modulated oscillator circuit is described which generates a wide range of frequencies, exhibiting random frequency modulation which is fast enough to reduce electromagnetic interference (EMI), yet able to be tracked reliably by a device using the oscillator circuit, such as a switching regulator.  
         [0016]      FIG. 2  is a diagram of an oscillator circuit  200 , in accordance with one embodiment of the invention. Oscillator circuit  200  includes a voltage controlled oscillator (VCO)  230 , whose output frequency is controlled by a DC source  220  which supplies a constant voltage for setting the mean output frequency of the VCO, and a random voltage source  210  which produces a random voltage signal to modulate, or “dither” the VCO frequency. While voltage signals are used in  FIG. 1  to control the oscillator, other signals may be used, such as currents or numbers.  
         [0017]     The generated random voltage signal is added to the supplied constant voltage signal to produce a control signal vo to control VCO  230 . A user selectable gain K may be used to vary the frequency of the oscillator over a wide range. A variable bandwidth lowpass filter  216  may be provided to limit the rate of change of the signal output by VCO  230 . Amplifier  212  and peak detector  214  ensure a constant peak-to-peak amplitude out of the filter as the bandwidth changes.  
         [0018]     An ideal VCO can be described by the equation fo=K*vo, where K is a constant. If the control voltage vo varies with time, then the frequency output of the VCO, fo, will vary with time, creating a frequency modulated signal. To generate a spread spectrum frequency modulated (SSFM) signal, in accord with the principles taught herein, (1) the rate of change of control voltage vo is kept small compared to the peak-to-peak change of vo, and (2) the probability distribution of the amplitude of vo is uniform. A slowly varying control voltage with uniformly distributed amplitude will result in a flat frequency spectrum, which is desirable for the reduction of EMI.  
         [0000]     Rate of Change of vo  
         [0019]     In frequency modulation systems, a useful term is β, or “modulation index.” The modulation index β may be defined as  
         β   ≡       Δ   ⁢           ⁢   fo       fo   m         ,       
 
 where Δfo is the deviation in the output frequency and fo m  is the maximum rate of change of the output frequency. For example, if the output frequency is nominally 1 MHz and deviates +10% at a 20 KHz rate, then  
       β   =           1.1   ⁢           ⁢   MHz     -     0.9   ⁢           ⁢   MHz         20   ⁢           ⁢   KHz       =   10         
 
 Since the output of the VCO is linearly related to the control voltage vo, the modulation index can be expressed as  
       β   ≡       Δ   ⁢           ⁢   vo       vo   m           
 
 where Δvo is the peak-to-peak change in the control voltage and vo m  is the maximum rate of change of the control voltage. 
 
         [0020]     As described previously in respect of  FIG. 2 , user selectable gain K may be used to vary the frequency of the oscillator. The bandwidth of lowpass filter  216  tracks the VCO frequency. Thus, the rate of frequency change increases in proportion to K. Furthermore, as the amplitude of the signal from the filter is held constant by peak detector  214  and amplifier  212  in closed loop control, an increase in K increases the peak deviation. Accordingly, β is held constant for all output frequencies.  
         [0000]     Uniform Amplitude Probability Density  
         [0021]     The random voltage signal produced by random voltage source  210  ensures that the probability density, of the control voltage vo is uniform. To illustrate the advantageous characteristics of random voltage signal generation in this context,  FIG. 3  depicts some common waveforms that may be used to frequency modulate a clock signal. At any instant in time, the amplitude of a square wave  310  has a 50% probability of being 0.2V and a 50% probability of being 0.9V. The amplitude of the a triangular waveform  320  has an equal probability of being any value between 0.4V and 0.9V. The amplitude of a random waveform  330  has an equal probability of being any value between 0.1V and 1.0V. Hence, a triangular waveform and a random waveform have uniform probability densities; a square wave waveform does not. However, as a triangular waveform is periodic, it will tend to generate undesirable amplitude components despite spreading.  
         [0022]      FIG. 4  depicts a SSFM oscillator circuit  400  according to another exemplary embodiment of the invention, in which the oscillator circuit includes a random voltage generator  420  for applying random modulation to the frequency signal generated by controlled oscillator  430 . While controlled oscillator  430  is depicted as a voltage and current controlled oscillator, other forms of two input port controlled oscillators may be used.  
         [0023]     Ignoring for the present random voltage generator  420 , oscillator circuit  400  is a closed loop circuit that comprises a controlled oscillator  430  whose frequency is controlled alternatively by input voltage vo and input current fo. As either input voltage or current (or both) is increased, the output frequency of the controlled oscillator  430  will increase. Control input voltage vo is produced by an integrator  408  composed of operational amplifier  406  and feedback capacitor  407 . The non-inverting input of integrator  408  receives a reference voltage that corresponds to the difference between the supply voltage V+ and a voltage drop across resistor R SET    402 , the value of which is user determined. The non-inverting input receives a dynamic voltage V SET  that corresponds to the difference between supply voltage V+ and a voltage drop across resistor R SW    410 , the resistance value of which tracks oscillator  430  frequency through divider  432 . Control input current io is produced by current sources  404  mirrored from input current Iset at various prescribed ratios, represented by coefficients α, for purpose that will become clear hereinafter.  
         [0024]     According to the illustrated example, resistor  410  is a frequency controlled resistor that may be implemented in a semiconductor circuit as a switched capacitor, that is, a capacitor whose effective resistance is related to the repetition rate of a switch connected to bypass it. The frequency controlled resistor  410  in this example may have a value  
         R   SW     =       10       C   SW     *     f   VCO         .         
 
         [0025]     As current flows through integrator  408 , the output vo of the integrator increases from zero volt or other initial value, causing controlled oscillator  430  to begin to oscillate. Further increases in the integrator output vo cause the output frequency f VCO  to increase. The controlled oscillator output signal is fed back to frequency controlled resistor  410  to cause the value of resistor  410  to track oscillator frequency. As the resistance of the described resistor  410  is inversely proportional to applied frequency, current I SW  flowing through the resistor increases as f VCO  increases. In equilibrium, current I SW  matches current I SET , and the output frequency of oscillator  430  may be determined by the equation  
         f   VCO     =         α   ⁢           ⁢   1   *   10         R   SET     *     C   SW         =     Fo     R   SET             
 
 where Fo is a constant determined by the capacitor value C SW  and the current mirror ratio α1. As this equation indicates, the output of integrator  408  thus will track to any voltage necessary for the loop to reach equilibrium, independently of oscillator input control, within the limits of the power supply. A user thus can accurately select a wide range of clock (oscillator) frequencies simply by changing the value of the user selectable resistor  402  (R SET ). 
 
         [0026]     Random voltage generator  420  is introduced to superimpose random variations on to the oscillator control quantities vo and io, so as to randomly modulate, or “dither”, the oscillator frequency. Random voltage generator  420  may, as shown, include a switched capacitor lowpass filter  422 , a digital-to-analog converter  424 , a random number generator  426 , and dividers  427  and  428  interconnected in a conventional manner. Random voltage generator  420  receives the output of controlled oscillator  430 . Divider  428  may be used to increase the ratio of the controlled oscillator frequency to filter bandwidth. Therefore, increasing the value of divider  428  increases the modulation index β. For a given value of divider  428 , the modulation index β remains constant for all values of VCO frequency. Switched capacitor filter  422  smoothes the output of digital-to-analog converter  424  to prevent abrupt frequency changes.  
         [0027]     Random voltage generator  420  generates a random voltage V N  to create an apparent random fluctuation in the user selectable resistor  402 , which in turn creates random fluctuations in the output frequency F VCO . A fraction of the current I SET  is multiplied by the random voltage V N . The new current from the user-selectable resistor  402  applied to alter the voltage applied to the summing junction I SET *(α1+V n *α2). The term I SET *α1 sets the output frequency of the oscillator; the term I SET *V N *α2 generates the random modulation component of output frequency.  
         [0028]     Integrator  408 , however, within the context of presenting random modulation to oscillator  430 , undesirably imparts additional filtering to the added random term. To compensate for this filtering, a second random signal is applied to the current input of the controlled oscillator  430 . The second signal is defined by the equation I SET *(α3+V N *α4).  
         [0029]     Any increase in either the voltage input or the current input of controlled oscillator  430  increases the output frequency of the oscillator, as described previously. The term I SET *V N *α4 generates random fluctuations in the control current applied to the control current input io of controlled oscillator  430 .  
         [0030]     Oscillator circuit  400  thus realizes a variable frequency clock source which has random modulation with a constant modulation index and a flat frequency spectrum. The output of the oscillator is adjustable over a large range of frequency by adjusting the value of user selectable resistor  402 . The random voltage generator provides a modulation signal with a uniform amplitude distribution. The random signal applied to the integrator adds slowly varying random modulation, while adding a random signal into control signal io of the controlled oscillator  430  generates rapidly varying random modulation. Combined, the rate of change in the random modulation is determined by lowpass filter  422 , allowing the modulation rate to track the output frequency for any user selectable output frequency. EMI may be reduced for any clock frequency.  
         [0031]     The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein, but is to be accorded the full scope consistent with the claims.