Abstract:
An apparatus and method for generating high-speed clock signals using a voltage-controlled-oscillator (VCO) device. The apparatus implements a linear variable gain amplifier rather than a non-linear hard limiter to remove unwanted signal modulation in VCO output signals. Implementation of the linear variable gain amplifier leads to the generation of amplitude modulation-free oscillation leading to the generation of jitter free high frequency clock signals.

Description:
BACKGROUND  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to the generation of high-speed clock signals using a voltage-controlled-oscillator (VCO) circuit and more particularly, to the use of a linear variable gain amplifier rather than a non-linear hard limiter to remove unwanted signal modulation in VCO output signals.  
         [0003]     2. Description of the Prior Art  
         [0004]     Voltage-controlled-oscillator (VCO) circuits are typically used to deliver signals that are modified to be used as clock signals for timing and control in an integrated circuit. As an example, phase-locked loop (PLL) circuits often employ a VCO to provide the clocking means. Many current VCO circuit designs provide VCO output signals that exhibit low frequency amplitude modulations. One such cause for amplitude modulation at the VCO output is tail current modulation which could have many sources. For example, all differential circuits have tail devices supplying current. These tail devices on the other hand refer to reference devices for their current multiplication and these reference devices on the other hand, refer to op-amp stabilized current sources. If there is some instability in these operational amplifier devices, one could have a cascade effect, which could manipulate itself as tail current modulation.  
         [0005]     An exaggerated example of amplitude modulation, due to tail current modulation is shown in  FIG. 1  which illustrates an example VCO output signal  10  provided at an output of an amplitude detection device (not shown). The example VCO output signal  10  depicted, includes a carrier waveform of 1 GHz. As further shown in  FIG. 1 , the high frequency carrier is periodically amplitude modulated.  
         [0006]     Prior to being used as a clock signal, this kind of amplitude modulation needs to be removed from the system or circuit in which the VCO is implemented.  
         [0007]     While most common VCO design concentrate on the provision of a hard limiter designed to suppress amplitude modulation (AM), if a sinusoidal input is supplied and the limiter has a linear transfer curve, then the output is sinusoidal. At large input amplitudes, the limiter cuts off the peaks of the sine wave. These waveforms exhibit different zero-crossing delays when fed through a circuit of finite bandwidth. This leads to amplitude-modulation-pulse width modulation (AM-PM) conversion. It is very hard to establish a low AM-PM conversion with a hard limiter, and this is only established at high bandwidth-to-input frequency ratios, which come at the expense of considerable power dissipation. This kind of PM leads to deterministic jitter that must be avoided in clock signals.  
       SUMMARY OF THE INVENTION  
       [0008]     It is thus an object of the invention to provide a system and method for processing VCO output signals used in the generation of clock signals in electronic devices, e.g., circuits, integrated circuits, etc. that removes unwanted amplitude modulation without generating pulse width modulation in the VCO output signals which results in the improved generation of jitter-free clock signals.  
         [0009]     According to the invention, there is provided implementation of a form of variable gain amplifier that removes amplitude modulation from VCO output signals without generating pulse width modulation in the VCO output signals.  
         [0010]     Thus, according to one aspect of the invention, there is provided an apparatus and method for removing low frequency amplitude modulation component of an oscillation signal comprising: 
        an operational amplifier means for receiving an oscillation signal having the low frequency modulation component at one input, and a feedback path connecting an output of the operation amplifier with the one input;     a means for generating a signal representing the low frequency amplitude modulation component of the oscillation signal; and,     a linear amplifier device located in the feedback path between the operational amplifier output and the one input for multiplying the signal representing the low frequency amplitude modulation component with a signal at the operational amplifier output, wherein the operational amplifier produces an output signal comprising the oscillation signal without the low frequency amplitude modulation.        
 
         [0014]     According to this aspect of the invention, the linear amplifier device located in the feedback path comprises a highly linear Gilbert cell multiplier.  
         [0015]     Advantageously, the apparatus and method implementing a variable gain amplifier of the present invention for controlling VCO output signals may be included in those integrated circuits, systems and devices used in the production of jitter free clock signals in the GHz frequency range. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  illustrates an exaggerated example of a VCO output signal exhibiting an amplitude modulation, e.g., due to tail current modulation in a VCO circuit according to prior art designs;  
         [0017]      FIG. 2  illustrates the novel linear variable gain amplifier designed to remove AM from VCO output signals according to the invention; and  
         [0018]      FIG. 3  illustrates a plot of waveforms including the modulated carrier waveform (V 1  signal of  FIG. 2 ), the modulation envelop waveform (V 2  signal of  FIG. 2 ); and, lastly, the waveform V 3 , which exhibits no modulation;  
         [0019]      FIG. 4  illustrates a technique for obtaining the modulation (envelop) waveform V 2 , as shown in  FIGS. 2 and 3 ; and,  
         [0020]      FIG. 5  illustrates a technique for obtaining the modulation (envelop) waveform V 2 , generated for input to the linear variable gain amplifier as shown in  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     The above-described AM-PM conversion originates from waveform changes which, according to the invention, are eliminated by inputting the VCO output signals into a linear variable gain amplifier device designed to remove AM instead of a nonlinear hard limiter as used in the prior art. An exemplary circuit diagram of the linear variable gain amplifier device implementation is shown in  FIG. 2 .  
         [0022]      FIG. 2  illustrates an exemplary circuit diagram  20  for achieving VCO output signal demodulation scheme employing a linear variable gain amplifier device according to the invention. As shown in  FIG. 2 , V 1  is the VCO signal output having the carrier signal V a  sin(2πf c t) where V a  is the peak amplitude and f C  is the carrier frequency, in addition to the modulation information sin(2πf m t) where f m  is the modulation frequency. The signal V 1  thus may be represented mathematically, for example, as a signal V a  sin(2πf c t)sin(2πf m t) and is input to a negative input terminal  22  of operational amplifier (op-amp)  25 . The positive terminal  23  of op-amp  25  is shown connected to a ground. The output of the op-amp  25 , signal V 3 , is input to a mixer device  30  such as a MOS Gilbert multiplier cell configured as a mixer device having high linearity and yielding linear products of both inputs (e.g., when configured in a double balanced mixer topology well-known to skilled artisans). A representative MOS Gilbert cell design may be found in commonly-owned U.S. Pat. No. 5,872,446, the whole disclosure and contents of which, including the references cited therein, being herein incorporated by reference as if fully set forth herein. Additionally, input to the Gilbert cell device  30  is a voltage V 2  corresponding to the waveform of modulation of the modulated carrier signal V 1  and represented mathematically, for example, by V m  sin(2πf m t), where V m  is the peak amplitude.  
         [0023]     It is understood that the Gilbert cell device  30  multiplies the inputs with a unity gain, and, the op-amp device may be programmed with a unity gain if the resistor ratio R 2 /R 1  is unity as shown in  FIG. 2 . However, the circuit of  FIG. 2  functions is a variable gain amplifier as the use of an op-amp in a negative feedback configuration in the topology shown in  FIG. 2  provides the variable gain amplifier characteristic.  
         [0024]     Assuming generally the Gilbert cell device  30  multiplies the inputs with a unity gain (gain of 1) then the output V 3  of the ideal op-amp in  FIG. 2  is given by equation 1) as follows:  
               V   ⁢           ⁢   3     =         -     V   a       ⁢     sin   ⁡     (     2   ⁢   π   ⁢           ⁢     f   c     ⁢   t     )       ⁢     sin   ⁡     (     2   ⁢   π   ⁢           ⁢     f   m     ⁢   t     )             V   m     ⁢     sin   ⁡     (     2   ⁢   π   ⁢           ⁢     f   m     ⁢   t     )                   1   )             
 
 where the modulated carrier waveform is V 1 , the modulation information is contained in V 2 , and Va and Vm are the peak amplitudes of their respective signals, V 1  and V 2 . As can be seen, the waveform V 3  is demodulated and the pure carrier waveform component is obtained. That is, due to the variable gain configuration of the op-amp device  25  in negative feedback configuration, V 3  is produced in the expression given in equation 1 where the sin(2πf m t) terms in the numerator and denominator get cancelled leaving just the carrier waveform of sin(2πf c t). This concept is verified and output waveforms plotted in  FIG. 3  which depicts: the modulated carrier waveform, i.e., the V 1  signal waveform  50 , the V 2  signal waveform  51 ; and, lastly, the V 3  signal waveform  52  which exhibits no modulation. Referring back to  FIG. 2 , this demodulated waveform V 3  may be input to a circuit  75  that is used in the production of jitter free clock signals for integrated circuit devices. For example, such a circuit  75  may include a hard limiter device as the input signal (V 3 ) does not have any AM-PM modulation. As there is no AM-PM modulation, jitter in the limiter device is virtually non-existent. 
 
         [0025]      FIG. 4  illustrates a technique for obtaining the modulation (envelop) waveform V 2 , as shown in  FIGS. 2 and 3 . Particularly, by passing the modulating carrier waveform signal V 1  through a low pass filter device  60 , as shown in  FIG. 4 , the envelope of the modulation is obtained.  
         [0026]      FIG. 5  depicts a plot of an example modulated carrier waveform V 1  and the output envelop of the modulation V 2  obtained after the waveform V 1  is passed through a low pass filter device  60  of  FIG. 4 . Using these two waveforms, the MOS Gilbert cell  30  and the op-amp device  25  configured as a variable gain amplifier circuit in accordance with the circuit shown in  FIG. 2  provide a clean carrier signal V 3  that enables the generation of jitter free digital clock signals for integrated circuits.  
         [0027]     It should be understood that there may be a different periodic function doing the modulation, however, the concept of demodulation once the envelope of the modulation is available, is contemplated according to the invention.  
         [0028]     While there has been shown and described what is considered to be preferred embodiments of the invention, it will, of course, be understood that various modifications and changes in form or detail could readily be made without departing from the spirit of the invention. It is therefore intended that the invention be not limited to the exact forms described and illustrated, but should be constructed to cover all modifications that may fall within the scope of the appended claims.