Abstract:
Apparatus and method for reducing clock skew. A compensator is connected to receive an uncorrected clock signal and delay the clock signal in accordance with a skew control voltage. The skew control voltage is derived from the signal to noise ratio of an analog signal produced by a device controlled by the clock signal. The skew control voltage changes step wise maintaining the system signal to noise at a minimum by reducing the clock skew.

Description:
FIELD OF THE INVENTION 
     The present invention provides a circuit and method for reducing skew between a clock signal and a reference signal. Specifically, the circuit and method reduces the skew between a reference signal and the clock signal produced from a phase lock loop which utilizes the reference signal. 
     BACKGROUND OF THE INVENTION 
     Radio frequency communication systems utilize phase lock loop (PLL) circuits to produce a clock signal which control digital devices such as frequency synthesizers and digital to analog conversion circuits. The well known PLL circuit receives a reference signal and generates from a voltage controlled oscillator (VCO) a higher power clock signal which is phase and frequency locked to the reference signal. 
     In higher frequency applications above 1-GHz, a time delay exists between the reference signal and the VCO output signal which is referred to as skew. The skew can produce errors in the analog to digital conversion process, as well as reduce system performance in base station communications operations. The skew is a result of dynamic mismatches between circuit elements of the phase lock loop. Specifically, mismatches of transistors forming the phase detector and charge pump which establishes a control voltage for the voltage controlled oscillator introduce a phase error between the input reference signal and the VCO signal. 
     In applications which require the conversion of digital signals to analog signals, a slight clock skew will contribute significantly to the noise in a high resolution digital to analog converters. Even when the passive and active elements in the phase lock loop are matched, at 2-GHz a clock skew of 30 PS may still exist between the reference signal and VCO signal. 
     The prior art includes various techniques for reducing clock skew for a multi-channel signal source. U.S. Pat. No. 5,384,781 describes a cross-coupled flip-flop calibration circuit and microprocessor which aligns the timing of a pair of signals from a multi-channel signal source. The flip-flop calibration circuit indicates which of a pair of signals is leading in phase, and the microprocessor uses the output of the flip-flop circuit to adjust a signal delay for one of the signal sources. By changing the delay of the leading signal, a calibrated value can be obtained wherein both signals have essentially the same timing. 
     U.S. Pat. No. 5,394,024 provides a circuit for eliminating off chip to on chip clock skew. An on chip clock signal is derived by phase delaying an off chip generated clock signal. First and second delay paths are connected to receive the off chip clock signal. A phase detector and filter circuit generates control signals to adjust the respective phase delay through each of the delay paths. A multiplexer selects one of the delay paths to produce the on chip clock signal. 
     The present invention is directed to providing a method for calibrating a VCO generated signal with respect to a sine wave reference signal to reduce the skew between the VCO signal and the sine reference signal of the phase lock loop. 
     SUMMARY OF THE INVENTION 
     An apparatus and method are provided for correcting clock signal skew. A calibration circuit comprising a skew compensator is connected to receive an uncorrected clock signal, and delays the clock signal in accordance with a skew control voltage. The skew control voltage is derived from the signal to noise ratio of an analog signal produced from a device controlled by the clock signal. In a preferred embodiment of the invention, a digital analog converter produces a signal to noise ratio measurement signal which is representative of the skew of the clock signal. The signal to noise ratio signal is supplied as a control signal to a skew compensator, which affectively delays the clock signal in accordance with the applied control signal. The clock signal is shifted in time to produce a signal to noise ratio signal having a minimum value, representing a zero skew condition. 
     In accordance with a preferred embodiment of the invention, a source of sine wave reference signals is used in a sine digital to analog converter. The digital to analog converter produces Sine pulses having an area representing the value of a digital input signal. A signal representing the signal to noise ratio of the analog signal is produced from the digital to analog converter, representing the skew between an input clock signal and the sine wave reference signal. A skew controller receives the signal to noise ratio signal and generates the control signal for a variable delay network. The variable delay network delays the input signal in accordance with the signal to noise ratio signal derived from the digital to analog converter to eliminate the clock skew. 
    
    
     DESCRIPTION OF THE FIGURES 
     FIG. 1 is a block diagram of a skew calibration circuit in accordance with a preferred embodiment. 
     FIG. 2 illustrates the timing of signals used to calibrate the clock signal. 
     FIG. 3 illustrates the architecture of the SIN digital to analog converter which is used to generate a signal to noise ratio signal. 
     FIG. 4 illustrates the effect of the system on the signal to noise performance. 
     FIG. 5 illustrates the skew controller circuit for adjusting the skew of the clock signal. 
     FIG. 6 illustrates the effect of compensating clock skew. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates (within dashed line) a phase lock loop circuit  9  which produces a clock signal phase locked to a source of sine wave signals  11 . The source of sine wave signals  11  constitutes a reference frequency which, in accordance with the principle of phase lock loop operation, controls the operating frequency and phase of voltage control oscillator (VCO)  14 . The voltage controlled oscillator  14  produces a clock signal which has a sufficient power output capability to drive a plurality of integrated circuit chips. As noted in the previous section, the phase of the clock signal produced by VCO  14  is delayed with respect to the phase of the reference signal oscillator  11 , producing a skew between the two signals. Eliminating skew is desirable in high frequency communications systems which utilize digital to analog conversion devices, as the skew is responsible for an increase in signal to noise ratio for the digital to analog converter (DAC) where high resolution is sought. As will be evident with respect to the discussion of FIG. 2, the Sin DAC architecture is used in applications requiring a low signal to noise ratio output and a reduced sensitivity to clock jitter. 
     The portion of the phase lock loop  9  which includes a phase detector  12 , low pass filter  13  and voltage control oscillator  14  is of a conventional design, having a loop bandwidth and performance selected for a specific application. Additionally, the phase lock loop  9  includes, in accordance with the present embodiment, a delay circuit  16  which introduces a fixed amount of delay between the clock signal from VCO  14  and the input of phase detector  12 . 
     The fixed delay is used to change the time origin for the clock signal, and permit the clock signal delay to be shifted in time by a delay circuit  17 , so that skew between the clock signal and reference signal can be reduced. The skew, or a relative time difference between the reference signal produced by Sine signal source  11 , and the clock signal output from VCO  14 , is shown in trace A and trace E of FIG. 2. A represents the sine reference signal from source  11 , and E represents the input to the phase detector  12 , which is a delayed version of D, the signal from voltage controlled oscillator  14 . The error signal produced by phase detector  12 , is filtered in filter  13  and is shown in C. The clock signal D produced from VCO  14  is illustrated as D in FIG. 2, and has a leading relationship with respect to the reference Sine signal A due to the delay imposed by delay circuit  16 . Delay circuit  17  is controlled by a control voltage which, in accordance with the preferred embodiment, reduces the time difference between the signal F produced from the output of the voltage controlled delay circuit  17  and the Sine reference signal from reference signal source  11 . 
     The delay circuit  16  similar to delay circuit  17  has the input control voltage set at a common potential, so that V con  is fixed, providing a fixed delay to the clock signal which is applied to phase detector  12 . As a result of the additional delay provided by delay circuit  16 , the time margin is shifted by a value TD zero. By selecting the variable delay of delay circuit  17 , TD0, to be less than the fixed delay of delay circuit  16 , the effective delay of the VCO  14  output provided by variable delay network  17  is TD-TD0. Accordingly, the phase of the signal shown in FIG. 2, as F, can be adjusted to that it coincides with the new time reference. The edge of the clock signal from VCO  14  can thereafter be adjusted, with respect to the new time reference, depending on the control signal V con  provided by the skew controller  19 . 
     The correction voltage for eliminating the skew between the sine reference signal A and delayed clock signal F is derived, in accordance with the preferred embodiment, from a skew controller  19  which receives an error signal, representing the skew between the sine reference signal and clock signal D. The skew controller  19  includes a skew calibrating algorithm which generates a correction voltage based on signal to noise (SNR) information derived from a Sin DAC 18. The Sin DAC  18  has at least a significant bit output, representing the signal to noise ratio of an analog quantity converted from a digital input. In practice, the digital input G is set to zero, and the digital to analog converter output changes in accordance with its least significant bit. 
     The architecture for the previously known Sin DAC  18  is shown more completely in FIG.  3 . Referring now to FIG. 3, a digital signal is shown as one input to a multiplier  20 , and a Sine wave signal with a d-c component, 1+cos(2πωt) is shown as a second input to the multiplier  20 . The output comprises the product of the digital data signal a(t) and the sine signal 1+cos(2πωt). The Sin DAC  18  provides pulse shaping, wherein the data signal is multiplied with the sine reference signal having the same frequency as the clock for the DAC. The output signal is a pulsed Sin wave, wherein at the zero crossing point, the pulse has a zero slope which minimizes the circuit sensitivity to clock jitter. The output of the multiplier  20  is filtered in a low pass filter  21  to produce an amplitude signal which is representative of the input digital data. 
     The Sin DAC  18  output voltage signal to noise ratio is adversely affected by clock skew. As the clock skew increases, the difference in phase between the clock signal applied to the Sine DAC  18  and reference Sine wave signal from Sine source  11 , will increase the signal to noise ratio of the analog signal produced by the sine DAC  18 . The measure of the signal to noise ratio for the Sine DAC  18  is, therefore, an indication of the measure of skew between the clock signal and reference signal from the Sine signal source  11 . In the embodiment shown of FIG. 1, the data input G is held to zero, representing an idle input condition and an output signal produced by the Sin DAC  18  is detected, as an indication of the signal to noise ratio and applied to the skew controller  19  wherein each value of SNR is determined as:            SNR        [   k   ]       =       1   M            ∑     i   =     k   -   M       k                   x   -&gt;     noise          2           ,                          
     where k is a current sample, M is a running average range and {right arrow over (x)} noise  is an analog to digital converted output vector of said digital to analog converter having an idle input. 
     The skew controller  19  produces a control voltage based on the input signal to noise ratio, or some other indicator of the clock skew. The algorithm used to derive a control voltage in accordance with the preferred embodiment, based on signal to noise ratio is given as follows: 
     
       
           V   con   [k+ 1]= V   con   [k]+μ·sign (Δ SNR[k ])· sign (Δ SNR[k− 1]) 
       
     
     where V con  is the control signal, SNR is a value of said signal to noise ratio signal, and Δis the difference between k samples of consecutive SNR values. 
     In the foregoing, each sample instant is represented by K, and the control voltage is updated each sample instant based on the change in signal to noise ratio from the previous sampling instant. A maximum control voltage step change, μ, forms part of the algorithm. The value of μ, the maximum step change size in control voltage, is selected as a tradeoff between resolution and performance. 
     FIG. 4 illustrates the relationship between the measurement of system performance, i.e., in the preferred embodiment, SNR, whereby position  1  depicts an initial starting point. Once the algorithm has calculated a control voltage based on the measured signal to noise ratio, the next calculated value of control voltage results in a signal to noise ratio of the Sin DAC  18  identified by position  2 . The step wise change from position  1  to position  2  is based on the value of μ and, the system will control the clock skew, such that it operates on either side of the signal to noise ratio peak, represented in FIG.  4 . The smaller the value of μ, the less isolation, and smaller the clock skew. To obtain the delay change, of +/− 10 ps, the step size μ is selected to be +/− 10 mv, with the relationship that 1 ps/mv produces the delay change. 
     The delay circuit  17  is shown, for example, more particularly in FIG.  5 . Referring now to FIG. 5, a differential amplifier is shown, receiving as inputs the clock signal V in+ , and inverted clock signal V in−  on the base connections of transistors  23  and  24 . The differential transistor pair  23  and  24  are connected in the conventional manner to receive a current from current source  25  which is divided between each of the differential transistors  23  and  24 . Control over the current source  25  by the skew calibration circuit increases and decreases the delay of signals V in+  and its complement V in− . Output driver amplifiers  26  and  27  provide the amplified outputs of a clock signal which has been delayed in accordance with the current level set by current source  25 . 
     FIG. 6 illustrates the result of controlling skew with respect to the signal to noise ratio. The solid line X shows the skew of approximately 30 picoseconds for a phase lock loop without compensation. The dashed line Y, shows how the skew can be reduced over time to zero, as the skew controller  19  operates to reduce the skew to zero. 
     Thus, there has been described with respect to one embodiment of the invention a circuit for calibrating skew. 
     The foregoing description of the invention illustrates and describes the present invention. Additionally, the disclosure shows and describes only the preferred embodiments of the invention but, as mentioned above, it is to be understood that the invention is capable of use in various other combinations, modifications, and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein, commensurate with the above teachings and/or the skill or knowledge of the relevant art. The embodiments described hereinabove are further intended to explain best modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with the various modifications required by the particular applications or uses of the invention. Accordingly, the description is not intended to limit the invention to the form disclosed herein. Also, it is intended that the apended claims be construed to include alternative embodiments.