Abstract:
A method and apparatus for converting a high precision digital word into a high precision analog signal is disclosed. A sigma delta modulator applies a digital input signal to a dither signal to generate a combined signal for sampling. A digital-to-analog converter quantizes the combined signal. An analog filter provides a cutoff at a bandwidth of interest to remove out of band quantization noise and signals. An I transfer function and a Q transfer function can be coupled between the sigma delta modulator and the digital-to-analog converter for mapping of the combined signal. The apparatus can also include a phase interpolator for receiving the output signal and outputting a clock recovery phase.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to phase-locked loops (PLL&#39;s) in timing recovery. More specifically, this invention relates to PLL&#39;s using sigma delta modulation in timing recovery to convert a high precision digital word into a high precision analog signal. 
     BACKGROUND OF THE INVENTION 
     Timing recovery units perform clock recovery and data retiming functions. Well known timing recovery schemes include a phase-locked loop (PLL). The PLL is basically a closed loop frequency control system, which operates by producing an oscillator frequency to match the frequency of an input signal. In this locked condition, any slight change in the frequency of the input signal first appears as a change in phase between the input signal frequency and the oscillator frequency. This phase shift then acts as an error signal to change the frequency of a local PLL oscillator to match the input signal frequency. Locking onto a phase relationship between the input signal frequency and the local oscillator accounts for the name phase-locked loop. 
       FIG. 1A  illustrates a prior art timing recovery unit  10  as part of a high-speed transceiver, which provides receiver clocks that are synchronized to an incoming signal. The timing recovery unit  10  comprises a clock multiplying unit  15  including a voltage-controlled oscillator (VCO)  20 , and a PLL loop  25  comprising a phase detector  30 , a loop filter  40  and a phase interpolator  50 . The clock multiplying unit  15  takes a reference clock and speeds it up to a high frequency. The VCO  20  is thus locked to the reference clock. A phase interpolator  50  takes the phase from the VCO  20  and shifts the phase either forward or backward to track frequency as well as phase from a received (Rcvd) signal. An output clock of the phase interpolator  50  drives a phase detector  30  which compares the output clock to the received signal and generates output current pulses proportional to the phase error between them. The output current pulses are filtered and integrated by a loop filter  40  to generate a voltage. 
       FIG. 1B  illustrates a prior art implementation of the timing recovery unit of  FIG. 1A . A quadrature low frequency analog control oscillator  60  is provided to control a phase interpolator  70 . The control oscillator  60 , which receives an input voltage, can operate from a positive frequency to a negative frequency. Phase interpolation is then performed to output a high speed recovery clock aligned with an incoming high speed signal. A benefit of this implementation is that the control elements are low frequency devices, and both the control oscillator  60  and the phase interpolator  70  are designed in analog form. A problem with this implementation, however, is that when the phase is “locked”, the control oscillator  60  has to maintain that phase perfectly in perpetuity. Due to variations in manufacturing and introduction over many units, mismatches and imperfections degrade performance. The control oscillator  60  can cause phase errors induced by supply or substrate noise that can accumulate over many clock cycles. At higher frequencies, overload and saturation of the control oscillator  60  can affect the stability of the system. Leakage, drifts in frequencies, and other sensitivities caused by analog devices in prior art PLL&#39;s degrade performance and bit error rate (BER) targets. 
     What is needed is a PLL-based timing recovery unit that avoids sensitive analog circuitry, increases yield, and provides digital programmability in the system and preferably on the same integrated circuit chip. What is also needed is a timing recovery unit using both analog and digital devices to modulate phase and achieve sub-picosecond phase resolution in a timing recovery unit. What is also needed is a timing recovery unit that uses both analog and digital techniques on a single integrated circuit to convert a high precision digital word into a high precision analog signal while preventing an overload or saturation condition. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to a method of and apparatus for converting a high precision digital word representing the desired phase into a high precision analog signal representing the desired phase using low precision components. This invention uses digital circuits to achieve deep sub-picosecond timing resolution using imprecise analog circuits. Additionally, the majority of the digital circuits can be synthesized, thereby reducing design time. This invention avoids sensitive analog circuitry, increases yield, recovers granularity and preserves precision through control of a phase interpolator which controls adjustment of the clock phase. The invention achieves higher precision for greater phase lock loop (PLL) accuracy. According to a simplified understanding of the invention, a scalar value of a dither signal is formed and combined with digital signal using sigma delta modulation which provides significant improvements over the prior art. 
     In accordance with an embodiment of the present invention, a method of converting a high precision digital word into a high precision analog signal is disclosed. The method comprises the steps of applying a digital input signal to a dither signal to generate a combined signal for sampling; convertizing the combined signal; and filtering the combined signal to generate an output signal. The method can also include the step of sampling the combined signal. The method can further include the step of oversampling the combined signal. 
     In accordance with the present invention, a scalar value of the dither signal is shifted before the dither signal is applied to the digital input signal. Preferably, the output signal is an interpolated value of the digital input signal. The digital signal can be generated by a phase accumulator. The dither signal, which can be a frequency shaped noise signal, is preferably generated by a sigma delta modulator. The combined signal can be mapped into an I transfer function and a Q transfer function. 
     In accordance with the present invention, the output signal is preferably an analog signal. The combined signal is preferably filtered by an analog reconstruction filter which filters out shaped phase noise. The filtered signal can be input to a phase interpolator. An output of the phase interpolator is fed back to a phase detector. 
     In accordance with an alternative embodiment of the present invention, an apparatus for converting a high precision digital word into a high precision analog signal is disclosed. The apparatus comprises a sigma delta modulator for applying a digital input signal to a dither signal to generate a combined signal for data conversion; a digital-to-analog converter for converting the combined signal; and an analog filter for filtering the combined signal to generate an output signal. The analog filter provides a cutoff at a bandwidth of interest which removes out of band quantization phase noise and phase signals. The apparatus can also include an I transfer function and a Q transfer function coupled between the sigma delta modulator and the digital-to-analog converter. The combined signal is preferably mapped into the I and Q transfer functions. The apparatus can further include a phase interpolator for receiving the output signal and outputting a clock with the desired phase. 
     In accordance with another embodiment of the present invention, a sigma delta modulator is disclosed. The sigma delta modulator comprises a twos-complement accumulator for generating a phase accumulator output; a subtractor for subtracting a quantization feedback signal for the phase accumulator output to generate a first subtracted signal; and a two-input multiplexer for combining the first subtracted signal and a modulator output feedback signal to generate a first combined signal. 
     Preferably, the modulator allows a wrap procedure of the phase accumulator output to be performed. The sigma delta modulator can also include a plurality of outputs and generate an I transfer function and a Q transfer function. The modulator can be at least one of a first order sigma delta modulator, a second order sigma delta modulator, and a n-order sigma delta modulator. 
     In accordance with another embodiment of the present invention, a sigma delta modulator is disclosed. The sigma delta modulator comprises a twos-complement accumulator for generating a phase accumulator output; a subtractor for subtracting a quantization feedback signal for the phase accumulator output to generate a first subtracted signal; a two-input multiplexer for combining the first subtracted signal and a modulator output feedback signal to generate a first combined signal; and means for performing a wrap procedure of the phase accumulator output. 
     In accordance with another embodiment of the present invention, a method of modulating phase in a timing recovery unit is disclosed. The method comprises the steps of generating a phase accumulator output; subtracting a quantization feedback signal from the phase accumulator output to produce a first subtracted signal; and combining the first subtracted signal and a modulator output feedback signal to generate a first combined signal. 
     In accordance with another embodiment of the present invention, a method of modulating phase in a timing recovery unit is disclosed. The method comprises the steps of generating a phase accumulator output; subtracting a quantization feedback signal from the phase accumulator output to produce a first subtracted signal; combining the first subtracted signal and a modulator output feedback signal to generate a first combined signal; and performing a wrap procedure of the phase accumulator output. 
     In accordance with another embodiment of the present invention, a timing recovery unit using sigma delta modulation is disclosed. The timing recovery unit comprises a sigma delta modulator for applying a digital input signal to a dither signal to generate a combined signal for sampling; an I transfer function and a Q transfer function coupled to the modulator for defining the phase interpolator control functions; a digital-to-analog converter for converting the combined analog signals to analog I and Q output values; and analog filters for filtering the analog signals to generate an output signal to control the phase interpolator. The timing recovery unit can also include a phase interpolator for receiving the output signal and output a clock with the recovered phase, wherein an output of the phase interpolator is fed back to a phase detector. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1A  illustrates a prior art timing recovery unit as part of a high-speed transceiver. 
         FIG. 1B  illustrates a prior art implementation of the timing recovery unit of  FIG. 1A . 
         FIG. 2  illustrates a block diagram of a PLL timing recovery unit, in accordance with the present invention. 
         FIG. 3  illustrates a block diagram of a sigma delta modulator, in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the preferred and alternative embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the specific embodiments, it will be understood that the invention is not limited to these embodiments. On the contrary, the invention covers alternatives, modifications and equivalents, which are included within the spirit and scope of the invention as defined by the appended claims as read in light of this specification. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth to provide a thorough understanding of the present invention. However, it should be noted that the present invention may be practiced without these specific details. In other instances, well known methods, procedures and components have not been described in detail as not to unnecessarily obscure aspects of the present invention. 
       FIG. 2  illustrates a block diagram of a phase-locked loop (PLL) timing recovery unit  100 , in accordance with one embodiment of the present invention. The timing recovery unit  100  comprises a phase detector  110 , a majority vote logic  120 , a digital loop filter  130 , a phase accumulator  140 , a sigma delta modulator  150 , I- and Q-conversion logic  160  and  161 , digital-analog converters  170  and  171 , analog reconstruction filters  180  and  181 , a phase interpolator  190 , and flip flops  195  and  196 . 
     The phase detector  110  is preferably a bang-bang phase detector. The phase detector  110  samples an input waveform at a zero crossing point and at a data point from a receiver  105 . Preferably, the receiver  105  and the phase detector  110  use the same circuits. Phase sampling is shifted by 90 degrees to sample the zero crossing of {1, −1} and {−1, 1}data transitions. Only phase samples of transitions are used. If a phase sample is before the zero crossing point, the phase detector  110  speeds up a recovered clock phase. If the phase sample is after the zero crossing point, the phase detector  110  slows down the lock clock signal. Input samples are processed to indicate the direction to move the loop phase. 
     The majority vote logic  120  receives a digital signal from the phase detector  110  and performs non-linear processing on the input data and decimates it. Eight “commands” are summed and limited to {1, 0, −1} representing an aggregate direction command. Decimation to 625 MHz from 2.5 Ghz permits synthesis of the digital portions of the PLL: the digital loop filter  130 , the phase accumulator  140 , the sigma delta modulator  150  and the I- and Q-conversion logic  160  and  161 . 
     The loop filter  130  is preferably a programmable first order digital filter. Design is simplified as the input is limited to (1, 0, −1) which represents loop phase directions. The loop filter  130  converts a digital word at low frequency to a high precision digital word that describes th required phase at the receiver input. An output of the loop filter  130  is coupled to an input of the phase accumulator  140 . 
     The phase accumulator  140  adds a second pole to ensure the PLL  100  tracks frequency and phase offset. An output of the phase accumulator  140  is preferably an 18-bit high precision digital word that determines a recovery phase of the phase interpolator  190 . The phase accumulator  140  is coupled to the sigma delta modulator  150 . 
     The sigma delta modulator  150  receives the high precision digital word and maps it into two transfer functions  160  and  161 , an I conversion logic  160  and a Q conversion logic  161 , and into at least one digital-analog converter (DAC)  170  and  171  where it is then filtered by at least one analog filter  180  and  181 . The combination of the sigma delta modulator  150 , the I and Q conversion logic  160  and  161 , the DACs  170  and  171  and the analog filters  180  and  181  turn the high precision digital word into a high precision analog signal. Phase interpolation is then performed by the phase interpolator  190  to output a clock recovery phase aligned with the incoming data. The phase interpolator  190  is essentially a CMOS four quadrant multiplier. The phase interpolator uses CMU 5 Ghz quadrature clocks to generate a differential 5 Ghz receiver clock. This is then digitally divided to generate two 2.5 Ghz quadrature clocks used by the receiver data and phase receivers. 
     An inventive feature of the present invention is adoption of a sigma delta modulator that applies dither to a high precision digital word to restore accuracy and granularity lost as a result of the DAC&#39;s  170  and  171 , which loses most of this precision to fit its available output levels. In one embodiment of the present invention, a dither signal is applied to the digital input signal before placing it in the DAC  170  and  171 . The dither signal, which is a frequency shaped signal, possesses a probability density function and is automatically added and spectrally shaped to provide minimum phase jitter. 
     A scalar value of the dither signal is shifted and applied to the digital input signal. The combined signal—the dither signal applied to the input signal—is then mapped and converted by the I and Q conversion logic  160  and  161  and the DACs  170  and  171  and filtered by the analog filters  180  and  181 . The analog filters  180  and  181  provide a cutoff at a bandwidth of interest which removes out of band quantization noise and signals. In this design, conversion logic will implement the I and Q conversion logic  160  and  161 . The analog filters  180  and  181  rebuild a waveform as close to the input analog signal as possible. The output signal of the analog filters  180  and  181  is preferably an interpolated value of the high precision digital word. 
     Each analog filter  180  and  181  is preferably a low pass filter. The DACs  170  and  171  can be one of a 1-bit DAC, a 2-bit DAC, a 3-bit DAC, a 4-bit DAC, a 5-bit DAC, and a 6-bit DAC. Preferably, the DACs  170  and  171  are 6-bit thermometer DACs. Thermometer DAC element values are preferably scaled to achieve a more sinusoidal transfer function from an output of the modulator  150  to inputs of the phase interpolator  190 . Though binary DACs with segmentation can be used, thermometer DACs ensure monotonicity. The phase interpolator  190  receives the output signal of the analog filters  180  and  181  and outputs a recovered clock phase. The clock recovery phase is fed back to the phase detector  110 . Thus, a precision filtered analog output controls the phase interpolator  190  to form the recovered clock. 
       FIG. 3  illustrates a block diagram of a sigma delta modulator  200 , in accordance with the present invention. The sigma delta modulator  200  includes a twos-complement accumulator  210  for generating a phase accumulator output; a quantization feedback signal for the phase accumulator output is subtracted in the twos-complement accumulator  210 ; a two-input multiplexer  220  for combining a subtracted signal and a modulator output feedback signal; an inverter  230  whose output generates the modulator output feedback signal; and a quantizer  240  whose output generates the quantization feedback signal. The sigma delta modulator  200  can be one of a first order, second order, and n-order modulator. The use of a sigma delta modulator and an analog phase interpolator to achieve extremely fine phase step resolution is an inventive feature of the present invention. A 6-bit phase step is hence resolved to ±60 fs; an increase in resolution of more than 50×. The six most significant output bits (MSBs) of the modulator  200  is used to control the DACs  170  and  171  ( FIG. 2 ). The modulator output feedback and the quantization feedback can be combined into a single feedback comprised of the least significant bits (LSBs) of the accumulator output. 
     The modulator  200  performs a wrap procedure of the phase accumulator output to prevent saturation or overload condition. The output of the phase accumulator  140  ( FIG. 2 ) is a twos complement digital representation of the output phase mapping the digital range of {1-2 −(N−1) , −1} to the phase range of {360/2 N , 360} degrees. 
     The “overload” condition is prevented by allowing the accumulator to overflow corresponding to a rotation of the phase by 2π. Modulation continues and the dither remains random and uniform across the phase “wrap”. When tracking a frequency deviation, the output “wraps” around the phase circle from quadrant IV to quadrant I. The modulator  200  emulates this behavior with appropriate added dither. This “wrap” discontinuity is mapped to a continuous function between all quadrants before analog filtering. In this case, it is mapped to orthogonal functions and other mappings to continuous functions and phase interpolators are possible. The mapping to continuous functions which are then explicitly filtered eliminates jumps at 2π. 
     The present invention has been described in terms of specific embodiments incorporating details to facilitate the understanding of the principles of construction and operation of the invention. As such, references herein to specific embodiments and details thereof are not intended to limit the scope of the claims appended hereto. It will be apparent to those skilled in the art that modifications can be made to the embodiments chosen for illustration without departing from the spirit and scope of the invention.