Abstract:
Method and apparatus for recovering a timing reference signal in an optical storage subsystem. The timing reference signal is recovered by passing it through a zonal bandpass filter coupled to a hard limiter, which feeds a phase locked loop that produces the reference signal.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention pertains to clock recovery in optical storage systems. In such systems, a laser is typically used to read and write data stored in a rotating medium. During these processes, light reflecting from the medium is used for other processes, such as tracking the optical head with respect to the medium, adjusting the rotational velocity of the medium, and recovering clock information. 
     Recovered clock information is used typically in a phase locked loop (PLL) which locks to a clock reference and generates a write data clock. One issue that arises with such a PLL is the inability to distinguish at high speed the differences between amplitude and phase differences. One approach to handling amplitude differences uses techniques such as automatic gain control (AGC) circuitry to normalize signal amplitude into the PLL. However, for applications such as DVD-RW and DVD-RAM, these AGC structures are too slow. This is especially true given the extreme changes in laser power and therefore in reflected light during the rapid and dramatic changes in laser power from the level needed to read marks up to the levels needed to write marks and back down again. 
     Another approach is a normalizer circuit which divides the various outputs of the optical detector by the sum of all the detector outputs. However, currently available normalizer designs are too slow to track the rapid changes between the read, write, and erase levels; the use of a normalized signal input to the PLL would introduce signal shifts resulting in phase errors. 
     To compensate for large signal level changes then, traditional AGC circuitry and normalizer designs are too slow or imprecise. What is needed is a way to stabilize the signal level to the PLL, adjusting for amplitude changes due to the rapid changes in laser power levels from read to write to erase. 
     SUMMARY OF THE INVENTION 
     A hard limiter in combination with a zonal bandpass filter is used to compensate for amplitude variations in the reference signal provided to the PLL in an optical data storage subsystem used to produce a timing reference signal. The zonal bandpass filter is placed at the input to the limiter to reduce or eliminate out of band noise that a hard limiter would alias into the passband of interest. The hard limiter then outputs a digital signal whose zero crossings accurately represent the zero crossings of the input reference signal. This signal is used as the reference to the PLL. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is described with respect to particular exemplary embodiments thereof and reference is made to the drawings in which: 
     FIG. 1 shows a PLL in an optical system, 
     FIG. 2 shows a diagram of a write pulse, 
     FIG. 3 shows a PLL using according to the present invention, and 
     FIG. 4 shows a diagram of the zonal band pass filter and limiter according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a digital PLL in an optical data storage system. Write controller  100  sends signals controlling the power level of laser  110 . Laser Radiation  120  is directed to storage media  130 , reflecting  140  to optical pickup  150 . The signal from optical pickup  150  is supplied through optional amplifier  160  to Phase Locked Loop subsystem  200 . The operation of Phase Locked Loops is understood in the art and is described for example in  The Art of Electronics  by Horowitz and Hill, 2nd Edition, Cambridge University Press, 1989, section 9.27, pp. 641-655. Digital phase detector  210  compares the input signal with the signal from VCO  230 , producing a phase error signal. This phase error signal is conditioned by loop filter  220  and used to control the input of voltage controlled oscillator (VCO)  230 . In phase locked loops using digital phase detectors it is common to run the VCO at a multiple of the input frequency. Digital phase detectors require input signals having close to a 50% duty cycle. By insuring that the frequency division process has as its final stage a divide by two, as shown with dividers  250  and  260 , a 50% duty cycle input to digital phase detector  210  is provided. 
     FIG. 2 shows the diagram of a write pulse, representing the different power levels generated by laser  110  of FIG.  1 . The lowest power level is read level  190  used in reading information from the storage medium, followed by erase level  180 , used to erase previously stored information, and the highest power, write level  170 , used for writing information. There is also a fourth state for the laser, off, not significant in the current invention. In operation, these different power levels of laser  110  result in rapid and dramatic changes in the level of reflected radiation  140  impinging upon optical pickup  150 . The voltage levels vary with detector configuration, but typical laser power levels for write, erase, and read are 10 milliwatts (mW), 5 mW, and 1 mW respectively. For a detector with linear response to incident light power the output voltage would swing over a 10 to 1 range. PLL  200  for best performance requires a fairly stable level of reference signal input to phase detector  210 . Automatic Gain Control (AGC) amplifiers and normalizers are too slow to deal with the rapid changes in power levels. 
     FIG. 3 shows in block diagram form the present invention. Optical pickup  150  supplies signal to zonal band pass filter  310 , which attenuates signals outside the frequency of interest. This prevents these frequencies from being aliased by the phase detector of PLL  200 , resulting in phase errors. Limiter  320  removes amplitude variations from the filtered signal, essentially retaining only zero crossings, sending this signal to PLL  200 . 
     FIG. 4 shows a diagram of zonal bandpass filter  310  and limiter  320  in greater detail. Buffer amplifier  410  may be required if the signal source is not capable of driving the zonal bandpass filter. Any high performance operational amplifier or buffer may be used, such as the CLC440 from National Semiconductor. For clarity, circuit details known to the art such as power supply connections, bypassing, offset trim and the like are not shown in FIG.  4 . While amplifier  410  is shown connected as a unity gain buffer, it may also be used to provide gain. 
     The zonal bandpass filter consists of the parallel resonant circuit made from inductor  450 , capacitor  440 , and Q setting resistor  430 . Capacitor  420  blocks any DC levels present at the output of buffer  410 . The zonal bandpass filter passes energy at the frequencies of interest by providing a high impedance at the resonant frequency of the LC combination  450  and  440 . The impedance of this LC network decreases away from the resonant frequency, attenuating signals out of the desired band. Resistor  430  sets the Q, or shape factor for the filter. In the preferred embodiment, the Q is approximately 3. 
     For a reference frequency of approximately 3 MHz, capacitor  420  is 100 nanofarads. Resistor  430  is 100 ohms. Capacitor  440  is 3.9 nanofarads and inductor  450  is 680 nanohenries. These component values may be scaled as is known in the art. 
     For a tracking filter needed for a CAV system the reference frequency shifts over a 2.5 to 1 range, and the resonant frequency of the zonal bandpass filter must change accordingly. This can be performed for example through the use of varactor diodes replacing or supplementing capacitor  440 . The bias on the diodes and therefore their capacitance are varied so that the resonant frequency tracks the data. 
     In the embodiment shown, a first order filter is used. Higher order filters may also be used, taking into consideration the variation in signal frequency that may occur as a result of the normal motor speed control process. The filter Q must be high enough to ring through missing pulses, but low enough so as to pass the range of frequencies expected due to motor speed variations. 
     The output of the zonal bandpass filter is sent to one input of comparator  460 . The other input of comparator  460  is provided by the simplified threshold setting network made of bypass capacitor  470 , 100 nanofarads in the preferred embodiment, and variable resistor  480 , 5K ohms. Comparator  460  may be any high performance comparator such as the AD96685 from Analog Devices. The AD96685 has a differential ECL output; additional circuitry may be needed to convert this differential output to a single ended output suitable for the phase detector of the PLL. 
     The foregoing detailed description of the present invention is provided for the purpose of illustration and is not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Accordingly the scope of the present invention is defined by the appended claims.