ADAPTIVE PHASE LOCK LOOP

An adaptive phase lock loop for jitter tracking. A filter in the adaptive PLL has coefficients that are automatically adjusted to minimize the error signal. The first block of the adaptive PLL is a phase shifter (13) that is used to eliminate the frequency offset and phase offset in the input signal (10). The output of the phase shifter (13) is received by a slicer (14) that makes an estimate of what the corrected signal should be. Then the difference between the input and output of the slicer (14) is multiplied (17) by the output of the slicer (14). The imaginary part (18) of the result is used as an estimate of the phase error. After passing through as an amplitude limiter (19) the signal is provided to a finite impulse response (FIR) filter (20) having adaptive coefficients. The FIR coefficients are adjusted to minimize the mean squared phase error using a gradient method. These coefficients are updated each baud. A second order loop is added to track any frequency offset present in the line signal. The signal is fed through a sin/cos table (27) back to the phase shifter (13).