U. Glaeser

10 −4 and 10 −2 . The steady-state jitter is examined in the DPLL output phase and the response of the
timing loop to a phase step in the data field. Figure 34.30 shows the transient phase response plots of
the SLT and MM DPLLs responding to a 0.1875T (12T/64) phase step in data field for the same LOW
p g and f g settings. Note that they have very similar responses. Table 34.2 shows the steady-state output
jitter of the two timing loops for various combinations of gains and noise levels corresponding to EERs
of 10 −4 and 10 −2 . The settled DPLL phases show some nonzero jitter without additive noise from quantization
effects. Timing jitter at the DPLL output is measured by measuring the standard deviation of
the DPLL phase. Again, observe that the two timing loops have very similar jitter numbers although the
MM timing loop jitter is slightly lower.
Finally, the Viterbi detector BER performance is examined instead of the timing loop jitter performance
for the read channel architecture of Fig. 34.31 employing the MM and SLT timing loops. Observe that
the BERs of the two systems are practically indistinguishable.
Conclusions
An overview of timing recovery methods for PRML magnetic recording channels, including interpolative
and traditional symbol rate VCO-based timing recovery methods, was provided. Also reviewed
was the MMSE timing recovery from first principles and its formulation in the framework of a SLTbased
timing gradient. A framework for analyzing the performance of the timing loops in terms of
output noise jitter was provided. The jitter calculation is based on obtaining linearized Z domain closed
loop transfer functions of the timing loop. Also compared was the output timing jitter, due to input noise,
© 2002 by CRC Press LLC
TABLE 34.2 Simulation-Based Timing Loop Output Jitter σ jt (Units of T/64)
Performance of SLT and MM Timing Loops for Final EERs of Zero (Noiseless),
10 −4 , and 10 −2
SLT MMPD
pg, fg GAINS EER 0 EER 10 −4
EER 10 −2
EER 0 EER 10 −4
EER 10 −2
LOW 0.49 1.30 2.18 0.45 1.16 1.86
MED 0.49 1.69 2.99 0.46 1.56 2.51
HGH 0.67 2.67 4.86 0.70 2.67 4.38
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FIGURE 34.30 SLT and MM DPLL reaction to 0.1875T (12T/64) phase step. Low p g, f g gains. No noise in this
simulation.