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U. Glaeser

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FIGURE 34.19 CTF BER degradation with respect to oversampled ideal system vs. number of taps (cbd = 2.4).<br />

FIGURE 34.20 Percentage of bad CTF settings vs. number of taps (cbd = 2.4).<br />

performance are shown in Fig. 34.21. The vertical axis is again the degradation with respect to the ideal<br />

system BER of 10 −5 . The programmable tap of the FIR is optimized to yield the best performance in each<br />

case. The main tap is placed roughly in the center. There is benefit in increasing the number of taps from<br />

4 to 6 to 10. Beyond 10 taps, however, there is more latency in the timing loop as the main tap position<br />

is more delayed. This causes increased phase errors to enter the timing loop and outweighs the benefit<br />

of enhanced equalization obtained with more taps. Although one could increase the number of taps<br />

while keeping the main tap location mostly fixed, the FIR will then not be able to cancel the precursor<br />

ISI as well with a CTF, which is not involved in equalization. Also shown (dashed plot) is the performance<br />

of a Type 2 equalizer (CTF, with its corner frequency optimized and with an optimized zero included to<br />

provide boost). Clearly the Type 2 equalizer outperforms the Type 3 equalizer.<br />

Actual Equalizer Architectures<br />

Various equalization architectures and examined their performance have been considered. Let us now<br />

examine what actual architectures read channel vendors are using. Table 34.1 summarizes some of the<br />

most commonly used architectures. For example, Agere Systems [Note: storage products unit of AT&T was<br />

© 2002 by CRC Press LLC<br />

OPTIMUM BER LOSS vs ID (db)<br />

% OF CONVERGED SETTNGS WITH BER > 4e-5<br />

0.8<br />

0.7<br />

0.6<br />

0.5<br />

0.4<br />

0.3<br />

0.2<br />

0.1<br />

BER LOSS OF OPTIMUM BER RELATIVE TO IDEAL SYSTEM<br />

ideal+qnt<br />

4th order Butterworth<br />

6th order Butterworth<br />

7th ord. lin. phs.<br />

7th ord. lin. phs., symm. zeros<br />

4th ord. mixed<br />

0<br />

2 3 4 5 6 7 8 9 10 11 12<br />

NUMBER OF FIR TAPS<br />

100<br />

90<br />

80<br />

70<br />

60<br />

50<br />

40<br />

30<br />

PERCENTAGE OF BAD CTF SETTINGS vs. NUMBER OF TAPS<br />

20<br />

10<br />

4th order Butterworth<br />

6th order Butterworth<br />

7th ord. lin. phs.<br />

7th ord. lin. phs., symm. zeros<br />

4th ord. mixed<br />

0<br />

2 3 4 5 6 7 8 9 10 11<br />

NUMBER OF FIR TAPS

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