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U. Glaeser

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strategies are reviewed, which have been popular historically and still being used in present day read<br />

channels. A quick review of the well-known least mean square (LMS) algorithm used for adaptive equalizers<br />

is also provided. Finally, the performance implications of selecting several different equalizer architectures<br />

is explored. This performance is measured in terms of bit error rate (BER) at the output of the read<br />

channel’s Viterbi detector.<br />

Equalization Architectures and Strategies<br />

In PRML channels the read back signal will be sampled at some point in the data path for further digital<br />

signal processing. A continuous time filter (CTF) with a low-pass characteristic will be present as an antialiasing<br />

filter [1] prior to the sampling operation so that high-frequency noise is not aliased into the<br />

signal band. This same CTF may also play a role in equalizing the read back signal to the target partial<br />

response. Various architectures can be used to perform the required equalization. The equalizer architecture<br />

can consist of a CTF, a finite impulse response filter (FIR), or both. The CTF parameters may be<br />

fixed, programmable, or adaptive. The FIR filter coefficients may be fixed, programmable, or adaptive.<br />

In addition, the FIR operation may occur in the sampled data analog domain or digital domain. Following<br />

equalization, the data are detected using a Viterbi detector. Of course, quantization by an analog-todigital<br />

converter (ADC) occurs at some point before the Viterbi detector.<br />

Figure 34.11 shows some examples of various equalizer architecture configurations. The first architecture<br />

(Type 1) consists of a CTF-only equalizer. The CTF is comprised of an all-pole low-pass filter section<br />

whose purpose is to reject high-frequency noise for anti-aliasing. One key parameter in the CTF is its<br />

low-pass bandwidth determined by its cutoff or corner frequency, fc. The type of CTF, fc, and its order<br />

(or the number of poles it contains) will determine its low-pass rolloff characteristic. If the CTF is expected<br />

to take part in equalization, it must also be able to provide some boost and does so by typically having<br />

one or two real zeros at some frequency fz in its transfer function. These parameters are noted in the figure.<br />

The second architecture (Type 2) is one where both the CTF and an analog FIR are involved in<br />

performing equalization. The third architecture (Type 3) is an analog FIR-only architecture in that the<br />

CTF design does not consist of any zeros, i.e., its main role is to perform anti-aliasing and not provide<br />

any boost for equalization. Finally, the last architecture (Type 4) is one where a CTF and FIR are both<br />

involved in equalization except that the FIR operation is done digitally.<br />

In general, there is a clear trade-off between the degree of flexibility of the equalizer and implementation<br />

complexity. The read-back signal characteristics change across the disk surface as manifested by somewhat<br />

FIGURE 34.11 Various equalizer architectures.<br />

© 2002 by CRC Press LLC<br />

(Type 1 ) CTF only equalizer<br />

CTF<br />

T<br />

ADC<br />

f<br />

c<br />

f<br />

z<br />

( Type 2 ) CTF + Analog FIR equalizer<br />

CTF<br />

f c<br />

f c<br />

f z<br />

CTF<br />

CTF<br />

f c<br />

f z<br />

T<br />

T<br />

T<br />

FIR ADC<br />

Viterbi<br />

Detector<br />

FIR ADC<br />

ADC FIR<br />

Viterbi<br />

Detector<br />

( Type 3 ) Analog FIR Only Equalizer (CTF has no boost but must perform anti-aliasing)<br />

( Type 4 ) CTF + Digital FIR equalizer<br />

Viterbi<br />

Detector<br />

Viterbi<br />

Detector

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