LeCroy DDA 7 Zi series, Руководство оператора

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Operator's Manual
WP700Zi-OM-E-RevA
494
The DDA does this work automatically: you set the desired -3 dB frequency, and the DDA calculates fc for the
current Boost and Group delay.
The frequency cutoff accuracy (alpha = 0) is < 2% of the setting.
Boost at fc
The pulse shape produced by a transition going under a head is nearly Lorentzian, but that is not the correct
shape for PRML. Boosting the gain at high frequencies makes the pulses slimmer and eliminates the long tails
they would otherwise have. The perfect (mathematically created) shape that an isolated pulse should have is
illustrated below, for PR4, EPR4 and E2PR4. Note that the vertical lines show when the channel samples the
waveform.
The boost
accuracy is
within 0.5 dB of the setting.
Group Delay
The delay through the filter of the lowest frequencies can be adjusted. The normal setting is 0.0% adjustment; that
is, flat response. This is used to compensate for group delay distortion before the filter. Group delay setting is
unaffected by
Train Filter .
The digital implementation of our equalizing filter does not have perfectly flat group delay. The non-flatness
increases with the ratio of -3 dB frequency to sampling rate and with boost. The DDA requires 5 samples per bit
cell, which means that the -3 dB frequency will be </= 10% of sampling rate. If this requirement is met, group
delay should be sufficiently flat. E2PR4 is especially sensitive to non-flat group delay. For E2PR4, especially if the
"5 samples per bit cell" requirement is not met, it will probably be beneficial to set the "Group Delay" field to a
small positive number, perhaps 6% or so. This helps flatten the group delay of our equalizer filter.