Once this timing error has been determined this can be feedback to the analog domain instructing
the oscillator to either speed up or slow down in order for the samples to have proper sampling. If a
timing offset exists then the receiver may want to use this for the present samples waiting inside the
memory. This would be accomplished through a feed forward path using an interpolator. Whether the
timing correction is feed back or feed forward, an LPF was inserted to reduce the effects of noise and
jitter.
In order to provide more details about the optimum sampling point we show an eye diagram in
Fig. 6.24 and highlight the possible sampling instances. This diagram assumes an overall raised
cosine response with a roll-off factor, 0.22. The optimal sampling point is indeed the center of
the eye diagram. This sampling time instance is optimal since there is zero ISI at this instance.
Next the roll-off factor was increased to 0.9 to use as an example. The overall eye diagram
can vary from one system to another and depending on the roll-off factor of the raised cosine filter,
timing requirements may differ (see Fig. 6.25).
In the subsections that follow we will briefly present some timing error estimators that can be used
in a variety of places.
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