The above block diagram produces a CQE that is inversely proportional to the received eye opening.
Consider the received signal to have much noise, interference, and ISI then the eye diagram that
would be considered to have a small opening, if not closed. This would create a large error signal
which is the difference between the decision device output and its input. In turn, this will create an
even larger error power signal. On the other hand, if the received eye opening is large, then the difference
between this signal and the output of the decision device is small, and so will be its power.
Hence we have a CQE value that is inversely proportional to the eye opening.
In these subsections we have presented a few CQE techniques that can be used to generally provide
a qualitative and quantitative measurement of the channel.
6.6 AUTOMATIC FREQUENCY CONTROL
When performing any spectral conversion operation, frequency offsets exist due to component tolerances,
temperature changes, estimation error, device aging, and so on. This frequency offset is defined
as the offset in frequency between the carrier frequency of the received signal and the locally generated
frequency. Let us recall the following functionality of the quadrature demodulator shown in Fig.
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