B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

364 PRINCIPLES OF DIGITAL DATA TRANSMISSION
Fi g ure 7.23
Timing
extraction.
a very efficient method of timing extraction or clock recovery because the timing is derived
from the received message signal itself. An example of the self-synchronization method will
be discussed here.
We have already shown that a digital signal, such as an on-off signal (Fig. 7 .3a), contains
a discrete component of the clock frequency itself (Fig. 7.3c). Hence, when the on-off binary
signal is applied to a resonant circuit tuned to the clock frequency, the output signal is the
desired clock signal.
Not all the binary signals contain a discrete component of the clock frequency. For example,
a bipolar signal has no discrete component of any frequency [see Eq. (7.21) or Fig. 7.9]. In such
cases, it may be possible to extract timing by using a nonlinear device to generate a frequency
tone that is related to the timing clock. In the bipolar case, for instance, a simple rectification
converts a bipolar signal to an on-off signal, which can readily be used to extract timing.
Small random deviations of the incoming pulses from their ideal location (known as timing
jitter) are always present, even in the most sophisticated systems. Although the source emits
pulses at the right instants, subsequent operations during transmission (e.g., Doppler shift)
tend to cause pulses to deviate from these original positions. The Q of the tuned circuit used
for timing extraction must be large enough to provide an adequate suppression of timing jitter,
yet small enough to meet the stability requirements. During the intervals in which there are
no pulses in the input, the oscillation continues because of the flywheel effect of the high-Q
circuit. But still the oscillator output is sensitive to the pulse pattern; for example, during a
long string of ls the output amplitude will increase, whereas during a long string of Os it will
decrease. This introduces additional jitter in the timing signal extracted.
The complete timing extractor and time pulse generator for a polar case in shown in
Fig. 7.23. The sinusoidal output of the oscillator (timing extractor) is passed through a phase
shifter that adjusts the phase of the timing signal so that the timing pulses occur at the maximum
points. This method is used to recover the clock at each of the regenerators in a PCM system. The
jitter introduced by successive regenerators adds up, and after a certain number of regenerators
it is necessary to use a regenerator with a more sophisticated clock recovery system such as a
phase-locked loop.
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