CLOCK EXTRACTION
In a digital
communication system, the output of the demodulator must be sampled
periodically at the bit rate, at the precise sampling time instants tm
= mTb + τ , where Tb is the bit interval and τ is a time delay that accounts for the
propagation time of the signal from the transmitter to the receiver. To perform
this periodic sampling, we require a clock signal at the receiver. The process
of extracting such a clock signal at the receiver is usually called symbol
synchronization or timing recovery.
Timing
recovery is one of the most critical functions that is performed at the
receiver of a synchronous digital communication system. We should note that the
receiver must know not only the frequency (1 /Tb), but also
where to take the samples within each bit interval. Timing recovery can be
accomplished in one of several ways.
Methods Of Timing Recovery
Derivation From A primary or A secondary Standard
The
transmitter and receiver clocks are synchronized to a master clock, which
provides a very precise timing signal. In this case, the receiver must estimate
and compensate for the relative time delay between the transmitted and received
signal. Such may be the case for radio communication systems that operate in
the very low frequency (VLF) band (below 30 kHz), where precise clock signals
are transmitted from a master radio station.
Transmitting A separate Synchronizing Signal (Pilot Clock)
The
transmitter simultaneously transmit The clock frequency 1/Tb
or a multiple of 1/Tb along with the information signal. The
receiver may simply employ a narrowband filter tuned to the transmitted clock
frequency and, thus, extract the clock signal for sampling. This approach has
the advantage of being simple to implement. The disadvantage, is that a small
fraction of the channel bandwidth must be allocated for the transmission of the
clock signal. This method is used in telephone transmission systems that employ
large bandwidths to transmit the signals of many users. In such a case, the
transmission of a clock signal is shared in the demodulation of the signals
among the many users.
Self Synchronization
A clock signal can also be extracted from the
received data signal. 
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