Clock recovery is a process in
serial communication used to extract timing information from a stream of serial data being sent in order to accurately determine payload sequence without separate clock information. It is widely used in
data communication
Data communication, including data transmission and data reception, is the transfer of data, transmitted and received over a point-to-point or point-to-multipoint communication channel. Examples of such channels are copper wires, optic ...
s; the similar concept used in analog systems like
color television
Color television (American English) or colour television (British English) is a television transmission technology that also includes color information for the picture, so the video image can be displayed in color on the television set. It improv ...
is known as
carrier recovery.
Basic concept
In serial communication data is normally transmitted and received as a series of pulses with well-defined timing constraints. In
asynchronous serial communication
Asynchronous serial communication is a form of serial communication in which the communicating endpoints' interfaces are not continuously synchronized by a common clock signal. Synchronization ( clock recovery) is done by data-embedded signal ...
this presents a problem for the receiving side: if their own clock is not precisely synchronized with the transmitter, they may sample the signal at the wrong time and thereby decode the signal incorrectly. Partially, the problem may be addressed with extremely accurate and stable clocks, like
atomic clock
An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...
s, but these are expensive and complex. More common low-cost clock systems, like
quartz oscillators, are accurate enough for this task over short periods of time, but over a period of minutes or hours the
clock drift
Clock drift refers to several related phenomena where a clock does not run at exactly the same rate as a reference clock. That is, after some time the clock "drifts apart" or gradually desynchronizes from the other clock. All clocks are subject to ...
in these systems will make timing too inaccurate for most tasks.
Clock recovery addresses this problem by embedding clock information into the data stream, allowing the transmitter's clock timing to be determined. This normally takes the form of short signals inserted into the data that can be easily seen and then used in a
phase-locked loop
A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is fixed relative to the phase of an input signal. Keeping the input and output phase in lockstep also implies keeping the input and ou ...
or similar adjustable oscillator to produce a receiver's clock signal that can be used to time the signal in the periods between the clock signals. The advantage of this approach is that a small drift in the transmitter's clock can be compensated as the receiver will always match it, within limits, and therefore sample received data correctly.
The term is most often used to describe digital data transmission, in which case the entire signal is suitable for clock recovery. For instance, in the case of early 300 bit/s
modem
The Democratic Movement (, ; MoDem ) is a centre to centre-right political party in France, whose main ideological trends are liberalism and Christian democracy, and that is characterised by a strong pro-Europeanist stance. MoDem was establis ...
s, the timing of the signal was recovered from the transitions between the two frequencies used to represent binary 1 and 0. As some data might not have any transitions, a long string of zeros for instance, additional bits are added to the signal, the start and stop bits. These ensure that there are at least two transitions every of a second, enough to allow the receiver to accurately set its local oscillator.
The basic concept is also used in a wider variety of fields, including non-digital uses. For instance, the pioneering
Wireless Set Number 10 used clock recovery to properly sample the analog
pulse-width modulation
Pulse-width modulation (PWM), also known as pulse-duration modulation (PDM) or pulse-length modulation (PLM), is any method of representing a signal as a rectangular wave with a varying duty cycle (and for some methods also a varying peri ...
(PWM) voice signals it carried.
Another example of this concept is used in
color television
Color television (American English) or colour television (British English) is a television transmission technology that also includes color information for the picture, so the video image can be displayed in color on the television set. It improv ...
systems. Color information is carried at a very specific frequency that can drift from station to station. In order for receivers to accurately match the transmitter's own
carrier frequency
In telecommunications, a carrier wave, carrier signal, or just carrier, is a periodic waveform (usually sinusoidal) that conveys information through a process called ''modulation''. One or more of the wave's properties, such as amplitude or fre ...
, the transmitter sends a short burst of the signal in the unused space before the start of a scan line. This
colorburst signal is used to feed a local oscillator in the television, which then uses that local signal to decode the color information in the line. In these examples, the concept is known as
carrier recovery.
Details
Some digital data streams, especially high-speed serial data streams (such as the raw stream of data from the magnetic head of a
disk drive and serial communication networks such as
Ethernet
Ethernet ( ) is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...
) are sent without an accompanying
clock signal
In electronics and especially synchronous digital circuits, a clock signal (historically also known as ''logic beat'') is an electronic logic signal (voltage or current) which oscillates between a high and a low state at a constant frequency and ...
. The receiver generates a clock from an approximate frequency reference, and then phase-aligns the clock to the transitions in the data stream with a
phase-locked loop
A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is fixed relative to the phase of an input signal. Keeping the input and output phase in lockstep also implies keeping the input and ou ...
(PLL). This is one method of performing a process commonly known as ''clock and data recovery'' (CDR). Other methods include the use of a
delay-locked loop and
oversampling
In signal processing, oversampling is the process of sampling (signal processing), sampling a signal at a sampling frequency significantly higher than the Nyquist rate. Theoretically, a bandwidth-limited signal can be perfectly reconstructed if ...
of the data stream.
Oversampling can be done ''blind'' using multiple phases of a free-running clock to create multiple samples of the input and then selecting the best sample. Or, a counter can be used that is driven by a sampling clock running at some multiple of the data stream frequency, with the counter reset on every transition of the data stream and the data stream sampled at some predetermined count. These two types of oversampling are sometimes called ''spatial'' and ''time'' respectively. The best
bit error ratio
In digital transmission, the number of bit errors is the number of received bits of a data stream over a communication channel that have been altered due to noise, interference, distortion or bit synchronization errors.
The bit error rate ( ...
(BER) is obtained when the samples are taken as far away as possible from any data stream transitions. While most oversampling designs using a counter employ a sampling clock frequency that is an even multiple of the data stream, an odd multiple is better able to create a sampling point further from any data stream transitions and can do so at nearly half the frequency of a design using an even multiple. In oversampling type CDRs, the signal used to sample the data can be used as the recovered clock.
Clock recovery is very closely related to the problem of
carrier recovery, which is the process of re-creating a phase-locked version of the carrier when a
suppressed carrier modulation scheme is used. These problems were first addressed in a 1956 paper, which introduced a clock-recovery method now known as the
Costas loop.
Since then many additional methods have been developed.
In order for this scheme to work, a data stream must transition frequently enough to correct for any drift in the PLL's oscillator. The limit for how long a clock-recovery unit can operate without a transition is known as its maximum consecutive identical digits (CID) specification. To ensure frequent transitions, some sort of
self-clocking signal is used, often a
run length limited encoding;
8b/10b encoding is very common, while
Manchester encoding
In telecommunications and data storage, Manchester code (also known as phase encoding, or PE) is a line code in which the encoding of each data bit is either low then high, or high then low, for equal time. It is a self-clocking signal with no ...
serves the same purpose in old revisions of
802.3 local area network
A local area network (LAN) is a computer network that interconnects computers within a limited area such as a residence, campus, or building, and has its network equipment and interconnects locally managed. LANs facilitate the distribution of da ...
s.
See also
*
64B/66B encoding
*
8B/10B encoding
*
B8ZS encoding
*
Burst mode clock and data recovery
*
Eight-to-Fourteen Modulation
*
HDB3 encoding
*
Jitter
In electronics and telecommunications, jitter is the deviation from true periodicity of a presumably periodic signal, often in relation to a reference clock signal. In clock recovery applications it is called timing jitter. Jitter is a signifi ...
*
Line code
In telecommunications, a line code is a pattern of voltage, current, or photons used to represent digital data transmission (telecommunications), transmitted down a communication channel or written to a storage medium. This repertoire of signal ...
*
Manchester code
In telecommunications and data storage, Manchester code (also known as phase encoding, or PE) is a line code in which the encoding of each data bit is either low then high, or high then low, for equal time. It is a self-clocking signal with no ...
*
Phase-locked loop
A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is fixed relative to the phase of an input signal. Keeping the input and output phase in lockstep also implies keeping the input and ou ...
References
{{reflist
Clock signal
Electrical circuits
Line codes
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