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Clock signal and legend. In electronics and especially synchronous digital circuits, a clock signal (historically also known as logic beat) [1] is an electronic logic signal (voltage or current) which oscillates between a high and a low state at a constant frequency and is used like a metronome to synchronize actions of digital circuits.
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 output frequencies the same, thus a phase-locked loop can also track an input frequency.
In signal processing, phase noise is the frequency-domain representation of random fluctuations in the phase of a waveform, corresponding to time-domain deviations from perfect periodicity . Generally speaking, radio-frequency engineers speak of the phase noise of an oscillator, whereas digital-system engineers work with the jitter of a clock.
The dynamic logic circuit requires two phases. The first phase, when Clock is low, is called the setup phase or the precharge phase, and the second phase, when Clock is high, is called the evaluation phase. In the setup phase, the output is driven high unconditionally (no matter the values of the inputs A and B).
A timing diagram can contain many rows, usually one of them being the clock. It is a tool commonly used in digital electronics, hardware debugging, and digital communications. Besides providing an overall description of the timing relationships, the digital timing diagram can help find and diagnose digital logic hazards.
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. [3]
Different clock domains have clocks which have a different frequency, a different phase (due to either differing clock latency or a different clock source), or both. [3] Either way the relationship between the clock edges in the two domains cannot be relied upon.
Reference clock jitter translates directly to the output, but this jitter is a smaller percentage of the output period (by the ratio above). Since the maximum output frequency is limited to /, the output phase noise at close-in offsets is always at least 6 dB below the reference clock phase noise. [6]