-timed and it avoids the need to send timing information via a separate channel. Timing and Synchronization Frequency Peak at 2.5 GHz Fig. 2.15 Spectrum of a 2.5-GHz signal. Frequency (GHz) For efficient clock extraction, coding is needed in the data signal to guarantee a clock component in the signal to be transmitted. The clock component must present frequency, phase, and amplitudes sufficiently consistent for recovery at the receiver. Therefore, this signal must be free of distortions and interference. In some cases, when a long transmission link must be designed, intermediate regenerators are used to recover the attenuated and out-of-phase signal. Clock extraction is executed at these stages to regenerate the signal and reach the destination in the transmission link. Clock extraction is also useful to decrease the amount of jitter in the signal, which is caused by the time fluctuations in the data patterns being transmitted. Jitter accumulation can be present when several repeaters are used. Therefore, correct clock extraction for data recovery will provide an improvement of the data received. All sequences of digital pulses present symmetry in their amplitude and phase when they are observed in the frequency domain. This symmetry is independent of the data sequence received. Figure 2.15 shows the spectrum of a 2.5-Gbit/s transmission link at the receiver point. Return-to-zero coding is used (different methods of codification have been discussed elsewhere). As observed from the spectrum, a clear peak at 2.5 GHz is produced from the binary data signal. This peak can be chosen by a clock-selecting filter, which will pass the clock at the data frequency and will reject as much as possible of the pattern-related spectrum and noise. Methods for extraction of this frequency component include Phase Lock Loop (PLL) circuits or a bandpass filter. When using a bandpass filter, the selection of the filter must be in agreement with the wanted quality, Q, of the circuit element. Q has two definitions. One definition is co