tion process are the main causes of jitter. Timing jitter can be caused by spectral components close to the desired clock frequency. The combination of wanted and unwanted spectral components produces a resultant output clock that exhibits both amplitude and phase modulation. From Figure 2.19(a) it can be seen that the spectrum of the wanted clock and the unwanted component are at different frequen- cies. Figure 2.19(b) is the phasor representation of these two components and their resultant, where the resultant is amplitude and phase modulated. The resulting waveform in the time domain is represented in Figure 2.19(c). Amplitude variations can be removed by a zero-crossing detector provided that the clock component is large enough. In general, an acceptable low-jitter performance can be Timing and Synchronization achieved if a proper combination of the following elements exists: careful transmission coding (to guarantee a tolerable clock component relative to the continuous spectrum) and received noise limiting and an appropriate choice of Q for the clock selecting filter. When timing variations are present at frequencies lower than 10 Hz, the jitter effect is known as wander, and it is caused by propagation delay and phase changes. According to the ANSI T1.403 standard, wander can be classified as long term and short term. Long-term wander includes variations within 24 hours due to temperature changes. Short-term wander refers to the variations produced within 15 minutes. Jitter Accumulation Overall growth of jitter will appear because of the jitter arising from the clock extraction and retiming process in a regenerator, which will be transmitted to subsequent regenerators where it might merge with any locally generated timing variations. Two different tools are used to assess the effect of this jitter accumulation in a self-time system: eye jitter and clock edge jitter (see Figure 2.18(b)). Eye jitter is a straight result of any timing variations already present on the received data signal. Clock edge jitter o