clock errors are measured over a sufficiently long time period to thoroughly understand the quality of a clock. To illustrate, most quartz watches gain/lose some time (seconds) daily. For such a device, a measurement done every few minutes or every few hours might not be a good indication of the quality. For such watches, a good time period might be one month, over which it could wander by up to (say) 10 seconds. For illustration purposes, Figure 5-13 shows only four clock periods or cycle times, Tc of a clock. These clock periods are shown as (n-1), (n), and (n + 1). Notice that the measured clock is not perfectly aligned to the reference clock, and that difference is called clock error or time error (TE). The error is marked as X(n-1) showing the error for instance (n-1) of the clock period, x (n) for period (n), and SO on. In the figure, the TEs at each successive instance of the clock period are marked as x(n-1), X(n), and x(n+1)- The TE is simply the time of the measured clock minus the time of the reference clock. In Figure 5-13, the measured clock signal at instance (n+1) and (n) lags (arrives later than) the reference clock signal and is therefore by convention a negative time error. And for the interval (n+1), the clock signal is leading the reference and is referred to as a positive time error. Of course, the time error measured at varying time periods varies and can be either negative or positive. TE measurements in the time domain are normally specified in seconds, or some fraction of, such as nanoseconds or microseconds. As mentioned previously, there could be positive or negative errors, and SO it is imperative to indicate this via a positive or negative sign. For example, a time error for a certain interval might be written as +40 ns, whereas another interval might be -10 ns. An engineer can measure the TE value (against the reference) at every edge (or significant instant) of the clock over an extended observation interval. If these values are plotted in a graph against time on the horizontal axis