hysical frequency im- proves the ability to maintain phase alignment. Just look at some rough calculations. An oscillator in holdover might be accurate to (say) 10 ppb (at least for a short while). After 1 S, this oscillator could be 10 ns off from the nominal frequency. And after the end of 2 S, the error could be another 10 ns off from the error at the end of 1 S. Every second of error builds upon the previous second. Yes, it could average out (one up and one down) to some extent, but the point is that the error is additive on top of the previous frequency error-at least to the limits of the free-running performance. But consider another oscillator that is also accurate (as an example) to 10 ppb but is locked to a SyncE reference. That means that after 1 second, this oscillator might be up to 10 ns off. But after 2 S, the oscillator will be still within 10 ns from the starting point (the required nominal frequency). After 1000 S, it will still be within 10 ppb of the (in this example) 20 MHz. In this case, the error is an absolute error compared to the requirement; in the other case, it is relative to the previous measurement. In both cases, the frequency error induces some phase error, but the phase error is the frequency er- ror integrated over time. You don't need to be mathematically inclined to understand that the longer the time interval the frequency is off from the nominal, the more the phase error accumulates. The holdover case without a frequency reference shows that there is nothing trying to return the fre- quency offset to the mean and SO the phase error rapidly piles up. Figure 9-34 illustrates the difference between the two cases and shows frequency error (the curved line) versus phase error (the shaded area under the frequency error curve). With SyncE Reference Without SyncE Reference Freq. Freq. error error + 10 ppb Phase error + 10 ppb + 10 ppb + 10 ppb Phase error Ideal freq. Ideal freq. -10 ppb Time -> Figure 9-34 Frequency and Phase Error With and Without a SyncE Signal Traceable to a Prima