 synchronized signal. When the PLL loop filter (LPF) blocks the signal from the input reference, the output signal is constructed using the local oscillator. So the process of a PLL filtering the noise from the input signal is substituting noise from the local oscillator. Taken to a theoretical corner case, if the clock bandwidth of an LPF is made zero, all that the PLL will output is the signal from the local oscillator-which defeats the pur- pose of having a reference signal. So, if using a very low cut-off frequency for the LPF, the PLL needs to be matched to a good- quality local oscillator, SO that the noise added by the local oscillator is reduced. For the hard- ware designer, this has obvious cost ramifications-to get better noise filtering, you need to spend more money on the oscillator. The time taken for a PLL to move from transient state to the steady (or locked) state depends on the clock bandwidth (as well as the input frequency and quality of the local oscillator). The nar- rower the clock bandwidth for the LPF, the longer time it takes for the PLL to move to the steady state. For example, a synchronization supply unit (SSU) type I clock or building integrated timing supply (BITS) stratum 2 clock with an LPF configured for bandwidth of 3 mHz will take sev- eral minutes to lock to an input signal. However, telecom networks are very widely distributed and can consist of long chains of clock nodes. If all the clocks in the chain had low bandwidth, the complete chain could take many hours to settle to a steady state. Similarly, it might take sev- eral hours for the last node of the chain to settle down after any disruption to the distribution of clock. It is for these reasons that a clock node with better filtering capabilities should have a good-quality oscillator and should be placed in a chain of clock nodes at selected locations. These factors also explain why SSU/BITS clock nodes (which have stratum 2-quality oscillators and better PLL cap- abilities) are recommended only after a certain number of 