Rationale Behind TDEV

To overcome the difficulties described in section 4.1, TDEV is utilised as a clock performance measure. To summarise the previous section, a clock performance measure is required which:

• converges when presented with noise sources present in phaselock loops
• has minimum dependence on the observation time
• it would also be useful if the measure can discriminate between the various different types of noise sources so that dominant noise sources may be ascertained

The general principle of TDEV arises from a statistical measure known as the Allan Variance proposed by an IEEE committee on frequency stability. Unlike the Allan Variance, TDEV has the important property in that it is able to discriminate between white phase modulation and flicker phase modulation, two of the dominant noise types in telecommunications networks. In the following section, by adapting the Allan Variance arguments presented in [23] and part IV of [18] the motivation behind TDEV will be explained.