Limiting Values of TDEV of Top Branch of Model

 In this section expressions for the TDEV produced by the top branch of the noise model will be obtained. Examining the behaviour of (6.1)

$$\mathrm{TDEV}_f(\tau)\approx K_f\cdot B \cdot 10^{-3}\sqrt{\frac{\tau^2}{0.3^2+B^2\tau^2}}[ns]$$

it is evident that

$$\lim_{\tau\to\infty}\sqrt{\frac{\tau^2}{0.3^2+B^2\tau^2}}=\frac{1}{B}$$

and hence  

$$\lim_{\tau\to\infty}\mathrm{TDEV}_f(\tau)\sim K_f\cdot 10^{-3}.$$ (52)

Thus the top branch produces approximately flicker phase noise at large $\tau$ at a level of approximately $K_f\cdot 10^{-3}$. It can be seen from the form of (6.1) that  

$$\mathrm{TDEV}_f(\tau)\to 0 \quad \mathrm{as} \quad \tau\to 0.$$ (53)