# White gaussian noise analysis deduction

I´m stuck in a deduction analysis of the variance of a gaussian white noise signal in a "integrate-and-dump detector" of a baseband data transmission receiver, where $$n(t)$$ is white noise with double-sided power spectral density $$N_0/2$$ [W/Hz] I can understand all the steps except when they deduce How do you get to this last expression? Thank you.

They say that $$n(t)$$ is white noise with a double-sided power spectral density (PSD) of $$N_0/2$$, i.e., the PSD is given by
$$S_n(f)=\frac{N_0}{2}\tag{1}$$
The auto-correlation function is the inverse Fourier transform of $$(1)$$, which is
$$R_n(\tau)=E\big\{n(t)n(t+\tau)\big\}=\mathcal{F}^{-1}\big\{S_n(f)\big\}=\frac{N_0}{2}\delta(\tau)\tag{2}$$
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