  The power spectrum of magnetic-field fluctuations in the fast solar wind
($V_{\rm SW}> 500 \mbox{ km} \mbox{ s}^{-1}$) at magnetohydrodynamic (MHD)
scales is characterized by two different power laws on either side of a break
frequency $f_{\rm b}$. The low-frequency range at frequencies $f$ smaller than
$f_{\rm b}$ is often viewed as the energy reservoir that feeds the turbulent
cascade at $f>f_{\rm b}$. At heliocentric distances $r$ exceeding $60$ solar
radii ($R_{\rm s}$), the power spectrum often has a $1/f$ scaling at $f<f_{\rm
b}$; i.e., the spectral index is close to $-1$. In this study, measurements
from the encounter $10$ of ${Parker Solar Probe}$ (PSP) with the Sun are used
to investigate the evolution of the magnetic-field power spectrum at $f< f_{\rm
b}$ at $r<60 R_{\rm s}$ during a fast radial scan of a single fast-solar-wind
stream. We find that the spectral index in the low-frequency part of the
spectrum decreases from approximately $-0.61$ to $-0.94$ as $r$ increases from
$17.4 $ to $45.7$ solar radii. Our results suggest that the $1/f $ spectrum
that is often seen at large $r$ in the fast solar wind is not produced at the
Sun, but instead develops dynamically as the wind expands outward from the
corona into the interplanetary medium.
