  We measure the power spectral density of frequency fluctuations in
nanocontact spin torque oscillators over time scales up to 50 ms. We use a
mixer to convert oscillator signals ranging from 10 GHz to 40 GHz into a band
near 70 MHz before digitizing the time domain waveform. We analyze the waveform
using both zero crossing time stamps and a sliding Fourier transform, discuss
the different limitations and advantages of these two methods, and combine them
to obtain a frequency noise spectrum spanning more than five decades of Fourier
frequency $f$. For devices having a free layer consisting of either a single
Ni$_{\text{}80}$Fe$_{\text{}20}$ layer or a Co/Ni multilayer we find a
frequency noise spectrum that is white at large $f$ and varies as \emph{$1/f$}
at small $f$. The crossover frequency ranges from $\approx\unit[10^{4}]{Hz}$ to
$\approx\unit[10^{6}]{Hz}$ and the $1/f$ component is stronger in the
multilayer devices. Through actual and simulated spectrum analyzer
measurements, we show that $1/f$ frequency noise causes both broadening and a
change in shape of the oscillator's spectral line as measurement time
increases. Our results indicate that the long term stability of spin torque
oscillators cannot be accurately predicted from models based on thermal (white)
noise sources.
