  We measure oscillator phase from the zero crossings of the voltage vs. time
waveform of a spin torque nanocontact oscillating in a vortex mode. The power
spectrum of the phase noise varies with Fourier frequency $f$ as $1/f^2$,
consistent with frequency fluctuations driven by a thermal source. The
linewidth implied by phase noise alone is about 70 % of that measured using a
spectrum analyzer. A phase-locked loop reduces the phase noise for frequencies
within its 3 MHz bandwidth.
