  Low-frequency gravitational-wave experiments require the highest timing
precision from an array of the most stable millisecond pulsars. Several known
sources of noise on short timescales in single radio-pulsar observations are
well described by a simple model of three components: template-fitting from a
finite signal-to-noise ratio, pulse phase/amplitude jitter from single-pulse
stochasticity, and scintillation errors from short-timescale interstellar
scattering variations. Currently template-fitting errors dominate, but as radio
telescopes push towards higher signal-to-noise ratios, jitter becomes the next
dominant term for most millisecond pulsars. Understanding the statistics of
jitter becomes crucial for properly characterizing arrival-time uncertainties.
We characterize the radio-frequency dependence of jitter using data on 48
pulsars in the North American Nanohertz Observatory for Gravitational Waves
(NANOGrav) timing program. We detect significant jitter in 43 of the pulsars
and test several functional forms for its frequency dependence; we find
significant frequency dependence for 30 pulsars. We find moderate correlations
of rms jitter with pulse width (R = 0.62) and number of profile components (R =
0.40); the single-pulse rms jitter is typically ~1% of pulse phase. The average
frequency dependence for all pulsars using a power-law model has index -0.42.
We investigate the jitter variations for the interpulse of PSR~B1937+21 and
find no significant deviations from the main pulse rms jitter. We also test the
time-variation of jitter in two pulsars and find that systematics likely bias
the results for high-precision pulsars. Pulsar timing array analyses must
properly model jitter as a significant component of the noise within the
detector.
