  We report on a study of microstructure and its quasi-periodicities of three
pulsars at 1.65 GHz with the S2 VLBI system at a resolution of 62.5 ns, by far
the highest for any such statistical study yet. For PSR B1929+10 we found in
the average cross-correlation function (CCF) broad microstructure with a
characteristic timescale of 95+-10 mcs and confirmed microstructure with
characteristic timescales between 100 and 450 mcs for PSRs B0950+08 and
B1133+16. On a finer scale PSRs B0950+08, B1133+16 (component II) and B1929+10
show narrow microstructure with a characteristic timescale in the CCFs of ~10
mcs, the shortest found in the average CCF or autocorrelation function (ACF)
for any pulsar, apart perhaps for the Crab pulsar. Histograms of microstructure
widths are skewed heavily toward shorter timescales but display a sharp cutoff.
The shortest micropulses have widths between 2 and 7 mcs. No nanopulses or
unresolved pulse spikes were detected. Cross-power spectra of single pulses
show a large range of complexity with single spectral features representing
classic quasi-periodicities and broad and overlapping features with essentially
no periodicities at all. Significant differences were found for the two
components of PSR B1133+16 in every aspect of our statistical analysis of
micropulses and their quasi-periodicities. Asymmetries in the magnetosphere and
the hollow cone of emission above the polar cap of the neutron star may be
responsible for these differences.
