  We have carried out a detailed study of single pulse emission from the pulsar
B2000+40 (J2002+4050), observed at 1.6 GHz frequencies using the Effelsberg
radio telescope. The pulsar has three components which are not well separated,
with the central component resembling core emission. We have investigated
modulations in single pulse behaviour using the fluctuation spectral analysis
which showed presence of two prominent periodicities, around 2.5$P$ and 40$P$,
respectively. The shorter periodicity was associated with the phenomenon of
subpulse drifting and was seen to be absent in central core component. Drifting
showed large phase variations in conal components. Additionally, the periodic
modulations had significant evolution with time, varying between very sharp and
highly diffuse features. In addition to drifting the pulsar also had presence
of nulling in the single pulse sequence. The longer periodic feature in the
fluctuation spectra was associated with nulling behaviour. The pulsar joins a
select group which shows the presence of phase modulated drifting as well as
periodic nulling in the presence of core emission. This provides further
evidence for the two phenomena to be distinct from each other with different
physical origin.
