Three sources of scintillation of radio sources:
ionised interstellar medium
Studying interplanetary scintillation at Cambridge
Hewish and Bell  observed the recordings
shown in Figure 2, period = 1.337 seconds.
from a spacecraft?
Figure 2: Discovery observations of the first pulsar. (a) The first recording of PSR 1919+21; the signal resembled the radio interference also seen on this chart. (b) Fast chart recording showing individual pulses as downward deflections of the trace. From Lyne & Graham-Smith 1990 .