An external timebase calibrator is a useful accessory for any oscilloscope it provides precise, visible time markers on the scopes horizontal sweep. Basically the circuit is a pulse generator with accurate time intervals between pulses i.e. the pulse repetition frequency. If the pulse width is made relatively small compared to the repetition rate and the pulse edges are steep then the output signal will look like a series of illuminated dots. These can be conveniently used to measure time periods on the screen just as you use the graduation marks on a ruler to measure length.