Digital storage oscilloscopes consist of a conventional analogue cathode ray oscilloscope
with the added facility that the measured analogue signal can be converted to
digital format and stored in computer memory within the instrument. This stored data
can then be reconverted to analogue form at the frequency necessary to refresh the
analogue display on the screen. This produces a non-fading display of the signal on
the screen.
The signal displayed by a digital oscilloscope consists of a sequence of individual
dots rather than a continuous line as displayed by an analogue oscilloscope. However,
as the density of dots increases, the display becomes closer and closer to a continuous
line, and the best instruments have displays that look very much like continuous traces.
The density of the dots is entirely dependent upon the sampling rate at which the
analogue signal is digitized and the rate at which the memory contents are read to
reconstruct the original signal. Inevitably, the speed of sampling etc. is a function of
cost, and the most expensive instruments give the best performance in terms of dot
density and the accuracy with which the analogue signal is recorded and represented.
Besides their ability to display the magnitude of voltage signals and other parameters
such as signal phase and frequency, some digital oscilloscopes can also compute signal
parameters such as peak values, mean values and r.m.s. values. They are also ideally
suited to capturing transient signals when set to single-sweep mode. This avoids the
problem of the very careful synchronization that is necessary to capture such signals
on an analogue oscilloscope. In addition, digital oscilloscopes often have facilities to
output analogue signals to devices like chart recorders and output digital signals in a
form that is compatible with standard interfaces like IEEE488 and RS232. Some now
even have floppy disk drives to extend their storage ability. Fuller details on digital
oscilloscopes can be found elsewhere Hickman, (1997).