Specifications

scope counterparts. Some offer isolated and floating inputs, which are advantages in industrial

environments because it means the scope can be used like a digital multimeter (i.e., you don't have to

worry that a probe's ground lead will e.g. short out a non-ground potential in a circuit).

For budget-minded hobbyists, there are numerous low-cost microcontroller-based projects that let you

construct your own oscilloscope.

Theory of operation

The architecture of a typical digital scope is as follows:

Figure 18

The input circuitry, attenuator, and parts of the trigger circuitry may be similar to an analog scope. The

fundamental difference from an analog scope is that the sections with the gray backgrounds are digital

in architecture and operation.

The key task of the digital scope is to digitize the waveform, which means to convert it to a sequence of

numbers. This is done by the A/D (analog to digital) converter that allows the digitized information to be

stored in digital memory, manipulated, displayed, and saved to a file. The A/D converter is

characterized by how many bits of resolution it has. A common resolution is 8 bits, giving 256 different

voltage levels that the scope can measure. Some scopes have higher resolutions.

This digitizing is characterized by the sampling rate, S, in samples per second. An important

relationship between the sampling rate and the measured signal is given by the following sampling

theorem:

If a function x(t) contains only frequencies less than B hertz, it is completely determined by

giving its ordinates at a series of points spaced 1/(2B) seconds apart.

In the real world of measurements, it is rare that the signal being input to the oscilloscope is band-

limited per the sampling theorem's assumption -- you usually don't know much about the signal's

frequency content. Because of this, you have to be aware that frequencies in the signal that are higher

than the sampling frequency can cause aliasing, something we'll look at in more detail below.

You can see the sampling frequency given as 250 MSa/s (megasamples per second) in Figure 17

above. A careful digital oscilloscope user is always conscious of what the sampling frequency is and its

impact on the signals he or she is examining.

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