Regular oscilloscope versus digital oscilloscope
Q. I have seen advertisements for digital oscilloscopes. What is the difference between a regular oscilloscope and a digital one?
A. In a conventional oscilloscope, the signal to be viewed is simply amplified and applied to the deflection plates in the CRT. A digital scope has a very fast electronic switch that is connected in series with the input. The switch is turned on for very short periods of time to “sample” the input signal. While the switch is on, a very fast analog-to-digital converter takes the sample and produces a digital signal.
The digital signal is a binary number whose value corresponds to the voltage of the sample. Successive samples are taken, so that the waveform of the signal is converted into a stream of binary numbers. Digital circuits in the CRT scanning electronics are used to light up individual dots on the screen called “pixels” Each pixel corresponds to a sample of the waveform. The value of the binary number for each sample determines the height of the pixel that represents it. The pixels are displayed in succession across the screen of the CRT. Together, the pixels forrn a line which represents the waveform of the signal.
There are also oscilloscopes which have electronic switches that break the signal up into analog samples that are instead simply amplified and applied to the deflection plates in a conventional scope CRT. These are called “sampling” scopes. They are used for high frequency signals.