What is Sampling?
Sampling is the taking of a specimen, or a part, to illustrate the whole. For example, when a ship’s cargo of sugar must be checked for the amount (%) of water in the sugar, specimens of the sugar are taken from different places in the ship. The more specimens taken, the more information is available about the quality of the cargo overall. To be 100% sure about the condition of the cargo, all the sugar present in the ship would have to be checked; for obvious reasons, this is not possible.
The same is applicable in oscilloscopy. For several reasons, it is hardly possible to display directly a very high frequency (VHF) signal (say, above 1000 MHz) on the CRT of a normal real-time oscilloscope. However, by means of sampling techniques, it is possible to display frequencies of over 10,000 MHz. As with the specimens of the ship’s cargo, samples of the VHF signal waveform can be taken and examined on the CRT. The more samples taken at different places on the signal waveform, the more information can be obtained about the shape of the signal.
In the case of taking samples of the ship’s sugar cargo, it goes without saying that the cargo is at hand every time a sample must be taken. The same applies to the sampling oscilloscope too; the signal must be present every time a sample is taken.
This is where the analogy with the ship’s cargo ends because with the sampling oscilloscope the samples can only be taken sequentially in time. This means that for every sample the next signal waveform must be present. In other words, the signal waveform must be repetitive (not necessarily periodic), and this again means that single-shot phenomena can never be displayed on a sampling oscilloscope, which is a drawback.
Often the sampling rate is low with respect to the rate of the signal waveform so that a sample is taken only every now and then on a waveform of the signal. For example, a sample may be taken of the 1st, 101st, 201st, and 301st waveforms. As a matter of fact, only the input circuits, sampling gates, and trigger inputs must be able to handle VHF signals. Once the samples are taken and stored, all further electronic circuits in the sampling oscilloscope can be, and often are, relatively low-frequency circuits.
However, it obviously does not matter to the user in which way the signal is displayed on the CRT, provided it can be studied on the screen. However, due to different setups, the sampling oscilloscope offers possibilities to the user which are hard to achieve with normal, real-time oscilloscopes. Consequently, the introduction of the sampling oscilloscope has led to new applications of oscilloscopy in general. Before discussing new applications, it is first necessary to know and to understand the setup of sampling systems. This will make the user more conversant with the oscilloscope and give him or her a better understanding of the results on the screen.