Sampling Oscilloscopes

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 various places in the ship. The more specimens are taken, the more information is available about the quality of the cargo overall. It is evident that to be 100% sure about the condition of the cargo, all the sugar present in the ship would have to be checked; however, this is not possible.

The same is applicable in oscilloscopy. For several reasons, it is hardly possible to display directly an extremely high frequency (VHF) signal (say, above 1000 MHz) on the CRT of a normal real-time oscilloscope. However, by using sampling methods, 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 are taken at different places on the signal waveform, the more information can be obtained about the shape of the signal.

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. For each sample, the next signal waveform must be present. The signal waveform must be repetitive (not necessarily periodic), which means that single-shot phenomena can never be shown on a sampling oscilloscope. This 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. Only the input circuits, sampling gates, and trigger inputs must be able to handle VHF signals. After the samples are taken and saved, all further electronic circuits in the sampling oscilloscope can be relatively low-frequency circuits.

Although, it does not matter to the user in which way the signal is displayed on the CRT, as long as it can be studied on the screen. But, due to different setups, the sampling oscilloscope offers possibilities to the user which are hard to accomplish with normal, real-time oscilloscopes. Thus, the introduction of the sampling oscilloscope has led to new applications of oscilloscopy in general.

Currently, the two sampling techniques most commonly applied are random sampling and sequential sampling. In the random-sampling technique, no time relationship exists between the timing-ramp voltage (trigger-source functioning) and the sampling instant. Therefore, the picture on the screen is built up with samples which appear at places scattered intermittently over the waveform. In the sequential-sampling technique, which is the technique most frequently utilized, the successive samples appear on the screen at adjacent places over the waveform because a comparison circuit links the sampling instants to the timing ramp voltages when triggered by the input signal.